COSE Working Group J. Schaad
Internet-Draft August Cellars
Intended status: Informational October 17, 2015
Expires: April 19, 2016
CBOR Encoded Message Syntax
draft-ietf-cose-msg-06
Abstract
Concise Binary Object Representation (CBOR) is data format designed
for small code size and small message size. There is a need for the
ability to have the basic security services defined for this data
format. This document specifies how to do signatures, message
authentication codes and encryption using this data format.
Contributing to this document
The source for this draft is being maintained in GitHub. Suggested
changes should be submitted as pull requests at <https://github.com/
cose-wg/cose-spec>. Instructions are on that page as well.
Editorial changes can be managed in GitHub, but any substantial
issues need to be discussed on the COSE mailing list.
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
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This Internet-Draft will expire on April 19, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Design changes from JOSE . . . . . . . . . . . . . . . . 5
1.2. Requirements Terminology . . . . . . . . . . . . . . . . 5
1.3. CBOR Grammar . . . . . . . . . . . . . . . . . . . . . . 6
1.4. CBOR Related Terminology . . . . . . . . . . . . . . . . 6
1.5. Document Terminology . . . . . . . . . . . . . . . . . . 7
1.6. Mandatory to Implement Algorithms . . . . . . . . . . . . 7
2. Basic COSE Structure . . . . . . . . . . . . . . . . . . . . 8
3. Header Parameters . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Common COSE Headers Parameters . . . . . . . . . . . . . 10
4. Signing Structure . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Externally Supplied Data . . . . . . . . . . . . . . . . 15
4.2. Signing and Verification Process . . . . . . . . . . . . 15
4.3. Computing Counter Signatures . . . . . . . . . . . . . . 17
5. Encryption objects . . . . . . . . . . . . . . . . . . . . . 18
5.1. Enveloped COSE structure . . . . . . . . . . . . . . . . 18
5.1.1. Recipient Algorithm Classes . . . . . . . . . . . . . 19
5.2. Encrypted COSE structure . . . . . . . . . . . . . . . . 20
5.3. Encryption Algorithm for AEAD algorithms . . . . . . . . 20
5.4. Encryption algorithm for AE algorithms . . . . . . . . . 21
6. MAC objects . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1. How to compute a MAC . . . . . . . . . . . . . . . . . . 23
7. Key Structure . . . . . . . . . . . . . . . . . . . . . . . . 24
7.1. COSE Key Common Parameters . . . . . . . . . . . . . . . 24
8. Signature Algorithms . . . . . . . . . . . . . . . . . . . . 27
8.1. ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1.1. Security Considerations . . . . . . . . . . . . . . . 29
9. Message Authentication (MAC) Algorithms . . . . . . . . . . . 30
9.1. Hash-based Message Authentication Codes (HMAC) . . . . . 30
9.1.1. Security Considerations . . . . . . . . . . . . . . . 31
9.2. AES Message Authentication Code (AES-CBC-MAC) . . . . . . 32
9.2.1. Security Considerations . . . . . . . . . . . . . . . 32
10. Content Encryption Algorithms . . . . . . . . . . . . . . . . 33
10.1. AES GCM . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1.1. Security Considerations . . . . . . . . . . . . . . 34
10.2. AES CCM . . . . . . . . . . . . . . . . . . . . . . . . 34
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10.2.1. Security Considerations . . . . . . . . . . . . . . 37
10.3. ChaCha20 and Poly1305 . . . . . . . . . . . . . . . . . 37
10.3.1. Security Considerations . . . . . . . . . . . . . . 38
11. Key Derivation Functions (KDF) . . . . . . . . . . . . . . . 38
11.1. HMAC-based Extract-and-Expand Key Derivation Function
(HKDF) . . . . . . . . . . . . . . . . . . . . . . . . . 39
11.2. Context Information Structure . . . . . . . . . . . . . 40
12. Recipient Algorithm Classes . . . . . . . . . . . . . . . . . 44
12.1. Direct Encryption . . . . . . . . . . . . . . . . . . . 44
12.1.1. Direct Key . . . . . . . . . . . . . . . . . . . . . 45
12.1.2. Direct Key with KDF . . . . . . . . . . . . . . . . 45
12.2. Key Wrapping . . . . . . . . . . . . . . . . . . . . . . 47
12.2.1. AES Key Wrapping . . . . . . . . . . . . . . . . . . 47
12.3. Key Encryption . . . . . . . . . . . . . . . . . . . . . 48
12.4. Direct Key Agreement . . . . . . . . . . . . . . . . . . 48
12.4.1. ECDH . . . . . . . . . . . . . . . . . . . . . . . . 49
12.5. Key Agreement with KDF . . . . . . . . . . . . . . . . . 53
12.5.1. ECDH . . . . . . . . . . . . . . . . . . . . . . . . 53
13. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
13.1. Elliptic Curve Keys . . . . . . . . . . . . . . . . . . 54
13.1.1. Double Coordinate Curves . . . . . . . . . . . . . . 54
13.2. Symmetric Keys . . . . . . . . . . . . . . . . . . . . . 55
14. CBOR Encoder Restrictions . . . . . . . . . . . . . . . . . . 56
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 56
15.1. CBOR Tag assignment . . . . . . . . . . . . . . . . . . 56
15.2. COSE Header Parameter Registry . . . . . . . . . . . . . 57
15.3. COSE Header Algorithm Label Table . . . . . . . . . . . 58
15.4. COSE Algorithm Registry . . . . . . . . . . . . . . . . 58
15.5. COSE Key Common Parameter Registry . . . . . . . . . . . 59
15.6. COSE Key Type Parameter Registry . . . . . . . . . . . . 60
15.7. COSE Elliptic Curve Registry . . . . . . . . . . . . . . 60
15.8. Media Type Registrations . . . . . . . . . . . . . . . . 61
15.8.1. COSE Security Message . . . . . . . . . . . . . . . 61
15.8.2. COSE Key media type . . . . . . . . . . . . . . . . 63
15.9. CoAP Content Format Registrations . . . . . . . . . . . 65
16. Security Considerations . . . . . . . . . . . . . . . . . . . 65
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 66
17.1. Normative References . . . . . . . . . . . . . . . . . . 66
17.2. Informative References . . . . . . . . . . . . . . . . . 66
Appendix A. CDDL Grammar . . . . . . . . . . . . . . . . . . . . 68
Appendix B. Three Levels of Recipient Information . . . . . . . 69
Appendix C. Examples . . . . . . . . . . . . . . . . . . . . . . 70
C.1. Examples of MAC messages . . . . . . . . . . . . . . . . 71
C.1.1. Shared Secret Direct MAC . . . . . . . . . . . . . . 71
C.1.2. ECDH Direct MAC . . . . . . . . . . . . . . . . . . . 72
C.1.3. Wrapped MAC . . . . . . . . . . . . . . . . . . . . . 73
C.1.4. Multi-recipient MAC message . . . . . . . . . . . . . 74
C.2. Examples of Encrypted Messages . . . . . . . . . . . . . 75
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C.2.1. Direct ECDH . . . . . . . . . . . . . . . . . . . . . 75
C.2.2. Direct plus Key Derivation . . . . . . . . . . . . . 76
C.3. Examples of Signed Message . . . . . . . . . . . . . . . 77
C.3.1. Single Signature . . . . . . . . . . . . . . . . . . 77
C.3.2. Multiple Signers . . . . . . . . . . . . . . . . . . 78
C.4. COSE Keys . . . . . . . . . . . . . . . . . . . . . . . . 78
C.4.1. Public Keys . . . . . . . . . . . . . . . . . . . . . 78
C.4.2. Private Keys . . . . . . . . . . . . . . . . . . . . 81
Appendix D. Document Updates . . . . . . . . . . . . . . . . . . 82
D.1. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 82
D.2. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 83
D.3. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 83
D.4. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 83
D.5. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 83
D.6. Version -01 to -2 . . . . . . . . . . . . . . . . . . . . 84
D.7. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 84
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 85
1. Introduction
There has been an increased focus on the small, constrained devices
that make up the Internet of Things (IOT). One of the standards that
has come out of this process is the Concise Binary Object
Representation (CBOR). CBOR extended the data model of the
JavaScript Object Notation (JSON) by allowing for binary data among
other changes. CBOR is being adopted by several of the IETF working
groups dealing with the IOT world as their encoding of data
structures. CBOR was designed specifically to be both small in terms
of messages transport and implementation size as well having a schema
free decoder. A need exists to provide basic message security
services for IOT and using CBOR as the message encoding format makes
sense.
The JOSE working group produced a set of documents
[RFC7515][RFC7516][RFC7517][RFC7518] that defined how to perform
encryption, signatures and message authentication (MAC) operations
for JSON documents and then to encode the results using the JSON
format [RFC7159]. This document does the same work for use with the
CBOR [RFC7049] document format. While there is a strong attempt to
keep the flavor of the original JOSE documents, two considerations
are taken into account:
o CBOR has capabilities that are not present in JSON and should be
used. One example of this is the fact that CBOR has a method of
encoding binary directly without first converting it into a base64
encoded string.
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o COSE is not a direct copy of the JOSE specification. In the
process of creating COSE, decisions that were made for JOSE were
re-examined. In many cases different results were decided on as
the criteria were not always the same as for JOSE.
1.1. Design changes from JOSE
o Define a top level message structure so that encrypted, signed and
MACed messages can easily identified and still have a consistent
view.
o Signed messages separate the concept of protected and unprotected
parameters that are for the content and the signature.
o Recipient processing has been made more uniform. A recipient
structure is required for all recipients rather than only for
some.
o MAC messages are separated from signed messages.
o MAC messages have the ability to do use all recipient algorithms
on the MAC authentication key.
o Use binary encodings for binary data rather than base64url
encodings.
o Combine the authentication tag for encryption algorithms with the
ciphertext.
o Remove the flattened mode of encoding. Forcing the use of an
array of recipients at all times forces the message size to be two
bytes larger, but one gets a corresponding decrease in the
implementation size that should compensate for this. [CREF1]
1.2. Requirements Terminology
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
[RFC2119].
When the words appear in lower case, their natural language meaning
is used.
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1.3. CBOR Grammar
There currently is no standard CBOR grammar available for use by
specifications. We therefore describe the CBOR structures in prose.
There is a version of a CBOR grammar in the CBOR Data Definition
Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl]. An
informational version of the CBOR grammar that reflects what is in
the prose can be found in Appendix A. CDDL has not been fixed, so
this grammar may will only work with the version of CDDL at the time
of publishing.
The document was developed by first working on the grammar and then
developing the prose to go with it. An artifact of this is that the
prose was written using the primitive type strings defined by early
versions CDDL. In this specification the following primitive types
are used:
bstr - byte string (major type 2).
int - an unsigned integer or a negative integer.
nil - a null value (major type 7, value 22).
nint - a negative integer (major type 1).
tstr - a UTF-8 text string (major type 3).
uint - an unsigned integer (major type 0).
Text from here to start of next section to be removed
NOTE: For the purposes of review, we are currently interlacing the
CDDL grammar into the text of document. This is being done for
simplicity of comparison of the grammar against the prose. The
grammar will be removed to an appendix during WGLC.
start = COSE_Untagged_Message / COSE_Tagged_Message /
COSE_Key / COSE_KeySet
1.4. CBOR Related Terminology
In JSON, maps are called objects and only have one kind of map key: a
string. In COSE, we use both strings and integers (both negative and
non-negative integers) as map keys, as well as data items to identify
specific choices. The integers (both positive and negative) are used
for compactness of encoding and easy comparison. (Generally, in this
document the value zero is going to be reserved and not used.) Since
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the work "key" is mainly used in its other meaning, as a
cryptographic key, we use the term "label" for this usage of either
an integer or a string to identify map keys and choose data items.
Text from here to start of next section to be removed
label = int / tstr
values = any
1.5. Document Terminology
In this document we use the following terminology: [CREF2]
Byte is a synonym for octet.
Key management is used as a term to describe how a key at level n is
obtained from level n+1 in encrypted and MACed messages. The term is
also used to discuss key life cycle management, this document does
not discuss key life cycle operations.
1.6. Mandatory to Implement Algorithms
One of the issues that needs to be addressed is a requirement that a
standard specify a set of algorithms that are required to be
implemented. [CREF3] This is done to promote interoperability as it
provides a minimal set of algorithms that all devices can be sure
will exist at both ends. However, we have elected not to specify a
set of mandatory algorithms in this document.
It is expected that COSE is going to be used in a wide variety of
applications and on a wide variety of devices. Many of the
constrained devices are going to be setup to use a small fixed set of
algorithms, and this set of algorithms may not match those available
on a device. We therefore have deferred to the application protocols
the decision of what to specify for mandatory algorithms.
Since the set of algorithms in an environment of constrained devices
may depend on what the set of devices are and how long they have been
in operation, we want to highlight that application protocols will
need to specify some type of discovery method of algorithm
capabilities. The discovery method may be as simple as requiring
preconfiguration of the set of algorithms to providing a discovery
method built into the protocol. S/MIME provided a number of
different ways to approach the problem:
o Advertising in the message (S/MIME capabilities) [RFC5751].
