Internet DRAFT - draft-barnes-jose-jsms
draft-barnes-jose-jsms
Network Working Group R. Barnes
Internet-Draft BBN Technologies
Intended status: Standards Track June 15, 2012
Expires: December 17, 2012
JavaScript Message Security Format
draft-barnes-jose-jsms-00.txt
Abstract
Many applications require the ability to send cryptographically
secured messages. While the IETF has defined a number of formats for
such messages (e.g. CMS) those formats use encodings which are not
easy to use in modern applications. This document describes the
JavaScript Message Security format (JSMS), a new cryptographic
message format which is based on JavaScript Object Notation (JSON)
and thus is easy for many applications to generate and parse.
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 http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 17, 2012.
Copyright Notice
Copyright (c) 2012 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
(http://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 extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Operational Modes . . . . . . . . . . . . . . . . . . . . 4
3.2. Design Principles . . . . . . . . . . . . . . . . . . . . 4
3.3. Certificate Processing . . . . . . . . . . . . . . . . . . 5
3.4. Certificate Discovery . . . . . . . . . . . . . . . . . . 5
4. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Data types . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Basic Types . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. SignedData . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3.1. Signature . . . . . . . . . . . . . . . . . . . . . . 7
4.3.2. Generating a SignedData Object . . . . . . . . . . . . 8
4.3.3. Verifying a SignedData Object . . . . . . . . . . . . 8
4.4. AuthenticatedData . . . . . . . . . . . . . . . . . . . . 9
4.4.1. Generating an AuthenticatedData Object . . . . . . . . 9
4.4.2. Verifying an AuthenticatedData Object . . . . . . . . 9
4.5. EncryptedData . . . . . . . . . . . . . . . . . . . . . . 10
4.5.1. Generating an EncryptedData Object . . . . . . . . . . 10
4.5.2. Decrypting a EncryptedData Object . . . . . . . . . . 11
4.6. Useful Objects . . . . . . . . . . . . . . . . . . . . . . 11
4.6.1. AlgorithmIdentifier . . . . . . . . . . . . . . . . . 11
4.6.2. PublicKey . . . . . . . . . . . . . . . . . . . . . . 14
4.6.3. WrappedKey . . . . . . . . . . . . . . . . . . . . . . 16
5. Compact Format . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. SignedData . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. AuthenticatedData . . . . . . . . . . . . . . . . . . . . 19
6.4. EncryptedData . . . . . . . . . . . . . . . . . . . . . . 20
7. Mapping to CMS . . . . . . . . . . . . . . . . . . . . . . . . 20
8. Comparison to JWS/JWE/JWK . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
12.1. Normative References . . . . . . . . . . . . . . . . . . . 23
12.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Many applications require the ability to send cryptographically
secured (encrypted, digitally signed, etc.) messages. While the IETF
has defined a number of formats for such messages, those formats are
widely viewed as being excessively complicated for the demands of Web
applications, which typically only need the ability to secure simple
messages. In addition, existing formats use encoding mechanisms
(e.g., ASN.1 DER) which are not congenial for many classes of
applications (e.g., Web applications). This presents an obstacle to
the deployment of strong security by such applications.
This document describes a new cryptographic message format,
JavaScript Message Security (JSMS). This format is intended to meet
the need of modern applications, including JavaScript-based Web
applications. While JSMS is modeled on existing formats --
principally CMS [RFC5652] -- it uses JavaScript Object Notation
(JSON) rather than ASN.1, making it far easier for applications to
handle. In the interest of simplicity, JSMS also omits many of less
commonly used CMS modes (such as password-based encryption).
2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In order to enable JSON to carry binary data, JSMS makes extensive
use of Base64 encoding [RFC4648]. Whenever this document refers to
Base64 encoding, we mean the URL-safe variant "base64url" encoding.
As stated in section 3.1 of [RFC4648], Base64 does not allow
linefeeds. Since linefeeds are not valid characters in a JSON
string, whenever a field is specified to be Base64-encoded in this
document, it MUST NOT include any line breaks. Base64-encoded fields
also MUST NOT include JSON-encoded linefeeds such as "\n". Any
linebreaks in the middle of Base64-encoded sections of the examples
in this document have been inserted in order to make the examples fit
on the page. Any trailing "=" characters SHOULD be removed. They
are not needed, because JSON strings have defined lengths (namely the
number of characters between unescaped '"' characters).
3. Overview
The JSMS message format is simply a JSON [RFC4627] object with an
appropriate collection of fields. Each operating mode will have a
separate set of fields, with a common field to distinguish between
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the modes.
3.1. Operational Modes
JSMS supports three operational modes:
Signed Data
A block of data signed by a single signer using his asymmetric key
and optionally carrying his certificate.
Authenticated Data
A block of data with authentication and integrity protection
provided using a symmetric-key Message Authentication Code (MAC).
The MAC key may be provided in encrypted form (as with Encryped
Data) or identified by name.
