Internet DRAFT - draft-dkg-openpgp-stateless-cli
draft-dkg-openpgp-stateless-cli
openpgp D. K. Gillmor
Internet-Draft ACLU
Intended status: Informational 1 March 2024
Expires: 2 September 2024
Stateless OpenPGP Command Line Interface
draft-dkg-openpgp-stateless-cli-10
Abstract
This document defines a generic stateless command-line interface for
dealing with OpenPGP messages, known as sop. It aims for a minimal,
well-structured API covering OpenPGP object security.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://dkg.gitlab.io/openpgp-stateless-cli/. Status information for
this document may be found at https://datatracker.ietf.org/doc/draft-
dkg-openpgp-stateless-cli/.
Discussion of this document takes place on the OpenPGP Working Group
mailing list (mailto:openpgp@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/openpgp/. Subscribe at
https://www.ietf.org/mailman/listinfo/openpgp/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/dkg/openpgp-stateless-cli/.
Status of This Memo
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This Internet-Draft will expire on 2 September 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Using sop in a Test Suite . . . . . . . . . . . . . . . . 5
1.4. Semantics vs. Wire Format . . . . . . . . . . . . . . . . 6
2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. sopv Subset . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. sopv Versioning . . . . . . . . . . . . . . . . . . . . . 7
4. Subcommands . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Meta Subcommands . . . . . . . . . . . . . . . . . . . . 8
4.1.1. version: Version Information . . . . . . . . . . . . 8
4.1.2. list-profiles: Describe Available Profiles . . . . . 10
4.2. Key and Certificate Management Subcommands . . . . . . . 10
4.2.1. generate-key: Generate a Secret Key . . . . . . . . . 10
4.2.2. change-key-password: Update a Key's Password . . . . 11
4.2.3. revoke-key: Create a Revocation Certificate . . . . . 12
4.2.4. extract-cert: Extract a Certificate from a Secret
Key . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. Messaging Subcommands . . . . . . . . . . . . . . . . . . 13
4.3.1. sign: Create Detached Signatures . . . . . . . . . . 13
4.3.2. verify: Verify Detached Signatures . . . . . . . . . 14
4.3.3. encrypt: Encrypt a Message . . . . . . . . . . . . . 15
4.3.4. decrypt: Decrypt a Message . . . . . . . . . . . . . 18
4.3.5. inline-detach: Split Signatures from an Inline-Signed
Message . . . . . . . . . . . . . . . . . . . . . . . 20
4.3.6. inline-verify: Verify an Inline-Signed Message . . . 22
4.3.7. inline-sign: Create an Inline-Signed Message . . . . 23
4.4. Transport Subcommands . . . . . . . . . . . . . . . . . . 24
4.4.1. armor: Convert Binary to ASCII . . . . . . . . . . . 24
4.4.2. dearmor: Convert ASCII to Binary . . . . . . . . . . 26
5. Input String Types . . . . . . . . . . . . . . . . . . . . . 26
5.1. DATE . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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5.2. USERID . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.3. SUBCOMMAND . . . . . . . . . . . . . . . . . . . . . . . 27
5.4. PROFILE . . . . . . . . . . . . . . . . . . . . . . . . . 27
6. Input/Output Indirect Types . . . . . . . . . . . . . . . . . 28
6.1. Special Designators for Indirect Types . . . . . . . . . 29
6.1.1. @ENV: Special Designator for Environment Variable . . 29
6.1.2. @FD: Special Designator for File Descriptor . . . . . 29
6.2. CERTS . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.3. KEYS . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.4. CIPHERTEXT . . . . . . . . . . . . . . . . . . . . . . . 30
6.5. INLINESIGNED . . . . . . . . . . . . . . . . . . . . . . 31
6.6. SIGNATURES . . . . . . . . . . . . . . . . . . . . . . . 32
6.7. SESSIONKEY . . . . . . . . . . . . . . . . . . . . . . . 32
6.8. MICALG . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.9. PASSWORD . . . . . . . . . . . . . . . . . . . . . . . . 32
6.10. VERIFICATIONS . . . . . . . . . . . . . . . . . . . . . . 33
6.11. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.12. PROFILELIST . . . . . . . . . . . . . . . . . . . . . . . 34
7. Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . 34
8. Known Implementations . . . . . . . . . . . . . . . . . . . . 36
9. Alternate Interfaces . . . . . . . . . . . . . . . . . . . . 38
10. Guidance for Implementers . . . . . . . . . . . . . . . . . . 38
10.1. One OpenPGP Message at a Time . . . . . . . . . . . . . 38
10.2. Simplified Subset of OpenPGP Message . . . . . . . . . . 38
10.3. Validate Signatures Only from Known Signers . . . . . . 39
10.4. OpenPGP Inputs can be either Binary or ASCII-armored . . 39
10.5. Complexities of the Cleartext Signature Framework . . . 40
10.6. Reliance on Supplied Certs and Keys . . . . . . . . . . 41
10.7. Text is always UTF-8 . . . . . . . . . . . . . . . . . . 41
10.8. Passwords are Human-Readable . . . . . . . . . . . . . . 42
10.8.1. Generating Material with Human-Readable Passwords . 42
10.8.2. Consuming Password-protected Material . . . . . . . 43
10.9. Be Careful with Special Designators . . . . . . . . . . 43
10.10. Nuances for Hardware-backed Secret Key Material . . . . 44
11. Guidance for Consumers . . . . . . . . . . . . . . . . . . . 45
11.1. Choosing Between --as=text and --as=binary . . . . . . . 45
11.2. Special Designators and Unusual Filenames . . . . . . . 46
12. Security Considerations . . . . . . . . . . . . . . . . . . . 46
12.1. Signature Verification . . . . . . . . . . . . . . . . . 46
12.2. Compression . . . . . . . . . . . . . . . . . . . . . . 48
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 48
13.1. Object Security vs. Transport Security . . . . . . . . . 48
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 48
14.1. Normative References . . . . . . . . . . . . . . . . . . 48
14.2. Informative References . . . . . . . . . . . . . . . . . 49
Appendix A. C Library API (Tentative) . . . . . . . . . . . . . 50
A.1. Design Choices for Library API . . . . . . . . . . . . . 62
A.2. Library Use Patterns . . . . . . . . . . . . . . . . . . 62
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A.3. libsopv C API Subset . . . . . . . . . . . . . . . . . . 63
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 64
Appendix C. Future Work . . . . . . . . . . . . . . . . . . . . 65
Appendix D. Document History . . . . . . . . . . . . . . . . . . 65
D.1. Substantive Changes between -09 and -10: . . . . . . . . 66
D.2. Substantive Changes between -08 and -09: . . . . . . . . 66
D.3. Substantive Changes between -07 and -08: . . . . . . . . 66
D.4. Substantive Changes between -06 and -07: . . . . . . . . 67
D.5. Substantive Changes between -05 and -06: . . . . . . . . 67
D.6. Substantive Changes between -04 and -05: . . . . . . . . 67
D.7. Substantive Changes between -03 and -04: . . . . . . . . 67
D.8. Substantive Changes between -02 and -03: . . . . . . . . 68
D.9. Substantive Changes between -01 and -02: . . . . . . . . 68
D.10. Substantive Changes between -00 and -01: . . . . . . . . 69
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 69
1. Introduction
Different OpenPGP implementations have many different requirements,
which typically break down in two main categories: key/certificate
management and object security.
The purpose of this document is to provide a "stateless" interface
that primarily handles the object security side of things, and
assumes that secret key management and certificate management will be
handled some other way.
Isolating object security from key/certificate management should make
it easier to provide interoperability testing for the object security
side of OpenPGP implementations, as described in Section 1.3.
This document defines a generic stateless command-line interface for
dealing with OpenPGP messages, known here by the placeholder sop. It
aims for a minimal, well-structured API.
An OpenPGP implementation should not name its executable sop to
implement this specification. It just needs to provide a program
that conforms to this interface.
A sop implementation should leave no trace on the system, and its
behavior should not be affected by anything other than command-line
arguments and input.
Obviously, the user will need to manage their secret keys (and their
peers' certificates) somehow, but the goal of this interface is to
separate out that task from the task of interacting with OpenPGP
messages.
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While this document identifies a command-line interface, the rough
outlines of this interface should also be amenable to relatively
straightforward library implementations in different languages.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
This document uses the term "key" to refer exclusively to OpenPGP
Transferable Secret Keys (see Section 11.2 of [RFC4880]).
It uses the term "certificate" to refer to OpenPGP Transferable
Public Key (see Section 11.1 of [RFC4880]).
"Stateless" in "Stateless OpenPGP" means avoiding secret key and
certificate state. The user is responsible for managing all OpenPGP
certificates and secret keys themselves, and passing them to sop as
needed. The user should also not be concerned that any state could
affect the underlying operations.
OpenPGP revocations can have "Reason for Revocation"
(Section 5.2.3.23 of [RFC4880]), which can be either "soft" or
"hard". The set of "soft" reasons is: "Key is superseded" and "Key
is retired and no longer used". All other reasons (and revocations
that do not state a reason) are "hard" revocations.
This document refers to a special verification-only subset of the sop
command-line interface as sopv (see Section 3 for more details).
1.3. Using sop in a Test Suite
If an OpenPGP implementation provides a sop interface, it can be used
to test interoperability (e.g.,
[OpenPGP-Interoperability-Test-Suite]).
Such an interop test suite can, for example, use custom code (_not_
sop) to generate a new OpenPGP object that incorporates new
primitives, and feed that object to a stable of sop implementations,
to determine whether those implementations can consume the new form.
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Or, the test suite can drive each sop implementation with a simple
input, and observe which cryptographic primitives each implementation
chooses to use as it produces output.
1.4. Semantics vs. Wire Format
The semantics of sop are deliberately simple and very high-level
compared to the vast complexity and nuance available within the
OpenPGP specification. This reflects the perspective of nearly every
piece of tooling that relies on OpenPGP to accomplish its task: most
toolchains don't care about the specifics, they just want the high-
level object security properties.
Given this framing, this document generally tries to avoid
overconstraining the details of the wire format objects emitted, or
what kinds of wire format structures should be acceptable or
unacceptable. This allows a test suite to evaluate and contrast the
wire format choices made by different implementations in as close to
their native configuration as possible. It also makes it easier to
promote interoperability by ensuring that the native wire formats
emitted by one implementation can be consumed by another, without
relying on their choices of wire format being constrained by this
draft.
Where this draft does identify specific wire format requirements,
that might be due to an ambiguity in the existing specifications
(which maybe needs fixing elsewhere), or to a bug in this
specification that could be improved.
2. Examples
These examples show no error checking, but give a flavor of how sop
might be used in practice from a shell.
The key and certificate files described in them (e.g., alice.sec)
could be for example those found in
[I-D.draft-bre-openpgp-samples-01].
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sop generate-key "Alice Lovelace <alice@openpgp.example>" > alice.sec
sop extract-cert < alice.sec > alice.pgp
sop generate-key "Bob Babbage <bob@openpgp.example>" > bob.sec
sop extract-cert < bob.sec > bob.pgp
sop sign --as=text alice.sec < statement.txt > statement.txt.asc
sop verify statement.txt.asc alice.pgp < statement.txt
sop encrypt --sign-with=alice.sec bob.pgp < msg.eml > ciphertext.asc
sop decrypt bob.sec < ciphertext.asc > cleartext.eml
See Section 7 for more information about errors and error handling.
3. sopv Subset
While the primary goal of this document is to provide a full sop
interface, as a special case, an implementer may choose to produce a
version of the command-line interface that only supports OpenPGP
signature verification. As a shorthand, this document refers to such
an interface as sopv, or "the sopv subset". This can be useful for
constrained environments where the only thing needed is signature
verification, for example, system installation or update media.
A full implementation of sop by definition provides sopv, of course.
Only the following subcommands and their associated options MUST be
implemented for sopv:
* version (Section 4.1.1)
* verify (Section 4.3.2)
* inline-verify (Section 4.3.6)
3.1. sopv Versioning
The abstract sopv interface is itself versioned using [SEMVER]. The
definition of the relevant subcommands and options specified in this
document is known as sopv version 1.0.
If backward-incompatible changes are made to the sopv subset, the
major version number will be increased. If the sopv subset is
extended without backward-incompatible changes, the minor version
number will be increased.
Elements of the CLI relevant to sopv are annotated in this document
with the sopv version in which they were introduced.
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4. Subcommands
sop uses a subcommand interface, similar to those popularized by
systems like git and svn.
If the user supplies a subcommand that sop does not implement, it
fails with UNSUPPORTED_SUBCOMMAND. If a sop implementation does not
handle a supplied option for a given subcommand, it fails with
UNSUPPORTED_OPTION.
All subcommands that produce OpenPGP material on standard output
produce ASCII-armored (Section 6 of
[I-D.ietf-openpgp-crypto-refresh-10]) objects by default (except for
sop dearmor). These subcommands have a --no-armor option, which
causes them to produce binary OpenPGP material instead.
All subcommands that accept OpenPGP material on input should be able
to accept either ASCII-armored or binary inputs (see Section 10.4)
and behave accordingly.
See Section 6 for details about how various forms of OpenPGP material
are expected to be structured.
4.1. Meta Subcommands
The subcommands grouped in this section are related to the sop
implementation itself.
4.1.1. version: Version Information
sop version [--backend|--extended|--sop-spec|--sopv]
* Standard Input: ignored
* Standard Output: version information
This subcommand emits version information as UTF-8-encoded text.