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o Advertising in the certificate (capabilities extension) [RFC4262]
o Minimum requirements for the S/MIME which have been updated over
time [RFC2633][RFC5751]
2. Basic COSE Structure
The COSE Message structure is designed so that there can be a large
amount of common code when parsing and processing the different
security messages. All of the message structures are built on a CBOR
array type. The first three elements of the array contains the same
basic information. The first element is a set of protected header
information. The second element is a set of unprotected header
information. The third element is the content of the message (either
as plain text or encrypted). Elements after this point are dependent
on the specific message type.
Identification of which message is present is done by one of two
methods:
o The specific message type is known from the context in which it is
placed. This may be defined by a marker in the containing
structure or by restrictions specified by the application
protocol.
o The message type is identified by a CBOR tag. This document
defines a CBOR tag for each of the message structures.
Text from here to start of next section to be removed
COSE_Untagged_Message = COSE_Sign /
COSE_enveloped /
COSE_encryptData /
COSE_Mac
COSE_Tagged_Message = COSE_Sign_Tagged /
COSE_Enveloped_Tagged /
COSE_EncryptedData_Tagged /
COSE_Mac_Tagged
3. Header Parameters
The structure of COSE has been designed to have two buckets of
information that are not considered to be part of the payload itself,
but are used for holding information about content, algorithms, keys,
or evaluation hints for the processing of the layer. These two
buckets are available for use in all of the structures in this
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document except for keys. While these buckets can be present, they
may not all be usable in all instances. For example, while the
protected bucket is defined as part of recipient structures, most of
the algorithms that are used for recipients do not provide the
necessary functionality to provide the needed protection and thus the
bucket should not be used.
Both buckets are implemented as CBOR maps. The map key is a 'label'
(Section 1.4). The value portion is dependent on the definition for
the label. Both maps use the same set of label/value pairs. The
integer and string values for labels has been divided into several
sections with a standard range, a private range, and a range that is
dependent on the algorithm selected. The defined labels can be found
in the 'COSE Header Parameters' IANA registry (Section 15.2).
Two buckets are provided for each layer:
protected: Contains parameters about the current layer that are to
be cryptographically protected. This bucket MUST be empty if it
is not going to be included in a cryptographic computation. This
bucket is encoded in the message as a binary object. This value
is obtained by CBOR encoding the protected map and wrapping it in
a bstr object. Senders SHOULD encode an empty protected map as a
zero length binary object (it is shorter). Recipients MUST accept
both a zero length binary value and a zero length map encoded in
the binary value. The wrapping allows for the encoding of the
protected map to be transported with a greater chance that it will
not be altered in transit. (Badly behaved intermediates could
decode and re-encode, but this will result in a failure to verify
unless the re-encoded byte string is identical to the decoded byte
string.) This finesses the problem of all parties needing to be
able to do a common canonical encoding.
unprotected: Contains parameters about the current layer that are
not cryptographically protected.
Only parameters that deal with the current layer are to be placed at
that layer. As an example of this, the parameter 'content type'
describes the content of the message being carried in the message.
As such this parameter is placed only in the content layer and is not
placed in the recipient or signature layers. In principle, one
should be able to process any given layer without reference to any
other layer. (The only data that should need to cross layers is the
cryptographic key.)
The buckets are present in all of the security objects defined in
this document. The fields in order are the 'protected' bucket (as a
CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map'
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type). The presence of both buckets is required. The parameters
that go into the buckets come from the IANA "COSE Header Parameters"
(Section 15.2). Some common parameters are defined in the next
section, but a number of parameters are defined throughout this
document.
Text from here to start of next section to be removed [CREF4]
header_map = {+ label => any }
Headers = (
protected : bstr, ; Contains a header_map
unprotected : header_map
)
3.1. Common COSE Headers Parameters
This section defines a set of common header parameters. A summary of
those parameters can be found in Table 1. This table should be
consulted to determine the value of label used as well as the type of
the value.
The set of header parameters defined in this section are:
alg This parameter is used to indicate the algorithm used for the
security processing. This parameter MUST be present at each level
of a signed, encrypted or authenticated message. The value is
taken from the 'COSE Algorithm Registry' (see Section 15.4).
crit This parameter is used to ensure that applications will take
appropriate action based on the values found. The parameter is
used to indicate which protected header labels an application that
is processing a message is required to understand. The value is
an array of COSE Header Labels. When present, this parameter MUST
be placed in the protected header bucket.
* Integer labels in the range of 0 to 10 SHOULD be omitted.
* Integer labels in the range -1 to -255 can be omitted as they
are algorithm dependent. If an application can correctly
process an algorithm, it can be assumed that it will correctly
process all of the parameters associated with that algorithm.
(The algorithm range is -1 to -65536, it is assumed that the
higher end will deal with more optional algorithm specific
items.)
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The header parameter values indicated by 'crit' can be processed
by either the security library code or by an application using a
security library, the only requirement is that the parameter is
processed. If the 'crit' value list includes a value for which
the parameter is not in the protected bucket, this is a fatal
error in processing the message.
content type This parameter is used to indicate the content type of
the data in the payload or ciphertext fields. Integers are from
the 'CoAP Content-Formats' IANA registry table. Strings are from
the IANA 'Media Types' registry. Applications SHOULD provide this
parameter if the content structure is potentially ambiguous.
kid This parameter one of the ways that can be used to find the key
to be used. The value of this parameter is matched against the
'kid' member in a COSE_Key structure. Applications MUST NOT
assume that 'kid' values are unique. There may be more than one
key with the same 'kid' value, it may be required that all of the
keys need to be checked to find the correct one. The internal
structure of 'kid' values is not defined and generally cannot be
relied on by applications. Key identifier values are hints about
which key to use, they are not directly a security critical field,
for this reason they can be placed in the unprotected headers
bucket.
nonce This parameter holds either a nonce or Initialization Vector
value. The value can be used either as a counter value for a
protocol or as an IV for an algorithm.
counter signature This parameter holds a counter signature value.
Counter signatures provide a method of having a second party sign
some data, the counter signature can occur as an unprotected
attribute in any of the following structures: COSE_Sign,
COSE_signature, COSE_enveloped, COSE_recipient,
COSE_encryptedData, COSE_mac. These structures all have the same
basic structure so that a consistent calculation of the counter
signature can be computed. Details on computing counter
signatures are found in Section 4.3.
creation time This parameter provides the time the content was
created. For signatures and recipient structures, this would be
the time that the signature or recipient key object was created.
For content structures, this would be the time that the content
was created. The unsigned integer value is the number of seconds,
excluding leap seconds; after midnight UTC, January 1, 1970.
sequence number This parameter provides a counter field. The use of
this parameter is application specific.
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+----------+-------+---------------+----------------+---------------+
| name | label | value type | value registry | description |
+----------+-------+---------------+----------------+---------------+
| alg | 1 | int / tstr | COSE Algorithm | Integers are |
| | | | Registry | taken from |
| | | | | table --POINT |
| | | | | TO REGISTRY-- |
| | | | | |
| crit | 2 | [+ label] | COSE Header | integer |
| | | | Label Registry | values are |
| | | | | from -- |
| | | | | POINT TO |
| | | | | REGISTRY -- |
| | | | | |
| content | 3 | tstr / int | CoAP Content- | Value is |
| type | | | Formats or | either a |
| | | | Media Types | Media Type or |
| | | | registry | an integer |
| | | | | from the CoAP |
| | | | | Content |
| | | | | Format |
| | | | | registry |
| | | | | |
| kid | 4 | bstr | | key |
| | | | | identifier |
| | | | | |
| nonce | 5 | bstr | | Nonce or Init |
| | | | | ialization |
| | | | | Vector (IV) |
| | | | | |
| counter | 6 | COSE_signatur | | CBOR encoded |
| signatur | | e | | signature |
| e | | | | structure |
| | | | | |
| creation | * | uint | | Time the |
| time | | | | content was |
| | | | | created |
| | | | | |
| sequence | * | uint | | Application |
| number | | | | specific |
| | | | | Integer value |
+----------+-------+---------------+----------------+---------------+
Table 1: Common Header Parameters
OPEN ISSUES:
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1. I am currently torn on the question "Should epk and iv/nonce be
algorithm specific or generic headers?" They are really specific
to an algorithm and can potentially be defined in different ways
for different algorithms. As an example, it would make sense to
defined nonce for CCM and GCM modes that can have the leading
zero bytes stripped, while for other algorithms this might be
undesirable.
2. We might want to define some additional items. What are they? A
possible example would be a sequence number as this might be
common. On the other hand, this is the type of things that is
frequently used as the nonce in some places and thus should not
be used in the same way. Other items might be challenge/response
fields for freshness as these are likely to be common.
4. Signing Structure
The signature structure allows for one or more signatures to be
applied to a message payload. There are provisions for parameters
about the content and parameters about the signature to be carried
along with the signature itself. These parameters may be
authenticated by the signature, or just present. Examples of
parameters about the content would be the type of content, when the
content was created, and who created the content. [CREF5] Examples
of parameters about the signature would be the algorithm and key used
to create the signature, when the signature was created, and counter-
signatures.
When more than one signature is present, the successful validation of
one signature associated with a given signer is usually treated as a
successful signature by that signer. However, there are some
application environments where other rules are needed. An
application that employs a rule other than one valid signature for
each signer must specify those rules. Also, where simple matching of
the signer identifier is not sufficient to determine whether the
signatures were generated by the same signer, the application
specification must describe how to determine which signatures were
generated by the same signer. Support of different communities of
recipients is the primary reason that signers choose to include more
than one signature. For example, the COSE_Sign structure might
include signatures generated with the RSA signature algorithm and
with the Elliptic Curve Digital Signature Algorithm (ECDSA) signature
algorithm. This allows recipients to verify the signature associated
with one algorithm or the other. (The original source of this text
is [RFC5652].) More detailed information on multiple signature
evaluation can be found in [RFC5752].
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The COSE_Sign structure is a CBOR array. The fields of the array in
order are:
protected is described in Section 3.
unprotected is described in Section 3.
payload contains the serialized content to be signed. If the
payload is not present in the message, the application is required
to supply the payload separately. The payload is wrapped in a
bstr to ensure that it is transported without changes. If the
payload is transported separately, then a nil CBOR object is
placed in this location and it is the responsibility of the
application to ensure that it will be transported without changes.
signatures is an array of signature items. Each of these items uses
the COSE_signature structure for its representation.
The COSE_signature structure is a CBOR array. The fields of the
array in order are:
protected is described in Section 3.
unprotected is described in Section 3.
signature contains the computed signature value. The type of the
field is a bstr.
Text from here to start of next section to be removed
COSE_Sign_Tagged = #6.999(COSE_Sign) ; Replace 999 with TBD1
COSE_Sign = [
Headers,
payload : bstr / nil,
signatures : [+ COSE_signature]
]
COSE_signature = [
Headers,
signature : bstr
]
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4.1. Externally Supplied Data
One of the features that we supply in the COSE document is the
ability for applications to provide additional data to be
authenticated as part of the security, but that is not carried as
part of the COSE object. The primary reason for supporting this can
be seen by looking at the CoAP message structure [RFC7252] where the
facility exists for options to be carried before the payload. An
example of data that can be placed in this location would be
transaction ids and nonces to check for replay protection. If the
data is in the options section, then it is available for routers to
help in performing the replay detection and prevention. However, it
may also be desired to protect these values so that they cannot be
modified in transit. This is the purpose of the externally supplied
data field.
This document describes the process for using a byte array of
externally supplied authenticated data, however the method of
constructing the byte array is a function of the application.
Applications which use this feature need to define how the externally
supplied authenticated data is to be constructed. Such a
construction needs to take into account the following issues:
o If multiple items are included, care needs to be taken that data
cannot bleed between the items. This is usually addressed by
making fields fixed width and/or encoding the length of the field.
Using options from CoAP as an example, these fields use a TLV
structure so they can be concatenated without any problems.
o If multiple items are included, a defined order for the items
needs to be defined. Using options from CoAP as an example, an
application could state that the fields are to be ordered by the
option number.
4.2. Signing and Verification Process
In order to create a signature, a consistent byte stream is needed in
order to process. This document uses a CBOR array to construct the
byte stream to be processed. The fields of the array in order are:
1. The body protected attributes. This is a bstr type containing
the protected attributes of the body.
2. The signature protected attributes. This is a bstr type
containing the protected attributes of the signature.
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3. The external protected attributes. This is a bstr type
containing the protected attributes external to the
COSE_Signature structure.
4. The payload to be signed. The payload is encoded in a bstr. The
payload is placed here independent of how it is transported.
How to compute a signature:
1. Create a CBOR array and populate it with the appropriate fields.
For body_protected and sign_protected, if the fields are not
present in their corresponding maps, a bstr of length zero is
used.
2. If the application has supplied external additional authenticated
data to be included in the computation, then it is placed in the
third field. If no data was supplied, then a zero length binary
value is used.
3. Create the value ToBeSigned by encoding the Sig_structure to a
byte string.
4. Call the signature creation algorithm passing in K (the key to
sign with), alg (the algorithm to sign with) and ToBeSigned (the
value to sign).
5. Place the resulting signature value in the 'signature' field of
the map.
How to verify a signature:
1. Create a Sig_structure object and populate it with the
appropriate fields. For body_protected and sign_protected, if
the fields are not present in their corresponding maps, a bstr of
length zero is used.
2. If the application has supplied external additional authenticated
data to be included in the computation, then it is placed in the
third field. If no data was supplied, then a zero length binary
value is used.
3. Create the value ToBeSigned by encoding the Sig_structure to a
byte string.
