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Versions: 00
Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track J. Hildebrand
Expires: September 8, 2011 Cisco Systems, Inc.
March 7, 2011
JavaScript Message Security Format
draft-rescorla-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
congenial for Web applications. This document describes a new
cryptographic message format which is based on JavaScript Object
Notation (JSON) and thus is easy for Web 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 September 8, 2011.
Copyright Notice
Copyright (c) 2011 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used In This Document . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Operational Modes . . . . . . . . . . . . . . . . . . . . 4
3.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Certificate Processing . . . . . . . . . . . . . . . . . . 6
3.4. Certificate Discovery . . . . . . . . . . . . . . . . . . 6
4. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Base64 Handling . . . . . . . . . . . . . . . . . . . . . 6
4.2. Content Object . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Common Elements . . . . . . . . . . . . . . . . . . . . . 7
4.4. Signed Data . . . . . . . . . . . . . . . . . . . . . . . 8
4.4.1. Signature Computation . . . . . . . . . . . . . . . . 9
4.4.2. Signature Verification . . . . . . . . . . . . . . . . 10
4.5. Encrypted Data . . . . . . . . . . . . . . . . . . . . . . 12
4.5.1. Message Encryption . . . . . . . . . . . . . . . . . . 13
4.5.2. Message Decryption . . . . . . . . . . . . . . . . . . 13
4.5.3. Key Derivation . . . . . . . . . . . . . . . . . . . . 14
4.5.4. CMK Encryption . . . . . . . . . . . . . . . . . . . . 14
4.6. Composition . . . . . . . . . . . . . . . . . . . . . . . 15
5. Version Processing . . . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. JSON Schema . . . . . . . . . . . . . . . . . . . . . 17
A.1. Message Contents Schema . . . . . . . . . . . . . . . . . 18
A.2. Common Elements Schema . . . . . . . . . . . . . . . . . . 19
A.3. Signed Message Schema . . . . . . . . . . . . . . . . . . 20
A.4. PKIX Certificate Chain Schema . . . . . . . . . . . . . . 21
A.5. Encrypted Message Schema . . . . . . . . . . . . . . . . . 21
A.6. Recipient Schema . . . . . . . . . . . . . . . . . . . . . 23
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 BER/DER) which are not congenial for 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) intended to meet the need of the
Web environment. While JSMS is modeled on existing formats --
principally CMS [RFC5652] -- it uses JavaScript Object Notation
(JSON) rather than ASN.1/BER/DER, making it far easier for Web
applications to handle. In the interest of simplicity, JSMS also
omits as many as possible of the CMS modes (multiple signatures,
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].
3. Overview
The JSMS message format is simply a JSON [RFC4627] dictionary with an
appropriate collection of fields. Each operating mode will have a
separate set of fields, with a common field to distinguish between
the modes.
3.1. Operational Modes
JSMS supports two operational modes:
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, either via a separate Message Authentication
Code (MAC) or an Authenticated Encryption with Associated Data
(AEAD) algorithm such as AES-GCM or AES-CCM.
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Signed Data
A block of data signed by a single signer using his asymmetric key
and optionally carrying his certificate. Multiple signatures are
not permitted in order to keep things simple.
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. (See Section 4.6 for more on composition.
3.2. Conventions
In general, JSMS follows the following structural conventions:
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
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.
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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. 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. In addition,
because many implementations will not want to do any ASN.1/BER
processing at all, we will define a Web Service which applications
can use for chain validation and translation to an easy-to-parse
format. (See [TODO]).
3.4. Certificate Discovery
JSMS will often be used in an online messaging environment 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
All of the field definitions in this section make use of JSON Schema
[I-D.zyp-json-schema]. For each of the fields that is designed to
hold an enumerated value, a registry will be created allowing other
values to be used in addition to the values enumerated in the schema.