With no arguments, the version string emitted should contain the name
of the sop implementation, followed by a single space, followed by
the version number. A sop implementation should use a version number
that respects an established standard that is easily comparable and
parsable, like [SEMVER].
If --backend is supplied, the implementation should produce a
comparable line of implementation and version information about the
primary underlying OpenPGP toolkit.
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If --extended is supplied, the implementation may emit multiple lines
of version information. The first line MUST match the information
produced by a simple invocation, but the rest of the text has no
defined structure.
If --sop-spec is supplied, the implementation should emit a single
line of text indicating the latest version of this draft that it
targets, for example, draft-dkg-openpgp-stateless-cli-06. If the
implementation targets a specific draft but the implementer knows the
implementation is incomplete, it should prefix the draft title with a
"~" (TILDE, U+007E), for example: ~draft-dkg-openpgp-stateless-cli-
06. The implementation MAY emit additional text about its
relationship to the targeted draft on the lines following the
versioned title.
If --sopv is supplied, the implementation should produce a single
line with the implemented [SEMVER] version of the sopv interface
subset (see Section 3) that this implementation provides complete
coverage for. If the implementation does not provide complete
coverage for any sopv interface, it should emit nothing on standard
out and return UNSUPPORTED_OPTION.
--backend, --extended, --sop-spec, and --sopv are mutually-exclusive
options.
Example:
$ sop version
ExampleSop 0.2.1
$ sop version --backend
LibExamplePGP 3.4.2
$ sop version --extended
ExampleSop 0.2.1
Running on MonkeyScript 4.5
LibExamplePGP 3.4.2
LibExampleCrypto 3.1.1
LibXCompression 4.0.2
See https://pgp.example/sop/ for more information
$ sop version --sop-spec
~draft-dkg-openpgp-stateless-cli-06
This implementation does not handle @FD: special designators for output.
$ sop version --sopv
1.0
$
The version subcommand and all of its options are part of the sopv
subset (see Section 3) starting at sopv version 1.0.
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4.1.2. list-profiles: Describe Available Profiles
sop list-profiles SUBCOMMAND
* Standard Input: ignored
* Standard Output: PROFILELIST (Section 6.12)
This subcommand emits a list of profiles supported by the identified
subcommand. The first profile listed is the default profile, as
described in Section 6.12.
If the indicated SUBCOMMAND does not accept a --profile option, it
returns UNSUPPORTED_PROFILE.
Example:
$ sop list-profiles generate-key
default: use the implementer's recommendations
rfc4880: use algorithms from RFC 4880
$
4.2. Key and Certificate Management Subcommands
The subcommands grouped in this section are primarily intended to
manipulate keys and certificates.
4.2.1. generate-key: Generate a Secret Key
sop generate-key [--no-armor]
[--with-key-password=PASSWORD]
[--profile=PROFILE]
[--signing-only]
[--] [USERID...]
* Standard Input: ignored
* Standard Output: KEYS (Section 6.3)
Generate a single default OpenPGP key with zero or more User IDs.
The generated secret key SHOULD be usable for as much of the sop
functionality as possible. In particular:
* It should be possible to extract an OpenPGP certificate from the
key in KEYS with sop extract-cert.
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* The key in KEYS should be able to create signatures (with sop
sign) that are verifiable by using sop verify with the extracted
certificate.
* Unless the --signing-only parameter is supplied, the key in KEYS
should be able to decrypt messages (with sop decrypt) that are
encrypted by using sop encrypt with the extracted certificate.
The detailed internal structure of the certificate is left to the
discretion of the sop implementation.
If the --with-key-password option is supplied, the generated key will
be password-protected (locked) with the supplied password. Note that
PASSWORD is an indirect data type from which the actual password is
acquired (Section 6). See also the guidance on ensuring that the
password is human-readable in Section 10.8.1.
If no --with-key-password option is supplied, the generated key will
be unencrypted.
If the --profile argument is supplied and the indicated PROFILE is
not supported by the implementation, sop will fail with
UNSUPPORTED_PROFILE.
The presence of the --signing-only option is intended to create a key
that is only capable of signing, not decrypting. This is useful for
deployments where only signing and verification are necessary.
If any of the USERID options are not valid UTF-8, sop generate-key
fails with EXPECTED_TEXT.
If the implementation rejects any USERID option that is valid UTF-8
(e.g., due to internal policy, see Section 5.2), sop generate-key
fails with BAD_DATA.
Example:
$ sop generate-key 'Alice Lovelace <alice@openpgp.example>' > alice.sec
$ head -n1 < alice.sec
-----BEGIN PGP PRIVATE KEY BLOCK-----
$
4.2.2. change-key-password: Update a Key's Password
sop change-key-password [--no-armor]
[--new-key-password=PASSWORD]
[--old-key-password=PASSWORD...]
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* Standard Input: KEYS (Section 6.3)
* Standard Output: KEYS (Section 6.3)
The output will be the same set of OpenPGP Transferable Secret Keys
as the input, but with all secret key material locked according to
the password indicated by the --new-key-password option (or, with no
password at all, if --new-key-password is absent). Note that --old-
key-password can be supplied multiple times, and each supplied
password will be tried as a means to unlock any locked key material
encountered. It will normalize a Transferable Secret Key to use a
single password even if it originally had distinct passwords locking
each of the subkeys.
If any secret key packet is locked but cannot be unlocked with any of
the supplied --old-key-password arguments, this subcommand should
fail with KEY_IS_PROTECTED.
Example:
# adding a password to an unlocked key:
$ sop change-key-password --new-key-password=@ENV:keypass < unlocked.key > locked.key
# removing a password:
$ sop change-key-password --old-key-password=@ENV:keypass < locked.key > unlocked.key
# changing a password:
$ sop change-key-password --old-key-password=@ENV:keypass --new-key-password=@ENV:newpass < locked.key > refreshed.key
$
4.2.3. revoke-key: Create a Revocation Certificate
sop revoke-key [--no-armor]
[--with-key-password=PASSWORD...]
* Standard Input: KEYS (Section 6.3)
* Standard Output: CERTS (Section 6.2)
Generate a revocation certificate for each Transferable Secret Key
found. See Section 10 of [I-D.ietf-openpgp-crypto-refresh-10] for a
discussion of common forms of revocation certificate.
Example:
$ sop revoke-key < alice.key > alice-revoked.pgp
$
4.2.4. extract-cert: Extract a Certificate from a Secret Key
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sop extract-cert [--no-armor]
* Standard Input: KEYS (Section 6.3)
* Standard Output: CERTS (Section 6.2)
The output should contain one OpenPGP certificate in CERTS per
OpenPGP Transferable Secret Key found in KEYS. There is no guarantee
what order the CERTS will be in.
sop extract-cert SHOULD work even if any of the keys in KEYS is
password-protected.
Example:
$ sop extract-cert < alice.sec > alice.pgp
$ head -n1 < alice.pgp
-----BEGIN PGP PUBLIC KEY BLOCK-----
$
4.3. Messaging Subcommands
The subcommands in this section handle OpenPGP messages: encrypting,
decrypting, signing, and verifying.
4.3.1. sign: Create Detached Signatures
sop sign [--no-armor] [--micalg-out=MICALG]
[--with-key-password=PASSWORD...]
[--as={binary|text}] [--] KEYS [KEYS...]
* Standard Input: DATA (Section 6.11)
* Standard Output: SIGNATURES (Section 6.6)
Exactly one signature will be made by each key in the supplied KEYS
arguments.
--as defaults to binary. If --as=text and the input DATA is not
valid UTF-8 (Section 10.7), sop sign fails with EXPECTED_TEXT.
--as=binary SHOULD result in OpenPGP signatures of type 0x00
("Signature of a binary document"). --as=text SHOULD result in
OpenPGP signatures of type 0x01 ("Signature of a canonical text
document"). See Section 5.2.1 of [RFC4880] for more details.
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When generating PGP/MIME messages ([RFC3156]), it is useful to know
what digest algorithm was used for the generated signature. When --
micalg-out is supplied, sop sign emits the digest algorithm used to
the specified MICALG file in a way that can be used to populate the
micalg parameter for the Content-Type (see Section 6.8). If the
specified MICALG file already exists in the filesystem, sop sign will
fail with OUTPUT_EXISTS. When --micalg-out is supplied, the DATA on
standard input should already be in canonical text form (7-bit clean,
CRLF line endings, no trailing whitespace), as specified in Section 3
of [RFC3156]. If the incoming DATA does not already meet these
requirements, sop sign will fail with EXPECTED_TEXT, regardless of
any argument supplied for --as.
When signing with multiple keys, sop sign SHOULD use the same digest
algorithm for every signature generated in a single run, unless there
is some internal constraint on the KEYS objects. If --micalg-out is
requested, and multiple incompatibly-constrained KEYS objects are
supplied, sop sign MUST emit the empty string to the designated
MICALG.
If the signing key material in any key in the KEYS objects is
password-protected, sop sign SHOULD try all supplied --with-key-
password options to unlock the key material until it finds one that
enables the use of the key for signing. If none of the PASSWORD
options unlock the key (or if no such option is supplied), sop sign
will fail with KEY_IS_PROTECTED. Note that PASSWORD is an indirect
data type from which the actual password is acquired (Section 6).
Note also the guidance for retrying variants of a non-human-readable
password in Section 10.8.2.
If any key in the KEYS objects is not capable of producing a
signature, sop sign will fail with KEY_CANNOT_SIGN.
sop sign MUST NOT produce any extra signatures beyond those from KEYS
objects supplied on the command line.
Example:
$ sop sign --as=text alice.sec < message.txt > message.txt.asc
$ head -n1 < message.txt.asc
-----BEGIN PGP SIGNATURE-----
$
4.3.2. verify: Verify Detached Signatures
sop verify [--not-before=DATE] [--not-after=DATE]
[--] SIGNATURES CERTS [CERTS...]
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* Standard Input: DATA (Section 6.11)
* Standard Output: VERIFICATIONS (Section 6.10)
--not-before and --not-after indicate that signatures with dates
outside certain range MUST NOT be considered valid.
--not-before defaults to the beginning of time. Accepts the special
value - to indicate the beginning of time (i.e., no lower boundary).
--not-after defaults to the current system time (now). Accepts the
special value - to indicate the end of time (i.e., no upper
boundary).
sop verify only returns OK if at least one certificate included in
any CERTS object made a valid signature in the time window specified
over the DATA supplied.
For details about the valid signatures, the user MUST inspect the
VERIFICATIONS output.
If no CERTS are supplied, sop verify fails with MISSING_ARG.
If no valid signatures are found, sop verify fails with NO_SIGNATURE.
See Section 12.1 for more details about signature verification.
Example:
(In this example, we see signature verification succeed first, and
then fail on a modified version of the message.)
$ sop verify message.txt.asc alice.pgp < message.txt
2019-10-29T18:36:45Z EB85BB5FA33A75E15E944E63F231550C4F47E38E EB85BB5FA33A75E15E944E63F231550C4F47E38E mode:text signed by alice.pgp
$ echo $?
0
$ tr a-z A-Z < message.txt | sop verify message.txt.asc alice.pgp
$ echo $?
3
$
The verify subcommand and all of its options are part of the sopv
subset (see Section 3) starting at sopv version 1.0.
4.3.3. encrypt: Encrypt a Message
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sop encrypt [--as={binary|text}]
[--no-armor]
[--with-password=PASSWORD...]
[--sign-with=KEYS...]
[--with-key-password=PASSWORD...]
[--profile=PROFILE]
[--session-key-out=SESSIONKEY]
[--] [CERTS...]
* Standard Input: DATA (Section 6.11)
* Standard Output: CIPHERTEXT (Section 6.4)
--as defaults to binary. The setting of --as corresponds to the one
octet format field found in the Literal Data packet at the core of
the output CIPHERTEXT. If --as is set to binary, the octet is b
(0x62). If it is text, the format octet is u (0x75).
--with-password enables symmetric encryption (and can be used
multiple times if multiple passwords are desired).
--sign-with creates exactly one signature by for each secret key
found in the supplied KEYS object (this can also be used multiple
times if signatures from keys found in separaate files are desired).
If any key in any supplied KEYS object is not capable of producing a
signature, sop sign will fail with KEY_CANNOT_SIGN. If any signing
key material in any supplied KEYS object is password-protected, sop
encrypt SHOULD try all supplied --with-key-password options to unlock
the key material until it finds one that enables the use of the key
for signing. If none of the --with-key-password=PASSWORD options can
unlock any locked signing key material (or if no such option is
supplied), sop encrypt will fail with KEY_IS_PROTECTED. All
signatures made must be placed inside the encryption produced by sop
encrypt.
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Note that both --with-password and --with-key-password supply
PASSWORD arguments, but they do so in different contexts which are
not interchangeable. A PASSWORD supplied for symmetric encryption
(--with-password) MUST NOT be used to try to unlock a signing key (--
with-key-password) and a PASSWORD supplied to unlock a signing key
MUST NOT be used to symmetrically encrypt the message. Regardless of
context, each PASSWORD argument is presented as an indirect data type
from which the actual password is acquired (Section 6). If sop
encrypt encounters a password which is not a valid UTF-8 string
(Section 10.7), or is otherwise not robust in its representation to
humans, it fails with PASSWORD_NOT_HUMAN_READABLE. If sop encrypt
sees trailing whitespace at the end of a password, it will trim the
trailing whitespace before using the password. See Section 10.8 for
more discussion about passwords.