Encrypted Data
A block of data encrypted under a random message encryption key
(MEK). The MEK is then separately encrypted for each recipient,
either via symmetric or asymmetric encryption. The data is always
integrity protected, through the use of an Authenticated
Encryption with Associated Data (AEAD) algorithm such as AES-GCM
or AES-CCM.
Any other desired security functions are provided by composition of
these modes. For instance, a signed and encrypted message is
produced by first creating a Signed message and then encrypting that
data.
3.2. Design Principles
In general, JSMS follows the following design principles.
Minimize implementation complexity
Wherever possible, protocol choices have been made such that the
time and effort required to implement the protocol in many
different programming languages will be minimized. This means
that optimizations for bandwidth, CPU, and memory utilization have
been explicitly avoided.
Base64 as the only encoding
Any data that does not have a straightforward string
representation (binary values, large integers, etc.) is base64-
encoded (see: [RFC4648]). In some cases, hexadecimal encodings
might be more convenient, but consistency is even more important
to reduce implementation complexity.
No canonicalization
In many cryptographic message formats, canonical encodings are
used to allow the same value to be computed at both sender and
recipient (e.g., for digital signatures). This is inconvenient in
JSON, which just views messages as a bundle of key/value pairs.
Instead, whenever canonicalization would be required, the relevant
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data is serialized and base64-encoded for transport, allowing both
sides to run computations over the same original set of octets.
In-memory processing
We assume that the entire message can fit in main memory and make
no effort to design a wire representation which can be handled in
small chunks in a single pass. This means, for instance, that
there is no need to have a message digest indicator at the
beginning of the message and then the signature at the end, as is
done in CMS. Fields are simply serialized in whatever order is
most convenient for the JSON implementation. The examples in this
document are generally shown in whatever order seems most readable
and are not normative.
Consistency with CMS
To simplify the adaptation of existing cryptographic modules and
the validation of JSMS implementations, changes from the CMS
cryptographic operations are minimized. JSMS is semantically
equivalent to a profile of CMS, as described in Section 7.
3.3. Certificate Processing
Experience has shown that certificate handling (path construction) is
one of the trickier parts of building a cryptographic system. While
JSMS supports PKIX certificates, its certificate processing is far
simpler than that of CMS. (It also supports the use of bare public
keys in order to avoid the use of X.509 altogether.) When a JSMS
agent provides its certificate, it must provide an ordered chain (as
in TLS [RFC5246]) terminating in its own certificate, thus removing
the need to construct certificate paths. The certificates MUST be
ordered with the end-entity certificate first and each certificate
that follows signing the certificate immediately preceding it.
3.4. Certificate Discovery
JSMS will often be used in an online messaging environments with
users that have an address of the form user@domain, such as email,
XMPP, or SIP. As such, protocols such as WebFinger
[I-D.hammer-webfinger] or an end-to-end protocol can be used to
retrieve appropriate certificates. Downstream uses of JSMS SHOULD
define a discovery mechanism suitable for the intended use.
4. Message Format
A JSMS object is a JSON object that encodes cryptographic informaton
related to a content byte string. This document specifies the set of
keys that must be present in a JSMS object, what the associated
values are, and how these values are generated and processed in order
to realize security features. In processing JSMS objects, unknown
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keys MUST be ignored.
JSMS defines three top-level types of secure object, each of which
provides a specific cryptographic protection to a byte string.
SignedData: Signature using a public-key digital signature algorithm
AuthenticatedData: Authentication using a Message Authentication
Code (MAC)
EncryptedData: Encryption and authentication using an Authenticated
Encryption with Associated Data (AEAD) algorithm
4.1. Data types
For each field in a JSON object, we define the type of information
that must be included in that field. At base are the object, array,
string, number types defined by JSON. We also use two special sub-
classes of strings: Fields with type "Token" contain a string drawn
from a defined list of strings (e.g., an IANA registry for algorithm
names). Fields with type "ByteString" contain a Base64-encoded byte
string (note the considerations related to Base64 encoding in
Section 2 above).
In addition to the primitive data types, Section 4.6 defines a
collection of useful object types that are used by the top-level JSMS
objects. These are simply referred to by name when they appear as a
field value in another object.
4.2. Basic Types
The following elements are common to all JSMS messages:
"version": REQUIRED Number. The version of JSMS used by this
objec.t. This field MUST be set to 1.
"type": REQUIRED Token. The type of this JSMS object. This field
MUST be set to one of the following values
"signed": SignedData object
"authenticated": AuthenticatedData object
"encrypted": EncryptedData object
"content": OPTIONAL ByteString. The content byte string, Base64-
encoded.
If the "content" key is not present in a given JSMS object, then the
JSMS object is "detached". In this case, the content must be
associated with the JSMS object through some out-of-band mechanism
before the JSMS object can be processed. Note that there is a risk
that detached JSMS object might become invalid if the content is
transformed, even if this transformation preserves the semantics of
the content. For example, if the content is a JSON object, and the
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object passes through an intermediate process that adds whitespace or
re-orders the fields in the object (neither of which changes the
meaning of the object), then the recipient will not be able to verify
the signature. For this reason, detached JSMS objects SHOULD NOT be
used unless there is a canonical form for the content being
processed.