4.1. Base64 Handling
As stated in section 3.1 of [RFC4648], Base64 does not require
linefeeds after a specific number of characters. 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 are unintended side-effects of the
production process.
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Implementation Note: Much existing Base64-encoding code will
generate linefeeds every 64 or 76 characters of output. Ensure
that these linefeeds are removed before inserting the output into
a JSON structure.
4.2. Content Object
JSMS operates by providing transformations on "Content" objects,
which are just mime-typed JSON objects. These objects are then
wrapped in a signed/encrypted wrapper with the following fields:
ContentType: A MIME [RFC2045] media type that MUST be included
indicating the type of the "Data" field.
Type: The constant string "content", to facilitate easy
determination that this is the target content. This is useful
(for example) in certain operating conditions where you must
continue to unwrap layers of signatures until you get to the
content. This field MUST be included.
Data: The data value MUST be included as a text encoded as Base64
(See: [RFC4648]).
ID: An OPTIONAL universally unique ID that identifies this message,
for use in detecting replay attacks.
Created: An OPTIONAL field describing the UTC date/time that the
content was encoded into JSON, formatted according to the "date-
time" production of [RFC3339].
Signing and encryption transform a "Content" object into "Signed" and
"Encrypted" objects respectively. Verification and decryption
transform "Signed" and "Encrypted" objects back into "Content"
objects. For example:
{
"ContentType":"text/plain; charset=UTF-8",
"Type":"content",
"Data":"SGVsbG8sIFdvcmxkCg==",
"ID":"746a4c9f-8e84-4313-b669-81590ee2949e",
"Created":"2011-03-07T16:17Z"
}
Figure 1: Content Example
4.3. Common Elements
A JSMS message is a JSON dictionary object containing a set of
specific values.
The following fields MUST be present in all messages:
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Version: The version number. For this specification this value MUST
be set to the string "1.0". See Section 5 for details on version
handling.
Type: The type of the message. MUST be either "signed" or
"encrypted", to indicate a signed message (Section 4.4) or an
encrypted message (Section 4.5) respectively.
4.4. Signed Data
A "signed" message contains a signed data block plus a digital
signature over that data. To simplify implementation, only one
signer is allowed. In addition to the required fields from
Section 4.3, the fields in a signature message are:
SignedData: This field MUST consist of a Base64-encoded "Content"
structure (see Section 4.2), which MUST have been encoded into
octets as UTF-8 prior to Base64-encoding. The signature is
computed over the UTF-8 octet stream before Base64-encoding to
ensure that the sender and receiver have the exact same
representation.
DigestAlgorithm: The message digest used to compute the signature.
This field MUST be present for RSA-based signatures but MAY be
omitted for future signatures which do not allow flexible digests.
For now, this field MUST have the value "SHA-256", meaning the
digest algorithm was SHA-256 [FIPS-180-3].
SignatureAlgorithm: The signature algorithm used to compute the
signature. This field MUST be present. For now, this field MUST
have the value "RSA-PKCS1-1.5", meaning the signature algorithm
was RSASSA-PKCS1-v1_5 as specified in [RFC3447].
Signer: The signer's identity, expressed as a URI [RFC3986]. This
field MUST be present.
CertChain: The signer's certificate chain, if any (see
Section 4.4.2.1).
Signature: The Base64-encoded signature, which MUST be included (see
Section 4.4.1).