4. Call the signature verification algorithm passing in K (the key
to verify with), alg (the algorithm to sign with), ToBeSigned
(the value to sign), and sig (the signature to be verified).
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In addition to performing the signature verification, one must also
perform the appropriate checks to ensure that the key is correctly
paired with the signing identity and that the appropriate
authorization is done.
Text from here to start of next section to be removed
The COSE structure used to create the byte stream to be signed uses
the following CDDL grammar structure:
Sig_structure = [
body_protected: bstr,
sign_protected: bstr,
external_aad: bstr,
payload: bstr
]
4.3. Computing Counter Signatures
Counter signatures provide a method of having a different signature
occur on some piece of content. This is normally used to provide a
signature on a signature allowing for a proof that a signature
existed at a given time. In this document we allow for counter
signatures to exist in a greater number of environments. A counter
signature can exist, for example, on a COSE_encryptedData object and
allow for a signature to be present on the encrypted content of a
message.
The creation and validation of counter signatures over the different
items relies on the fact that the structure all of our objects have
the same structure. The first element may be a message type, this is
followed by a set of protected attributes, a set of unprotected
attributes and a body in that order. This means that the
Sig_structure can be used for in a uniform manner to get the byte
stream for processing a signature. If the counter signature is going
to be computed over a COSE_encryptedData structure, the
body_protected and payload items can be mapped into the Sig_structure
in the same manner as from the COSE_Sign structure.
While one can create a counter signature for a COSE_Sign structure,
there is not much of a point to doing so. It is equivalent to create
a new COSE_signature structure and placing it in the signatures
array. It is strongly suggested that it not be done, but it is not
banned.
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5. Encryption objects
COSE supports two different encryption structures. OOSE_enveloped is
used when the key needs to be explicitly identified. This structure
supports the use of recipient structures to allow for random content
encryption keys to be used. COSE_encrypted is used when a recipient
structure is not needed because the key to be used is known
implicitly.
5.1. Enveloped COSE structure
The enveloped structure allows for one or more recipients of a
message. There are provisions for parameters about the content and
parameters about the recipient information to be carried in the
message. The parameters associated with the content can be
authenticated by the content encryption algorithm. The parameters
associated with the recipient can be authenticated by the recipient
algorithm (when the algorithm supports it). Examples of parameters
about the content are the type of the content, when the content was
created, and the content encryption algorithm. Examples of
parameters about the recipient are the recipient's key identifier,
the recipient encryption algorithm.
In COSE, the same techniques and structures for encrypting both the
plain text and the keys used to protect the text. This is different
from the approach used by both [RFC5652] and [RFC7516] where
different structures are used for the content layer and for the
recipient layer.
The COSE_encrypt structure is a CBOR array. The fields of the array
in order are:
protected is described in Section 3.
unprotected is described in Section 3.
ciphertext contains the encrypted plain text encoded as a bstr. If
the ciphertext is to be transported independently of the control
information about the encryption process (i.e. detached content)
then the field is encoded as a null object.
recipients contains an array of recipient information structures.
The type for the recipient information structure is a
COSE_recipient.
The COSE_recipient structure is a CBOR array. The fields of the
array in order are:
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protected is described in Section 3.
unprotected is described in Section 3.
ciphertext contains the encrypted key encoded as a bstr. If there
is not an encrypted key, then this field is encoded as a nil type.
recipients contains an array of recipient information structures.
The type for the recipient information structure is a
COSE_recipient. If there are no recipient information structures,
this element is absent.
Text from here to start of next section to be removed
COSE_Enveloped_Tagged = #6.998(COSE_enveloped) ; Replace 998 with TBD32
COSE_enveloped = [
COSE_encrypt_fields
recipients: [+COSE_recipient]
]
COSE_encrypt_fields = (
Headers,
ciphertext: bstr / nil,
)
COSE_recipient = [
COSE_encrypt_fields
? recipients: [+COSE_recipient]
]
5.1.1. Recipient Algorithm Classes
A typical encrypted message consists of an encrypted content and an
encrypted CEK for one or more recipients. The content-encryption key
is encrypted for each recipient, using a key specific to that
recipient. The details of this encryption depends on which class the
recipient algorithm falls into. Specific details on each of the
classes can be found in Section 12. A short summary of the six
recipient algorithm classes is:
none: The CEK is the same as the identified previously distributed
symmetric key or derived from a previously distributed secret.
symmetric key-encryption keys: The CEK is encrypted using a
previously distributed symmetric key-encryption key.
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key agreement: the recipient's public key and a sender's private key
are used to generate a pairwise secret, a KDF is applied to derive
a key, and then the CEK is either the derived key or encrypted by
the derived key.
key transport: the CEK is encrypted in the recipient's public key
passwords: the CEK is encrypted in a key-encryption key that is
derived from a password.
5.2. Encrypted COSE structure
The encrypted structure does not have the ability to specify
recipients of the message. The structure assumes that the recipient
of the object will already know the identity of the key to be used in
order to decrypt the message. If a key needs to be identified to the
recipient, the enveloped structure is used.
The CDDL grammar structure for encrypted data is:
COSE_EncryptedData_Tagged = #6.997(COSE_encryptData) ; Replace 997 with TBD3
COSE_encryptData = [
COSE_encrypt_fields
]
The COSE_encryptedData structure is a CBOR array. The fields of the
array in order are:
protected is described in Section 3.
unprotected is described in Section 3.
ciphertext contains the encrypted plain text. If the ciphertext is
to be transported independently of the control information about
the encryption process (i.e. detached content) then the field is
encoded as a null object.
5.3. Encryption Algorithm for AEAD algorithms
The encryption algorithm for AEAD algorithms is fairly simple. In
order to get a consistent encoding of the data to be authenticated,
the Enc_structure is used to have canonical form of the AAD.
1. Copy the protected header field from the message to be sent.
2. If the application has supplied external additional authenticated
data to be included in the computation, then it is placed in the
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'external_aad' field. If no data was supplied, then a zero
length binary value is used. (See Section 4.1 for application
guidance on constructing this field.)
3. Encode the Enc_structure using a CBOR Canonical encoding
Section 14 to get the AAD value.
4. Determine the encryption key. This step is dependent on the
class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm
and key at the current level.
Direct and Direct Key Agreement: The key is determined by the
key and algorithm in the recipient structure. The encryption
algorithm and size of the key to be used are inputs into the
KDF used for the recipient. (For direct, the KDF can be
thought of as the identity operation.)
Other: The key is randomly generated.
5. Call the encryption algorithm with K (the encryption key to use),
P (the plain text) and AAD (the additional authenticated data).
Place the returned cipher text into the 'ciphertext' field of the
structure.
6. For recipients of the message, recursively perform the encryption
algorithm for that recipient using the encryption key as the
plain text.
Text from here to start of next section to be removed
Enc_structure = [
protected: bstr,
external_aad: bstr
]
5.4. Encryption algorithm for AE algorithms
1. Verify that the 'protected' field is absent.
2. Verify that there was no external additional authenticated data
supplied for this operation.
3. Determine the encryption key. This step is dependent on the
class of recipient algorithm being used. For:
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No Recipients: The key to be used is determined by the algorithm
and key at the current level.
Direct and Direct Key Agreement: The key is determined by the
key and algorithm in the recipient structure. The encryption
algorithm and size of the key to be used are inputs into the
KDF used for the recipient. (For direct, the KDF can be
thought of as the identity operation.)
Other: The key is randomly generated.
4. Call the encryption algorithm with K (the encryption key to use)
and the P (the plain text). Place the returned cipher text into
the 'ciphertext' field of the structure.
5. For recipients of the message, recursively perform the encryption
algorithm for that recipient using the encryption key as the
plain text.
6. MAC objects
In this section we describe the structure and methods to be used when
doing MAC authentication in COSE. This document allows for the use
of all of the same classes of recipient algorithms as are allowed for
encryption.
When using MAC operations, there are two modes in which it can be
used. The first is just a check that the content has not been
changed since the MAC was computed. Any class of recipient algorithm
can be used for this purpose. The second mode is to both check that
the content has not been changed since the MAC was computed, and to
use recipient algorithm to verify who sent it. The classes of
recipient algorithms that support this are those that use a pre-
shared secret or do static-static key agreement (without the key wrap
step). In both of these cases the entity MACing the message can be
validated by a key binding. (The binding of identity assumes that
there are only two parties involved and you did not send the message
yourself.)
The COSE_Mac structure is a CBOR array. The fields of the array in
order are:
protected is described in Section 3.
unprotected is described in Section 3.
payload contains the serialized content to be MACed. If the payload
is not present in the message, the application is required to
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supply the payload separately. The payload is wrapped in a bstr
to ensure that it is transported without changes. If the payload
is transported separately, then a null CBOR object is placed in
this location and it is the responsibility of the application to
ensure that it will be transported without changes.
tag contains the MAC value.
recipients contains the recipient information. See the description
under COSE_Encryption for more info.
Text from here to start of next section to be removed
COSE_Mac_Tagged = #6.996(COSE_Mac) ; Replace 996 with TBD4
COSE_Mac = [
Headers,
payload: bstr / nil,
tag: bstr,
recipients: [+COSE_recipient]
]
6.1. How to compute a MAC
How to compute a MAC:
1. Create a MAC_structure and copy the protected and payload fields
from the COSE_Mac structure.
2. If the application has supplied external authenticated data,
encode it as a binary value and place in the MAC_structure. If
there is no external authenticated data, then use a zero length
'bstr'. (See Section 4.1 for application guidance on
constructing this field.)
3. Encode the MAC_structure using a canonical CBOR encoder. The
resulting bytes is the value to compute the MAC on.
4. Compute the MAC and place the result in the 'tag' field of the
COSE_Mac structure.
5. Encrypt and encode the MAC key for each recipient of the message.
Text from here to start of next section to be removed
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MAC_structure = [
protected: bstr,
external_aad: bstr,
payload: bstr
]
7. Key Structure
A COSE Key structure is built on a CBOR map object. The set of
common parameters that can appear in a COSE Key can be found in the
IANA registry 'COSE Key Common Parameter Registry' (Section 15.5).
Additional parameters defined for specific key types can be found in
the IANA registry 'COSE Key Type Parameters' (Section 15.6).
A COSE Key Set uses a CBOR array object as its underlying type. The
values of the array elements are COSE Keys. A Key Set MUST have at
least one element in the array.
The element "kty" is a required element in a COSE_Key map.
Text from here to start of next section to be removed
The CDDL grammar describing a COSE_Key and COSE_KeySet is: [CREF6]
COSE_Key = {
key_kty => tstr / int,
? key_ops => [+ (tstr / int) ],
? key_alg => tstr / int,
? key_kid => bstr,
* label => values
}
COSE_KeySet = [+COSE_Key]
7.1. COSE Key Common Parameters
This document defines a set of common parameters for a COSE Key
object. Table 2 provides a summary of the parameters defined in this
section. There are also a set of parameters that are defined for a
specific key type. Key type specific parameters can be found in
Section 13.
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+---------+-------+-------------+-------------+---------------------+
| name | label | CBOR type | registry | description |
+---------+-------+-------------+-------------+---------------------+
| kty | 1 | tstr / int | COSE | Identification of |
| | | | General | the key type |
| | | | Values | |
| | | | | |
| key_ops | 4 | [* | | Restrict set of |
| | | (tstr/int)] | | permissible |
| | | | | operations |
| | | | | |
| alg | 3 | tstr / int | COSE | Key usage |
| | | | Algorithm | restriction to this |
| | | | Values | algorithm |
| | | | | |
| kid | 2 | bstr | | Key Identification |
| | | | | value - match to |
| | | | | kid in message |
| | | | | |
| use | * | tstr | | deprecated - don't |
| | | | | use |
+---------+-------+-------------+-------------+---------------------+
Table 2: Key Map Labels
kty: This parameter is used to identify the family of keys for this
structure, and thus the set of key type specific parameters to be
found. The set of values can be found in Table 18. This
parameter MUST be present in a key object. Implementations MUST
verify that the key type is appropriate for the algorithm being
processed. The key type MUST be included as part of a trust
decision process.
alg: This parameter is used to restrict the algorithms that are to
be used with this key. If this parameter is present in the key
structure, the application MUST verify that this algorithm matches
the algorithm for which the key is being used. If the algorithms
do not match, then this key object MUST NOT be used to perform the
cryptographic operation. Note that the same key can be in a
different key structure with a different or no algorithm
specified, however this is considered to be a poor security
practice.
kid: This parameter is used to give an identifier for a key. The
identifier is not structured and can be anything from a user
provided string to a value computed on the public portion of the
key. This field is intended for matching against a 'kid'
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parameter in a message in order to filter down the set of keys
that need to be checked.
key_ops: This parameter is defined to restrict the set of operations
that a key is to be used for. The value of the field is an array
of values from Table 3.
+---------+-------+-------------------------------------------------+
| name | value | description |
+---------+-------+-------------------------------------------------+
| sign | 1 | The key is used to create signatures. Requires |
| | | private key fields. |
| | | |
| verify | 2 | The key is used for verification of signatures. |
| | | |
| encrypt | 3 | The key is used for key transport encryption. |
| | | |
| decrypt | 4 | The key is used for key transport decryption. |
| | | Requires private key fields. |
| | | |
| wrap | 5 | The key is used for key wrapping. |
| key | | |
| | | |
| unwrap | 6 | The key is used for key unwrapping. Requires |
| key | | private key fields. |
| | | |
| key | 7 | The key is used for key agreement. |
| agree | | |
+---------+-------+-------------------------------------------------+
Table 3: Key Operation Values
Text from here to start of next section to be removed
The following provides a CDDL fragment which duplicates the
assignment labels from Table 2 and Table 3.