4.3. SignedData
A SignedData object MUST have a "type" field set to "signed". In
addition, a SignedData object contains the following keys:
"digestAlgorithm": REQUIRED AlgorithmIdentifier. The digest
algorithm used in signing the content.
"signatures": REQUIRED Array of Signature. One or more digital
signatures over the content.
"certificates": OPTIONAL Array of String. A certificate chain
associating the signer's public key with an identifier. Each
element in the array is a string containing the Base64-encoded
representation of a DER-formatted certificate. The certificates
MUST be ordered with the end-entity certificate first and each
certificate that follows signing the certificate immediately
preceding it.
"certificatesURI": OPTIONAL String. An HTTP or HTTPS URI referring
to a certificate chain. The referenced resource MUST have type
"application/json" and contain an array of certificates in the
same format as the "certificates" element above, including the
ordering constraint.
4.3.1. Signature
A Signature object represents the signature over the content in the
SignedData object by a specific key pair. A Signature objec can
contain the following keys:
"signatureAlgorithm": REQUIRED AlgorithmIdentifier. The signature
algorithm used in signing the content.
"key" REQUIRED PublicKey. The public key identifier for the signer,
represented as a PublicKey object (see Section 4.6.2)
"signature" REQUIRED ByteString. The Base64-encoded signature value
If the "key" value represents the public key as an identifier, then a
certificate for the signer MUST be provided by setting either the
"certificates" or "certificatesURL" fields. The subject key in the
end-entity certificate MUST match the identifier in the "key" value;
the certificate SHOULD contain the subjectKeyIdentifier field, with a
value matching the "key" value. (Note that this implies that when
there are multiple signers, only one key can be represented by ID.)
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4.3.2. Generating a SignedData Object
The inputs to the process of generating a SignedData object are:
o The content, as a byte string
o A digest algorithm
o One or more signature algorithms and asymmetric key pairs
To generate the signature for SignedData object, the originator takes
the following steps:
1. Compute the message digest by applying the digest algorithm to
the content.
2. For each signing key pair, compute the signature by using the
signature algorithm to sign the message digest with the private
key from the asymmetric key pair.
The originator then encodes the SignedData object by including the
appropriate AlgorithmIdentifiers for the digest algorithms, a
Signature object for each signature, and (optionally) the content.
4.3.3. Verifying a SignedData Object
To verify a SignedData object, the recipient takes the following
steps:
1. Verify that the digest and signature algorithms are supported.
Otherwise, report an error and fail.
2. Compute the content byte string by decoding the "content" value
of the JSMS object. If the JSMS object does not contain a
"content" field, retrieve the content by other means.
3. Compute the message digest by applying the digest algorithm to
the content.
4. Compute the signature by decoding the "signature" value of the
JSMS object.
5. Compute the public key:
* If the key is represented directly, then decode it according
to the rules specified by the algorithm name.
* If the key is represented by an ID, then retrieve the
corresponding subject public key from the end-entity
certificate . If no "certificates" or "certificatesURI" value
is present, then report an error and fail.
* If the key is represented by a URI, retrieve the public key
from the URI.
6. Verify the signature by using the signature algorithm to verify
the message digest with the public key.
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4.4. AuthenticatedData
An AuthenticatedData object MUST have a "type" field set to
"authenticated". In addition, an AuthenticatedData object contains
the following keys:
"algorithm": REQUIRED AlgorithmIdentifier. The MAC algorithm used
to authenticate the content.
"mac": REQUIRED ByteString. The MAC value
"keys": OPTIONAL Array of WrappedKey. Wrapped versions of the
symmetric key used for this MAC. Each element in the array MUST
be a WrappedKey object (see below)
"keyId": OPTIONAL ByteString. An opaque identifier for a pre-shared
MAC key
An AuthenticatedData object MUST contain either the "key" field or
the "keyId" field, so that the recipient knows which key to use to
verify the MAC.
4.4.1. Generating an AuthenticatedData Object
The inputs to the process of generating a AuthenticatedData object
are:
o The content, as a byte string
o A MAC algorithm
o A MAC key and key identifier, or
o One or more recipient keys and key encipherment algorithms
If the recipient key is specified rather than the MAC key directly,
then a random MAC key is generated and encoded in a WrappedKey
objects for each recipient (see Section 4.6.3). Once the MAC key has
been determined, the originator uses the MAC algorithm and MAC key to
compute the MAC over the content byte string.
The originator then encodes the AuthenticatedData object by including
the appropriate AlgorithmIdentifier for the MAC algorithm and the
Base64 representations of the MAC value and (optionally) the content.
If the MAC key was specified directly, then the Base64 representation
of the key identifier is set as the "keyId" value; otherwise, the
WrappedKey objects are collected in an array and set as the "keys"
value.