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{
"SignedData":"ewogICAgIkNvbnRlbnRUeXBlIjoidGV4dC9wbGFpbjsgY2hhcn
NldD1VVEYtOCIsCiAgICAiVHlwZSI6ImNvbnRlbnQiLAogICAg
IkRhdGEiOiJTR1ZzYkc4c0lGZHZjbXhrQ2c9PSIsCiAgICAiSU
QiOiI3NDZhNGM5Zi04ZTg0LTQzMTMtYjY2OS04MTU5MGVlMjk0
OWUiLAogICAgIkNyZWF0ZWQiOiIyMDExLTAzLTA3VDE2OjE3Wi
IKfQ==",
"DigestAlgorithm":"SHA-256",
"SignatureAlgorithm":"RSA-PKCS1-1.5",
"Signer":"xmpp:romeo@example.net",
"Signature":"sNsxJltUaz4pSzAtJiPZagUMV4SwWugWexGbffK/WJRDi2uq7TxN
/V9SwG/kvQ7CaTABbeUuc6cKGO5YxnH5hME3bHB5L9PKPWSjxzxo
68RPxQyPli2YJDDHKVPbofEa86CLqYcwTF5qrcL7fQFvlRSOVxpS
SJfIdiAJNA+nEnk="
}
Figure 2: Signed Message Example
4.4.1. Signature Computation
The signature is computed over the string prior to base64 encoding.
I.e., the processing order for encoding is:
1. Serialize the inner "Content" value into a UTF8-encoded octet
series X.
2. Compute the signature value over X, and call the result Y. (In
the case of signatures which use digests, this means feed the
literal octets of the signature into the digest function.)
3. Compute the Base64 representation of X and insert it into the
"SignedData" field of the message.
4. Compute the Base64 representation of Y, and insert the result
into the "Signature" field.
This procedure removes dependencies on the exact serialization
algorithm; variation in spacing, field order, etc. do not affect
signature validity since the Base64 representation preserves them on
the wire and protects them from modification by intermediaries.
Note: An alternative algorithm would be to compute the signature on
the base64 representation itself, but this has two disadvantages:
(1) any intermediaries which change spacing/line breaks would
break the signature. (2) it is inconsistent with the algorithm for
encryption (Section 4.5), which is designed to avoid multiple
base64 encoding.
This procedure only specifies the input to the signature computation.
The details of the computation depend on the signature algorithm
itself. The mapping from code points to algorithms is found in
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Section 6.
4.4.2. Signature Verification
In order to verify the signature, the steps of the previous section
are reversed.
1. Process the provided "Signer" and "CertChain" fields as described
in Section 4.4.2.1 in order to determine the sender's public key.
2. Base64 decode the "SignedData" field in order to recover a string
X.
3. Verify the "Signature" field against X using the sender's public
key and the "SignatureAlgorithm" and "DigestAlgorithm" fields.
If the signature fails, return an error.
4. Deserialize X to recover the inner "Content" value.
5. Check any "ID" or "Created" fields for replay.
6. Using the value of the "ContentType" field to give MIME type
context, Base64-decode the "Data" field to retrieve the intended
message.
4.4.2.1. Certificate Processing
JSMS uses the "CertChain" element to carry certificate chains. For
the moment, each certificate in the chain is expected to be a PKIX
certificate BER-encoded then Base64-encoded. Future versions of this
document will likely specify other valid certificate formats, since
one of the goals of this format is to avoid . The meaning of the
fields is described below:
Type: The type of the certificate chain. The only defined value is
"PKIX", referring to PKIX [RFC5280] certificates.
Chain: An array of certificate values. In the case of "PKIX"
certificates this is a list of base64-encoded DER/BER PKIX
certificate values. PKIX certificates MUST be represented in
order with each certificate certifying the next and the final
certificate representing the end-entity.