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;key_labels
key_kty=1
key_kid=2
key_alg=3
key_ops=4
;key_ops values
key_ops_sign=1
key_ops_verify=2
key_ops_encrypt=3
key_ops_decrypt=4
key_ops_wrap=5
key_ops_unwrap=6
key_ops_agree=7
8. Signature Algorithms
There are two basic signature algorithm structures that can be used.
The first is the common signature with appendix. In this structure,
the message content is processed and a signature is produced, the
signature is called the appendix. This is the message structure used
by our common algorithms such as ECDSA and RSASSA-PSS. (In fact the
SSA in RSASSA-PSS stands for Signature Scheme with Appendix.) The
basic structure becomes:
signature = Sign(message content, key)
valid = Verification(message content, key, signature)
The second is a signature with message recovery. (An example of such
an algorithm is [PVSig].) In this structure, the message content is
processed, but part of is included in the signature. Moving bytes of
the message content into the signature allows for an effectively
smaller signature, the signature size is still potentially large, but
the message content is shrunk. This has implications for systems
implementing these algorithms and for applications that use them.
The first is that the message content is not fully available until
after a signature has been validated. Until that point the part of
the message contained inside of the signature is unrecoverable. The
second is that the security analysis of the strength of the signature
is very much based on the structure of the message content. Messages
which are highly predictable require additional randomness to be
supplied as part of the signature process, in the worst case it
becomes the same as doing a signature with appendix. Thirdly, in the
event that multiple signatures are applied to a message, all of the
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signature algorithms are going to be required to consume the same
number of bytes of message content.
signature, message sent = Sign(message content, key)
valid, message content = Verification(message sent, key, signature)
At this time, only signatures with appendixes are defined for use
with COSE, however considerable interest has been expressed in using
a signature with message recovery algorithm due to the effective size
reduction that is possible. Implementations will need to keep this
in mind for later possible integration.
8.1. ECDSA
ECDSA [DSS] defines a signature algorithm using ECC.
The ECDSA signature algorithm is parameterized with a hash function
(h). In the event that the length of the hash function output is
greater than group of the key, the left most bytes of the hash output
are used.
The algorithms defined in this document can be found in Table 4.
+-------+-------+---------+------------------+
| name | value | hash | description |
+-------+-------+---------+------------------+
| ES256 | -7 | SHA-256 | ECDSA w/ SHA-256 |
| | | | |
| ES384 | -8 | SHA-384 | ECDSA w/ SHA-384 |
| | | | |
| ES512 | -9 | SHA-512 | ECDSA w/ SHA-512 |
+-------+-------+---------+------------------+
Table 4: ECDSA Algorithm Values
This document defines ECDSA to work only with the curves P-256, P-384
and P-521. This document requires that the curves be encoded using
the 'EC2' key type. Implementations need to check that the key type
and curve are correct when creating and verifying a signature. Other
documents can defined it to work with other curves and points in the
future.
In order to promote interoperability, it is suggested that SHA-256 be
used only with curve P-256, SHA-384 be used only with curve P-384 and
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SHA-512 be used with curve P-521. This is aligned with the
recommendation in Section 4 of [RFC5480].
The signature algorithm results in a pair of integers (R, S). These
integers will be of the same order as length of the key used for the
signature process. The signature is encoded by converting the
integers into byte strings of the same length as the key size. The
length is rounded up to the nearest byte and is left padded with zero
bits to get to the correct length. The two integers are then
concatenated together to form a byte string that is the resulting
signature.
Using the function defined in [RFC3447] the signature is:
Signature = I2OSP(R, n) | I2OSP(S, n)
where n = ceiling(key_length / 8)
8.1.1. Security Considerations
The security strength of the signature is no greater than the minimum
of the security strength associated with the bit length of the key
and the security strength of the hash function.
System which have poor random number generation can leak their keys
by signing two different messages with the same value of 'k'.
[RFC6979] provides a method to deal with this problem by making 'k'
be deterministic based on the message content rather than randomly
generated. Applications which specify ECDSA should evaluate the
ability to get good random number generation and require this when it
is not possible. Note: Use of this technique a good idea even when
good random number generation exists. Doing so both reduces the
possibility of having the same value of 'k' in two signature
operations, but allows for reproducible signature values which helps
testing.
There are two substitution that can theoretically be mounted against
the ECDSA signature algorithm.
o Changing the curve used to validate the signature: If one changes
the curve used to validate the signature, then potentially one
could have a two messages with the same signature each computed
under a different curve. The only requirement on the new curve is
that its order be the same as the old one and it be acceptable to
the client. An example would be to change from using the curve
secp256r1 (aka P-256) to using secp256k1. (Both are 256 bit
curves.) We current do not have any way to deal with this version
of the attack except to restrict the overall set of curves that
can be used.
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o Change the hash function used to validate the signature: If one
has either two different hash functions of the same length, or one
can truncate a hash function down, then one could potentially find
collisions between the hash functions rather than within a single
hash function. (For example, truncating SHA-512 to 256 bits might
collide with a SHA-256 bit hash value.) This attack can be
mitigated by including the signature algorithm identifier in the
data to be signed.
9. Message Authentication (MAC) Algorithms
Message Authentication Codes (MACs) provide data authentication and
integrity protection. They provide either no or very limited data
origination. (One cannot, for example, be used to prove the identity
of the sender to a third party.)
MACs are designed in the same basic structure as signature with
appendix algorithms. The message content is processed and an
authentication code is produced, the authentication code is
frequently called a tag. The basic structure becomes:
tag = MAC_Create(message content, key)
valid = MAC_Verify(message content, key, tag)
MAC algorithms can be based on either a block cipher algorithm (i.e.
AES-MAC) or a hash algorithm (i.e. HMAC). This document defines a
MAC algorithm for each of these two constructions.
9.1. Hash-based Message Authentication Codes (HMAC)
The Hash-base Message Authentication Code algorithm (HMAC)
[RFC2104][RFC4231] was designed to deal with length extension
attacks. The algorithm was also designed to allow for new hash
algorithms to be directly plugged in without changes to the hash
function. The HMAC design process has been vindicated as, while the
security of hash algorithms such as MD5 has decreased over time, the
security of HMAC combined with MD5 has not yet been shown to be
compromised [RFC6151].
The HMAC algorithm is parameterized by an inner and outer padding, a
hash function (h) and an authentication tag value length. For this
specification, the inner and outer padding are fixed to the values
set in [RFC2104]. The length of the authentication tag corresponds
to the difficulty of producing a forgery. For use in constrained
environments, we define a set of HMAC algorithms that are truncated.
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There are currently no known issues when truncating, however the
security strength of the message tag is correspondingly reduced in
strength. When truncating, the left most tag length bits are kept
and transmitted.
The algorithm defined in this document can be found in Table 5.
+-----------+-------+---------+--------+----------------------------+
| name | value | Hash | Length | description |
+-----------+-------+---------+--------+----------------------------+
| HMAC | * | SHA-256 | 64 | HMAC w/ SHA-256 truncated |
| 256/64 | | | | to 64 bits |
| | | | | |
| HMAC | 4 | SHA-256 | 256 | HMAC w/ SHA-256 |
| 256/256 | | | | |
| | | | | |
| HMAC | 5 | SHA-384 | 384 | HMAC w/ SHA-384 |
| 384/384 | | | | |
| | | | | |
| HMAC | 6 | SHA-512 | 512 | HMAC w/ SHA-512 |
| 512/512 | | | | |
+-----------+-------+---------+--------+----------------------------+
Table 5: HMAC Algorithm Values
Some recipient algorithms carry the key while others derive a key
from secret data. For those algorithms which carry the key (i.e.
RSA-OAEP and AES-KeyWrap), the size of the HMAC key SHOULD be the
same size as the underlying hash function. For those algorithms
which derive the key, the derived key MUST be the same size as the
underlying hash function.
If the key obtained from a key structure, the key type MUST be
'Symmetric'. Implementations creating and validating MAC values MUST
validate that the key type, key length and algorithm are correct and
appropriate for the entities involved.
9.1.1. Security Considerations
HMAC has proved to be resistant even when used with weakening hash
algorithms. The current best method appears to be a brute force
attack on the key. This means that key size is going to be directly
related to the security of an HMAC operation.
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9.2. AES Message Authentication Code (AES-CBC-MAC)
AES-CBC-MAC is defined in [MAC].
AES-CBC-MAC is parameterized by the key length, the authentication
tag length and the IV used. For all of these algorithms, the IV is
fixed to all zeros. We provide an array of algorithms for various
key lengths and tag lengths. The algorithms defined in this document
are found in Table 6.
+-------------+-------+----------+----------+-----------------------+
| name | value | key | tag | description |
| | | length | length | |
+-------------+-------+----------+----------+-----------------------+
| AES-MAC | * | 128 | 64 | AES-MAC 128 bit key, |
| 128/64 | | | | 64-bit tag |
| | | | | |
| AES-MAC | * | 256 | 64 | AES-MAC 256 bit key, |
| 256/64 | | | | 64-bit tag |
| | | | | |
| AES-MAC | * | 128 | 128 | AES-MAC 128 bit key, |
| 128/128 | | | | 128-bit tag |
| | | | | |
| AES-MAC | * | 256 | 128 | AES-MAC 256 bit key, |
| 256/128 | | | | 128-bit tag |
+-------------+-------+----------+----------+-----------------------+
Table 6: AES-MAC Algorithm Values
Keys may be obtained either from a key structure or from a recipient
structure. If the key obtained from a key structure, the key type
MUST be 'Symmetric'. Implementations creating and validating MAC
values MUST validate that the key type, key length and algorithm are
correct and appropriate for the entities involved.
9.2.1. Security Considerations
A number of attacks exist against CBC-MAC that need to be considered.
o A single key must only be used for messages of a fixed and known
length. If this is not the case, an attacker will be able to
generate a message with a valid tag given two message, tag pairs.
This can be addressed by using different keys for different length
messages. (CMAC mode also addresses this issue.)
o If the same key is used for both encryption and authentication
operations, using CBC modes an attacker can produce messages with
a valid authentication code.
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o If the IV can be modified, then messages can be forged. This is
addressed by fixing the IV to all zeros.
10. Content Encryption Algorithms
Content Encryption Algorithms provide data confidentiality for
potentially large blocks of data using a symmetric key. They provide
either no or very limited data origination. (One cannot, for
example, be used to prove the identity of the sender to a third
party.) The ability to provide data origination is linked to how the
symmetric key is obtained.
We restrict the set of legal content encryption algorithms to those
which support authentication both of the content and additional data.
The encryption process will generate some type of authentication
value, but that value may be either explicit or implicit in terms of
the algorithm definition. For simplicity sake, the authentication
code will normally be defined as being appended to the cipher text
stream. The basic structure becomes:
ciphertext = Encrypt(message content, key, additional data)
valid, message content = Decrypt(cipher text, key, additional data)
Most AEAD algorithms are logically defined as returning the message
content only if the decryption is valid. Many but not all
implementations will follow this convention. The message content
MUST NOT be used if the decryption does not validate.
10.1. AES GCM
The GCM mode is a generic authenticated encryption block cipher mode
defined in [AES-GCM]. The GCM mode is combined with the AES block
encryption algorithm to define an AEAD cipher.
The GCM mode is parameterized with by the size of the authentication
tag. The size of the authentication tag is limited to a small set of
values. For this document however, the size of the authentication
tag is fixed at 128-bits.
The set of algorithms defined in this document are in Table 7.
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+---------+-------+-----------------------------+
| name | value | description |
+---------+-------+-----------------------------+
| A128GCM | 1 | AES-GCM mode w/ 128-bit key |
| | | |
| A192GCM | 2 | AES-GCM mode w/ 192-bit key |
| | | |
| A256GCM | 3 | AES-GCM mode w/ 256-bit key |
+---------+-------+-----------------------------+
Table 7: Algorithm Value for AES-GCM
Keys may be obtained either from a key structure or from a recipient
structure. If the key obtained from a key structure, the key type
MUST be 'Symmetric'. Implementations creating and validating MAC
values MUST validate that the key type, key length and algorithm are
correct and appropriate for the entities involved.
10.1.1. Security Considerations
When using AES-CCM the following restrictions MUST be enforced:
o The key and nonce pair MUST be unique for every message encrypted.
o The total amount of data encrypted MUST NOT exceed 2^39 - 256
bits. An explicit check is required only in environments where it
is expected that it might be exceeded.
10.2. AES CCM
Counter with CBC-MAC (CCM) is a generic authentication encryption
block cipher mode defined in [RFC3610]. The CCM mode is combined
with the AES block encryption algorithm to define a commonly used
content encryption algorithm used in constrained devices.
The CCM mode has two parameter choices. The first choice is M, the
size of the authentication field. The choice of the value for M
involves a trade-off between message expansion and the probably that
an attacker can undetectably modify a message. The second choice is
L, the size of the length field. This value requires a trade-off
between the maximum message size and the size of the Nonce.