4.4.2. Verifying an AuthenticatedData Object
To verify a AuthenticatedData object, the recipient takes the
following steps:
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1. Verify that the MAC algorithm is supported. If not, report an
error and fail.
2. Compute the content byte string by decoding the "content" value
of the JSMS object. If the JSMS object does not contain a
"content" field, retrieve the content by other means.
3. Compute the MAC key:
* If the "keyId" value is present and represents a known key,
use the identified key.
* If the "keys" value is present, check each WrappedKey object
to determine if it matches a known key for this recipient. If
any of the wrapped keys matches, unwrap the key from the first
one and use it (see Section Section 4.6.3). Otherwise, report
an error and fail.
4. Use the MAC algorithm and MAC key to compute the MAC over the
content byte string
5. Decode the MAC value from the "mac" field.
6. Verify that the computed MAC matches the MAC from the object.
4.5. EncryptedData
An EncryptedData object MUST have a "type" field set to "encrypted".
Note also that in an EncryptedData object, the "content" field
contains the encrypted form of the content, not the content itself
(as plaintext). An EncryptedData object contains the following keys
in addition to any common fields:
"algorithm": REQUIRED AlgorithmIdentifier. The encryption algorithm
used to encrypt the content
"keys": REQUIRED Array of WrappedKey. Wrapped versions of the
symmetric key used to encrypt the content. Each element in the
array MUST be a WrappedKey object (see Section 4.6.3).
"mac": OPTIONAL ByteString. The MAC value, if required by the
algorithm
Note that although the "mac" field is optional, an EncryptedData
object always has an integrity check. All of the encryption
algorithms used in JSMS are "Authenticated Encryption with Associated
Data" algorithms, which include an authentication / integrity fuction
by definition. The MAC field is optional because some AEAD
algorithms have a separate MAC value (e.g., GCM), while others
incorporate the MAC value into the ciphertext (e.g., CCM).
4.5.1. Generating an EncryptedData Object
The inputs to the process of generating a SignedData object are:
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o The content, as a byte string
o An encryption algorithm
o One or more recipient keys and key encipherment algorithms
The originator generates a random encryption key of a length suitable
for the encryption algorithm, then encodes it in a WrappedKey object
for each recipient (see Section 4.6.3). The content is then
encrypted using the generated encryption key and the specified
encryption algorithm.
The originator then encodes the EncryptedData object by including the
appropriate AlgorithmIdentifier for the encryption algorithm, an
array containing the WrappedKey objects, and (optionally) the Base64
representation of the content.
4.5.2. Decrypting a EncryptedData Object
To decrypt an EncryptedData object, the recipient takes the following
steps:
1. Verify that the encryption algorithm is supported. If not,
report an error and fail.
2. Compute the content byte string by decoding the "content" value
of the JSMS object. If the JSMS object does not contain a
"content" field, retrieve the content by other means.
3. Locate the encryption key: Check each WrappedKey object to
determine if it matches a known key for this recipient. If any
of the wrapped keys matches, unwrap the key from the first one
and use it (see Section 4.6.3). Otherwise, report an error and
fail.
4. Decrypt the content using the encryption key and the specified
encryption algorithm.
5. Verify that the integrity check in the AEAD decryption was
successful. If not, report an error and fail.
6. Return the decrypted content.
4.6. Useful Objects
In this section we define some common object types that are used
across the top-level objects above.
4.6.1. AlgorithmIdentifier
An AlgorithmIdentifier object names a cryptographic algorithm and
specifies any associated parameters such as nonces or initialization
vectors (IVs). If the algorithm has no parameters, then the
AlgorithmIdentifier object is simply a token representing the name of
the algorithm, drawn from an IANA registry of algorithm names.
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If the algorithm specifies parameters, the AlgorithmIdentifier object
is a JSON object. There is only one required field, "name". Any
other fields are specified in the algorithm definition.
"name": REQUIRED Token. The name of the algorithm, chosen from one
of the IANA registries defined by this document.
The following table summarizes the algorithms to be used with JSMS.
[[ More detail to be added later, in a separate document ]]
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Name Parameters Reference Example
===================================================================
SIGNING
rsa no [RFC3447] "rsa"
dsa yes (p,q,g) [FIPS186] {name:"dsa",
p:1, q:2, g:3}
ecdsa yes (curve) [RFC6090] {name:"ecdsa",
curve:"P-256"}
-------------------------------------------------------------------
DIGEST
sha1 no [FIPS180-1] "sha1"
sha256 no [FIPS180-3] "sha256"
sha384 no [FIPS180-3] "sha384"
sha512 no [FIPS180-3] "sha512"
-------------------------------------------------------------------
MAC
hs1 no [FIPS180-1] "hs1"
hs256 no [FIPS180-3] "hs256"
hs384 no [FIPS180-3] "hs384"
hs512 no [FIPS180-3] "hs512"
-------------------------------------------------------------------
ENCRYPTION
aes128-ccm yes (n,M) [RFC3610] {name:"aes128-ccm",
n:"ZONce...lU-g",
m:8}
aes128-gcm yes (iv) [McGrew & Viega] {name:"aes128-gcm",
iv:"ZONce...lU-g"}
-------------------------------------------------------------------
KEY ENCIPHERMENT
aes no [RFC3394] "aes"
rsaes-oaep no [RFC3447] "rsaes-oaep"
-------------------------------------------------------------------
KEY AGREEMENT
dh-es yes (group) [RFC2631] {name:"dh-es",
group: 14}
ecdh-es yes (curve) [RFC6090] {name:"ecdh-es",
curve:"P-256"}
===================================================================
Obviously, there will be more detail needed beyond the above, and
some IANA considerations to create the necessary registries. For
some algorithms, there will be specific notes about how they are to
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be used with JSMS, for example:
o The signature value produced by DSA is comprised of two integers.