If --as is set to binary, then --sign-with will sign as a binary
document (OpenPGP signature type 0x00).
If --as is set to text, then --sign-with will sign as a canonical
text document (OpenPGP signature type 0x01). In this case, if the
input DATA is not valid UTF-8 (Section 10.7), sop encrypt fails with
EXPECTED_TEXT.
If --sign-with is supplied for input DATA that is not valid UTF-8,
sop encrypt MAY sign as a binary document (OpenPGP signature type
0x00).
sop encrypt MUST NOT produce any extra signatures beyond those from
KEYS objects identified by --sign-with.
The resulting CIPHERTEXT should be decryptable by the secret keys
corresponding to every certificate included in all CERTS, as well as
each password given with --with-password.
If no CERTS or --with-password options are present, sop encrypt fails
with MISSING_ARG.
If at least one of the identified certificates requires encryption to
an unsupported asymmetric algorithm, sop encrypt fails with
UNSUPPORTED_ASYMMETRIC_ALGO.
If at least one of the identified certificates is not encryption-
capable (e.g., revoked, expired, no encryption-capable flags on
primary key and valid subkeys), sop encrypt fails with
CERT_CANNOT_ENCRYPT.
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If the --profile argument is supplied and the indicated PROFILE is
not supported by the implementation, sop will fail with
UNSUPPORTED_PROFILE. The use of a profile for this subcommand allows
an implementation faced with parametric or algorithmic choices to
make a decision coarsely guided by the operator. For example, when
encrypting with a password, there is no knowledge about the
capabilities of the recipient, and an implementation may prefer
cryptographically modern algorithms, or it may prefer more broad
compatibility. In the event that a known recipient (i.e., one of the
CERTS) explicitly indicates a lack of support for one of the features
preferred by the indicated profile, the implementation SHOULD conform
to the recipient's advertised capabilities where possible.
If --session-key-out argument is supplied, the session key generated
for this encrypted will be written to the indicated location. This
can be useful, for example, when Alice encrypts a message to Bob, but
also wants to retain the ability to read it without having any of her
own secret key material available (see Section 9.1 of
[I-D.ietf-lamps-e2e-mail-guidance-11]).
If sop encrypt fails for any reason, it emits no CIPHERTEXT.
Example:
(In this example, bob.bin is a file containing Bob's binary-formatted
OpenPGP certificate. Alice is encrypting a message to both herself
and Bob.)
$ sop encrypt --as=text --sign-with=alice.key alice.asc bob.bin < message.eml > encrypted.asc
$ head -n1 encrypted.asc
-----BEGIN PGP MESSAGE-----
$
4.3.4. decrypt: Decrypt a Message
sop decrypt [--session-key-out=SESSIONKEY]
[--with-session-key=SESSIONKEY...]
[--with-password=PASSWORD...]
[--with-key-password=PASSWORD...]
[--verifications-out=VERIFICATIONS
[--verify-with=CERTS...]
[--verify-not-before=DATE]
[--verify-not-after=DATE] ]
[--] [KEYS...]
* Standard Input: CIPHERTEXT (Section 6.4)
* Standard Output: DATA (Section 6.11)
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The caller can ask sop for the session key discovered during
decryption by supplying the --session-key-out option. If the
specified file already exists in the filesystem, sop decrypt will
fail with OUTPUT_EXISTS. When decryption is successful, sop decrypt
writes the discovered session key to the specified file.
--with-session-key enables decryption of the CIPHERTEXT using the
session key directly against the SEIPD packet. This option can be
used multiple times if several possible session keys should be tried.
SESSIONKEY is an indirect data type from which the actual sessionkey
value is acquired (Section 6).
--with-password enables decryption based on any SKESK (Section 5.3 of
[I-D.ietf-openpgp-crypto-refresh-10]) packets in the CIPHERTEXT.
This option can be used multiple times if the user wants to try more
than one password.
--with-key-password lets the user use password-protected (locked)
secret key material. If the decryption-capable secret key material
in any key in the KEYS objects is password-protected, sop decrypt
SHOULD try all supplied --with-key-password options to unlock the key
material until it finds one that enables the use of the key for
decryption. If none of the --with-key-password options unlock the
key (or if no such option is supplied), and the message cannot be
decrypted with any other KEYS, --with-session-key, or --with-password
options, sop decrypt will fail with KEY_IS_PROTECTED.
Note that the two kinds of PASSWORD options are for different
domains: --with-password is for unlocking an SKESK, and --with-key-
password is for unlocking secret key material in KEYS. sop decrypt
SHOULD NOT apply the --with-key-password argument to any SKESK, or
the --with-password argument to any KEYS.
Each PASSWORD argument is an indirect data type from which the actual
password is acquired (Section 6). If sop decrypt tries and fails to
use a password supplied by a PASSWORD, and it observes that there is
trailing UTF-8 whitespace at the end of the password, it will retry
with the trailing whitespace stripped. See Section 10.8.2 for more
discussion about consuming password-protected key material.
--verifications-out produces signature verification status to the
designated file. If the designated file already exists in the
filesystem, sop decrypt will fail with OUTPUT_EXISTS.
The return code of sop decrypt is not affected by the results of
signature verification. The caller MUST check the returned
VERIFICATIONS to confirm signature status. An empty VERIFICATIONS
output indicates that no valid signatures were found.
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--verify-with identifies a set of certificates whose signatures would
be acceptable for signatures over this message.
If the caller is interested in signature verification, both --
verifications-out and at least one --verify-with must be supplied.
If only one of these options is supplied, sop decrypt fails with
INCOMPLETE_VERIFICATION.
--verify-not-before and --verify-not-after provide a date range for
acceptable signatures, by analogy with the options for sop verify
(see Section 4.3.2). They should only be supplied when doing
signature verification.
See Section 12.1 for more details about signature verification.
If no KEYS or --with-password or --with-session-key options are
present, sop decrypt fails with MISSING_ARG.
If unable to decrypt, sop decrypt fails with CANNOT_DECRYPT.
sop decrypt only emits cleartext to Standard Output that was
successfully decrypted.
Example:
(In this example, Alice stashes and re-uses the session key of an
encrypted message.)
$ sop decrypt --session-key-out=session.key alice.sec < ciphertext.asc > cleartext.out
$ ls -l ciphertext.asc cleartext.out
-rw-r--r-- 1 user user 321 Oct 28 01:34 ciphertext.asc
-rw-r--r-- 1 user user 285 Oct 28 01:34 cleartext.out
$ sop decrypt --with-session-key=session.key < ciphertext.asc > cleartext2.out
$ diff cleartext.out cleartext2.out
$
4.3.4.1. Historic Options for sop decrypt
The sop decrypt option --verifications-out used to be named --verify-
out. An implementation SHOULD accept either form of this option, and
SHOULD produce a deprecation warning to standard error if the old
form is used.
4.3.5. inline-detach: Split Signatures from an Inline-Signed Message
sop inline-detach [--no-armor] --signatures-out=SIGNATURES
* Standard Input: INLINESIGNED
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* Standard Output: DATA (the message without any signatures)
In some contexts, the user may expect an inline-signed message of
some form or another (INLINESIGNED, see Section 6.5) rather than a
message and its detached signature. sop inline-detach takes such an
inline-signed message on standard input, and splits it into:
* the potentially signed material on standard output, and
* a detached signature block to the destination identified by --
signatures-out
Note that no cryptographic verification of the signatures is done by
this subcommand. Once the inline-signed message is separated,
verification of the detached signature can be done with sop verify.
If no --signatures-out is supplied, sop inline-detach fails with
MISSING_ARG.
Note that there may be more than one Signature packet in an inline-
signed message. All signatures found in the inline-signed message
will be emitted to the --signatures-out destination.
If the inline-signed message uses the Cleartext Signature Framework,
it may be dash-escaped (see Section 7.1 of [RFC4880]). The output of
sop detach-inband-signature-and-message will have any dash-escaping
removed.
If the input is not an INLINESIGNED message, sop inline-detach fails
with BAD_DATA. If the input contains more than one object that could
be interpreted as an INLINESIGNED message, sop inline-detach also
fails with BAD_DATA. A sop implementation MAY accept (and discard)
leading and trailing data when the incoming INLINESIGNED message uses
the Cleartext Signature Framework.
If the file designated by --signatures-out already exists in the
filesystem, sop detach-inband-signature-and-message will fail with
OUTPUT_EXISTS.
Note that --no-armor here governs the data written to the --
signatures-out destination. Standard output is always the raw
message, not an OpenPGP packet.
Example:
$ sop inline-detach --signatures-out=Release.pgp < InRelease >Release
$ sop verify Release.pgp archive-keyring.pgp < Release
$
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4.3.6. inline-verify: Verify an Inline-Signed Message
sop inline-verify [--not-before=DATE] [--not-after=DATE]
[--verifications-out=VERIFICATIONS]
[--] CERTS [CERTS...]
* Standard Input: INLINESIGNED (Section 6.5)
* Standard Output: DATA (Section 6.11)
This command is similar to sop verify (Section 4.3.2) except that it
takes an INLINESIGNED message (see Section 6.5) and produces the
message body (without signatures) on standard output. It is also
similar to sop inline-detach (Section 4.3.5) except that it actually
performs signature verification.
--not-before and --not-after indicate that signatures with dates
outside certain range MUST NOT be considered valid.
--not-before defaults to the beginning of time. Accepts the special
value - to indicate the beginning of time (i.e., no lower boundary).
--not-after defaults to the current system time (now). Accepts the
special value - to indicate the end of time (i.e., no upper
boundary).
sop inline-verify only returns OK if INLINESIGNED contains at least
one valid signature made during the time window specified by a
certificate included in any CERTS object.
For details about the valid signatures, the user MUST inspect the
VERIFICATIONS output.
If no CERTS are supplied, sop inline-verify fails with MISSING_ARG.
If no valid signatures are found, sop inline-verify fails with
NO_SIGNATURE and emits nothing on standard output.
See Section 12.1 for more details about signature verification.
Example:
(In this example, we see signature verification succeed first, and
then fail on a modified version of the message.)
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$ sop inline-verify -- alice.pgp < message.txt
Hello, world!
$ echo $?
0
$ sed s/Hello/Goodbye/ < message.txt | sop inline-verify -- alice.pgp
$ echo $?
3
$
The inline-verify subcommand and all of its options are part of the
sopv subset (see Section 3) starting at sopv version 1.0.
4.3.7. inline-sign: Create an Inline-Signed Message
sop inline-sign [--no-armor]
[--with-key-password=PASSWORD...]
[--as={binary|text|clearsigned}]
[--] KEYS [KEYS...]
* Standard Input: DATA (Section 6.11)
* Standard Output: INLINESIGNED (Section 6.5)
Exactly one signature will be made by each key in the supplied KEYS
arguments.
The generated output stream will be an inline-signed message, by
default producing an OpenPGP "Signed Message" packet stream.
--as defaults to binary. If --as= is set to either text or
clearsigned, and the input DATA is not valid UTF-8 (Section 10.7),
sop inline-sign fails with EXPECTED_TEXT.
--as=binary SHOULD result in OpenPGP signatures of type 0x00
("Signature of a binary document"). --as=text SHOULD result in an
OpenPGP signature of type 0x01 ("Signature of a canonical text
document"). See Section 5.2.1 of [RFC4880] for more details.
--as=clearsigned SHOULD behave the same way as --as=text except that
it produces an output stream using the Cleartext Signature Framework
(see Section 7 of [RFC4880] and Section 10.5).
If both --no-armor and --as=clearsigned are supplied, sop inline-sign
fails with INCOMPATIBLE_OPTIONS.
If the signing key material in any key in the KEYS objects is
password-protected, sop inline-sign SHOULD try all supplied --with-
key-password options to unlock the key material until it finds one
that enables the use of the key for signing. If none of the PASSWORD
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options unlock the key (or if no such option is supplied), sop
inline-sign will fail with KEY_IS_PROTECTED. Note that PASSWORD is
an indirect data type from which the actual password is acquired
(Section 6). Note also the guidance for retrying variants of a non-
human-readable password in Section 10.8.2.
If any key in the KEYS objects is not capable of producing a
signature, sop inline-sign will fail with KEY_CANNOT_SIGN.
sop inline-sign MUST NOT produce any extra signatures beyond those
from KEYS objects supplied on the command line.
Example:
$ sop inline-sign --as=clearsigned alice.sec < message.txt > message-signed.txt
$ head -n5 < message-signed.txt
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
This is the message.
-----BEGIN PGP SIGNATURE-----
$
4.4. Transport Subcommands
The commands in this section handle manipulating OpenPGP objects for
transport: armoring and dearmoring for 7-bit cleanness and
compactness, respectively.
4.4.1. armor: Convert Binary to ASCII
sop armor
* Standard Input: OpenPGP material (SIGNATURES, KEYS, CERTS,
CIPHERTEXT, or INLINESIGNED)
* Standard Output: the same material with ASCII-armoring added, if
not already present
sop armor inspects the input and chooses the label appropriately,
based on the OpenPGP packets encountered. If the type of the first
OpenPGP packet is:
* 0x05 (Secret-Key), the packet stream should be parsed as a KEYS
input (with Armor Header BEGIN PGP PRIVATE KEY BLOCK).
* 0x06 (Public-Key), the packet stream should be parsed as a CERTS
input (with Armor Header BEGIN PGP PUBLIC KEY BLOCK).