It is unfortunate that the specification for CCM specified L and M as
a count of bytes rather than a count of bits. This leads to possible
misunderstandings where AES-CCM-8 is frequently used to refer to a
version of CCM mode where the size of the authentication is 64-bits
and not 8-bits. These values have traditionally been specified as
bit counts rather than byte counts. This document will follow the
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tradition of using bit counts so that it is easier to compare the
different algorithms presented in this document.
We define a matrix of algorithms in this document over the values of
L and M. Constrained devices are usually operating in situations
where they use short messages and want to avoid doing recipient
specific cryptographic operations. This favors smaller values of M
and larger values of L. Less constrained devices do will want to be
able to user larger messages and are more willing to generate new
keys for every operation. This favors larger values of M and smaller
values of L. (The use of a large nonce means that random generation
of both the key and the nonce will decrease the chances of repeating
the pair on two different messages.)
The following values are used for L:
16-bits (2) limits messages to 2^16 bytes (64 KiB) in length. This
sufficiently long for messages in the constrained world. The
nonce length is 13 bytes allowing for 2^(13*8) possible values of
the nonce without repeating.
64-bits (8) limits messages to 2^64 bytes in length. The nonce
length is 7 bytes allowing for 2^56 possible values of the nonce
without repeating.
The following values are used for M:
64-bits (8) produces a 64-bit authentication tag. This implies that
there is a 1 in 2^64 chance that a modified message will
authenticate.
128-bits (16) produces a 128-bit authentication tag. This implies
that there is a 1 in 2^128 chance that a modified message will
authenticate.
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+--------------------+-------+----+-----+-----+---------------------+
| name | value | L | M | k | description |
+--------------------+-------+----+-----+-----+---------------------+
| AES-CCM-16-64-128 | 10 | 16 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, 64-bit |
| | | | | | tag, 13-byte nonce |
| | | | | | |
| AES-CCM-16-64-256 | 11 | 16 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, 64-bit |
| | | | | | tag, 13-byte nonce |
| | | | | | |
| AES-CCM-64-64-128 | 30 | 64 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, 64-bit |
| | | | | | tag, 7-byte nonce |
| | | | | | |
| AES-CCM-64-64-256 | 31 | 64 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, 64-bit |
| | | | | | tag, 7-byte nonce |
| | | | | | |
| AES-CCM-16-128-128 | 12 | 16 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-16-128-256 | 13 | 16 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-64-128-128 | 32 | 64 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, 7-byte |
| | | | | | nonce |
| | | | | | |
| AES-CCM-64-128-256 | 33 | 64 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, 7-byte |
| | | | | | nonce |
+--------------------+-------+----+-----+-----+---------------------+
Table 8: Algorithm Values for AES-CCM
Keys may be obtained either from a key structure or from a recipient
structure. If the key obtained from a key structure, the key type
MUST be 'Symmetric'. Implementations creating and validating MAC
values MUST validate that the key type, key length and algorithm are
correct and appropriate for the entities involved.
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10.2.1. Security Considerations
When using AES-CCM the following restrictions MUST be enforced:
o The key and nonce pair MUST be unique for every message encrypted.
o The total number of times the AES block cipher is used MUST NOT
exceed 2^61 operations. This limitation is the sum of times the
block cipher is used in computing the MAC value and in performing
stream encryption operations. An explicit check is required only
in environments where it is expected that it might be exceeded.
[RFC3610] additionally calls out one other consideration of note. It
is possible to do a pre-computation attack against the algorithm in
cases where the portions encryption content is highly predictable.
This reduces the security of the key size by half. Ways to deal with
this attack include adding a random portion to the nonce value and/or
increasing the key size used. Using a portion of the nonce for a
random value will decrease the number of messages that a single key
can be used for. Increasing the key size may require more resources
in the constrained device. See sections 5 and 10 of [RFC3610] for
more information.
10.3. ChaCha20 and Poly1305
ChaCha20 and Poly1305 combined together is a new AEAD mode that is
defined in [RFC7539]. This is a new algorithm defined to be a cipher
which is not AES and thus would not suffer from any future weaknesses
found in AES. These cryptographic functions are designed to be fast
in software only implementations.
The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no
parameterization. It takes a 256-bit key and a 96-bit nonce as well
as the plain text and additional data as inputs and produces the
cipher text as an option. We define one algorithm identifier for
this algorithm in Table 9.
+-------------------+-------+----------------------------------+
| name | value | description |
+-------------------+-------+----------------------------------+
| ChaCha20/Poly1305 | 11 | ChaCha20/Poly1305 w/ 256-bit key |
+-------------------+-------+----------------------------------+
Table 9: Algorithm Value for AES-GCM
Keys may be obtained either from a key structure or from a recipient
structure. If the key obtained from a key structure, the key type
MUST be 'Symmetric'. Implementations creating and validating MAC
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values MUST validate that the key type, key length and algorithm are
correct and appropriate for the entities involved.
10.3.1. Security Considerations
The pair of key, nonce MUST be unique for every invocation of the
algorithm. Nonce counters are considered to be an acceptable way of
ensuring that they are unique.
11. Key Derivation Functions (KDF)
Key Derivation Functions (KDFs) are used to take some secret value
and generate a different one. The original secret values come in
three basic flavors:
o Secrets which are uniformly random: This is the type of secret
which is created by a good random number generator.
o Secrets which are not uniformly random: This is type of secret
which is created by operations like key agreement.
o Secrets which are not random: This is the type of secret that
people generate for things like passwords.
General KDF functions work well with the first type of secret, can do
reasonable well with the second type of secret and generally do
poorly with the last type of secret. None of the KDF functions in
this section are designed to deal with the type of secrets that are
used for passwords. Functions like PBSE2 [RFC2898] need to be used
for that type of secret.
Many functions are going to handle the first two type of secrets
differently. The KDF function defined in Section 11.1 can use
different underlying constructions if the secret is uniformly random
than if the secret is not uniformly random. This is reflected in the
set of algorithms defined for HKDF.
When using KDF functions, one component that is generally included is
context information. Context information is used to allow for
different keying information to be derived from the same secret. The
use of context based keying material is considered to be a good
security practice. This document defines a single context structure
and a single KDF function.
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11.1. HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
The HKDF key derivation algorithm is defined in [RFC5869].
The HKDF algorithm is defined to take a number of inputs. These
inputs are:
secret - a shared value that is secret. Secrets may be either
previously shared or derived from operations like a DH key
agreement.
salt - an optional public value that is used to change the
generation process. If specified, the salt is carried using the
'salt' algorithm parameter. While [RFC5869] suggests that the
length of the salt be the same as the length of the underlying
hash value, any amount of salt will improve the security as
different key values will be generated. A parameter to carry the
salt is defined in Table 11. This parameter is protected by being
included in the key computation and does not need to be separately
authenticated.
length - the number of bytes of output that need to be generated.
context information - Information that describes the context in
which the resulting value will be used. Making this information
specific to the context that the material is going to be used
ensures that the resulting material will always be unique. The
context structure used is encoded into the algorithm identifier.
hash function - The underlying hash function to be used in the
HKDF algorithm. The hash function is encoded into the HKDF
algorithm selection.
HKDF is defined to use HMAC as the underlying PRF. However, it is
possible to use other functions in the same construct to provide a
different KDF function that may be more appropriate in the
constrained world. Specifically, one can use AES-CBC-MAC as the PRF
for the expand step, but not for the extract step. When using a good
random shared secret of the correct length, the extract step can be
skipped. The extract cannot be skipped if the secret is not
uniformly random, for example if it is the result of an ECDH key
agreement step.
The algorithms defined in this document are found in Table 10
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+-------------+-------------+----------+----------------------------+
| name | hash | Skip | context |
| | | extract | |
+-------------+-------------+----------+----------------------------+
| HKDF | SHA-256 | no | XXX |
| SHA-256 | | | |
| | | | |
| HKDF | SHA-512 | no | XXX |
| SHA-512 | | | |
| | | | |
| HKDF AES- | AES-CBC-128 | yes | HKDF using AES-MAC as the |
| MAC-128 | | | PRF w/ 128-bit key |
| | | | |
| HKDF AES- | AES-CBC-128 | yes | HKDF using AES-MAC as the |
| MAC-256 | | | PRF w/ 256-bit key |
+-------------+-------------+----------+----------------------------+
Table 10: HKDF algorithms
+------+-------+------+-------------+
| name | label | type | description |
+------+-------+------+-------------+
| salt | -20 | bstr | Random salt |
+------+-------+------+-------------+
Table 11: HKDF Algorithm Parameters
11.2. Context Information Structure
The context information structure is used to ensure that the derived
keying material is "bound" to the context of the transaction. The
context information structure used here is based on that defined in
[SP800-56A]. By using CBOR for the encoding of the context
information structure, we automatically get the same type of type and
length separation of fields that is obtained by the use of ASN.1.
This means that there is no need to encode the lengths for the base
elements as it is done by the JOSE encoding. [CREF7]
The context information structure refers to PartyU and PartyV as the
two parties which are doing the key derivation. Unless the
application protocol defines differently, we assign PartyU to the
entity that is creating the message and PartyV to the entity that is
receiving the message. By doing this association, different keys
will be derived for each direction as the context information is
different in each direction.
Application protocols are free to define the roles differently. For
example, they could assign the PartyU role to the entity that
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initiates the connection and allow directly sending multiple messages
over the connection in both directions without changing the role
information.
The use of a transaction identifier, either in one of the
supplemental fields or as the salt if one is using HKDF, ensures that
a unique key is generated for each set of transactions. Combining
nonce fields with the transaction identifier provides a method so
that a different key is used for each message in each direction.
The context structure is built from information that is known to both
entities. Some of the information is known only to the two entities,
some is implied based on the application and some is explicitly
transported as part of the message. The information that can be
carried in the message, parameters have been defined and can be found
in Table 12. These parameters are designed to be placed in the
unprotected bucket of the recipient structure. (They do not need to
be in the protected bucket since they already are included in the
cryptographic computation by virtue of being included in the context
structure.)
We encode the context specific information using a CBOR array type.
The fields in the array are:
AlgorithmID This field indicates the algorithm for which the key
material will be used. This field is required to be present and
is a copy of the algorithm identifier in the message. The field
exists in the context information so that if the same environment
is used for different algorithms, then completely different keys
will be generated each of those algorithms. (This practice means
if algorithm A is broken and thus can is easier to find, the key
derived for algorithm B will not be the same as the key for
algorithm B.)
PartyUInfo This field holds information about party U. The
PartyUInfo is encoded as a CBOR structure. The elements of
PartyUInfo are encoded in the order presented, however if the
element does not exist no element is placed in the array. The
elements of the PartyUInfo array are:
identity This contains the identity information for party U. The
identities can be assigned in one of two manners. Firstly, a
protocol can assign identities based on roles. For example,
the roles of "client" and "server" may be assigned to different
entities in the protocol. Each entity would then use the
correct label for the data they send or receive. The second
way is for a protocol to assign identities is to use a name
based on a naming system (i.e. DNS, X.509 names).
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We define an algorithm parameter 'PartyU identity' that can be
used to carry identity information in the message. However,
identity information is often known as part of the protocol and
can thus be inferred rather than made explicit. If identity
information is carried in the message, applications SHOULD have
a way of validating the supplied identity information. The
identity information does not need to be specified and can be
left as absent.
The identity value supplied will be integrity checked as part
of the key derivation process. If the identity string is
wrong, then the wrong key will be created.
nonce This contains a one time nonce value. The nonce can either
be implicit from the protocol or carried as a value in the
unprotected headers.
We define an algorithm parameter 'PartyU nonce' that can be
used to carry this value in the message However, the nonce
value could be determined by the application and the value
determined from elsewhere.
This item is optional and can be absent.
other This contains other information that is defined by the
protocol.
This item is optional and can be absent.
PartyVInfo M00TODO: Copy down from PartyUInfo when that text is
ready.
SuppPubInfo This field contains public information that is mutually
known to both parties.
keyDataLength This is set to the number of bits of the desired
output value. (This practice means if algorithm A can use two
different key lengths, the key derived for longer key size will
not contain the key for shorter key size as a prefix.)
protected This field contains the protected parameter field.
other The field other is for free form data defined by the
application. An example is that an application could defined
two different strings to be placed here to generate different
keys for a data stream vs a control stream. This field is
optional and will only be present if the application defines a
structure for this information. Applications that define this
SHOULD use CBOR to encode the data so that types and lengths
are correctly include.
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SuppPrivInfo This field contains private information that is
mutually known information. An example of this information would
be a pre-existing shared secret. The field is optional and will
only be present if the application defines a structure for this
information. Applications that define this SHOULD use CBOR to
encode the data so that types and lengths are correctly include.
+---------------+-------+-----------+-------------------------------+
| name | label | type | description |
+---------------+-------+-----------+-------------------------------+
| PartyU | -21 | bstr | Party U identity Information |
| identity | | | |
| | | | |
| PartyU nonce | -22 | bstr / | Party U provided nonce |
| | | int | |
| | | | |
| PartyU other | -23 | bstr | Party U other provided |
| | | | information |
| | | | |
| PartyV | -24 | bstr | Party V identity Information |
| identity | | | |
| | | | |
| PartyV nonce | -25 | bstr / | Party V provided nonce |
| | | int | |
| | | | |
| PartyV other | -26 | bstr | Party V other provided |
| | | | information |
+---------------+-------+-----------+-------------------------------+
Table 12: Context Algorithm Parameters
Text from here to start of next section to be removed
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COSE_KDF_Context = [
AlgorithmID : int / tstr,
PartyUInfo : [
? nonce : bstr / int,
? identity : bstr,
? other : bstr
],
PartyVInfo : [
? nonce : bstr,
? identity : bstr / tstr,
? other : bstr
],
SuppPubInfo : [
keyDataLength : uint,
protected : bstr,
? other : bstr
],
? SuppPrivInfo : bstr
]
12. Recipient Algorithm Classes
Recipient algorithms can be defined into a number of different
classes. COSE has the ability to support many classes of recipient
algorithms. In this section, a number of classes are listed and then
a set of algorithms are specified for each of the classes. The names
of the recipient algorithm classes used here are the same as are
defined in [RFC7517]. Other specifications use different terms for
the recipient algorithm classes or do not support some of our
recipient algorithm classes.