The byte string to be filled in the "signature" field is the two-
element JSON array containing two integers, "[r,s]"
o RSAES-OAEP is always used with SHA-256 and the default MGF1
masking generation function
o Elliptic curves may only be specified by name, not by directly
specifying curve parameters. [[ We may define our own registry, or
re-use the ones from TLS/IKE. ]]
o AEAD algorithms are only used for authenticated encryption; there
is never associated data. Further AEAD algorithms may be defined
using [draft-mcgrew-aead-aes-cbc-hmac-sha1]
4.6.2. PublicKey
A PublicKey object describes the public key used by a signer. The
key may be specified as a JSON structure directly, as a URI, or as an
identifier. A PublicKey object has the following fields:
"type" OPTIONAL Token. The name of the algorithm with which this
key is to be used
"id" OPTIONAL ByteString. An identifier for the key
"uri" OPTIONAL String. A URI pointing to a direct form of the key
If the key is specified directly, then the "type" key MUST be
present; the "id" and "uri" fields MAY be present. Subsequent
entries in the array specify the elements of the key, in a manner
determined by the algorithm. Formats for RSA and ECDH/ECDSA public
keys are specified below.
If the key is provided as a URI, then the "uri" field MUST be
present, containing a URI where the key can be retrieved, in the JSON
format described above. The method that the recipient of a JSMS
object uses to retrieve the key will depend on the URI scheme. For
HTTP URIs, the relying party MUST issue an HTTP request with the GET
method and an Accept header including the MIME type for JSMS
PublicKey object, "[[MIMETYPE-TBD]]". For MAILTO, SIP, and XMPP
URIs, the recipient MAY use the WebFinger protocol
[I-D.hammer-webfinger] to retrieve a public key for the user.
If the key is referenced by an opaque identifier or "fingerprint",
then the "id" field MUST be present, and contain the Base64-encoded
SHA-1 hash of the public key, represented as a DER-encoded
subjectPublicKeyInfo data structure. (This fingerprint value is the
same as the one commonly included in the subjectKeyIdentifier field
in an X.509 certificate.)
The recipient of a JSMS object can determine which of the above cases
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a given key falls into by seeking the three fields in sequence. If a
"type" field is present, then the key is represented directly. If a
"uri" field is present, then the key is represented directly, but
must be retreived from the URI. Finally, if the "id" field is the
only one of the three present, then the key is represented by ID
only, and must be retrieved from somewhere else (e.g., from a
certificate in the JSMS object).
Example: {"id": "i1LbR8FCEw-aiFcAAfUvpp75wdY="}
Example: {"uri": "xmpp:juliet@example.com"}
4.6.2.1. RSA Public Key
An RSA public key comprises two additional parameters in addition to
the algorithm identifier "rsa".
"n": REQUIRED ByteString. The modulus, represented as an integer in
network byte order (big-endian)
"e": REQUIRED Integer. The public exponent, represented as an
integer in network byte order (big-endian)
Example: {"type":"rsa", "n":98739...04251, "e": 3}
4.6.2.2. Elliptic-Curve Public Key
Public keys for several types of elliptic curve algorithms, including
ECDSA and ECDH, have the same format, namely an point on a specified
elliptic curve. In an elliptic curve PublicKey object, the curve
parameters are specified in the algorithm identifier, and there are
two additional fields that specify the point on the curve:
"x": REQUIRED ByteString. The x coordinate of the point
"y": REQUIRED Integer. The y coordinate of the point. MUST be
equal to 0 or 1.
These coordinates correspond to the compressed form of an elliptic
curve point, as specified in [[SEC01]]. In terms of the calculation
specified in section 2.3.3 of [[SEC01]], the "x" coordinate is the
byte string X and the "y" coordinate is the reduced y coordinate (or,
equivalently, Y mod 2).