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* 0x01 (Public-key Encrypted Session Key) or 0x03 (Symmetric-key
Encrypted Session Key), the packet stream should be parsed as a
CIPHERTEXT input (with Armor Header BEGIN PGP MESSAGE).
* 0x04 (One-Pass Signature), the packet stream should be parsed as
an INLINESIGNED input (with Armor Header BEGIN PGP MESSAGE).
* 0x02 (Signature), the packet stream may be either a SIGNATURES
input or an INLINESIGNED input. If the packet stream contains
only Signature packets, it should be parsed as aSIGNATURES input
(with Armor Header BEGIN PGP SIGNATURE). If it contains any
packet other than a Signature packet, it should be parsed as an
INLINESIGNED input (with Armor Header BEGIN PGP MESSAGE).
If the input packet stream does not match any expected sequence of
packet types, sop armor fails with BAD_DATA.
Since sop armor accepts ASCII-armored input as well as binary input,
this operation is idempotent on well-structured data. A caller can
use this subcommand blindly to ensure that any well-formed OpenPGP
packet stream is 7-bit clean.
FIXME: what to do if the input is a CSF INLINESIGNED message? Three
choices:
* Leave it untouched -- this violates the claim about blindly
ensuring 7-bit clean, since UTF-8-encoded message text is not
necessarily 7-bit clean.
* Convert to ASCII-armored INLINESIGNED -- this requires synthesis
of OPS packet (from signatures block) and Literal Data packet
(from the message body).
* Raise a specific error.
Example:
$ sop armor < bob.bin > bob.pgp
$ head -n1 bob.pgp
-----BEGIN PGP PUBLIC KEY BLOCK-----
$
4.4.1.1. Historic Options for sop armor
sop armor used to be specified as having a --label option, with an
argument that took one of the following values: auto, sig, key, cert,
or message, which allowed the user to specify the label used in the
header and tail of the armoring.
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The default value for --label was auto, which matches the currently
specified behavior. This option is now deprecated, as it offers no
useful functionality.
4.4.2. dearmor: Convert ASCII to Binary
sop dearmor
* Standard Input: OpenPGP material (SIGNATURES, KEYS, CERTS,
CIPHERTEXT, or INLINESIGNED)
* Standard Output: the same material with any ASCII-armoring removed
If the input packet stream does not match any of the expected
sequence of packet types, sop dearmor fails with BAD_DATA. See also
Section 10.4.
Since sop dearmor accepts binary-formatted input as well as ASCII-
armored input, this operation is idempotent on well-structured data.
A caller can use this subcommand blindly ensure that any well-formed
OpenPGP packet stream is in its standard binary representation.
FIXME: what to do if the input is a CSF INLINESIGNED? Three choices:
* Leave it untouched -- output data is not really in binary format.
* Convert to binary-format INLINESIGNED -- this requires synthesis
of OPS packet (from CSF Hash header) and Literal Data packet (from
the message body).
* Raise a specific error.
Example:
$ sop dearmor < message.txt.asc > message.txt.sig
$
5. Input String Types
Some material is passed to sop directly as a string on the command
line.
5.1. DATE
An ISO-8601 formatted timestamp with time zone, or the special value
now to indicate the current system time.
Examples:
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* now
* 2019-10-29T12:11:04+00:00
* 2019-10-24T23:48:29Z
* 20191029T121104Z
In some cases where used to specify lower and upper boundaries, a
DATE value can be set to - to indicate "no time limit".
A flexible implementation of sop MAY accept date inputs in other
unambiguous forms.
Note that whenever sop emits a timestamp (e.g., in Section 6.10) it
MUST produce only a UTC-based ISO-8601 compliant representation with
a resolution of one second, using the literal Z suffix to indicate
timezone.
5.2. USERID
This is an arbitrary UTF-8 string (Section 10.7). By convention,
most User IDs are of the form Display Name
<email.address@example.com>, but they do not need to be.
By internal policy, an implementation MAY reject a USERID if there
are certain UTF-8 strings it declines to work with as a User ID. For
example, an implementation may reject the empty string, or a string
with characters in it that it considers problematic. Of course,
refusing to create a particular User ID does not prevent an
implementation from encountering such a User ID in its input.
5.3. SUBCOMMAND
This is an ASCII string that matches the name of one of the
subcommands listed in Section 4.
5.4. PROFILE
Some sop subcommands can accept a --profile option, which takes as an
argument the name of a profile.
A profile name is a UTF-8 string that has no whitespace in it.
Which profiles are available depends on the sop implementation.
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Similar to OpenPGP Notation names, profile names are divided into two
namespaces: the IETF namespace and the user namespace. A profile
name in the user namespace ends with the @ character (0x40) followed
by a DNS domain name. A profile name in the IETF namespace does not
have an @ character.
A profile name in the user space is owned and controlled by the owner
of the domain in the suffix. A sop implementation that implements a
user profile but does not own the domain in question SHOULD hew as
closely as possible to the semantics described by the owner of the
domain.
A profile name in the IETF namespace that begins with the string rfc
should have semantics that hew as closely as possible to the
referenced RFC. Similarly, a profile name in the IETF namespace that
begins with the string draft- should have semantics that hew as
closely as possible to the referenced Internet Draft.
The reserved profile name default in the IETF namespace simply refers
to the implementation's default choices. It is not mandatory to name
the default profile default. The first profile listed in the list-
profiles output is considered the default configuration, as specified
in Section 6.12.
Note that this profile mechanism is intended to provide a limited way
for an implementation to select among a small set of options that the
implementer has vetted and is satisfied with. It is not intended to
provide an arbitrary channel for complex configuration, and a sop
implementation MUST NOT use it in that way.
6. Input/Output Indirect Types
Some material is passed to sop indirectly, typically by referring to
a filename containing the data in question. This type of data may
also be passed to sop on Standard Input, or delivered by sop to
Standard Output.
If any input data is specified explicitly to be read from a file that
does not exist, sop will fail with MISSING_INPUT.
If any input data does not meet the requirements described below, sop
will fail with BAD_DATA.
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6.1. Special Designators for Indirect Types
An indirect argument or parameter that starts with "@" (COMMERCIAL
AT, U+0040) is not treated as a filename, but is reserved for special
handling, based on the prefix that follows the @. We describe two of
those prefixes (@ENV: and @FD:) here. A sop implementation that
receives such a special designator but does not know how to handle a
given prefix in that context MUST fail with
UNSUPPORTED_SPECIAL_PREFIX.
See Section 10.9 for more details about safe handling of these
special designators.
6.1.1. @ENV: Special Designator for Environment Variable
If the filename for any indirect material used as input has the
special form @ENV:xxx, then contents of environment variable $xxx is
used instead of looking in the filesystem. @ENV is for input only: if
the prefix @ENV: is used for any output argument, sop fails with
UNSUPPORTED_SPECIAL_PREFIX.
The sopv subset (see Section 3) MUST be capable of supporting the
@ENV special designator for all relevant inputs starting at sopv
version 1.0.
6.1.2. @FD: Special Designator for File Descriptor
If the filename for any indirect material used as either input or
output has the special form @FD:nnn where nnn is a decimal integer,
then the associated data is read from file descriptor nnn.
On platforms which support file descriptors, the sopv subset (see
Section 3) MUST be capable of supporting the @FD special designator
for all relevant inputs and outputs starting at sopv version 1.0.
6.2. CERTS
One or more OpenPGP certificates (Section 10.1 of
[I-D.ietf-openpgp-crypto-refresh-10]), aka "Transferable Public Key".
May be armored (see Section 10.4).
Although some existing workflows may prefer to use one CERTS object
with multiple certificates in it (a "keyring"), supplying exactly one
certificate per CERTS input will make error reporting clearer and
easier.
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If any CERTS input contains secret key material, sop MUST fail with
BAD_DATA. This strictness is intended to keep the consumer of the
sop interface clear about what material they are dealing with in what
locations. This should reduce the consumer's risk of accidentally
exposing secret key materal where they meant to expose a CERTS
object.
6.3. KEYS
One or more OpenPGP Transferable Secret Keys (Section 10.2 of
[I-D.ietf-openpgp-crypto-refresh-10]). May be armored (see
Section 10.4).
Secret key material is often locked with a password to ensure that it
cannot be simply copied and reused. If any secret key material is
locked with a password and no --with-key-password option is supplied,
sop may fail with error KEY_IS_PROTECTED. However, when a cleartext
secret key (that is, one not locked with a password) is available,
sop should always be able to use it, whether a --with-key-password
option is supplied or not.
Although some existing workflows may prefer to use one KEYS object
with multiple keys in it (a "secret keyring"), supplying exactly one
key per KEYS input will make error reporting clearer and easier.
6.4. CIPHERTEXT
sop accepts only a restricted subset of the arbitrarily-nested
grammar allowed by the OpenPGP Messages definition (Section 10.3 of
[I-D.ietf-openpgp-crypto-refresh-10]).
In particular, it accepts and generates only:
An OpenPGP message, consisting of a sequence of PKESKs (Section 5.1
of [I-D.ietf-openpgp-crypto-refresh-10]) and SKESKs (Section 5.3 of
[I-D.ietf-openpgp-crypto-refresh-10]), followed by one SEIPD
(Section 5.13 of [I-D.ietf-openpgp-crypto-refresh-10]).
The SEIPD can decrypt into one of two things:
* "Maybe Signed Data" (see below), or
* Compressed data packet that contains "Maybe Signed Data"
"Maybe Signed Data" is a sequence of:
* N (zero or more) one-pass signature packets, followed by
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* zero or more signature packets, followed by
* one Literal data packet, followed by
* N signature packets (corresponding to the outer one-pass
signatures packets)
FIXME: does any tool do compression inside signing? Do we need to
handle that?
May be armored (see Section 10.4).
6.5. INLINESIGNED
An inline-signed message may take any one of three different forms:
* A binary sequence of OpenPGP packets that matches a subset of the
"Signed Message" element in the grammar in Section 10.3 of
[I-D.ietf-openpgp-crypto-refresh-10]
* The same sequence of packets, but ASCII-armored (see Section 10.4)
* A message using the Cleartext Signature Framework described in
Section 7 of [I-D.ietf-openpgp-crypto-refresh-10]
The subset of the packet grammar expected in the first two forms
consists of either:
* a series of Signature packets followed by a Literal Data packet
* a series of One-Pass Signature (OPS) packets, followed by one
Literal Data packet, followed by an equal number of Signature
packets corresponding to the OPS packets
When the message is in the third form (Cleartext Signature
Framework), it has the following properties:
* The stream SHOULD consist solely of UTF-8 text
* Every Signature packet found in the stream SHOULD have Signature
Type 0x01 (canonical text document).
* It SHOULD NOT contain leading text (before the -----BEGIN PGP
SIGNED MESSAGE----- cleartext header) or trailing text (after the
-----END PGP SIGNATURE----- armor tail).
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While some OpenPGP implementations MAY produce more complicated
inline signed messages, a sop implementation SHOULD limit itself to
producing these straightforward forms.
6.6. SIGNATURES
One or more OpenPGP Signature packets. May be armored (see
Section 10.4).
6.7. SESSIONKEY
This documentation uses the GnuPG defacto ASCII representation:
ALGONUM:HEXKEY
where ALGONUM is the decimal value associated with the OpenPGP
Symmetric Key Algorithms (Section 9.3 of
[I-D.ietf-openpgp-crypto-refresh-10]) and HEXKEY is the hexadecimal
representation of the binary key.
Example AES-256 session key:
9:FCA4BEAF687F48059CACC14FB019125CD57392BAB7037C707835925CBF9F7BCD
A sop implementation SHOULD produce session key data in this format.
When consuming such a session key, sop SHOULD be willing to accept
either upper or lower case hexadecimal digits, and to gracefully
ignore any trailing whitespace.
6.8. MICALG
This output-only type indicates the cryptographic digest used when
making a signature. It is useful specifically when generating signed
PGP/MIME objects, which want a micalg= parameter for the multipart/
signed content type as described in Section 5 of [RFC3156].
It will typically be a string like pgp-sha512, but in some situations
(multiple signatures using different digests) it will be the empty
string. If the user of sop is assembling a PGP/MIME signed object,
and the MICALG output is the empty string, the user should omit the
micalg= parameter entirely.
6.9. PASSWORD
This input-only is expected to be a UTF-8 string (Section 10.7), but
for sop decrypt, any bytestring that the user supplies will be
accepted. Note the details in sop encrypt and sop decrypt about
trailing whitespace!
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See also Section 10.8 for more discussion.
6.10. VERIFICATIONS
This output-only type consists of one line per successful signature
verification. Each line has three structured fields delimited by a
single space, followed by arbitrary text to the end of the line that
forms a message describing the verification.
* ISO-8601 UTC datestamp of the signature, to one second precision,
using the Z suffix
* Fingerprint of the signing key (may be a subkey)
* Fingerprint of primary key of signing certificate (if signed by
primary key, same as the previous field)
* a string describing the mode of the signature, either mode:text or
mode:binary
* message describing the verification (free form)
Note that while Section 5.1 permits a sop implementation to accept
other unambiguous date representations, its date output here MUST be
a strict ISO-8601 UTC date timestamp. In particular:
* the date and time fields MUST be separated by T, not by
whitespace, since whitespace is used as a delimiter
* the time MUST be emitted in UTC, with the explicit suffix Z
* the time MUST be emitted with one-second precision
Example:
2019-10-24T23:48:29Z C90E6D36200A1B922A1509E77618196529AE5FF8 C4BC2DDB38CCE96485EBE9C2F20691179038E5C6 mode:binary certificate from dkg.asc
6.11. DATA
Cleartext, arbitrary data. This is either a bytestream or UTF-8
text.