12.1. Direct Encryption
The direct encryption class algorithms share a secret between the
sender and the recipient that is used either directly or after
manipulation as the content key. When direct encryption mode is
used, it MUST be the only mode used on the message.
The COSE_encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be a zero length item if it is not used
in the computation of the content key.
o The 'alg' parameter MUST be present.
o A parameter identifying the shared secret SHOULD be present.
o The 'ciphertext' field MUST be a zero length item.
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o The 'recipients' field MUST be absent.
12.1.1. Direct Key
This recipient algorithm is the simplest, the supplied key is
directly used as the key for the next layer down in the message.
There are no algorithm parameters defined for this algorithm. The
algorithm identifier value is assigned in Table 13.
When this algorithm is used, the protected field MUST be zero length.
The key type MUST be 'Symmetric'.
+--------+-------+-------------------+
| name | value | description |
+--------+-------+-------------------+
| direct | -6 | Direct use of CEK |
+--------+-------+-------------------+
Table 13: Direct Key
12.1.1.1. Security Considerations
This recipient algorithm has several potential problems that need to
be considered:
o These keys need to have some method to be regularly updated over
time. All of the content encryption algorithms specified in this
document have limits on how many times a key can be used without
significant loss of security.
o These keys need to be dedicated to a single algorithm. There have
been a number of attacks developed over time when a single key is
used for multiple different algorithms. One example of this is
the use of a single key both for CBC encryption mode and CBC-MAC
authentication mode.
o Breaking one message means all messages are broken. If an
adversary succeeds in determining the key for a single message,
then the key for all messages is also determined.
12.1.2. Direct Key with KDF
These recipient algorithms take a common shared secret between the
two parties and applies the HKDF function (Section 11.1) using the
context structure defined in Section 11.2 to transform the shared
secret into the necessary key. Either the 'salt' parameter of HKDF
or the partyU 'nonce' parameter of the context structure MUST be
present. This parameter can be generated either randomly or
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deterministically, the requirement is that it be a unique value for
the key pair in question.
If the salt/nonce value is generated randomly, then it is suggested
that the length of the random value be the same length as the hash
function underlying HKDF. While there is no way to guarantee that it
will be unique, there is a high probability that it will be unique.
If the salt/nonce value is generated deterministically, it can be
guaranteed to be unique and thus there is no length requirement.
A new IV must be used if the same key is used in more than one
message. The IV can be modified in a predictable manner, a random
manner or an unpredictable manner. One unpredictable manner that can
be used is to use the HKDF function to generate the IV. If HKDF is
used for generating the IV, the algorithm identifier is set to "IV-
GENERATION".
When these algorithms are used, the key type MUST be 'symmetric'.
The set of algorithms defined in this document can be found in
Table 14.
+---------------------+-------+-------------+-----------------------+
| name | value | KDF | description |
+---------------------+-------+-------------+-----------------------+
| direct+HKDF-SHA-256 | * | HKDF | Shared secret w/ HKDF |
| | | SHA-256 | and SHA-256 |
| | | | |
| direct+HKDF-SHA-512 | * | HKDF | Shared secret w/ HKDF |
| | | SHA-512 | and SHA-512 |
| | | | |
| direct+HKDF-AES-128 | * | HKDF AES- | Shared secret w/ AES- |
| | | MAC-128 | MAC 128-bit key |
| | | | |
| direct+HKDF-AES-256 | * | HKDF AES- | Shared secret w/ AES- |
| | | MAC-256 | MAC 256-bit key |
+---------------------+-------+-------------+-----------------------+
Table 14: Direct Key
12.1.2.1. Security Considerations
The shared secret need to have some method to be regularly updated
over time. The shared secret is forming the basis of trust, although
not used directly it should still be subject to scheduled rotation.
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12.2. Key Wrapping
In key wrapping mode, the CEK is randomly generated and that key is
then encrypted by a shared secret between the sender and the
recipient. All of the currently defined key wrapping algorithms for
JOSE (and thus for COSE) are AE algorithms. Key wrapping mode is
considered to be superior to direct encryption if the system has any
capability for doing random key generation. This is because the
shared key is used to wrap random data rather than data has some
degree of organization and may in fact be repeating the same content.
The COSE_encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be absent if the key wrap algorithm is
an AE algorithm.
o The 'recipients' field is normally absent, but can be used.
Applications MUST deal with a recipients field present, not being
able to decrypt that recipient is an acceptable way of dealing
with it. Failing to process the message is not an acceptable way
of dealing with it.
o The plain text to be encrypted is the key from next layer down
(usually the content layer).
o At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the shared
secret.
12.2.1. AES Key Wrapping
The AES Key Wrapping algorithm is defined in [RFC3394]. This
algorithm uses an AES key to wrap a value that is a multiple of
64-bits, as such it can be used to wrap a key for any of the content
encryption algorithms defined in this document. The algorithm
requires a single fixed parameter, the initial value. This is fixed
to the value specified in Section 2.2.3.1 of [RFC3394]. There are no
public parameters that vary on a per invocation basis.
Keys may be obtained either from a key structure or from a recipient
structure. If the key obtained from a key structure, the key type
MUST be 'Symmetric'. Implementations creating and validating MAC
values MUST validate that the key type, key length and algorithm are
correct and appropriate for the entities involved.
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+--------+-------+----------+-----------------------------+
| name | value | key size | description |
+--------+-------+----------+-----------------------------+
| A128KW | -3 | 128 | AES Key Wrap w/ 128-bit key |
| | | | |
| A192KW | -4 | 192 | AES Key Wrap w/ 192-bit key |
| | | | |
| A256KW | -5 | 256 | AES Key Wrap w/ 256-bit key |
+--------+-------+----------+-----------------------------+
Table 15: AES Key Wrap Algorithm Values
12.2.1.1. Security Considerations for AES-KW
The shared secret need to have some method to be regularly updated
over time. The shared secret is forming the basis of trust, although
not used directly it should still be subject to scheduled rotation.
12.3. Key Encryption
Key Encryption mode is also called key transport mode in some
standards. Key Encryption mode differs from Key Wrap mode in that it
uses an asymmetric encryption algorithm rather than a symmetric
encryption algorithm to protect the key. This document defines one
Key Encryption mode algorithm.
When using a key encryption algorithm, the COSE_encrypt structure for
the recipient is organized as follows:
o The 'protected' field MUST be absent.
o The plain text to be encrypted is the key from next layer down
(usually the content layer).
o At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the
asymmetric key.
12.4. Direct Key Agreement
The 'direct key agreement' class of recipient algorithms uses a key
agreement method to create a shared secret. A KDF is then applied to
the shared secret to derive a key to be used in protecting the data.
This key is normally used as a CEK or MAC key, but could be used for
other purposes if more than two layers are in use (see Appendix B).
The most commonly used key agreement algorithm used is Diffie-
Hellman, but other variants exist. Since COSE is designed for a
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store and forward environment rather than an on-line environment,
many of the DH variants cannot be used as the receiver of the message
cannot provide any key material. One side-effect of this is that
perfect forward security is not achievable, a static key will always
be used for the receiver of the COSE message.
Two variants of DH that are easily supported are:
- Ephemeral-Static DH: where the sender of the message creates a
one time DH key and uses a static key for the recipient. The use
of the ephemeral sender key means that no additional random input
is needed as this is randomly generated for each message.
Static-Static DH: where a static key is used for both the sender
and the recipient. The use of static keys allows for recipient to
get a weak version of data origination for the message. When
static-static key agreement is used, then some piece of unique
data is require to ensure that a different key is created for each
message
In this specification, both variants are specified. This has been
done to provide the weak data origination option for use with MAC
operations.
When direct key agreement mode is used, there MUST be only one
recipient in the message. This method creates the key directly and
that makes it difficult to mix with additional recipients. If
multiple recipients are needed, then the version with key wrap needs
to be used.
The COSE_encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be absent.
o At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the
recipient's asymmetric key.
o The 'unprotected' field MUST contain the 'epk' parameter.
12.4.1. ECDH
The basic mathematics for Elliptic Curve Diffie-Hellman can be found
in [RFC6090].
ECDH is parameterized by the following:
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o Curve Type/Curve: The curve selected controls not only the size of
the shared secret, but the mathematics for computing the shared
secret. The curve selected also controls how a point in the curve
is represented and what happens for the identity points on the
curve. In this specification we allow for a number of different
curves to be used. The curves are defined in Table 19.
Since the only the math is changed by changing the curve, the
curve is not fixed for any of the algorithm identifiers we define,
instead it is defined by the points used.
o Ephemeral-static or static-static: The key agreement process may
be done using either a static or an ephemeral key at the sender's
side. When using ephemeral keys, the sender MUST generate a new
ephemeral key for every key agreement operation. The ephemeral
key is placed in the 'ephemeral key' parameter and MUST be present
for all algorithm identifiers which use ephemeral keys. When
using static keys, the sender MUST either generate a new random
value placed in either in the KDF parameters or the context
structure. For the KDF functions used, this means either in the
'salt' parameter for HKDF (Table 11) or in the 'PartyU nonce'
parameter for the context structure (Table 12) MUST be present.
(Both may be present if desired.) The value in the parameter MUST
be unique for the key pair being used. It is acceptable to use a
global counter which is incremented for every static-static
operation and use the resulting value. When using static keys,
the static key needs to be identified to the recipient. The
static key can be identified either by providing the key ('static
key') or by providing a key identifier for the static key ('static
key id'). Both of these parameters are defined in Table 17
o Key derivation algorithm: The result of an ECDH key agreement
process does not provide a uniformly random secret, as such it
needs to be run through a KDF in order to produce a usable key.
Processing the secret through a KDF also allows for the
introduction of both context material, how the key is going to be
used, and one time material in the even to of a static-static key
agreement.
o Key Wrap algorithm: The key wrap algorithm can be 'none' if the
result of the KDF is going to be used as the key directly. This
option, along with static-static, should be used if knowledge
about the sender is desired. If 'none' is used then the content
layer encryption algorithm size is value fed to the context
structure. Support is also provided for any of the key wrap
algorithms defined in Section 12.2.1. If one of these options is
used, the input key size to the key wrap algorithm is the value
fed into the context structure as the key size.
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The set of algorithms direct ECDH defined in this document are found
in Table 16.
+-------------+------+-------+----------------+--------+------------+
| name | valu | KDF | Ephemeral- | Key | descriptio |
| | e | | Static | Wrap | n |
+-------------+------+-------+----------------+--------+------------+
| ECDH-ES + | 50 | HKDF | yes | none | ECDH ES w/ |
| HKDF-256 | | - SHA | | | HKDF - |
| | | -256 | | | generate |
| | | | | | key |
| | | | | | directly |
| | | | | | |
| ECDH-ES + | 51 | HKDF | yes | none | ECDH ES w/ |
| HKDF-512 | | - SHA | | | HKDF - |
| | | -256 | | | generate |
| | | | | | key |
| | | | | | directly |
| | | | | | |
| ECDH-SS + | 52 | HKDF | no | none | ECDH ES w/ |
| HKDF-256 | | - SHA | | | HKDF - |
| | | -256 | | | generate |
| | | | | | key |
| | | | | | directly |
| | | | | | |
| ECDH-SS + | 53 | HKDF | no | none | ECDH ES w/ |
| HKDF-512 | | - SHA | | | HKDF - |
| | | -256 | | | generate |
| | | | | | key |
| | | | | | directly |
| | | | | | |
| ECDH- | 54 | HKDF | yes | A128KW | ECDH ES w/ |
| ES+A128KW | | - SHA | | | Concat KDF |
| | | -256 | | | and AES |
| | | | | | Key wrap |
| | | | | | w/ 128 bit |
| | | | | | key |
| | | | | | |
| ECDH- | 55 | HKDF | yes | A192KW | ECDH ES w/ |
| ES+A192KW | | - SHA | | | Concat KDF |
| | | -256 | | | and AES |
| | | | | | Key wrap |
| | | | | | w/ 192 bit |
| | | | | | key |
| | | | | | |
| ECDH- | 56 | HKDF | yes | A256KW | ECDH ES w/ |
| ES+A256KW | | - SHA | | | Concat KDF |
| | | -256 | | | and AES |
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| | | | | | Key wrap |
| | | | | | w/ 256 bit |
| | | | | | key |
| | | | | | |
| ECDH- | 57 | HKDF | no | A128KW | ECDH SS w/ |
| SS+A128KW | | - SHA | | | Concat KDF |
| | | -256 | | | and AES |
| | | | | | Key wrap |
| | | | | | w/ 128 bit |
| | | | | | key |
| | | | | | |
| ECDH- | 58 | HKDF | no | A192KW | ECDH SS w/ |
| SS+A192KW | | - SHA | | | Concat KDF |
| | | -256 | | | and AES |
| | | | | | Key wrap |
| | | | | | w/ 192 bit |
| | | | | | key |
| | | | | | |
| ECDH- | 59 | HKDF | no | A256KW | ECDH SS w/ |
| SS+A256KW | | - SHA | | | Concat KDF |
| | | -256 | | | and AES |
| | | | | | Key wrap |
| | | | | | w/ 256 bit |
| | | | | | key |
+-------------+------+-------+----------------+--------+------------+
Table 16: ECDH Algorithm Values
+-----------+-------+----------+-----------+------------------------+
| name | label | type | algorithm | description |
+-----------+-------+----------+-----------+------------------------+
| ephemeral | -1 | COSE_Key | ECDH-ES | Ephemeral Public key |
| key | | | | for the sender |
| | | | | |
| static | -2 | COSE_Key | ECDH-ES | Static Public key for |
| key | | | | the sender |
| | | | | |
| static | -3 | bstr | ECDH-SS | Static Public key |
| key id | | | | identifier for the |
| | | | | sender |
+-----------+-------+----------+-----------+------------------------+
Table 17: ECDH Algorithm Parameters
This document defines these algorithms to be used with the curves
P-256, P-384, P-521. Implementations MUST verify that the key type
and curve are correct, different curves are restricted to different
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key types. Implementations MUST verify that the curve and algorithm
are appropriate for the entities involved.