Example: {"type":"ecdh",
"x":"IIIs_x1m6Na6xKN37vOwvy7AvFeG9HhBN2EN3u5EZQ4", "y": 1}
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4.6.3. WrappedKey
In JSMS objects that use symmetric keys (for MAC or encryption), it
is necessary for the originator to convey the symmetric key used for
in JSMS computations to the recipient. The WrappedKey object is a
JSON object that allows these keys to be provided either using key
transport or key agreement. The following fields may be present in a
WrappedKey object:
"type": REQUIRED TOKEN The type of wrapping being done. This
document defines the following values for this field:
"encryption": Symmetric key transport. The "KEKIdentifier" field
MUST be present. Any other non-required fields MUST be
ignored.
"transport": Asymmetric key transport. The "recipientKey" field
MUST be present. Any other non-required fields MUST be
ignored.
"agreement": Key agreement. The "originatorKey" and
"recipientKey" MUST be present, and the "userKeyMaterial" field
MAY be present. Any other non-required fields MUST be ignored.
"algorithm": REQUIRED AlgorithmIdentifier The algorithm used to
encrypt the symmetric key
"encryptedKey": REQUIRED BYTES The symmetric key, encrypted
according to the algorithm indicated by the "algorithm" value
"KEKIdentifier": OPTIONAL BYTES An opaque identifier for the
symmtric key encryption key
"originatorKey": OPTIONAL PublicKey The public key of the originator
"recipientKey": OPTIONAL PublicKey The public key of the recipient
"userKeyMaterial": OPTIONAL BYTES User key material
The techniques used for wrapping and unwrapping the encrypted key is
determined by "type" and "algorithm" fields. In general, the options
are the same as for CMS [RFC5280], without the option for password-
based key wrapping.
"encryption": The key is encrypted under a pre-shared symmetric
key encryption key identified by the "KEKIdentifier" field
"transport": The key is encrypted under the recipient's public key,
identified in the "recipientKey" field.
"agreement": The key is encrypted under a shared secret derived
using a key agreement algorithm combining the originator's private
key and the recipient's public key, corresponding to the
"originatorKey" and "recipientKey", respectively. The value
provided in the "userKeyMaterial" field may be used to provide
additional entropy.
[[ More detail to be added. ]]
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5. Compact Format
The compact JSON format of a JSMS object is identical to the normal
JSMS format, except that field names are replaced with shorter
equivalent field names. Translations for the field names above are
given in the table below. In a given JSMS object, field names MUST
either all be in long form or all be in short form. An
implementation MUST reject a JSMS object with mixed long and short
names as improperly formatted.
Common Signature
-------------------------- --------------------------
version v sign atureAlgorithm sa
type t key k
signed s signature sg
authenticated au --------------------------
encrypted en
content c
-------------------------- AlgorithmIdentifier
--------------------------
name nm
SignedData --------------------------
--------------------------
digestAlgorithm da
signatures ss PublicKey
certificates ce --------------------------
certificatesURI cu type t
-------------------------- id i
uri u
--------------------------
AuthenticatedData
--------------------------
algorithm a WrappedKey
mac mac --------------------------
keys ks type t
keyId ki encryption ec
-------------------------- transport tr
agreement ag
algorithm a
EncryptedData encryptedKey ek
-------------------------- KEKIdentifier i
algorithm a originatorKey o
keys ks recipientKey r
mac mac userKeyMaterial uk
-------------------------- --------------------------
In applications where a JSMS object is required to be URL-safe, it is
RECOMMENDED that it be rendered in the compact serialization, then
Base64-encoded.
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[[ If there is a desire to avoid double-base64url-encoding things,
then we could define a mechanism for moving some fields out of the
object. ]]
6. Examples
This section contains complete examples of all three JSMS types. All
white space is for readability only, and must be removed before the
examples can be considered valid JSMS objects.
6.1. Parameters
RSA key:
{
"type": "rsa",
"n": "AfWGinFrdktMCi4LkD_vcIsqc0m4JSS0rNDk_5Zdi8fwja_qH0M7d3
U4tPUw7L0gP1iSMakdTKX0S7uTV_v9FeY8_WrxDgbphrH9Zaz0PvTL
OuiKfRkMWK5A6nzl_PdP7_ujDWkvHKhWcJtM7irdn9K059X21EDtuq
GJyq7_v_c_",
"e": "AQAB",
"d": "EMwfyOqzfJQgZyhl_W40k8SpNdfgDpmqjBiPYubhLqIk7LZns6XDO3
7ZuLiZxT_WP04uMZ7UmV5URwUJVlxEpmfozhtLooCTP1oWtRQQjhTa
Pz1f5nRKoHsO8e3PZY7O44ut2prRWNNxYxDk52rH9GTECqGAmDNb1f
he6zX4KJk="
}
Key Tag: HK1RA8AQwcI=
Symmetric key: rQS8Dx6WQ_xDWTER8mAHnw==
Content:
"Attack at dawn!"
6.2. SignedData
In this object the content is signed under the specified RSA key
pair, using SHA256 as the digest.
