Internet DRAFT - draft-nwjw-openpgp-cert-d
draft-nwjw-openpgp-cert-d
openpgp N. Widdecke
Internet-Draft J. Winter
Intended status: Informational Sequoia PGP
Expires: 2 December 2022 31 May 2022
Shared OpenPGP Certificate Directory
draft-nwjw-openpgp-cert-d-00
Abstract
This document defines a generic OpenPGP certificate store that can be
shared between implementations. It also defines a way to root trust,
and a way to associate petnames with certificates. Sharing
certificates and trust decisions increases security by enabling more
applications to take advantage of OpenPGP. It also improves privacy
by reducing the required certificate discoveries that go out to the
network.
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://sequoia-
pgp.gitlab.io/pgp-cert-d. Status information for this document may
be found at https://datatracker.ietf.org/doc/draft-nwjw-openpgp-cert-
d/.
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/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/sequoia-pgp/pgp-cert-d.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Related work . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1. OpenPGP keyrings . . . . . . . . . . . . . . . . . . 4
1.3.2. X.509 certificate stores . . . . . . . . . . . . . . 5
1.3.3. Maildir . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Addressing of certs . . . . . . . . . . . . . . . . . . . 6
2.2. Trust root . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Petname mapping . . . . . . . . . . . . . . . . . . . . . 7
2.4. Trusted introducers . . . . . . . . . . . . . . . . . . . 7
3. Implementation . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Default store's location . . . . . . . . . . . . . . . . 8
3.2. Mapping names to paths . . . . . . . . . . . . . . . . . 8
3.2.1. Fingerprints . . . . . . . . . . . . . . . . . . . . 8
3.2.2. Special names . . . . . . . . . . . . . . . . . . . . 9
3.3. Locking the store for writes . . . . . . . . . . . . . . 9
3.4. How to insert or update certs . . . . . . . . . . . . . . 9
3.5. Rooting trust . . . . . . . . . . . . . . . . . . . . . . 10
3.5.1. Trust root . . . . . . . . . . . . . . . . . . . . . 10
3.5.2. Petname mapping . . . . . . . . . . . . . . . . . . . 10
3.5.3. Trusted introducers . . . . . . . . . . . . . . . . . 11
3.6. Proprietary and experimental extensions . . . . . . . . . 11
3.7. Reserved filenames . . . . . . . . . . . . . . . . . . . 11
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3.8. Platform-specific conventions . . . . . . . . . . . . . . 12
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Reference implementation . . . . . . . . . . . . . . . . . . 12
5.1. Opening the store . . . . . . . . . . . . . . . . . . . . 13
5.2. Certificate lookup . . . . . . . . . . . . . . . . . . . 13
5.3. Certificate update . . . . . . . . . . . . . . . . . . . 13
5.4. Store enumeration . . . . . . . . . . . . . . . . . . . . 14
5.5. Input/Output Types . . . . . . . . . . . . . . . . . . . 14
5.5.1. NAME . . . . . . . . . . . . . . . . . . . . . . . . 14
5.5.2. TAG . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.5.3. CERT . . . . . . . . . . . . . . . . . . . . . . . . 14
5.5.4. KEY . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.6. Failure Modes . . . . . . . . . . . . . . . . . . . . . . 14
6. Guidance for Implementers . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Document Considerations . . . . . . . . . . . . . . . . . . . 15
8.1. Document History . . . . . . . . . . . . . . . . . . . . 15
8.2. Future Work . . . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
Using OpenPGP for encryption requires a certificate for each
communication partner. Likewise, verification of an OpenPGP
signature requires the signer's certificate.
An OpenPGP certificate must be discovered before it can be used.
There are a number of ways to do that, for example via
[keys.openpgp.org] or [I-D.draft-koch-openpgp-webkey-service-12].
Furthermore, an OpenPGP certificate evolves over time. The
certificate itself or one of its components may be revoked; a User ID
may be added; certificate subkeys may be rotated, and meta-data
stored on signatures updated. Crucially, the security of OpenPGP
depends on distributing each update to every involved party. A
certificate update may be passively collected (e.g. by consuming an
[Autocrypt] header), or actively sought out using the key discovery
options mentioned above.