12.5. Key Agreement with KDF
Key Agreement with Key Wrapping uses a randomly generated CEK. The
CEK is then encrypted using a Key Wrapping algorithm and a key
derived from the shared secret computed by the key agreement
algorithm.
The COSE_encrypt structure for the recipient is organized as follows:
o The 'protected' field is fed into the KDF context structure.
o The plain text to be encrypted is the key from next layer down
(usually the content layer).
o The 'alg' parameter MUST be present in the layer.
o A parameter identifying the recipient's key SHOULD be present. A
parameter identifying the sender's key SHOULD be present.
12.5.1. ECDH
These algorithms are defined in Table 16.
13. Keys
The COSE_Key object defines a way to hold a single key object, it is
still required that the members of individual key types be defined.
This section of the document is where we define an initial set of
members for specific key types.
For each of the key types, we define both public and private members.
The public members are what is transmitted to others for their usage.
We define private members mainly for the purpose of archival of keys
by individuals. However, there are some circumstances where private
keys may be distributed by various entities in a protocol. Examples
include: Entities which have poor random number generation.
Centralized key creation for multi-cast type operations. Protocols
where a shared secret is used as a bearer token for authorization
purposes.
Key types are identified by the 'kty' member of the COSE_Key object.
In this document we define four values for the member.
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+-----------+-------+--------------------------------------------+
| name | value | description |
+-----------+-------+--------------------------------------------+
| EC2 | 2 | Elliptic Curve Keys w/ X,Y Coordinate pair |
| | | |
| Symmetric | 4 | Symmetric Keys |
| | | |
| Reserved | 0 | This value is reserved |
+-----------+-------+--------------------------------------------+
Table 18: Key Type Values
13.1. Elliptic Curve Keys
Two different key structures could be defined for Elliptic Curve
keys. One version uses both an x and a y coordinate, potentially
with point compression. This is the traditional EC point
representation that is used in [RFC5480]. The other version uses
only the x coordinate as the y coordinate is either to be recomputed
or not needed for the key agreement operation Currently no algorithms
are defined using this key structure.
+-------+----------+-------+------------------------------------+
| name | key type | value | description |
+-------+----------+-------+------------------------------------+
| P-256 | EC2 | 1 | NIST P-256 also known as secp256r1 |
| | | | |
| P-384 | EC2 | 2 | NIST P-384 also known as secp384r1 |
| | | | |
| P-521 | EC2 | 3 | NIST P-521 also known as secp521r1 |
+-------+----------+-------+------------------------------------+
Table 19: EC Curves
13.1.1. Double Coordinate Curves
The traditional way of sending EC curves has been to send either both
the x and y coordinates, or the x coordinate and a sign bit for the y
coordinate. The latter encoding has not been recommended in the IETF
due to potential IPR issues. However, for operations in constrained
environments, the ability to shrink a message by not sending the y
coordinate is potentially useful.
For EC keys with both coordinates, the 'kty' member is set to 2
(EC2). The key parameters defined in this section are summarized in
Table 20. The members that are defined for this key type are:
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crv contains an identifier of the curve to be used with the key.
The curves defined in this document for this key type can be found
in Table 19. Other curves may be registered in the future and
private curves can be used as well.
x contains the x coordinate for the EC point. The integer is
converted to an octet string as defined in [SEC1]. Zero octets
MUST NOT be removed from the front of the octet string.
y contains either the sign bit or the value of y coordinate for the
EC point. For the value, the integer is converted to an octet
string as defined in [SEC1]. Zero octets MUST NOT be removed from
the front of the octet string. For the sign bit, the value is
true if the value of y is positive.
d contains the private key.
For public keys, it is REQUIRED that 'crv', 'x' and 'y' be present in
the structure. For private keys, it is REQUIRED that 'crv' and 'd'
be present in the structure. For private keys, it is RECOMMENDED
that 'x' and 'y' also be present, but they can be recomputed from the
required elements and omitting them saves on space.
+------+-------+-------+---------+----------------------------------+
| name | key | value | type | description |
| | type | | | |
+------+-------+-------+---------+----------------------------------+
| crv | 2 | -1 | int / | EC Curve identifier - Taken from |
| | | | tstr | the COSE General Registry |
| | | | | |
| x | 2 | -2 | bstr | X Coordinate |
| | | | | |
| y | 2 | -3 | bstr / | Y Coordinate |
| | | | bool | |
| | | | | |
| d | 2 | -4 | bstr | Private key |
+------+-------+-------+---------+----------------------------------+
Table 20: EC Key Parameters
13.2. Symmetric Keys
Occasionally it is required that a symmetric key be transported
between entities. This key structure allows for that to happen.
For symmetric keys, the 'kty' member is set to 3 (Symmetric). The
member that is defined for this key type is:
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k contains the value of the key.
This key structure contains only private key information, care must
be taken that it is never transmitted accidentally. For public keys,
there are no required fields. For private keys, it is REQUIRED that
'k' be present in the structure.
+------+----------+-------+------+-------------+
| name | key type | value | type | description |
+------+----------+-------+------+-------------+
| k | 4 | -1 | bstr | Key Value |
+------+----------+-------+------+-------------+
Table 21: Symmetric Key Parameters
14. CBOR Encoder Restrictions
There has been an attempt to limit the number of places where the
document needs to impose restrictions on how the CBOR Encoder needs
to work. We have managed to narrow it down to the following
restrictions:
o The restriction applies to the encoding the Sig_structure, the
Enc_structure, and the MAC_structure.
o The rules for Canonical CBOR (Section 3.9 of RFC 7049) MUST be
used in these locations. The main rule that needs to be enforced
is that all lengths in these structures MUST be encoded such that
they are encoded using definite lengths and the minimum length
encoding is used.
o All parsers used SHOULD fail on both parsing and generation if the
same label is used twice as a key for the same map.
15. IANA Considerations
15.1. CBOR Tag assignment
It is requested that IANA assign the following tags from the "Concise
Binary Object Representation (CBOR) Tags" registry. It is requested
that the tags be assigned in the 24 to 255 value range.
The tags to be assigned are:
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+-----------+-----------------------+-------------------------------+
| Tag Value | Data Item | Semantics |
+-----------+-----------------------+-------------------------------+
| TBD1 | COSE_Sign | COSE Signed Data Object |
| | | |
| TBD2 | COSE_enveloped | COSE Enveloped Data Object |
| | | |
| TBD3 | COSE_encryptData | COSE Encrypted Data Object |
| | | |
| TBD4 | COSE_Mac | COSE Mac-ed Data Object |
| | | |
| TBD5 | COSE_Key, COSE_KeySet | COSE Key or COSE Key Set |
| | | Object |
+-----------+-----------------------+-------------------------------+
15.2. COSE Header Parameter Registry
It is requested that IANA create a new registry entitled "COSE Header
Parameters".
The columns of the registry are:
name The name is present to make it easier to refer to and discuss
the registration entry. The value is not used in the protocol.
Names are to be unique in the table.
label This is the value used for the label. The label can be either
an integer or a string. Registration in the table is based on the
value of the label requested. Integer values between 1 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from 256 to 65535 and strings of length
2 are designated as Specification Required. Integer values of
greater than 65535 and strings of length greater than 2 are
designated as first come first server. Integer values in the
range -1 to -65536 are delegated to the "COSE Header Algorithm
Label" registry. Integer values beyond -65536 are marked as
private use.
value This contains the CBOR type for the value portion of the
label.
value registry This contains a pointer to the registry used to
contain values where the set is limited.
description This contains a brief description of the header field.
specification This contains a pointer to the specification defining
the header field (where public).
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The initial contents of the registry can be found in Table 1. The
specification column for all rows in that table should be this
document.
Additionally, the label of 0 is to be marked as 'Reserved'.
15.3. COSE Header Algorithm Label Table
It is requested that IANA create a new registry entitled "COSE Header
Algorithm Labels".
The columns of the registry are:
name The name is present to make it easier to refer to and discuss
the registration entry. The value is not used in the protocol.
algorithm The algorithm(s) that this registry entry is used for.
This value is taken from the "COSE Algorithm Value" registry.
Multiple algorithms can be specified in this entry. For the
table, the algorithm, label pair MUST be unique.
label This is the value used for the label. The label is an integer
in the range of -1 to -65536.
value This contains the CBOR type for the value portion of the
label.
value registry This contains a pointer to the registry used to
contain values where the set is limited.
description This contains a brief description of the header field.
specification This contains a pointer to the specification defining
the header field (where public).
The initial contents of the registry can be found in: Table 11,
Table 12, Table 17. The specification column for all rows in that
table should be this document.
15.4. COSE Algorithm Registry
It is requested that IANA create a new registry entitled "COSE
Algorithm Registry".
The columns of the registry are:
value The value to be used to identify this algorithm. Algorithm
values MUST be unique. The value can be a positive integer, a
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negative integer or a string. Integer values between 0 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from 256 to 65535 and strings of length
2 are designated as Specification Required. Integer values of
greater than 65535 and strings of length greater than 2 are
designated as first come first server. Integer values in the
range -1 to -65536 are delegated to the "COSE Header Algorithm
Label" registry. Integer values beyond -65536 are marked as
private use.
description A short description of the algorithm.
specification A document where the algorithm is defined (if publicly
available).
The initial contents of the registry can be found in the following:
Table 8, Table 7, Table 9, Table 4, Table 5, Table 6, Table 13,
Table 14, Table 15, Table 16. The specification column for all rows
in that table should be this document.
15.5. COSE Key Common Parameter Registry
It is requested that IANA create a new registry entitled "COSE Key
Common Parameter" Registry.
The columns of the registry are:
name This is a descriptive name that enables easier reference to the
item. It is not used in the encoding.
label The value to be used to identify this algorithm. Key map
labels MUST be unique. The label can be a positive integer, a
negative integer or a string. Integer values between 0 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from 256 to 65535 and strings of length
2 are designated as Specification Required. Integer values of
greater than 65535 and strings of length greater than 2 are
designated as first come first server. Integer values in the
range -1 to -65536 are used for key parameters specific to a
single algorithm delegated to the "COSE Key Parameter Label"
registry. Integer values beyond -65536 are marked as private use.
CBOR Type This field contains the CBOR type for the field
registry This field denotes the registry that values come from, if
one exists.
description This field contains a brief description for the field
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specification This contains a pointer to the public specification
for the field if one exists
This registry will be initially populated by the values in
Section 7.1. The specification column for all of these entries will
be this document.
15.6. COSE Key Type Parameter Registry
It is requested that IANA create a new registry "COSE Key Type
Parameters".
The columns of the table are:
key type This field contains a descriptive string of a key type.
This should be a value that is in the COSE General Values table
and is placed in the 'kty' field of a COSE Key structure.
name This is a descriptive name that enables easier reference to the
item. It is not used in the encoding.
label The label is to be unique for every value of key type. The
range of values is from -256 to -1. Labels are expected to be
reused for different keys.
CBOR type This field contains the CBOR type for the field
description This field contains a brief description for the field
specification This contains a pointer to the public specification
for the field if one exists
This registry will be initially populated by the values in Table 20,
and Table 21. The specification column for all of these entries will
be this document.
15.7. COSE Elliptic Curve Registry
It is requested that IANA create a new registry "COSE Elliptic Curve
Parameters".
The columns of the table are:
name This is a descriptive name that enables easier reference to the
item. It is not used in the encoding.
value This is the value used to identify the curve. These values
MUST be unique. The integer values from -256 to 255 are
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designated as Standards Track Document Required. The integer
values from 256 to 65535 and -65536 to -257 are designated as
Specification Required. Integer values over 65535 are designated
as first come first serve. Integer values less than -65536 are
marked as private use.
key type This designates the key type(s) that can be used with this
curve.
description This field contains a brief description of the curve.
specification This contains a pointer to the public specification
for the curve if one exists.
This registry will be initially populated by the values in Table 18.
The specification column for all of these entries will be this
document.