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{
"Type":"PKIX",
"Chain":[
"MIICPjCCAaegAwIBAgIBETANBgkqhkiG9w0BAQUFADBDMRMwEQ
YKCZImiZPyLGQBGRYDY29tMRcwFQYKCZImiZPyLGQBGRYHZXhh
bXBsZTETMBEGA1UEAxMKRXhhbXBsZSBDQTAeFw0wNDA0MzAxND
I1MzRaFw0wNTA0MzAxNDI1MzRaMEMxEzARBgoJkiaJk/IsZAEZ
FgNjb20xFzAVBgoJkiaJk/IsZAEZFgdleGFtcGxlMRMwEQYDVQ
QDEwpFeGFtcGxlIENBMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCB
iQKBgQDC15dtKHCqW88jLoBwOe7bb9Ut1WpPejQt+SJyR3Ad74
DpyjCMAMSabltFtG6l5myUDfqR6UD8JZ3Ht2gZVo8RcGrX8ckR
Tzp+P5mNbnaldF9epFVT5cdoNlPHHTsSpoX+vW6hyt81UKwI17
m0flz+4qMs0SOEqpjAm2YYmmhH6QIDAQABo0IwQDAdBgNVHQ4E
FgQUCGivhTPIOUp6+IKTjnBqSiCELDIwDgYDVR0PAQH/BAQDAg
EGMA8GA1UdEwEB/wQFMAMBAf8wDQYJKoZIhvcNAQEFBQADgYEA
bPgCdKZh4mQEplQMbHITrTxH+/ZlE6mFkDPqdqMm2fzRDhVfKL
fvk7888+I+fLlS/BZuKarh9Hpv1X/vs5XK82aIg06hNUWEy7yb
uMitxV5G2QsOjYDhMyvcviuSfkpDqWrvimNhs25HOL7oDaNnXf
P6kYE8krvFXyUl63zn2KE=",
"MIICcTCCAdqgAwIBAgIBEjANBgkqhkiG9w0BAQUFADBDMRMwEQ
YKCZImiZPyLGQBGRYDY29tMRcwFQYKCZImiZPyLGQBGRYHZXhh
bXBsZTETMBEGA1UEAxMKRXhhbXBsZSBDQTAeFw0wNDA5MTUxMT
Q4MjFaFw0wNTAzMTUxMTQ4MjFaMEMxEzARBgoJkiaJk/IsZAEZ
FgNjb20xFzAVBgoJkiaJk/IsZAEZFgdleGFtcGxlMRMwEQYDVQ
QDEwpFbmQgRW50aXR5MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCB
iQKBgQDhauQDMJcCPPQQ87UeTX8Ue/b10HjppIrwo3Xs7bZWln
+ImYWa8j5od4frntGfwLQX3KuJI6QdfhYjTE+oTfUxuHyq4xpJ
CfRLJtsnZzCCEgFK6Rq2wQxTi2z8L3pD7DM2fjKye9WqzwEUxh
LsE/ItFHqLIVgUE0xGo5ryFpX/IwIDAQABo3UwczAhBgNVHREE
GjAYgRZlbmQuZW50aXR5QGV4YW1wbGUuY29tMB0GA1UdDgQWBB
QXe5Iw/0TWZuGQECJsFk/AjkHdbTAfBgNVHSMEGDAWgBQIaK+F
M8g5Snr4gpOOcGpKIIQsMjAOBgNVHQ8BAf8EBAMCBsAwDQYJKo
ZIhvcNAQEFBQADgYEAACAoNFtoMgG7CjYOrXHFlRrhBM+urcdi
FKQbNjHA4gw92R7AANwQoLqFb0HLYnq3TGOBJl7SgEVeM+dwRT
s5OyZKnDvyJjZpCHm7+5ZDd0thi6GrkWTg8zdhPBqjpMmKsr9z
1E3kWORi6rwgdJKGDs6EYHbpc7vHhdORRepiXc0="
]
}
Figure 3: PKIX CertChain Example
The recipient MUST verify the certificate chain (in the case of PKIX
certificates according to [RFC5280]). If any validation failure
occurs, the implementation MUST abort processing and return an error.
Once the certificate chain is validated, the end-entity certificate
must contain an identity which matches the "Signer" field. In the
case of PKIX certificates, the certificate MUST contain a
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subjectAltName field of type "uniformResourceIdentifier". This field
MUST be equivalent to the URI in the "Signer" field. If not, an
error MUST be returned.