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However, actively reaching out to a network source leaks information
about the expected communication partner or partners, so requests
should be kept to a minimum. Now, if a user has more than one
application supporting OpenPGP, then every application has to
discover certificates and updates, increasing the meta-data leakage.
The obvious solution here is to provide a way to share the
certificates instead. This is the purpose of this specification.
Looking at X.509, we can see that on most systems, there is a shared
store of root certificates. Now, this root certificate store solves
a different problem: X.509 certificates do not need to be discovered.
Instead, the shared store ensures that every application uses the
same set of trust roots, which is also desirable for OpenPGP. The
important aspect we want to point out is that the store is shared
across different applications and TLS implementations. We will come
back to the differences later in this text.
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", or "cert" for short, to refer to
OpenPGP Transferable Public Key (see section 11.1 of [RFC4880]).
1.3. Related work
1.3.1. OpenPGP keyrings
The classic way of sharing data between OpenPGP implementations is
via a keyring (see section 3.6 of [RFC4880]). The only defined
format is simply a sequence of certs, stored in binary (not ASCII-
armored) format. The advantage is that only OpenPGP data structures
are used, and hence support for keyrings is widespread in OpenPGP
implementations.
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But, because an OpenPGP keyring does not have an index, this data
structure scales badly: Both lookups and updates take O(N) time,
where N is the number of certs. Worse, if the keyring contains a
flooded certificate, it will negatively affect the performance of
every operation, not just operations on the flooded cert itself.
Because of these limitations, an OpenPGP keyring as defined in
[RFC4880] should really only be used as interchange format (i.e. for
import and export), not for continuous sharing of certs between
applications and implementations.
1.3.2. X.509 certificate stores
Looking at X.509, we can see that on most systems, there is a shared
store of root certificates (see e.g. [FedoraSharedX.509CertStore],
[WindowsSharedX.509CertStore], [macOSSharedX.509CertStore]). Now,
this root certificate store solves a slightly different problem: It
ensures that every application uses the same set of trust roots.
During the Transport Layer Security ([TLS]) handshake, the party that
wants to authenticate itself (usually the server) presents the
certificate, along with all intermediate certificates in the
authentication chain up to a root certificate. In this setting, we
don't need to discover any certificates. Instead, the store is used
to check if the presented root certificate is in the set of trusted
root certificates. Additionally, the store may contain certificate
revocations.
In both OpenPGP and X.509, trust must be rooted. While the
predominant trust model in X.509 uses a fixed set of vendor-specified
trusted third parties, in OpenPGP the user is expected to provide
this set. See Section 2.4 for how this is modeled in this spec.
The main takeaway here is that to ensure a consistent behavior and
user experience, the certificate store with all its information
should be shared across all applications that use OpenPGP to
authenticate communication partners.
1.3.3. Maildir
Maildir is an on-disk data structure that is designed to allow
concurrent access by programs storing mails into and retrieving them
(see [Maildir]). It allows lock-free operations by relying on the
atomicity of rename(2). It is supported by a wide range of mail
servers, delivery agents, and mail user agents.
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Maildir is mainly concerned with storing blobs and orchestrating
concurrent access to the store. It does not provide any indices.
For example, if you need efficient full text search, you will need to
construct an index on top of the maildir, and keep it up to date (see
[Notmuch]).
Maildir's design and success is a major inspiration for this spec.
2. Requirements
This specification is motivated by the following requirements:
* The performance should not be affected by the number of
certificates in the store, or by the size of individual
certificates.
* We expect a read-heavy workload. As such, readers should not have
to synchronize with each other or with writers.
* Updates must be robust, i.e., they must not lose information in
the event of concurrent updates.
* No extra data structures besides the file system and OpenPGP
should be used to facilitate adoption by OpenPGP implementations.
Furthermore, the following requirements are required for secure and
ergonomic use of OpenPGP. Since any application using OpenPGP needs
to behave consistently so as not to jeopardize security and
ergonomics, this information needs to be shared well. Hence the
ideal place is the certificate store:
* Address book-like mapping from petnames to certificates.