It MUST only be UTF-8 text in the case of input supplied to sop sign
--as=text or sop encrypt --as=text. If sop receives DATA containing
non-UTF-8 octets in this case, it will fail (see Section 10.7) with
EXPECTED_TEXT.
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6.12. PROFILELIST
This output-only type consists of simple UTF-8 textual output, with
one line per profile. Each line consists of the profile name
optionally followed by a colon (0x31), a space (0x20), and a brief
human-readable description of the intended semantics of the profile.
Each line may be at most 1000 bytes, and no more than 4 profiles may
be listed.
These limits are intended to force sop implementers to make hard
decisions and to keep things simple.
The first profile MAY be explicitly named default. If it is not
named default, then default is an alias for the first profile listed.
No profile after the first listed may be named default.
See Section 5.4 for more discussion about the namespace and intended
semantics of each profile.
7. Failure Modes
sop return codes have both mnemonics and numeric values.
When sop succeeds, it will return 0 (OK) and emit nothing to Standard
Error. When sop fails, it fails with a non-zero return code, and
emits one or more warning messages on Standard Error. Known return
codes include:
+=======+=============================+============================+
| Value | Mnemonic | Meaning |
+=======+=============================+============================+
| 0 | OK | Success |
+-------+-----------------------------+----------------------------+
| 3 | NO_SIGNATURE | No acceptable signatures |
| | | found (sop verify) |
+-------+-----------------------------+----------------------------+
| 13 | UNSUPPORTED_ASYMMETRIC_ALGO | Asymmetric algorithm |
| | | unsupported (sop encrypt) |
+-------+-----------------------------+----------------------------+
| 17 | CERT_CANNOT_ENCRYPT | Certificate not |
| | | encryption-capable (e.g., |
| | | expired, revoked, |
| | | unacceptable usage flags) |
| | | (sop encrypt) |
+-------+-----------------------------+----------------------------+
| 19 | MISSING_ARG | Missing required argument |
+-------+-----------------------------+----------------------------+
| 23 | INCOMPLETE_VERIFICATION | Incomplete verification |
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| | | instructions (sop decrypt) |
+-------+-----------------------------+----------------------------+
| 29 | CANNOT_DECRYPT | Unable to decrypt (sop |
| | | decrypt) |
+-------+-----------------------------+----------------------------+
| 31 | PASSWORD_NOT_HUMAN_READABLE | Non-UTF-8 or otherwise |
| | | unreliable password (sop |
| | | encrypt, sop generate-key) |
+-------+-----------------------------+----------------------------+
| 37 | UNSUPPORTED_OPTION | Unsupported option |
+-------+-----------------------------+----------------------------+
| 41 | BAD_DATA | Invalid data type (no |
| | | secret key where KEYS |
| | | expected, secret key where |
| | | CERTS expected, etc) |
+-------+-----------------------------+----------------------------+
| 53 | EXPECTED_TEXT | Non-text input where text |
| | | expected |
+-------+-----------------------------+----------------------------+
| 59 | OUTPUT_EXISTS | Output file already exists |
+-------+-----------------------------+----------------------------+
| 61 | MISSING_INPUT | Input file does not exist |
+-------+-----------------------------+----------------------------+
| 67 | KEY_IS_PROTECTED | A KEYS input is password- |
| | | protected (locked), and |
| | | sop cannot unlock it with |
| | | any of the --with-key- |
| | | password (or --old-key- |
| | | password) options |
+-------+-----------------------------+----------------------------+
| 69 | UNSUPPORTED_SUBCOMMAND | Unsupported subcommand |
+-------+-----------------------------+----------------------------+
| 71 | UNSUPPORTED_SPECIAL_PREFIX | An indirect parameter is a |
| | | special designator (it |
| | | starts with @) but sop |
| | | does not know how to |
| | | handle the prefix |
+-------+-----------------------------+----------------------------+
| 73 | AMBIGUOUS_INPUT | A indirect input parameter |
| | | is a special designator |
| | | (it starts with @), and a |
| | | filename matching the |
| | | designator is actually |
| | | present |
+-------+-----------------------------+----------------------------+
| 79 | KEY_CANNOT_SIGN | Key not signature-capable |
| | | (e.g., expired, revoked, |
| | | unacceptable usage flags) |
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| | | (sop sign and sop encrypt |
| | | with --sign-with) |
+-------+-----------------------------+----------------------------+
| 83 | INCOMPATIBLE_OPTIONS | Options were supplied that |
| | | are incompatible with each |
| | | other |
+-------+-----------------------------+----------------------------+
| 89 | UNSUPPORTED_PROFILE | The requested profile is |
| | | unsupported (sop generate- |
| | | key, sop encrypt), or the |
| | | indicated subcommand does |
| | | not accept profiles (sop |
| | | list-profiles) |
+-------+-----------------------------+----------------------------+
| 97 | NO_HARDWARE_KEY_FOUND | The sop implementation |
| | | supports some form of |
| | | hardware-backed secret |
| | | keys, but could not |
| | | identify the hardware |
| | | device (see Section 10.10) |
+-------+-----------------------------+----------------------------+
| 101 | HARDWARE_KEY_FAILURE | The sop implementation |
| | | tried to use a hardware- |
| | | backed secret key, but the |
| | | cryptographic hardware |
| | | refused the operation for |
| | | some reason other than a |
| | | bad PIN or password (see |
| | | Section 10.10) |
+-------+-----------------------------+----------------------------+
Table 1: Error return codes
If a sop implementation fails in some way not contemplated by this
document, it MAY return any non-zero error code, not only those
listed above.
8. Known Implementations
The following implementations are known at the time of this draft:
+==========+=============================================================+===========+===========+
|Project |URL |cli name |notes |
|name | | | |
+==========+=============================================================+===========+===========+
|Sequoia |https://gitlab.com/sequoia-pgp/sequoia-sop |sqop |Implemented|
|SOP | | |in Rust |
| | | |using the |
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| | | |sequoia- |
| | | |openpgp |
| | | |crate |
+----------+-------------------------------------------------------------+-----------+-----------+
|gosop |https://github.com/ProtonMail/gosop |gosop |Implemented|
| | | |in golang |
| | | |(Go) using |
| | | |GOpenPGP |
+----------+-------------------------------------------------------------+-----------+-----------+
|PGPainless|https://codeberg.org/PGPainless/pgpainless/src/branch/master/|pgpainless-|Implemented|
|SOP |pgpainless-sop |cli |in Java |
| | | |using |
| | | |PGPainless |
+----------+-------------------------------------------------------------+-----------+-----------+
|sopgpy |https://gitlab.com/sequoia-pgp/openpgp-interoperability-test-|sopgpy |Implemented|
| |suite/-/blob/main/glue/sopgpy | |in Python |
| | | |using PGPy |
+----------+-------------------------------------------------------------+-----------+-----------+
|sop- |https://github.com/openpgpjs/sop-openpgpjs |sop-openpgp|Implemented|
|openpgp.js| | |in |
| | | |JavaScript |
| | | |using |
| | | |OpenPGP.js |
+----------+-------------------------------------------------------------+-----------+-----------+
|gpgme-sop |https://gitlab.com/sequoia-pgp/gpgme-sop |gpgme-sop |A Rust |
| | | |wrapper |
| | | |around the |
| | | |gpgme C |
| | | |library |
+----------+-------------------------------------------------------------+-----------+-----------+
|RNP-sop |https://gitlab.com/sequoia-pgp/rnp-sop |rnp-sop |A Rust |
| | | |wrapper |
| | | |around the |
| | | |librnp C |
| | | |library |
+----------+-------------------------------------------------------------+-----------+-----------+
|dkg-sop |https://git.savannah.nongnu.org/cgit/dkgpg.git/tree/tools/ |dkg-sop |Implemented|
| |dkg-sop.cc | |in C++ |
| | | |using the |
| | | |LibTMCG |
| | | |library |
+----------+-------------------------------------------------------------+-----------+-----------+
Table 2: Known implementations
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9. Alternate Interfaces
This draft primarily defines a command line interface, but future
versions may try to outline a comparable idiomatic interface for C or
some other widely-used programming language.
Comparable idiomatic interfaces are already active in the wild for
different programming languages, in particular:
* Rust: [RUST-SOP]
* Java: [SOP-JAVA]
* Python: [PYTHON-SOP]
These programmatic interfaces are typically coupled with a wrapper
that can automatically generate a command-line tool compatible with
this draft.
An implementation that uses one of these languages should target the
corresponding idiomatic interface for ease of development and
interoperability.
10. Guidance for Implementers
sop uses a few assumptions that implementers might want to consider.
10.1. One OpenPGP Message at a Time
sop is intended to be a simple tool that operates on one OpenPGP
object at a time. It should be composable, if you want to use it to
deal with multiple OpenPGP objects.
FIXME: discuss what this means for streaming. The stdio interface
doesn't necessarily imply streamed output.
10.2. Simplified Subset of OpenPGP Message
While the formal grammar for OpenPGP Message is arbitrarily nestable,
sop constrains itself to what it sees as a single "layer" (see
Section 6.4).
This is a deliberate choice, because it is what most consumers
expect. Also, if an arbitrarily-nested structure is parsed with a
recursive algorithm, this risks a denial of service vulnerability.
sop intends to be implementable with a parser that defensively
declines to do recursive descent into an OpenPGP Message.
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Note that an implementation of sop decrypt MAY choose to handle more
complex structures, but if it does, it should document the other
structures it handles and why it chooses to do so. We can use such
documentation to improve future versions of this spec.
10.3. Validate Signatures Only from Known Signers
There are generally only a few signers who are relevant for a given
OpenPGP message. When verifying signatures, sop expects that the
caller can identify those relevant signers ahead of time.
10.4. OpenPGP Inputs can be either Binary or ASCII-armored
OpenPGP material on input can be in either ASCII-armored or binary
form. This is a deliberate choice because there are typical
scenarios where the program can't predict which form will appear.
Expecting the caller of sop to detect the form and adjust accordingly
seems both redundant and error-prone.
The simple way to detect possible ASCII-armoring is to see whether
the high bit of the first octet is set: Section 4.2 of [RFC4880]
indicates that bit 7 is always one in the first octet of an OpenPGP
packet. In standard ASCII-armor, the first character is "-" (HYPHEN-
MINUS, U+002D), so the high bit should be cleared.
When considering an input as ASCII-armored OpenPGP material, sop MAY
reject an input based on any of the following variations (see
Section 6.2 of [RFC4880] for precise definitions):
* An unknown Armor Header Line
* Any text before the Armor Header Line
* Malformed lines in the Armor Headers section
* Any non-whitespace data after the Armor Tail
* Any Radix-64 encoded line with more than 76 characters
* Invalid characters in the Radix-64-encoded data
* An invalid Armor Checksum
* A mismatch between the Armor Header Line and the Armor Tail
* More than one ASCII-armored object in the input
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For robustness, sop SHOULD be willing to ignore whitespace after the
Armor Tail.
For any plural data type (i.e.,SIGNATURES, CERTS, or KEYS), the
unarmored form is trivially concatenatable with another object of the
same type (e.g., with Unix's cat utility). But the armored forms are
not concatenatable without first dearmoring. To avoid inconsistent
behavior, a sop implementation SHOULD reject anything that appears to
be a concatenated series of ASCII-armored objects.
When considering OpenPGP material as input, regardless of whether it
is ASCII-armored or binary, sop SHOULD reject any material that
doesn't produce a valid stream of OpenPGP packets. For example, sop
SHOULD raise an error if an OpenPGP packet header is malformed, or if
there is trailing garbage after the end of a packet.
For a given type of OpenPGP input material (i.e., SIGNATURES, CERTS,
KEYS, INLINESIGNED, or CIPHERTEXT), sop SHOULD also reject any input
that does not conform to the expected packet stream. See Section 6
for the expected packet stream for different types.
10.5. Complexities of the Cleartext Signature Framework
sop prefers a detached signature as the baseline form of OpenPGP
signature, but provides affordances for dealing with inline-signed
messages (see INLINESIGNED, Section 6.5) as well.
The most complex form of inline-signed messages is the Cleartext
Signature Framework (CSF). Handling the CSF structure requires
parsing to delimit the multiple parts of the document, including at
least:
* any preamble before the message
* the inline message header (delimiter line, OpenPGP headers)
* the message itself
* the divider between the message and the signature (including any
OpenPGP headers there)
* the signature
* the divider that terminates the signature
* any suffix after the signature
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Note also that the preamble or the suffix might be arbitrary text,
and might themselves contain OpenPGP messages (whether signatures or
otherwise).
If the parser that does this split differs in any way from the parser
that does the verification, or parts of the message are confused, it
would be possible to produce a verification status and an actual
signed message that don't correspond to one another.
Blurred boundary problems like this can produce ugly attacks similar
to those found in [EFAIL].
A user of sop that receives an inline-signed message (whether the
message uses the CSF or not) can detach the signature from the
message with sop inline-detach (see Section 4.3.5).
Alternately, the user can send the message through sop inline-verify
to confirm required signatures, and then (if signatures are valid)
supply its output to the consumer of the signed message.
10.6. Reliance on Supplied Certs and Keys
A truly stateless implementation may find that it spends more time
validating the internal consistency of certificates and keys than it
does on the actual object security operations.