4.5. Encrypted Data
An "encrypted" message contains an encrypted "Content" block. All
"encrypted" messages contain a symmetric integrity check, either via
a MAC or via an AEAD [RFC5116] algorithm such as Galois/Counter Mode
(GCM: [GCM]). A message may be encrypted to an arbitrary number of
recipients. Each recipient is represented by a "Recipient" block,
which contains a copy of the keying material encrypted for that
recipient. Both symmetric and asymmetric key establishment is
supported. In order to support both integrity and encryption, what
is carried in the Recipient block is a Content Master Key (CMK) which
is then used with a Key Derivation Function (KDF) to generate the
Content Encryption Key (CEK) used to encrypt the message and the
Content Integrity Key (CIK) used with the MAC. In addition to the
required fields from Section 4.3 the fields in an encrypted message
are:
Recipients: The list of recipients. This is an array of Recipient
objects, each of which establishes the CMK for that recipient.
KDF: Specifies the key derivation function used to generate the CEK
and the CIK from the CMK. This field MAY be absent if an AEAD
algorithm is used, in which case the CEK is derived by copying the
CMK.
Encryption: Specifies the properties of the encryption. The
Algorithm field MUST contain the encryption algorithm and the IV
field specifies the initialization vector (if required for the
algorithm). This field MUST be present.
Integrity: Specifies the properties of the integrity check. The
Algorithm field MUST contain the MAC algorithm and the Value field
MUST contain the MAC. This field MAY be absent if no integrity
check is used.
Data: Contains the ciphertext.
Each Recipient object provides an encrypted copy of the CMK for a
single recipient. The meaning of the fields is described below:
KEKidentifier Describes the key encrypting key (KEK) used to encrypt
the CMK. Either a "RecipientName" or a "KeyIdentifier" MUST be
provided. If the "RecipientName" is provided, then a
"CertificateDigest" SHOULD be provided.
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RecipientName: Provides the recipient's name in URI form.
CertificateDigest: For now, the SHA-1 fingerprint of the PKIX
certificate associated with the recipient.
KeyIdentifier The name of a shared symmetric key known to both
sender and recipient. This need not be globally unique as long
as it is unique within the recipient's context.
Algorithm: The algorithm used to encrypt the CMK. For now, one of
"RSA-PKCS1-1.5" (meaning RSASSA-PKCS1-v1_5 as specified in
[RFC3447]) or "AES-256-CBC" (meaning [FIPS-180-3]). Note the JSMS
only supports key transport and not key agreement (since key
agreement can always be turned into key transport).
Value: The CMK encrypted under the specified algorithm and key.
4.5.1. Message Encryption
The message encryption process is as follows.
1. Generate a random CMK. The CMK MUST have a length at least equal
to that of the larger of the required integrity or encryption
keys and MUST be generated randomly. See [RFC4086] for
considerations on generating random values. [[ TODO - we need a
section on generating randomness in browsers - it's easy to screw
up ]]
2. Encrypt the CMK for each recipient (see Section 4.5.4)
3. Generate a random IV (if required for the algorithm).
4. Run the key derivation algorithm (see Section 4.5.3) to generate
the CEK and CIK (if not using an AEAD algorithm).
5. Serialize the content into a bitstring M.
6. Encrypt M using the CEK and IV to form the bitstring C.
7. Set the Value element equal to the base64-encoded representation
of C.
8. If not using an AEAD algorithm, compute the function I = MAC(CIK,
C) using the chosen integrity algorithm. Note that this is EtA
encryption which is considered the best cryptographic choice
(See: [krawczyk-ate]). Set the Integrity.Value element equal to
the base64-encoded representation of I.
4.5.2. Message Decryption
The message decryption process is the reverse of the encryption
process.
1. Identify a Recipient block which appears to reference a key known
to the recipient.
2. Decrypt the CMK. If this fails and another Recipient block
appears plausible, that MAY be tried.
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3. Run the key derivation algorithm (see Section 4.5.3) to generate
the CEK and CIK (if not using an AEAD algorithm).