* Configuring a set of trusted introducers.
We also like to have some non-functional properties:
* The data structure should be efficient to backup.
* The data structure should be efficient to synchronize between
machines.
2.1. Addressing of certs
Conceptually, the cert store is a name-value store. We use cert
fingerprints as names, as well as a set of special names. This is
accomplished by mapping names to paths, then relying on the
filesystem for efficient lookups.
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2.2. Trust root
One certificate in the store is used to root trust. It is used for
mapping petnames to certs (see Section 2.3), and to designate trusted
introducers (see Section 2.4).
2.3. Petname mapping
Petnames span a namespace that is secure and human-meaningful, but
not distributed. A common example of a petname scheme are address
books in mobile phones that securely map human-meaningful names to
numbers (which are secure and distributed, but not human-meaningful).
See [Zookos-Triangle] for a more in-depth discussion.
Using petnames, we can securely map human-meaningful names, like
"Mom" or "juliett@example.org", to OpenPGP certificates. In contrast
to many other trust models, this is a concept that most users are
already familiar with. Therefore, it should be easy to train users,
increasing the chance that they will use it in a secure manner.
The petname mapping can also be used to integrate into existing
address book-like functionality provided by the platform.
2.4. Trusted introducers
To improve the ergonomics of public-key systems, users often delegate
questions about the identity of a communications partner to some set
of trusted third parties.
In X.509, these decisions are delegated to a fixed set of vendor-
specified trusted third parties known as root certification
authorities (see [X509-PKI]). These trusted third parties then
usually certify intermediate certification authorities, which in turn
certify the binding between a peer's key and its identity. The trust
relation forms a polyforest (i.e., a directed graph with multiple
roots).
Using this trust relation as a client during the [TLS] handshake is
straightforward: The server presents its certificate along with the
chain of all intermediate certificates up to the root. The client
simply checks if all links in the chain are valid, and whether the
terminal certificate is in its set of root certification authorities.
If so, the server's certificate is authenticated.
In contrast, the trust relation in OpenPGP forms a directed graph.
Any certificate can certify that a cert belongs to an identity.
Furthermore, the user is expected to provide not only the set of
trust roots (the equivalent of X.509's root certification
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authorities), but also to identify acceptable intermediate
authorities, which are known as "trusted introducers" in OpenPGP
parlance.
Traditionally, OpenPGP implementations have used idiosyncratic
mechanisms to configure both the trust roots and the trusted
introducers. That has the downside of being a proprietary mechanism
that cannot easily be shared between implementations. In contrast,
this specification uses a single distinguished certificate as a trust
root that delegates authority to the trusted introducers.
3. Implementation
This section describes in detail how to interact with a certificate
store. Note that we also provide a library that abstracts this away
behind a simple-to-use API.
3.1. Default store's location
If not explicitly requested otherwise, an application SHOULD use the
default store. The location is platform specific, see Section 3.8
for details.
The default store may be overridden by the user by setting the
environment variable PGP_CERT_D.
The application may explicitly choose to use a different location
entirely. Note, however, that this should be done only with good
reasons, because it jeopardizes security, privacy, and ergonomics.
The location of the store MUST be a directory. If it does not exist,
it MAY be created on demand.
3.2. Mapping names to paths
Names are either fingerprints or special names.
3.2.1. Fingerprints
The store is indexed by fingerprint. This is achieved by using the
file system as a dictionary, storing each certificate using a path
derived from the cert's fingerprint.
To compute the path to the certificate file:
* compute the cert's fingerprint,
* format it using lowercase hex digits,
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* take a two-digit prefix as sub-directory name,
* use the remaining digits as the filename.
For example, the certificate with the fingerprint
eb85bb5fa33a75e15e944e63f231550c4f47e38e will be stored at
${BASEPATH}/eb/85bb5fa33a75e15e944e63f231550c4f47e38e.
3.2.2. Special names
There is a set of special names that can be used to address
certificates in the store. The names map to fixed locations in the
store.