For performance reasons, an implementation may choose to ignore
validation on certificate and key material supplied to it. The
security implications of doing so depend on how the certs and keys
are managed outside of sop.
10.7. Text is always UTF-8
Various places in this specification require UTF-8 [RFC3629] when
encoding text. sop implementations SHOULD NOT consider textual data
in any other character encoding.
OpenPGP Implementations MUST already handle UTF-8, because various
parts of [RFC4880] require it, including:
* User ID
* Notation name
* Reason for revocation
* ASCII-armor Comment: header
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Dealing with messages in other charsets leads to weird security
failures like [Charset-Switching], especially when the charset
indication is not covered by any sort of cryptographic integrity
check. Restricting textual data to UTF-8 universally across the
OpenPGP ecosystem eliminates any such risk without losing
functionality, since UTF-8 can encode all known characters.
10.8. Passwords are Human-Readable
Passwords are generally expected to be human-readable, as they are
typically recorded and transmitted as human-visible, human-
transferable strings. However, they are used in the OpenPGP protocol
as bytestrings, so it is important to ensure that there is a reliable
bidirectional mapping between strings and bytes. The maximally
robust behavior here is for sop encrypt and sop generate-key (that
is, commands that use a password to encrypt) to constrain the choice
of passwords to strings that have such a mapping, and for sop decrypt
and sop sign (and sop inline-sign, as well assop encrypt when
decrypting a signing key; that is, commands that use a password to
decrypt) to try multiple plausible versions of any password supplied
by PASSWORD.
10.8.1. Generating Material with Human-Readable Passwords
When generating material based on a password, sop encrypt and sop
generate-key enforce that the password is actually meaningfully
human-transferable. In particular, an implementation generating
material based on a new paasword SHOULD apply the following
considerations to the supplied password:
* require UTF-8
* trim trailing whitespace
Some sop encrypt and sop generate-key implementations may make even
more strict requirements on input to ensure that they are
transferable between humans in a robust way.
For example, a more strict sop encrypt or sop generate-key MAY also:
* forbid leading whitespace
* forbid non-printing characters other than SPACE (U+0020), such as
ZERO WIDTH NON-JOINER (U+200C) or TAB (U+0009)
* require the password to be in Unicode Normal Form C
([UNICODE-NORMALIZATION])
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Violations of these more-strict policies SHOULD result in an error of
PASSWORD_NOT_HUMAN_READABLE.
A sop encrypt or sop generate-key implementation typically SHOULD NOT
attempt enforce a minimum "password strength", but in the event that
some implementation does, it MUST NOT represent a weak password with
PASSWORD_NOT_HUMAN_READABLE.
10.8.2. Consuming Password-protected Material
When sop decrypt receives a PASSWORD input, either from a --with-key-
password or --with-password option, it sees its content as a
bytestring. sop sign also sees the content of any PASSWORD input
supplied to its --with-key-password option as a bytestring. If the
bytestring fails to work as a password, but ends in UTF-8 whitespace,
it will try again with the trailing whitespace removed. This handles
a common pattern of using a file with a final newline, for example.
The pattern here is one of robustness in the face of typical errors
in human-transferred textual data.
A more robust sop decrypt or sop sign implementation that finds
neither of the above two attempts work for a given PASSWORD MAY try
additional variations if they produce a different bytestring, such
as:
* trimming any leading whitespace, if discovered
* trimming any internal non-printable characters other than SPACE
(U+0020)
* converting the supplied PASSWORD into Unicode Normal Form C
([UNICODE-NORMALIZATION])
A sop decrypt or sop sign implementation that stages multiple
decryption attempts like this SHOULD consider the computational
resources consumed by each attempt, to avoid presenting an attack
surface for resource exhaustion in the face of a non-standard
PASSWORD input.
10.9. Be Careful with Special Designators
As documented in Section 6.1, special designators for indirect inputs
like @ENV: and @FD: (and indirect outputs using @FD:) warrant some
special/cautious handling.
For one thing, it's conceivable that the filesystem could contain a
file with these literal names. If sop receives an indirect output
parameter that starts with an "@" (COMMERCIAL AT, U+0040) it MUST NOT
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write to the filesystem for that parameter. A sop implementation
that receives such a parameter as input MAY test for the presence of
such a file in the filesystem and fail with AMBIGUOUS_INPUT to warn
the user of the ambiguity and possible confusion.
These special designators are likely to be used to pass sensitive
data (like secret key material or passwords) so that it doesn't need
to touch the filesystem. Given this sensitivity, sop should be
careful with such an input, and minimize its leakage to other
processes. In particular, sop SHOULD NOT leak any environment
variable identified by @ENV: or file descriptor identified by @FD: to
any subprocess unless the subprocess specifically needs access to
that data.
10.10. Nuances for Hardware-backed Secret Key Material
There are a number of limitations and nuances to be aware of for
hardware-backed secret key support in this interface. Some sop
implementations will simply not support hardware-backed secret key
material. Other implementations might support only a single kind of
hardware-backing (e.g., an OpenPGP Smartcard [OPENPGP-SMARTCARD] but
not a TPM, or vice versa).
There is no formally adopted OpenPGP standard for identifying that a
given secret key is backed by hardware based on the OpenPGP wire
format. [I-D.dkg-openpgp-hardware-secrets] proposes one simple and
straightforward approach for how the wire format could cover this use
case. This simple mechanism is deliberately agnostic about the
specific kind of cryptographic hardware, but it does imply a sort of
rough shape of what the interface to the hardware would permit. In
particular, it will work best with hardware that has the following
properties:
* The hardware does specific asymmetric secret key operations, using
secret keys that it does not release.
* The user can ask the hardware to provide a list of corresponding
public key material (or OpenPGP key fingerprints) for any of the
secret keys held by the device.
* The hardware MAY require the provision of a PIN or password to
enable secret key operation, but does not require a PIN or
password for the list of public key material.
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The sop interface does not currently provide for provisioning
cryptographic hardware with secret key material, or for changing the
PIN or password for the cryptographic hardware. Users of
cryptographic hardware need to do provisioning and PIN or password
setting outside of sop.
If a user has two attached hardware tokens that both hold the same
secret key, and they are both password-locked, and they use different
passwords, sop offers no way for the user to clearly indicate which
password belongs to which device. Some cryptographic hardware is
designed to lock the device if the wrong password is entered too many
times, so users in this configuration are at risk of accidental
lockout. The easiest resolution for this is for the user to detach
any duplicate devices before invoking sop.
Note that some OpenPGP implementations use the private codepoint
ranges in the OpenPGP specification within an OpenPGP Transferable
Secret Key (e.g., [GNUPG-SECRET-STUB]) to indicate that the secret
key can be found on a smartcard.
While hardware-backed secret key operations can be significantly
slower than modern computers, and physical affordances like button-
presses or NFC tapping can themselves incur delay, it's bad form for
an invocation of sop to hang forever. This specification doesn't
define a specific maximum allowable delay, but if an implementation
calls into a hardware device either for public key listing or for
secret key operations, it should not allow the cryptographic hardware
to take an arbitrary amount of time to respond.
11. Guidance for Consumers
While sop is originally conceived of as an interface for
interoperability testing, it's conceivable that an application that
uses OpenPGP for object security would want to use it.
FIXME: more guidance for how to use such a tool safely and
efficiently goes here.
FIXME: if an encrypted OpenPGP message arrives without metadata, it
is difficult to know which signers to consider when decrypting. How
do we do this efficiently without invoking sop decrypt twice, once
without --verify-* and again with the expected identity material?
11.1. Choosing Between --as=text and --as=binary
A program that invokes sop to generate an OpenPGP signature typically
needs to decide whether it is making a text or binary signature.
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By default, sop will make a binary signature. The caller of sop sign
should choose --as=text only when it knows that:
* the data being signed is in fact textual, and encoded in UTF-8,
and
* the signed data might be transmitted to the recipient (the
verifier of the signature) over a channel that has the propensity
to transform line-endings.
Examples of such channels include FTP ([RFC0959]) and SMTP
([RFC5321]).
11.2. Special Designators and Unusual Filenames
In some cases, a user of sop might want to pass all the files in a
given directory as positional parameters (e.g., a list of CERTS files
to test a signature against).
If one of the files has a name that starts with --, it might be
confused by sop for an option. If one of the files has a name that
starts with @, it might be confused by sop as a special designator
(Section 6.1).
If the user wants to deliberately refer to such an ambiguously-named
file in the filesystem, they should prefix the filename with ./ or
use an absolute path.
Any specific @FD: special designator SHOULD NOT be supplied more than
once to an invocation of sop. If a sop invocation sees multiple
copies of a specific @FD:n input (e.g., sop sign @FD:3 @FD:3), it MAY
fail with MISSING_INPUT even if file descriptor 3 contains a valid
KEYS, because the bytestream for the KEYS was consumed by the first
argument. Doubling up on the same @FD: for output (e.g., sop decrypt
--session-key-out=@FD:3 --verifications-out=@FD:3) also results in an
ambiguous data stream.
12. Security Considerations
The OpenPGP object security model is typically used for
confidentiality and authenticity purposes.
12.1. Signature Verification
In many contexts, an OpenPGP signature is verified to prove the
origin and integrity of an underlying object.
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When sop checks a signature over data (e.g., via sop verify or sop
decrypt --verify-with), it MUST NOT consider it to be verified unless
all of these conditions are met:
* The signature must be made by a signing-capable public key that is
present in one of the supplied certificates
* The certificate and signing subkey must have been created before
or at the signature time
* The certificate and signing subkey must not have been expired at
the signature time
* The certificate and signing subkey must not be revoked with a
"hard" revocation
* If the certificate or signing subkey is revoked with a "soft"
revocation, then the signature time must predate the revocation
* The signing subkey must be properly bound to the primary key, and
cross-signed
* The signature (and any dependent signature, such as the cross-sig
or subkey binding signatures) must be made with strong
cryptographic algorithms (e.g., not MD5 or a 1024-bit RSA key)
* The signature must be of type 0x00 ("Signature of a binary
document") or 0x01 ("Signature of a canonical text document");
other signature types are inappropriate for data signatures
Implementers MAY also consider other factors in addition to the
origin and authenticity, including application-specific information.
For example, consider the application domain of checking software
updates. If software package Foo version 13.3.2 was signed on
2019-10-04, and the user receives a copy of Foo version 12.4.8 that
was signed on 2019-10-16, it may be authentic and have a more recent
signature date. But it is not an upgrade (12.4.8 < 13.3.2), and
therefore it should not be applied automatically.
In such cases, it is critical that the application confirms that the
other information verified is _also_ protected by the relevant
OpenPGP signature.
Signature validity is a complex topic (see for example the discussion
at [DISPLAYING-SIGNATURES]), and this documentation cannot list all
possible details.
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12.2. Compression
The interface as currently specified does not allow for control of
compression. Compressing and encrypting data that may contain both
attacker-supplied material and sensitive material could leak
information about the sensitive material (see the CRIME attack).
Unless an application knows for sure that no attacker-supplied
material is present in the input, it should not compress during
encryption.
13. Privacy Considerations
Material produced by sop encrypt may be placed on an untrusted
machine (e.g., sent through the public SMTP network). That material
may contain metadata that leaks associational information (e.g.,
recipient identifiers in PKESK packets (Section 5.1 of
[I-D.ietf-openpgp-crypto-refresh-10])). FIXME: document things like
PURBs and --hidden-recipient)
13.1. Object Security vs. Transport Security
OpenPGP offers an object security model, but says little to nothing
about how the secured objects get to the relevant parties.
When sending or receiving OpenPGP material, the implementer should
consider what privacy leakage is implicit with the transport.
14. References
14.1. Normative References
[I-D.ietf-openpgp-crypto-refresh-10]
Wouters, P., Huigens, D., Winter, J., and N. Yutaka,
"OpenPGP", Work in Progress, Internet-Draft, draft-ietf-
openpgp-crypto-refresh-10, 21 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-
crypto-refresh-10>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156,
DOI 10.17487/RFC3156, August 2001,
<https://www.rfc-editor.org/rfc/rfc3156>.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/rfc/rfc4880>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
14.2. Informative References
[Charset-Switching]
Gillmor, D. K., "Inline PGP Considered Harmful", 24
February 2014,
<https://dkg.fifthhorseman.net/notes/inline-pgp-harmful/>.
[DISPLAYING-SIGNATURES]
Brunschwig, P., "On Displaying Signatures", n.d.,
<https://admin.hostpoint.ch/pipermail/enigmail-
users_enigmail.net/2017-November/004683.html>.
[EFAIL] Poddebniak, D. and C. Dresen, "Efail: Breaking S/MIME and
OpenPGP Email Encryption using Exfiltration Channels",
n.d., <https://efail.de>.
[GNUPG-SECRET-STUB]
Koch, W., "GNU Extensions to the S2K algorithm", 4 July
2023,
<https://dev.gnupg.org/source/gnupg/browse/master/doc/
DETAILS;gnupg-2.4.3$1511>.
[I-D.dkg-openpgp-hardware-secrets]
Gillmor, D. K., "OpenPGP Hardware-Backed Secret Keys",
Work in Progress, Internet-Draft, draft-dkg-openpgp-
hardware-secrets-00, 28 December 2023,
<https://datatracker.ietf.org/doc/html/draft-dkg-openpgp-
hardware-secrets-00>.