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{
"version": 1,
"type": "signed",
"digestAlgorithm": "sha256",
"content": "QXR0YWNrIGF0IGRhd24h",
"signatures": [{
"signatureAlgorithm": "rsa",
"key": {
"type": "rsa",
"n": "AfWGinFrdktMCi4LkD_vcIsqc0m4JSS0rNDk_5Zdi8
fwja_qH0M7d3U4tPUw7L0gP1iSMakdTKX0S7uTV_v9
FeY8_WrxDgbphrH9Zaz0PvTLOuiKfRkMWK5A6nzl_P
dP7_ujDWkvHKhWcJtM7irdn9K059X21EDtuqGJyq7_
v_c_",
"e": "AQAB",
},
"signature": "AJll1tVYsRtGeHaJenAU-U3x4LxXklNoGrFwyu
xJWnYIeLZL16Ib7ZPvD79peMiSQAHAdLKcI8e-
CpU6HNQ-MxeE-tEXvaXOxuNZfVG9LBP9hq_ZwX
SguffHHzS9lLtVB0OzrXeszXtqD5igmeco1A0E
8eabzujA4bdN6Umyc7rA"
}]
}
6.3. AuthenticatedData
In this object the content is authenticated with a MAC under a
randomly-generated key (AuthenticatedData Key above), wrapped using
the key encryption key above, identified by the above key tag.
{
"version": 1,
"type": "authenticated",
"algorithm": "hs256",
"content": "QXR0YWNrIGF0IGRhd24h",
"mac": "990xwhrsX-COXUN0uF09HUHLU2CjdneeMqTtM4sGVDY=",
"keys": [{
"type": "encryption",
"algorithm": "aes",
"encryptedKey": "Dbf2O_ZIX0_Zfj-0aU6zQjn3xixj6vm7LVX
XFDdX4xqie5bZUS1nnstIPYOyzxNx9Udt-J
LZZh-zM8A_FbsZ8zAibdJ3EPyd",
"KEKIdentifier": "HK1RA8AQwcI="
}]
}
As another example, the following object is a detached MAC (over the
same content string) in the compact encoding. Here we use the key as
the MAC key directly (instead of as a key encryption key). The
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object is shown both in raw JSON form and in the Base64 encoding.
{"v": 1,"t": "au","a": "hs256","ki": "HK1RA8AQwcI=",
"mac": "PMVmhmrgbj-KNybfMqHu4ySJ0GnVrwe11MKpiuuGlIQ="}
eyJ2IjogMSwidCI6ICJhdSIsImEiOiAiaHMyNTYiLCJraSI6ICJISzFSQThBUXdj
ST0iLA0KICAibWFjIjogIlBNVm1obXJnYmotS055YmZNcUh1NHlTSjBHblZyd2Ux
MU1LcGl1dUdsSVE9In0=
6.4. EncryptedData
In this object, the content is encrypted under the general AEAD
algorithm using AES-128-CBC for encryption and HMAC-SHA1 for
authentication. The keys are described above as "EncryptedData Key
(E)" and "EncryptedData Key (A)", respectively. The temporary keys
are wrapped using the PKCS#1 wrapping, under the RSA key pair above.
{
"version": 1,
"type": "encrypted",
"algorithm": {
"name": "aes128-ccm",
"n": "LTR8s7KKbd1QlQ==",
"m": 8
},
"content": "0nkXCLOVxM2oNJOsDCwASLTODIMVZQE=",
"keys": [{
"type": "transport",
"algorithm": "rsaes-oaep",
"encryptedKey": "AbAxRnd_u7lICJlBskq3kgQVs54RLMgOjNmALXF
JjKqsQ4kLNL60VAoEswGOd2arGfcxoMCw9wMeSP
FOIvOXGvSt2wJXR_6kwzOJv_YyTC_eZUJHpcLNr
jKxB7Zf2_ap24W6JqcOYYVy2DhECcPgyvVRA_Ql
ZNHFYdqaImgOKJv-",
"recipientKey": {
"type": "rsa",
"n": "AfWGinFrdktMCi4LkD_vcIsqc0m4JSS0rNDk_5Zdi8fwja
_qH0M7d3U4tPUw7L0gP1iSMakdTKX0S7uTV_v9FeY8_Wrx
DgbphrH9Zaz0PvTLOuiKfRkMWK5A6nzl_PdP7_ujDWkvHK
hWcJtM7irdn9K059X21EDtuqGJyq7_v_c_",
"e": "AQAB",
}
}]
}
7. Mapping to CMS
The JSMS message format is semantically equivalent to a profile of
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the Cryptographic Message Syntax (CMS), and mirrors a fair bit of its
syntactical structure as well. The top-level message types each map
to top-level CMS types: SignedData to SignedData, AuthenticatedData
to AuthenticatedData, and EncryptedData to AuthEnvelopedData
[RFC5083]. The main difference other than encoding is that many
optional fields have been removed, for example the protected and
unprotected attributes.
This similarity also applies to the secondary objects. Just as in
CMS, AlgorithmIdentifier objects carry an identifier for the
algorithm (here a name instead of an OID) and any related parameters.