+==============+============+
| Special name | Location |
+==============+============+
| trust-root | trust-root |
+--------------+------------+
Table 1
3.3. Locking the store for writes
Before a cert can be inserted or updated, you MUST acquire an
exclusive lock on the store. Note that this lock only synchronizes
writers: Concurrent readers can continue to use the store, and will
always see consistent certs.
The lock to the store is represented by a file located at
${BASEPATH}/writelock which does not contain any data. If that file
does not exist, the store SHOULD be assumed unlocked and the file
MUST be created before any locking operation. The locking is
achieved with file descriptors using platform specific means, see
Section 3.8 for details.
3.4. How to insert or update certs
The following procedure MUST be followed to ensure that concurrent
readers are not disturbed:
* First, acquire an exclusive lock. See Section 3.3.
* Then, look up the cert you want to insert or update in the store.
* If the store contains a copy of the cert, merge it with your copy.
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* Write the cert to a temporary file. This file MUST be on the same
filesystem if the platform requires this for atomic replacement in
the next step (e.g. on POSIX, rename(2) fails if the rename
crosses filesystem boundaries).
* Atomically replace the existing cert with the temporary file (i.e.
using rename(2) on POSIX).
* Release the exclusive lock.
If a certificate is stored using a fingerprint as name, the name MUST
match the certificate's fingerprint.
3.5. Rooting trust
3.5.1. Trust root
The trust root is an OpenPGP certificate that is stored under the
special name trust-root.
The certificate:
* MUST be certification capable.
* SHOULD have a User ID to increase compatibility.
* SHOULD NOT have any subkeys.
* SHOULD use direct key signatures or binding signatures that are
marked as non-exportable.
* MAY have a secret key, password protected or not.
If the certificate has a secret key, then any conforming OpenPGP
implementation can use it to add a petname or a trusted introducer.
Otherwise, only an implementation with access to the secret key
material can do so.
3.5.2. Petname mapping
To add a petname to a certificate, create a User ID with the desired
petname, and bind it to the target certificate using the trust root.
The binding signature SHOULD be marked as non-exportable.
To remove a petname from a certificate, revoke the User ID using the
trust root. The revocation signature SHOULD be marked as non-
exportable.
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To look up certificates by petname, iterate over the store returning
all certificates that contain the petname as User ID bound by the
trust root.
This lookup SHOULD be facilitated using an index data structure.
Currently, we do not define such an index structure, but we define an
extension mechanism so that the index can be stored in the store (see
Section 3.6).
3.5.3. Trusted introducers
To mark a certificate as trusted introducer, create a direct key
signature for the trusted introducer using the trust root, with a
subpacket marking it as trust signature. The trust signature MAY be
scoped. The signature SHOULD be marked as non-exportable.
To rescind a trust delegation, create a new direct key signature for
the trusted introducer using the trust root, without a subpacket
marking it as trust signature. The signature SHOULD be marked as
non-exportable.
The trust root can be used in conjunction with the default OpenPGP
trust model to authenticate nicknames attached to certificates. To
look up certificates by nickname, explore the trust relation of certs
in the store starting with the trust root. Return all certificates
that contain the desired nickname as User ID which are corroborated
by a path from the root to the certificate.
This lookup should be facilitated using an index data structure.
Currently, we do not define such an index structure, but we define an
extension mechanism so that the index can be stored in the store (see
Section 3.6).
3.6. Proprietary and experimental extensions
Files or directories in the toplevel directory starting with an
underscore (_) may be freely used for proprietary and experimental
extensions. Please use a unique and descriptive prefix to minimize
the chance of collisions, e.g. _foopgp_subkey_map.sqlite.
Unknown or unsupported extensions MUST be ignored.
3.7. Reserved filenames
Any files or directories in the toplevel directory other than
* fingerprints mapped to paths (see Section 3.2)
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* known special names mapped to paths (see Section 3.2.2)
* files starting with an underscore (_) (see Section 3.6)
are reserved for future extensions and MUST be ignored.