[I-D.draft-bre-openpgp-samples-01]
Einarsson, B. R., "juga", and D. K. Gillmor, "OpenPGP
Example Keys and Certificates", Work in Progress,
Internet-Draft, draft-bre-openpgp-samples-01, 20 December
2019, <https://datatracker.ietf.org/doc/html/draft-bre-
openpgp-samples-01>.
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[I-D.ietf-lamps-e2e-mail-guidance-11]
Gillmor, D. K., Hoeneisen, B., and A. Melnikov, "Guidance
on End-to-End E-mail Security", Work in Progress,
Internet-Draft, draft-ietf-lamps-e2e-mail-guidance-11, 8
August 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-lamps-e2e-mail-guidance-11>.
[OpenPGP-Interoperability-Test-Suite]
"OpenPGP Interoperability Test Suite", 25 October 2021,
<https://tests.sequoia-pgp.org/>.
[OPENPGP-SMARTCARD]
Pietig, A., "Functional Specification of the OpenPGP
application on ISO Smart Card Operating Systems, Version
3.4", 18 March 2020, <https://www.gnupg.org/ftp/specs/
OpenPGP-smart-card-application-3.4.pdf>.
[PYTHON-SOP]
Gillmor, D., "SOP for python", n.d.,
<https://pypi.org/project/sop/>.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985,
<https://www.rfc-editor.org/rfc/rfc959>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/rfc/rfc5321>.
[RUST-SOP] Winter, J., "A Rust implementation of the Stateless
OpenPGP Protocol", n.d.,
<https://sequoia-pgp.gitlab.io/sop-rs/>.
[SEMVER] Preston-Werner, T., "Semantic Versioning 2.0.0", 18 June
2013, <https://semver.org/>.
[SOP-JAVA] Schaub, P., "Stateless OpenPGP Protocol for Java.", n.d.,
<https://github.com/pgpainless/sop-java>.
[UNICODE-NORMALIZATION]
Whistler, K., "Unicode Normalization Forms", 4 February
2019, <https://unicode.org/reports/tr15/>.
Appendix A. C Library API (Tentative)
As specified in this draft, SOP is a command-line tool.
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However, it can also be useful to have a comparable API exposed as a
C library. This library can be implemented as a shared object (e.g.,
.so, .dll, or .dylib depending on the platform) or as a statically
linked object. This interface can be reused in many different
places, as most modern programming languages offer "bindings" to C
libraries.
A proposed interface to a C library follows here as a C header file.
The primary goal of this shared object interface is to make it easy
to implement the command-line interface described in this document.
That said, it is also intended to be relatively ergonomic to use in
plausible OpenPGP workflows where the caller has access to all of the
explicit state.
If there is a plausible OpenPGP workflow that is not supported by
this library API, please propose improvements and explain the
specific workflow.
#ifndef __SOP_H__
#define __SOP_H__
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <string.h>
#include <limits.h>
/* C API for Stateless OpenPGP */
/* Depends on C99 */
/* statically-defined, non-opaque definitions */
typedef enum {
SOP_OK = 0,
SOP_INTERNAL_ERROR = 1, /* Not part of sop CLI */
SOP_INVALID_ARG = 2, /* Not part of sop CLI */
SOP_NO_SIGNATURE = 3,
SOP_OPERATION_ALREADY_EXECUTED = 4, /* Not part of sop CLI */
SOP_UNSUPPORTED_ASYMMETRIC_ALGO = 13,
SOP_CERT_CANNOT_ENCRYPT = 17,
SOP_MISSING_ARG = 19,
SOP_INCOMPLETE_VERIFICATION = 23,
SOP_CANNOT_DECRYPT = 29,
SOP_PASSWORD_NOT_HUMAN_READABLE = 31,
SOP_UNSUPPORTED_OPTION = 37,
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SOP_BAD_DATA = 41,
SOP_EXPECTED_TEXT = 53,
SOP_OUTPUT_EXISTS = 59,
SOP_MISSING_INPUT = 61,
SOP_KEY_IS_PROTECTED = 67,
SOP_UNSUPPORTED_SUBCOMMAND = 69,
SOP_UNSUPPORTED_SPECIAL_PREFIX = 71,
SOP_AMBIGUOUS_INPUT = 73,
SOP_KEY_CANNOT_SIGN = 79,
SOP_INCOMPATIBLE_OPTIONS = 83,
SOP_UNSUPPORTED_PROFILE = 89,
SOP_NO_HARDWARE_KEY_FOUND = 97,
SOP_HARDWARE_KEY_FAILURE = 101,
/* ensures a stable size for the enum -- do not use! */
SOP_MAX_ERR = INT_MAX,
} sop_err;
typedef enum {
SOP_SIGN_AS_BINARY = 0,
SOP_SIGN_AS_TEXT = 1,
/* ensures a stable size for the enum -- do not use! */
SOP_SIGN_AS_MAX = INT_MAX,
} sop_sign_as;
typedef enum {
SOP_INLINE_SIGN_AS_BINARY = 0,
SOP_INLINE_SIGN_AS_TEXT = 1,
SOP_INLINE_SIGN_AS_CLEARSIGNED = 2,
/* ensures a stable size for the enum -- do not use! */
SOP_INLINE_SIGN_AS_MAX = INT_MAX,
} sop_inline_sign_as;
typedef enum {
SOP_ENCRYPT_AS_BINARY = 0,
SOP_ENCRYPT_AS_TEXT = 1,
/* ensures a stable size for the enum -- do not use! */
SOP_ENCRYPT_AS_MAX = INT_MAX,
} sop_encrypt_as;
/* FIXME: timestamps */
/* time_t is 32-bit on some architectures; we also want this to be
able to represent a "none" value as well as a "now" value without
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removing some value from the range of time_t */
typedef time_t sop_time;
#define sop_time_none ((sop_time)0)
#define sop_time_now ((sop_time)-1)
/* Context object
*
* Each SOP object is bound back to a context object, and, when used
* in combination with other SOP objects, all SOP objects should come
* from the same context.
*
* A SOP context object need not be thread-safe; it should probably
* not be used across multiple threads. See "Zero global state" in
* https://git.kernel.org/pub/scm/linux/kernel/git/kay/libabc.git/plain/README
*/
struct sop_ctx_st;
typedef struct sop_ctx_st sop_ctx;
sop_ctx*
sop_ctx_new ();
void
sop_ctx_free (sop_ctx *sop);
/* Logging: */
typedef enum {
SOP_LOG_NEVER = 0,
SOP_LOG_ERROR = 1,
SOP_LOG_WARNING = 2,
SOP_LOG_INFO = 3,
SOP_LOG_DEBUG = 4,
/* ensures a stable size for the enum -- do not use! */
SOP_LOG_MAX = INT_MAX,
} sop_log_level;
static inline const char *
sop_log_level_name (sop_log_level log_level) {
#define rep(x) if (log_level == SOP_LOG_ ## x) return #x
rep(ERROR);
rep(WARNING);
rep(INFO);
rep(DEBUG);
#undef rep
return "Unknown";
}
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/* Handle warnings and other feedback.
*
* A SOP implementation that is capable of producing log messages will
* invoke the requested function with the log level of the message,
* and a NULL-terminated UTF-8 human-readable string with no trailing
* whitespace.
*
* the "passthrough" pointer is supplied by the library user via
* sop_set_log_level.
*/
typedef void (*sop_log_func) (sop_log_level log_level, void *passthrough, const char *);
sop_err
sop_set_log_function (sop_ctx *sop, sop_log_func func, void *passthrough);
/* Set the logging verbosity.
*
* Only log warnings up to max_level. (by default, max_level is
* SOP_LOG_WARNING, meaning SOP_LOG_INFO and SOP_LOG_DEBUG will be
* suppressed).
*/
sop_err
sop_set_log_level (sop_ctx *sop, sop_log_level max_level);
/* Information about the library: */
/* The name and version of the implementation of the C API (simple
* NUL-terminated string, no newlines), or NULL if there is an error
* producing the version. */
const char *
sop_version (sop_ctx *sop);
/* The name and version of the primary underlying OpenPGP toolkit (or
* NULL if there is no backend, or if there was an error producing the
* backend version) */
const char *
sop_version_backend (sop_ctx *sop);
/* Any arbitrary extended version information other than sop_ctx_version.
Version info should be UTF-8 text, separated by newlines (a
NUL-terminated string, no trailing newline). Can return NULL if
there is nothing more to report beyond sop_version. */
const char *
sop_version_extended (sop_ctx *sop);
/* note: there is nothing comparable to sop version --sop-spec because
* that should be visible based on the exported symbols in the shared
* object */
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/* PROFILE objects: */
/* These describe a profile (e.g. for generate-key or encrypt). This
* use used when the implementation might legitimately want to offer
* the user some minimal amount of control over what is done. The
* profile-listing functions return blocks of four profiles. A
* sop_profile value of NULL represents no profile at all. In a list
* of sop_profile objects, once a NULL profile appears, no non-NULL
* profiles may follow.
*/
struct sop_profile_st;
typedef struct sop_profile_st sop_profile;
/* the NUL-terminated string returned by sop_profile_name MUST be a
UTF-8 encoded string, and MUST NOT include any whitespace or colon
(`:`) characters. It MUST NOT vary depending on locale. */
const char *
sop_profile_name (const sop_profile *profile);
/* The NUL-terminated string returned by sop_profile_description
cannot contain any newlines, and it MAY vary depending on
locale(7) if the implementation is internationalized. */
const char *
sop_profile_description (const sop_profile *profile);
#define SOP_MAX_PROFILE_COUNT 4
typedef struct {
sop_profile *profile[SOP_MAX_PROFILE_COUNT];
} sop_profiles;
static inline int
sop_profiles_count(const sop_profiles profiles) {
for (int i = 0; i < SOP_MAX_PROFILE_COUNT; i++)
if (profiles.profile[i] == NULL)
return i;
return SOP_MAX_PROFILE_COUNT;
}
/* Return a list of profiles supported by the library for generating
* keys.
*/
sop_err
sop_list_profiles_generate_key (sop_ctx *sop, sop_profiles *out);
/* CLEARTEXT (and other raw data): */
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/* This is a standard buffer for bytestrings produced by sop. Users
never create this kind of object, but it is sometimes returned from
the library. */
struct sop_buf_st;
typedef struct sop_buf_st sop_buf;
void
sop_buf_free (sop_buf *buf);
size_t
sop_buf_size (const sop_buf *buf);
const uint8_t *
sop_buf_data (const sop_buf *buf);
/* KEYS objects: */
struct sop_keys_st;
typedef struct sop_keys_st sop_keys;
sop_err
sop_keys_from_bytes (sop_ctx *sop,
const uint8_t* data, size_t len,
sop_keys **out);
sop_err
sop_keys_to_bytes (const sop_keys *keys,
bool armor, sop_buf **out);
void
sop_keys_free (sop_keys *keys);
/* Generate a new, minimal OpenPGP Transferable secret key. `profile`
can be NULL to mean the default profile. */
sop_err
sop_generate_key_with_profile (sop_ctx *sop,
sop_profile *profile,
bool sign_only,
sop_keys **out);
static inline sop_err
sop_generate_key (sop_ctx *sop, sop_keys **out) {
return sop_generate_key_with_profile (sop, NULL, false, out);
}
/* For each key in the sop_keys object, add the given user ID, and
return a new sop_keys object containing the updated keys. If the
supplied user ID is not valid UTF-8 text, this call will fail and
return SOP_EXPECTED_TEXT.
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If the implementation rejects the user ID string by policy for any
other reason, this call will fail and return SOP_BAD_DATA.
*/
sop_err
sop_keys_add_uid (const sop_keys *keys, const char *uid, sop_keys **out);
/* returns true if any of the secret key material is currently locked
with a password */
sop_err
sop_keys_locked (const sop_keys *keys, bool *out);
/* return a new sop_keys object with any secret key material encrypted
with `password` unlocked, Returns SOP_OK if all keys have now been
unlocked.
If any locked key material could not be unlocked, return
SOP_KEY_IS_PROTECTED, while also unlocking what key material can be
unlocked.
This allows the user to try an arbitrary bytestream as a password.
Most users will just invoke the inlined sop_keys_unlock, below.
An implementation MUST NOT reject proposed passwords by policy
during unlock, but rather should try them as requested.
*/
sop_err
sop_keys_unlock_raw (const sop_keys *keys,
const uint8_t *raw_password, size_t len,
sop_keys **out);
static inline sop_err
sop_keys_unlock (const sop_keys *keys, const char *password, sop_keys **out) {
return sop_keys_unlock_raw (keys,
(const uint8_t *)password,
strlen (password),
out);
}
/* return a new sop_keys object where all secret key material is
locked with `password` where possible.
During locking, a safety-oriented implementation MAY reject the
supplied password by policy for any number of reasons. This helps
libsop ensure that the proposed password can be successfully
re-supplied during some future unlock attempt.
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If the implementation requires passwords to be UTF-8 text and the
supplied password is not valid UTF-8, the implementation will fail,
returning SOP_EXPECTED_TEXT. If an implementation rejects a
supplied password for some other reason (for example, if it
contains an NUL, unprintable, or otherwise forbidden character),
this call will fail and return SOP_BAD_DATA.
If any key material is already locked, it does nothing and returns
SOP_KEY_IS_PROTECTED.
Upon a successful locking, the user probably wants to use
sop_keys_free to free the original keys object.