4. If not using an AEAD algorithm, compute the integrity check value
I' on the binary representation of the Value element using the
indicated integrity check. If the Integrity.Value does not match
I', then an error MUST be reported and processing MUST be
aborted.
5. Decrypt the binary representation of the Value element and output
the result
4.5.3. Key Derivation
The key derivation process converts the CMK into a CEK. It assumes
as a primitive a Key Derivation Function (KDF) which notionally takes
three arguments:
MasterKey: The master key used to compute the individual use keys
Label: The use key label, used to differentiate individual use keys
Length: The length of the desired use key
The only real KDF specified in this document is the TLS PRF, which is
invoked as PRF(MasterKey, Label) with an empty seed and produces an
arbitrary length output. The appropriate number of bits (Length) is
simply extracted from the beginning of the output. The KDF name
"P_XXX" in this document refers the the TLS [RFC5246] PRF using P_XXX
as the underlying P_hash function.
To compute the CEK from the CMK, the label "Encryption" is used.
To compute the CIK from the CMK, the label "Integrity" is used.
When AEAD algorithms are used the KDF element MUST NOT be present.
When they are not used, it MUST be present.
4.5.4. CMK Encryption
JSMS supports two forms of CMK encryption:
o Asymmetric encryption under the recipient's public key.
o Symmetric encryption under a shared key.
4.5.4.1. Asymmetric Encryption
In the asymmetric encryption mode, the CMK is encrypted under the
recipient's public key. The only currently defined asymmetric
encryption mode is RSA-PKCS1-1.5, which refers to [RFC3447] RSAES-
PKCS1-v1_5.
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4.5.4.2. Symmetric Encryption
In the symmetric encryption mode, the CMK is encrypted under a
symmetric key shared between the sender and receiver. All such modes
MUST provide integrity for the CMK. This document defines four such
modes: AES-128-CBC, AES-256-CBC referring to the [RFC5649] AES key
wrapping modes and AES-128-GCM, AES-256-GCM, referring to AES
encryption with GCM. For GCM the random 64-bit IV is prepended to
the ciphertext.
4.6. Composition
This document does not specify a combination signed and encrypted
mode. However, because the contents of a message can be arbitrary,
and encryption and data origin authentication can be provided by
recursively encapsulating multiple JSMS messages. In general,
senders SHOULD sign the message and then encrypt the result (thus
encrypting the signature). This prevents attacks in which the
signature is stripped, leaving just an encrypted message, as well as
providing privacy for the signer.
5. Version Processing
For the moment, all version numbers in the protocol MUST be 1.0.
Receivers MUST return an error for any other version number. More
interesting version processing will be defined in the future.
6. IANA Considerations
[TODO]
o Register MIME types
o Registries for signature, encryption, MAC
o Well known HTTP URLs
7. Security Considerations
Much more to follow here.
8. References
8.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
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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
(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.
[I-D.zyp-json-schema]
Zyp, K. and G. Court, "A JSON Media Type for Describing
the Structure and Meaning of JSON Documents",
draft-zyp-json-schema-03 (work in progress),
November 2010.
[FIPS-180-3]
National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-3,
October 2008.
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8.2. 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.
[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.
[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. JSON Schema
The following schemas formally define various namespaces used in this
document, in conformance with [I-D.zyp-json-schema]. Because
validation of JSON documents is optional, these schemas are not
normative and are provided for descriptive purposes only.