The PublicKey object format is an amalgam of the SubjectKeyIdentifier
from CMS and the SubjectPublicKeyInfo from X.509. PublicKey objects
can be mapped to CMS constructs by converting them to
SubjectKeyIdentifier objects (using the appropriate hash) and
including a certificate containing the public key. The WrappedKey
object format maps directly to the CMS RecipientInfo structure, with
the above considerations related to public keys, and without the
option for password-based wrapping.
The major way in which JSMS diverges from CMS is that it allows the
use of static MAC keys, referenced by an identifier. CMS requires
the use of random MAC keys, encrypted in a RecipientInfo (i.e., a
WrappedKey) for each recipient. JSMS allows the use of random keys,
but also includes the "keyId" field to reference static MAC keys
directly. The security implications of this change are discussed in
Section 10.
In fact, it should be possible to translate JSMS objects back and
forth to CMS without changing any values (simply reformatting), with
only a couple of exception cases:
o JSMS objects that use static MAC keys cannot be translated to CMS
because CMS does not allow this keying mechanism.
o JSMS objects using general AEAD algorithms (according to
[[draft-mcgrew-aead-aes-cbc-hmac-sha1]]) because the required
algorithm identifiers have not been defined for CMS.
o CMS objects using features that are not supported in JSMS (e.g.,
password-based key wrapping) cannot be translated to JSMS.
8. Comparison to JWS/JWE/JWK
The overall JSMS structure covers the integrity, authentication, and
encryption use cases as the JSON Web Encryption (JWE) and JSON Web
Signature (JWS) specifications. Most of the fields in JWS and JWE
map conceptually to JSMS fields, with a couple of exceptions. The
major differences are as follows:
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o The signature and MAC functions of the JWS object are separated
into SignedData and AuthenticatedData JSMS objects.
o JSMS is pure JSON, whereas in JWE and JWS only the header
parameters are represented in JSON.
o JSMS parameters are not integrity-protected, as they are in JWE
and JWS.
o JSMS allows for full algorithm agility in key agreement, while JWE
only allows ECDH-ES.
o JSMS supports multiple recipients for EncryptedData and
AuthenticatedData objects via the inclusion of multiple WrappedKey
objects. Sending a JWE to multiple recipients requires re-
encryption of the entire object for each recipient.
o The "typ" and "zip" parameters are not defined in JSMS, but could
be added without significant change.
o JSMS requires that recipients MUST ignore unknown header
parameters, in order to facilitate extensibility.
The PublicKey structure is analogous to the JSON Web Key (JWK) (with
the public key parameters specified in the JSON Web Algorithms (JWA)
document). The JWK "use" and "kid" parameters are not defined in
JSMS, but could be added without significant change.
9. IANA Considerations
TODO:
o Register MIME types
o Registries for algorithms (signing, hash, MAC, encrypion,
encipherment, agreement)
10. Security Considerations
Much more to follow here.
[[ Given the CMS mapping above, import CMS security considerations.
]]
[[ Notes on identity for SignedData and AuthenticatedData: It is
important to note that the above verification process only checks
that the JSMS object was signed with a given public key. In order
for this information to be useful to an applications, it is usually
necessary to bind the public key to an application-layer identifier.
If the "certificates" or "certificatesURI" value is present, then the
recipient SHOULD verify that the chain is valid, and that the the
end-entity certificate chains to a trust anchor. In this case, the
recipient can consider the identity asserted in the end-entity
certificate to be bound to the public key. Applications using this
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specification without certificates will need to specify an
alternative mechanism for binding public keys to identifiers. ]]
[[ Notes on the security of static-key MACs. Need to periodically
refresh keys. ]]
[[ For multiple signatures, the considerations of RFC 4853. ]]
11. Acknowledgements
The inspirataion and starting point for this document was
draft-rescorla-jsms-00. Thanks to Eric Rescorla and Joe Hildebrand
for allowing me to re-use a fair bit of their document, and for some
helpful early reviews.
12. References
12.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
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(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] 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, May 2008.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
September 2009.
[RFC5840] Grewal, K., Montenegro, G., and M. Bhatia, "Wrapped
Encapsulating Security Payload (ESP) for Traffic
Visibility", RFC 5840, April 2010.
[FIPS-180-3]
National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-3,
October 2008.
12.2. Informative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[I-D.hammer-webfinger]
Hammer-Lahav, E., Fitzpatrick, B., and B. Cook, "The
WebFinger Protocol", draft-hammer-webfinger-00 (work in
progress), October 2009.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
November 2007.
[I-D.ietf-jose-json-web-signature]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", draft-ietf-jose-json-web-signature-02
(work in progress), May 2012.
[krawczyk-ate]
Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (or: How Secure Is SSL?)",
Advances in cryptology--CRYPTO 2001 August 2001.
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[GCM] National Institute of Standards and Technology (NIST),
"Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC", SP 800-38D,
November 2007.
Appendix A. Acknowledgments
[TODO]
Author's Address
Richard Barnes
BBN Technologies
1300 N. 17th St.
Arlington, VA 22209
USA
Email: rbarnes@bbn.com
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