*/
sop_err
sop_keys_lock_raw (const sop_keys *keys,
const uint8_t *password, size_t len,
sop_keys **out);
static inline sop_err
sop_keys_lock (const sop_keys *keys, const char *password, sop_keys **out) {
return sop_keys_lock_raw (keys,
(const uint8_t *)password,
strlen (password),
out);
}
/* CERTS objects: */
struct sop_certs_st;
typedef struct sop_certs_st sop_certs;
sop_err
sop_certs_from_bytes (sop_ctx *sop,
const uint8_t* data, size_t len,
sop_certs **out);
sop_err
sop_certs_to_bytes (const sop_certs *certs,
bool armor, sop_buf **out);
void
sop_certs_free (sop_certs *certs);
/* Return the OpenPGP certificates ("Transferable Public Keys") that
correspond to the OpenPGP Transferable Secret Keys. */
sop_err
sop_keys_extract_certs (const sop_keys *keys, sop_certs **out);
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/* Return an OpenPGP revocation certificate for each Transferable
Secret Key found in the input. */
sop_err
sop_keys_revoke_keys (const sop_keys *keys, sop_certs **out);
/* SIGNATURES objects: */
struct sop_sigs_st;
typedef struct sop_sigs_st sop_sigs;
sop_err
sop_sigs_from_bytes (sop_ctx *sop,
const uint8_t* data, size_t len,
sop_sigs **out);
sop_err
sop_sigs_to_bytes (const sop_sigs *sigs,
bool armor, sop_buf **out);
void
sop_sigs_free (sop_sigs *sigs);
/* VERIFICATIONS (output only, describes valid, verified signatures): */
struct sop_verifications_st;
typedef struct sop_verifications_st sop_verifications;
void
sop_verifications_free (sop_verifications *verifs);
sop_err
sop_verifications_count (const sop_verifications *verifs, int *out);
/* textual representations of verifications, in the form described by
VERIFICATIONS in the CLI */
sop_err
sop_verifications_to_text (const sop_verifications *verifs,
sop_buf **out);
/* returns SOP_INTERNAL_ERROR if count is out of bounds. */
sop_err
sop_verifications_get_time (const sop_verifications *verifs,
int count, sop_time *out);
/* returns SOP_INTERNAL_ERROR if count is out of bounds. If the
signature is neither type 0x00 nor 0x01, this should probably not
be considered a valid, verified signature. */
sop_err
sop_verifications_get_mode (const sop_verifications *verifs,
int count, sop_sign_as *out);
/* FIXME: (do we want to get more detailed info programmatically?
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each verification should also have an issuing key fingerprint, a
primary key fingerprint, and a trailing text string) */
/* create detached signatures: */
struct sop_op_sign_st;
typedef struct sop_op_sign_st sop_op_sign;
sop_err
sop_op_sign_new (sop_ctx *sop, sop_op_sign** out);
void
sop_op_sign_free (sop_op_sign *sign);
sop_err
sop_op_sign_use_keys (sop_op_sign *sign, const sop_keys *keys);
sop_err
sop_op_sign_detached_execute (sop_op_sign *sign,
sop_sign_as sign_as,
const uint8_t *msg,
size_t sz,
sop_buf **micalg_out,
sop_sigs **out);
/* verify detached signatures: */
struct sop_op_verify_st;
typedef struct sop_op_verify_st sop_op_verify;
sop_err
sop_op_verify_new (sop_ctx *sop, sop_op_verify** out);
void
sop_op_verify_free (sop_op_verify *verify);
sop_err
sop_op_verify_not_before (sop_op_verify *verify, sop_time when);
sop_err
sop_op_verify_not_after (sop_op_verify *verify, sop_time when);
sop_err
sop_op_verify_add_signers (sop_op_verify *verify,
const sop_certs *signers);
/* if no verifications are possible with the set of signers, this
returns SOP_NO_SIGNATURE, and *out is set to NULL */
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sop_err
sop_op_verify_detached_execute (sop_op_verify *verify,
const sop_sigs *sigs,
const uint8_t *msg,
size_t sz,
sop_verifications **out);
/* INLINESIGNED object: */
struct sop_inlinesigned_st;
typedef struct sop_inlinesigned_st sop_inlinesigned;
sop_err
sop_inlinesigned_from_bytes (sop_ctx *sop,
const uint8_t* data, size_t len,
sop_inlinesigned **out);
/* if the inlinesigned object uses the Cleartext Signing framework,
* the armor parameter is ignored.
*/
sop_err
sop_inlinesigned_to_bytes (const sop_inlinesigned *inlinesigned,
bool armor, sop_buf **out);
void
sop_inlinesigned_free (sop_inlinesigned *inlinesigned);
/* sop inline-sign */
sop_err
sop_op_sign_inline_execute (sop_op_sign *sign,
sop_inline_sign_as sign_as,
const uint8_t *msg,
size_t sz,
sop_inlinesigned **out);
/* sop inline-verify */
sop_err
sop_op_verify_inline_execute (sop_op_verify *verify,
const sop_inlinesigned *msg,
sop_verifications **verifications_out,
sop_buf **msg_out);
/* sop inline-detach */
sop_err
sop_inlinesigned_detach (const sop_inlinesigned *msg,
sop_sigs **sigs_out,
sop_buf **msg_out);
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#endif // __SOP_H__
This proposed interface currently deals only with signing.
Encryption and decryption will be added in a future revision.
A.1. Design Choices for Library API
The library is deliberately minimal, with data types and
functionality corresponding to the SOP CLI. The interface itself
should expose no dependencies beyond libc.
All datatypes are opaque structs. Library implementations MUST NOT
expose library users to the memory layout of the underlying objects.
The library deals with data that is all in RAM, and produces data in
RAM. For simplicity, it does not currently expose a streaming
interface.
It should be fairly straightforward to implement the SOP CLI on top
of such a library.
A.2. Library Use Patterns
There are two main kinds of data structures: operations (e.g.,
sop_op_sign and sop_op_verify) and datatypes (e.g., sop_keys and
sop_certs).
Operation objects are one-shot objects. They are used in the
following pattern:
* create an operations object (sop_op_*_new)
* adjust it to behave in certain ways (e.g., sop_op_sign_use_keys,
sop_op_verify_not_before)
* execute it (with some specific sop_op_*_execute function)
* dispose of it (sop_op_*_free)
The library user MUST NOT execute the same operation object more than
once. When a single operation object is executed more than once, it
should fail with SOP_OPERATION_ALREADY_EXECUTED. FIXME: if a use
case arises with a reasonable need to re-execute an already adjusted
object, we could extend the API to allow the user to clone an object.
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Datatype objects are reusable objects. For example, it is fine for a
library user to pass the same sop_certs to multiple sop_op_*
operation objects, as long as the sop_certs object is not freed
before the execution of all the operation objects it has been passed
to.
Datatype objects are also immutable. Any function which modifies a
datatype object always creates a new copy of the object, with the
specific change applied. This immutability avoids any ambguity about
what should happen when a datatype object is adjusted after it was
passed to an operation object but before it was executed.
A.3. libsopv C API Subset
A minimalist library subset that only does OpenPGP signature
verification might be called libsopv. This library is useful
wherever the use case is just OpenPGP signature verification.
Such a library MUST implement the following functions from the C API:
* sop_ctx_new
* sop_ctx_free
* sop_set_log_function
* sop_set_log_level
* sop_version
* sop_version_backend
* sop_version_extended
* sop_buf_size
* sop_buf_data
* sop_buf_free
* sop_certs_from_bytes
* sop_certs_free
* sop_sigs_from_bytes
* sop_sigs_free
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* sop_verifications_count
* sop_verifications_get_time
* sop_verifications_get_mode
* sop_verifications_to_text
* sop_verifications_free
* sop_inlinesigned_from_bytes
* sop_op_verify_new
* sop_op_verify_not_before
* sop_op_verify_not_after
* sop_op_verify_add_signers
* sop_op_verify_detached_execute
* sop_op_verify_inline_execute
* sop_op_verify_free
This minimal library interface should be sufficient to implement the
sopv subset (see Section 3).
Appendix B. Acknowledgements
This work was inspired by Justus Winter's
[OpenPGP-Interoperability-Test-Suite].
The following people contributed helpful feedback and considerations
to this draft, but are not responsible for its problems:
* Allan Nordhøy
* Antoine Beaupré
* Edwin Taylor
* Guillem Jover
* Heiko Schaefer
* Jameson Rollins
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* Justus Winter
* Paul Schaub
* Vincent Breitmoser
Appendix C. Future Work
* certificate transformation into popular publication forms:
- WKD
- DANE OPENPGPKEY
- Autocrypt
* sop encrypt -- specify compression? (see Section 12.2)
* sop encrypt -- specify padding policy/mechanism?
* sop decrypt -- how can it more safely handle zip bombs?
* sop decrypt -- what should it do when encountering weakly-
encrypted (or unencrypted) input?
* sop encrypt -- minimize metadata (e.g., --throw-keyids)?
* specify an error if a DATE arrives as input without a time zone?
* add considerations about what it means for armored CERTS to
contain multiple certificates -- multiple armorings? one big
blob?
* do we need an interface or option (for performance?) with the
semantics that sop doesn't validate certificates internally, it
just accepts whatever's given as legit data? (see Section 10.6)
* do we need to be able to convert a message with a text-based
signature to a CSF INLINESIGNED message? I'd rather not, given
the additional complications.
* add encryption and decryption to C Library API
Appendix D. Document History
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D.1. Substantive Changes between -09 and -10:
* drop @HARDWARE: special designator in favor of the simple
[I-D.dkg-openpgp-hardware-secrets] or other magic
* drop hardware-specific C API function
* define SemVer-versioned sopv subset of CLI
* sop version: add --sopv option
* define libsopv subset of C API
* explicitly require BAD_DATA failure when KEYS are passed as CERTS
D.2. Substantive Changes between -08 and -09:
* enable the use of hardware-backed secret key material via the
@HARDWARE: special designator
* C API: clarify design goals and usage patterns
* C API: major overhaul and normalization:
- allow passthrough "cookie" for logging
- allow NULL return from sop_version_*
- explicitly offer SOP_LOG_NEVER
- use *_from_bytes and *_to_bytes instead of *_import and
*_export
- datatype objects are now immutable
- operation objects are one-shot
- always return sop_err, even at a slight cost to C caller
ergonomics
D.3. Substantive Changes between -07 and -08:
* revoke-key, change-key-password: add --no-armor option
* generate-key: should fail on non-UTF-8 USERID
* generate-key: acknowledge that implementations MAY reject USERIDs
that seem bad
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* armor: drop --label option
* encrypt: add --session-key-out option
* ASCII-armored objects should not be concatenated
* signature verification should only work for sigtypes 0x00 (binary)
and 0x01 (canonical text)
* sign: Constrain input when --micalg-out is present for alignment
with [RFC3156]
* propose simple C API for signing and verification
D.4. Substantive Changes between -06 and -07:
* generate-key: add --signing-only option
* new key management subcommand: change-key-password
* new key management subcommand: revoke-key
D.5. Substantive Changes between -05 and -06:
* version: add --sop-spec argument
* encrypt: add --profile argument
D.6. Substantive Changes between -04 and -05:
* decrypt: change --verify-out to --verifications-out
* encrypt: add missing --with-key-password
* add the concept of "profiles", use with generate-key
* include table of known implementations
* VERIFICATIONS can now indicate the type of the signature
(mode:text or mode:binary)
D.7. Substantive Changes between -03 and -04:
* Reinforce that PASSWORD and SESSIONKEY are indirect data types
* encrypt: remove --as=mime option
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* Handle password-locked secret key material: add --with-key-
password options to generate-key, sign, and decrypt.
* Introduce INLINESIGNED message type (Section 6.5)
* Rename detach-inband-signature-and-message to inline-detach,
clarify its possible inputs
* Add inline-verify
* Add inline-sign
D.8. Substantive Changes between -02 and -03:
* Added --micalg-out parameter to sign
* Change from KEY to KEYS (permit multiple secret keys in each blob)
* New error code: KEY_CANNOT_SIGN
* version now has --backend and --extended options
D.9. Substantive Changes between -01 and -02:
* Added mnemonics for return codes
* decrypt should fail when asked to output to a pre-existing file
* Removed superfluous --armor option
* Much more specific about what armor --label=auto should do
* armor and dearmor are now fully idempotent, but work only well-
formed OpenPGP streams
* Dropped armor --allow-nested
* Specified what encrypt --as= means
* New error code: KEY_IS_PROTECTED
* Documented expectations around human-readable, human-transferable
passwords
* New subcommand: detach-inband-signature-and-message
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* More specific guidance about special designators like @FD: and
@ENV:, including new error codes UNSUPPORTED_SPECIAL_PREFIX and
AMBIGUOUS_INPUT
D.10. Substantive Changes between -00 and -01:
* Changed generate subcommand to generate-key
* Changed convert subcommand to extract-cert
* Added "Input String Types" section as distinct from indirect I/O
* Made implicit arguments potentially explicit (e.g., sop armor
--label=auto)
* Added --allow-nested to sop armor to make it idempotent by default
* Added fingerprint of signing (sub)key to VERIFICATIONS output
* Dropped --mode and --session-key arguments for sop encrypt (no
plausible use, not needed for interop)
* Added --with-session-key argument to sop decrypt to allow for
session-key-based decryption
* Added examples to each subcommand
* More detailed error codes for sop encrypt
* Move from CERT to CERTS (each CERTS argument might contain
multiple certificates)
Author's Address
Daniel Kahn Gillmor
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Email: dkg@fifthhorseman.net
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