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A.1. Message Contents Schema
{
"description":"Message Contents",
"type":"object",
"properties":{
"ContentType":{
"description":"A MIME content type",
"type":"string",
"required":true
},
"Type":{
"description":"Dictionary type",
"type":"string",
"enum":["content"],
"required":true
},
"Data":{
"description":"The underlying data",
"type":"string",
"required":true
},
"ID":{
"description":"(optional) unique ID for this message",
"type":"string"
},
"Created":{
"description":"(optional) time the message was created",
"type":"string",
"format":"date-time"
}
}
}
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A.2. Common Elements Schema
{
"description":"The basic schema for a JSMS message",
"type":"object",
"properties":{
"Type":{
"description":"Message type",
"type":"string",
"enum":["signed", "encrypted"]
},
"Version":{
"description":"Version number for the message",
"type":"string",
"enum":["1.0"]
}
}
}
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A.3. Signed Message Schema
{
"description":"A signed message",
"type":"object",
"extends":message_schema,
"properties":{
"Signature":{
"description":"The signature over the SignedData",
"type":"object",
"properties":{
"SignedData":{
"description":"content to be signed, Base64",
"type":"string",
"required":true
},
"DigestAlgorithm":{
"description":"",
"type":"string",
"enum":["SHA-256"]
},
"SignatureAlgorithm":{
"description":"",
"type":"string",
"enum":["RSA-PKCS1-1.5"]
},
"Signer":{
"description":"",
"type":"string",
"format":"uri",
"required":true
},
"CertChain": {
"description":"the signer's cert chain",
"type":"PKIXcertchain"
},
"Signature":{
"description":"the signature",
"type":"string",
"required":true
}
}
}
}
}
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A.4. PKIX Certificate Chain Schema
{
"description":"A chain of PKIX certificates",
"id":"PKIXcertchain",
"properties":{
"Type":{
"description":"The type of certificate chain",
"type":"string",
"enum":["PKIX"] },
"Chain":{
"description":"PKIX certs ordered from root to end",
"type":"array",
"items":{
"description":"A base64-encoded BER certificate",
"type":"string"
}
}
}
}
A.5. Encrypted Message Schema
{
"description":"An encrypted object",
"type":"object",
"extends":message_schema,
"properties":{
"Recipients":{
"description":"The list of recipient blocks",
"type":"array",
"required":true,
"items":{
"description":"A single recipient block",
"type":"Recipient"
}
},
"KDF":{
"description":
"The KDF used to derive the MAC and encryption keys",
"type":"string",
"enum":["P_SHA256"]
},
"Encryption":{
"description":"Encryption control information",
"type":"object",
"required":true,
"properties":{
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"Algorithm":{
"description":"The algorithm used to encrypt",
"type":"string",
"enum":["AES-256-CBC"]
},
"IV":{
"description":"Initialization vector (base64)",
"type":"string"
}
}
},
"Integrity":{
"description":"The integrity control information",
"type":"object",
"properties":{
"Algorithm":{
"description":"The MAC algorithm",
"type":"string",
"enum":["HMAC-SHA-256"]
},
"Value":{
"description":"The MAC value (base64-encoded)",
"type":"string",
"required":true
}
}
},
"Data":{
"description":"The ciphertext (Base64-encoded)",
"type":"string",
"required":true
}
}
}
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A.6. Recipient Schema
{
"description":"The recipient of an encrypted object",
"type":"object",
"id":"Recipient",
"properties":{
"KEKidentifier":{
"type":"object",
"description":"Identifies the key encrypting key",
"properties":{
"RecipientName":{
"type":"string",
"description":"The recipient's name",
"format":"uri"
},
"CertificateDigest":{
"type":"string",
"description":"Recipient's cert fingerprint"
},
"KeyIdentifier":{
"type":"string",
"description": "Shared symmetric key (opaque)"
}
}
},
"Algorithm":{
"description":"The algorithm used to protect the CMK",
"type":"string",
"enum":["RSA-PKCS1-1.5", "AES-256-CBC"]
},
"Value":{
"description": "Base64 of the encrypted CMK",
"type":"string"
}
}
}
Appendix B. Acknowledgments
[TODO]
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Authors' Addresses
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@rtfm.com
Joe Hildebrand
Cisco Systems, Inc.
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA
Email: jhildebr@cisco.com
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