3.8. Platform-specific conventions
+==========+======================================+==============+
| Platform | Default store location | Locking |
| | | mechanism |
+==========+======================================+==============+
| POSIX | $XDG_DATA_HOME/pgp.cert.d | flock(2) |
| | | with LOCK_EX |
+----------+--------------------------------------+--------------+
| macOS | $HOME/Library/Application Support/ | flock(2) |
| | pgp.cert.d | with LOCK_EX |
+----------+--------------------------------------+--------------+
| Windows | {FOLDERID_RoamingAppData}/pgp.cert.d | LockFile |
| | | (fileapi.h) |
+----------+--------------------------------------+--------------+
Table 2
4. Examples
Importing the certificates described
[I-D.draft-bre-openpgp-samples-00] yields the following certificate
store:
$ export PGP_CERT_D=$(mktemp -d)
$ pgp-cert-d import < alice.pgp
$ (cd $PGP_CERT_D ; find -type f)
./eb/85bb5fa33a75e15e944e63f231550c4f47e38e
$ pgp-cert-d import < bob.pgp
$ (cd $PGP_CERT_D ; find -type f)
./eb/85bb5fa33a75e15e944e63f231550c4f47e38e
./d1/a66e1a23b182c9980f788cfbfcc82a015e7330
5. Reference implementation
We provide a reference implementation in the form of a library
implemented in Rust (see [reference-implementation-api]). This
library also has a C API, so it is easy to use from other languages.
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The library deals with the low-level mechanics of accessing the
store, and computing the fingerprints of inserted certs. It does not
concern itself with emergent features like petname and authenticated
nickname lookups.
5.1. Opening the store
There are two ways to open a store. The first one uses the default
location, the second takes a path to the store's location.
function new() -> Store;
function open(Path) -> Store;
5.2. Certificate lookup
Looking up a certificate returns the certs data and a tag if the
certificate exists in the store, or a special value indicating that
the cert was not found.
The tag can be used in subsequent lookups to quickly check if the
cert has actually changed. This can be used to efficiently update
index data structures.
Usually, this function returns a CERT (Section 5.5.3), but if NAME
(Section 5.5.1) is a special name, it may return a KEY
(Section 5.5.4).
function Store::get(NAME) -> Maybe(TAG, CERT-or-KEY);
function Store::get_if_changed(TAG, NAME) -> Maybe(TAG, CERT-or-KEY);
5.3. Certificate update
Inserting or updating a cert requires the CERT (Section 5.5.3) and a
callback function.
The callback is invoked with the existing cert data (if any), and
SHOULD merge the two copies of the certificate together. The
function MAY decide to omit (parts of) the existing data, but this
should be done with great care as not to lose any vital information.
The insertion method returns the merged certificate data and the tag
for the new state.
Locking is handled by the library.
function Store::insert(CERT, Merge) -> (TAG, CERT)
where Merge is
function(CERT, Maybe(CERT)) -> CERT;
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5.4. Store enumeration
The user can iterate over all certificates in the store. The
iterator returns tuples of fingerprints and tags, which can be used
to efficiently update index data structures.
Note: The iterator does not return any special names like the trust
root (see Section 3.2.2).
function Store::iter() -> Iterator over (NAME, TAG, CERT);
5.5. Input/Output Types
5.5.1. NAME
A string representing a fingerprint or a special name (see
Section 3.2).
5.5.2. TAG
An opaque value corresponding to a cert in store. If the cert is
updated, its tag will change. This can be used to quickly determine
if an index data structure must be updated.
5.5.3. CERT
Exactly one OpenPGP certificate (section 11.1 of [RFC4880]), aka
"Transferable Public Key". The certificate MUST NOT be ASCII
Armored.
5.5.4. KEY
Exactly one OpenPGP Transferable Secret Key (section 11.2 of
[RFC4880]). The certificate MUST NOT be ASCII Armored.
5.6. Failure Modes
+=============+=======================================+
| Mnemonic | Meaning |
+=============+=======================================+
| OK | Success |
+-------------+---------------------------------------+
| BAD_NAME | The name was neither a valid |
| | fingerprint, nor a known special name |
+-------------+---------------------------------------+
| NOT_A_STORE | The base directory cannot possibly |
| | contain a store |
+-------------+---------------------------------------+
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| BAD_DATA | The data was not valid OpenPGP cert |
| | or key in binary format |
+-------------+---------------------------------------+
| IO_ERROR | Unspecified I/O error occurred |
+-------------+---------------------------------------+
Table 3
6. Guidance for Implementers
Despite the fact that this spec is designed with ease of
implementation in mind, and we explicitly invite reimplementations,
please consider using our reference implementation.
This is a list of implementation considerations that interoperating
implementations need to follow:
* The exclusive lock MUST be released in a timely manner.
* When exporting artifacts from the store, non-exportable signatures
and certificate components MUST be omitted.
7. Security Considerations
XXX
8. Document Considerations
8.1. Document History
This is a first draft that has not been published.
8.2. Future Work
OpenPGP requires efficient lookup by subkey fingerprint and keyids.
This is currently not provided by this spec, hence implementations
need to build their own index on top of this store. Future revisions
may specify a way to do this natively.
Collecting usage information for TOFU-like trust models creates a
write-heavy workload during normal usage, and requires more complex
data structures that are not easily expressed using file-system
operations and OpenPGP data structures. Future revisions of this
spec may define suitable mechanisms to keep a record of certificate
uses.
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This spec contains platform-specific conventions (see Section 3.8),
like default store locations and locking mechanisms. Porting to new
platforms requires amending the spec.
9. Acknowledgements
10. References
10.1. Normative References
[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/info/rfc2119>.
[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/info/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/info/rfc8174>.
10.2. Informative References
[Autocrypt]
"Convenient End-to-End Encryption for E-Mail", 14 June
2021, <https://autocrypt.org/>.
[FedoraSharedX.509CertStore]
"Shared System Certificates", 14 June 2021,
<https://fedoraproject.org/wiki/Features/
SharedSystemCertificates>.
[I-D.draft-bre-openpgp-samples-00]
Einarsson, B. R., "juga", and D. K. Gillmor, "OpenPGP
Example Keys and Certificates", Work in Progress,
Internet-Draft, draft-bre-openpgp-samples-00, 15 October
2019, <https://www.ietf.org/archive/id/draft-bre-openpgp-
samples-00.txt>.
[I-D.draft-koch-openpgp-webkey-service-12]
Koch, W., "OpenPGP Web Key Directory", Work in Progress,
Internet-Draft, draft-koch-openpgp-webkey-service-12, 17
May 2021, <https://www.ietf.org/archive/id/draft-koch-
openpgp-webkey-service-12.txt>.
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[keys.openpgp.org]
"A GDPR-conforming, validating keyserver", 14 June 2021,
<https://keys.openpgp.org/>.
[macOSSharedX.509CertStore]
"Lists of available trusted root certificates in macOS", 8
December 2018, <https://support.apple.com/en-us/HT202858>.
[Maildir] "Using maildir format", 14 June 2021,
<https://cr.yp.to/proto/maildir.html>.
[Notmuch] "Notmuch -- Just an email system", 14 June 2021,
<https://notmuchmail.org/>.
[reference-implementation-api]
"API documentation for the reference implementation", 15
June 2021, <https://sequoia-pgp.gitlab.io/pgp-cert-
d/pgp_cert_d/index.html>.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[WindowsSharedX.509CertStore]
"Managing Certificates with Certificate Stores", 14 June
2021, <https://docs.microsoft.com/en-
us/windows/win32/seccrypto/managing-certificates-with-
certificate-stores>.
[X509-PKI] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[Zookos-Triangle]
"Names: Distributed, Secure, Human-Readable: Choose Two",
17 June 2021, <https://web.archive.org/web/20011020191610/
http://zooko.com/distnames.html>.
Authors' Addresses
Nora Widdecke
Sequoia PGP
Email: nora@sequoia-pgp.org
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Justus Winter
Sequoia PGP
Email: justus@sequoia-pgp.org
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