15.8. Media Type Registrations
15.8.1. COSE Security Message
This section registers the "application/cose" and "application/
cose+cbor" media types in the "Media Types" registry. [CREF8] These
media types are used to indicate that the content is a COSE_MSG.
[CREF9]
Type name: application
Subtype name: cose
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
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Additional information:
* Magic number(s): N/A
* File extension(s): cbor
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
Type name: application
Subtype name: cose+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): cbor
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* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
15.8.2. COSE Key media type
This section registers the "application/cose+json" and "application/
cose-set+json" media types in the "Media Types" registry. These
media types are used to indicate, respectively, that content is a
COSE_Key or COSE_KeySet object.
Type name: application
Subtype name: cose-key+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): cbor
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* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
Type name: application
Subtype name: cose-key-set+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of RFC TBD.
Interoperability considerations: N/A
Published specification: RFC TBD
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): cbor
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg@ietf.org
Intended usage: COMMON
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Restrictions on usage: N/A
Author: Jim Schaad, ietf@augustcellars.com
Change Controller: IESG
Provisional registration? No
15.9. CoAP Content Format Registrations
This section registers a set of content formats for CoAP. ID
assignment in the 24-255 range requested.
+--------------------------+----------+-------+-----------------+
| Media Type | Encoding | ID | Reference |
+--------------------------+----------+-------+-----------------+
| application/cose | | TBD10 | [This Document] |
| | | | |
| application/cose-key | | TBD11 | [This Document] |
| | | | |
| application/cose-key-set | | TBD12 | [This Document |
+--------------------------+----------+-------+-----------------+
16. Security Considerations
There are security considerations:
1. Protect private keys
2. MAC messages with more than one recipient means one cannot figure
out who sent the message
3. Use of direct key with other recipient structures hands the key
to other recipients.
4. Use of direct ECDH direct encryption is easy for people to leak
information on if there are other recipients in the message.
5. Considerations about protected vs unprotected header fields.
6. Need to verify that: 1) the kty field of the key matches the key
and algorithm being used. 2) the kty field needs to be included
in the trust decision as well as the other key fields. 3) the
algorithm be included in the trust decision.
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17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
17.2. Informative References
[AES-GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", Nov 2007.
[DSS] U.S. National Institute of Standards and Technology,
"Digital Signature Standard (DSS)", July 2013.
[I-D.greevenbosch-appsawg-cbor-cddl]
Vigano, C., Birkholz, H., and R. Sun, "CBOR data
definition language: a notational convention to express
CBOR data structures.", draft-greevenbosch-appsawg-cbor-
cddl-05 (work in progress), March 2015.
[MAC] NiST, N., "FIPS PUB 113: Computer Data Authentication",
May 1985.
[MultiPrimeRSA]
Hinek, M. and D. Cheriton, "On the Security of Multi-prime
RSA", June 2006.
[PVSig] Brown, D. and D. Johnson, "Formal Security Proofs for a
Signature Scheme with Partial Message Recover", February
2000.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2633] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, DOI 10.17487/
RFC2898, September 2000,
<http://www.rfc-editor.org/info/rfc2898>.
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[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC
4231, December 2005.
[RFC4262] Santesson, S., "X.509 Certificate Extension for Secure/
Multipurpose Internet Mail Extensions (S/MIME)
Capabilities", RFC 4262, December 2005.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, March 2009.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5752] Turner, S. and J. Schaad, "Multiple Signatures in
Cryptographic Message Syntax (CMS)", RFC 5752, January
2010.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, May 2010.
[RFC5990] Randall, J., Kaliski, B., Brainard, J., and S. Turner,
"Use of the RSA-KEM Key Transport Algorithm in the
Cryptographic Message Syntax (CMS)", RFC 5990, September
2010.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.
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[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <http://www.rfc-editor.org/info/rfc6979>.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, May 2015.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, May 2015.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, May 2015.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, May
2015.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", May 2009.
[SP800-56A]
Barker, E., Chen, L., Roginsky, A., and M. Smid, "NIST
Special Publication 800-56A: Recommendation for Pair-Wise
Key Establishment Schemes Using Discrete Logarithm
Cryptography", May 2013.
Appendix A. CDDL Grammar
For people who prefer using a formal language to describe the syntax
of the CBOR, in this section a CDDL grammar is given that corresponds
to [I-D.greevenbosch-appsawg-cbor-cddl]. This grammar is
informational, in the event of differences between this grammar and
the prose, the prose is considered to be authoritative.
The collected CDDL can be extracted from the XML version of this
document via the following XPath expression below. (Depending on the
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XPath evaluator one is using, it may be necessary to deal with >
as an entity.)
//artwork[@type='CDDL']/text()
Appendix B. Three Levels of Recipient Information
All of the currently defined recipient algorithms classes only use
two levels of the COSE_Encrypt structure. The first level is the
message content and the second level is the content key encryption.
However, if one uses a recipient algorithm such as RSA-KEM (see
Appendix A of RSA-KEM [RFC5990], then it make sense to have three
levels of the COSE_Encrypt structure.
These levels would be:
o Level 0: The content encryption level. This level contains the
payload of the message.
o Level 1: The encryption of the CEK by a KEK.
o Level 2: The encryption of a long random secret using an RSA key
and a key derivation function to convert that secret into the KEK.
This is an example of what a triple layer message would look like.
The message has the following layers:
o Level 0: Has a content encrypted with AES-GCM using a 128-bit key.
o Level 1: Uses the AES Key wrap algorithm with a 128-bit key.
o Level 2: Uses ECDH Ephemeral-Static direct to generate the level 1
key.
In effect this example is a decomposed version of using the ECDH-
ES+A128KW algorithm.
Size of binary file is 216 bytes
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998( [
h'a10101',
{
5: h'02d1f7e6f26c43d4868d87ce'
},
h'64f84d913ba60a76070a9a48f26e97e863e285295a44320878caceb0763a3
34806857c67',
[
[
h'',
{
1: -3
},
h'5a15dbf5b282ecb31a6074ee3815c252405dd7583e078188',
[
[
h'',
{
1: 50,
4: h'6d65726961646f632e6272616e64796275636b406275636b
6c616e642e6578616d706c65',
-1: {
1: 2,
-1: 1,
-2: h'b2add44368ea6d641f9ca9af308b4079aeb519f11e9b8
a55a600b21233e86e68',
-3: h'1a2cf118b9ee6895c8f415b686d4ca1cef362d4a7630a
31ef6019c0c56d33de0'
}
},
h''
]
]
]
]
])
Appendix C. Examples
The examples can be found at https://github.com/cose-wg/Examples.
The file names in each section correspond the same file names in the
repository. I am currently still in the process of getting the
examples up there along with some control information for people to
be able to check and reproduce the examples.
Examples may have some features that are in questions but not yet
incorporated in the document.
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To make it easier to read, the examples are presented using the
CBOR's diagnostic notation rather than a binary dump. A ruby based
tool exists to convert between a number of formats. This tool can be
installed with the command line:
gem install cbor-diag
The diagnostic notation can be converted into binary files using the
following command line:
diag2cbor < inputfile > outputfile
The examples can be extracted from the XML version of this document
via an XPath expression as all of the artwork is tagged with the
attribute type='CBORdiag'.
C.1. Examples of MAC messages
C.1.1. Shared Secret Direct MAC
This example users the following:
o MAC: AES-CMAC, 256-bit key, truncated to 64 bits
o Recipient class: direct shared secret
o File name: Mac-04
Size of binary file is 73 bytes
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996( [
h'a1016f4145532d434d41432d3235362f3634',
{
},
h'546869732069732074686520636f6e74656e742e',
h'd9afa663dd740848',
[
[
h'',
{
1: -6,
4: h'6f75722d736563726574'
},
h''
]
]
])
C.1.2. ECDH Direct MAC
This example uses the following:
o MAC: HMAC w/SHA-256, 256-bit key
o Recipient class: ECDH key agreement, two static keys, HKDF w/
context structure
Size of binary file is 217 bytes
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996( [
h'a10104',
{
},
h'546869732069732074686520636f6e74656e742e',
h'2ba937ca03d76c3dbad30cfcbaeef586f9c0f9ba616ad67e9205d38576ad9
930',
[
[
h'',
{
1: 52,
4: h'6d65726961646f632e6272616e64796275636b406275636b6c61
6e642e6578616d706c65',
-3: h'706572656772696e2e746f6f6b407475636b626f726f7567682
e6578616d706c65',
"apu": h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d19558ccf
ec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a58368b01
7e7f2a9e5ce4db5'
},
h''
]
]
])
C.1.3. Wrapped MAC
This example uses the following:
o MAC: AES-MAC, 128-bit key, truncated to 64 bits
o Recipient class: AES keywrap w/ a pre-shared 256-bit key
Size of binary file is 124 bytes
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996( [
h'a1016e4145532d3132382d4d41432d3634',
{
},
h'546869732069732074686520636f6e74656e742e',
h'6d1fa77b2dd9146a',
[
[
h'',
{
1: -5,
4: h'30313863306165352d346439622d343731622d626664362d6565
66333134626337303337'
},
h'711ab0dc2fc4585dce27effa6781c8093eba906f227b6eb0'
]
]
])
C.1.4. Multi-recipient MAC message
This example uses the following:
o MAC: HMAC w/ SHA-256, 128-bit key
o Recipient class: Uses three different methods
1. ECDH Ephemeral-Static, Curve P-521, AES-Key Wrap w/ 128-bit
key
2. AES-Key Wrap w/ 256-bit key
Size of binary file is 374 bytes
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996( [
h'a10104',
{
},
h'546869732069732074686520636f6e74656e742e',
h'7aaa6e74546873061f0a7de21ff0c0658d401a68da738dd893748651983ce
1d0',
[
[
h'',
{
1: 55,
4: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65',
-1: {
1: 2,
-1: 3,
-2: h'43b12669acac3fd27898ffba0bcd2e6c366d53bc4db71f909
a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2d61
3574e7dc242f79c3',
-3: h'812dd694f4ef32b11014d74010a954689c6b6e8785b333d1a
b44f22b9d1091ae8fc8ae40b687e5cfbe7ee6f8b47918a07bb04e9f5b1a51a334
a16bc09777434113'
}
},
h'f20ad9c96134f3c6be4f75e7101c0ecc5efa071ff20a87fd1ac285109
41ee0376573e2b384b56b99'
],
[
h'',
{
1: -5,
4: h'30313863306165352d346439622d343731622d626664362d6565
66333134626337303337'
},
h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a518e7736549e99
8370695e6d6a83b4ae507bb'
]
]
])
C.2. Examples of Encrypted Messages
C.2.1. Direct ECDH
This example uses the following:
o CEK: AES-GCM w/ 128-bit key
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o Recipient class: ECDH Ephemeral-Static, Curve P-256
Size of binary file is 184 bytes
998( [
h'a10101',
{
5: h'c9cf4df2fe6c632bf7886413'
},
h'45fce2814311024d3a479e7d3eed063850f3f0b9f3f948677e3ae9869bcf9
ff4e1763812',
[
[
h'',
{
1: 50,
4: h'6d65726961646f632e6272616e64796275636b406275636b6c61
6e642e6578616d706c65',
-1: {
1: 2,
-1: 1,
-2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf05
4e1c7b4d91d6280',
-3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d
924b7e03bf822bb'
}
},
h''
]
]
])
C.2.2. Direct plus Key Derivation
This example uses the following:
o CEK: AES-CCM w/128-bit key, truncate the tag to 64-bits
o Recipient class: Use HKDF on a shared secret with the following
implicit fields as part of the context.
* APU identity: "lighting-client"
* APV identity: "lighting-server"
* Supplementary Public Other: "Encryption Example 02"
Size of binary file is 97 bytes
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998( [
h'a1010a',
{
5: h'89f52f65a1c580933b5261a7'
},
h'7b9dcfa42c4e1d3182c402dc18ef8b5637de4fb62cf1dd156ea6e6e0',
[
[
h'',
{
1: "dir+kdf",
4: h'6f75722d736563726574',
-20: h'61616262636364646565666667676868'
},
h''
]
]
])
C.3. Examples of Signed Message
C.3.1. Single Signature
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1
Size of binary file is 105 bytes
999( [
h'',
{
},
h'546869732069732074686520636f6e74656e742e',
[
[
h'a10126',
{
4: h'3131'
},
h'4358e9e92b46d45134548b6e3b4eae3d2f801bce85236c7aab42968ad
8e3e92400873ed761735222a6d1f442c4bb3a3151946b16900048572455e65451
d89aaba7'
]
]
])
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C.3.2. Multiple Signers
This example uses the following:
o Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1
o Signature Algorithm: ECDSA w/ SHA-512, Curve P-521
Size of binary file is 277 bytes
999( [
h'',
{
},
h'546869732069732074686520636f6e74656e742e',
[
[
h'a10126',
{
4: h'3131'
},
h'0dc1c5e62719d8f3cce1468b7c881eee6a8088b46bf836ae956dd38fe
931991900823ea760648f2425b96c39e23ddc4b7faed56d4a9bd0f3752cfdc628
254ed0f2'
],
[
h'',
{
1: -9,
4: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65'
},
h'012ce5b1dfe8b5aa6eaa09a54c58a84ad0900e4fdf2759ec22d1c861c
ccd75c7e1c4025a2da35e512fc2874d6ac8fd862d09ad07ed2deac297b897561e
04a8d42476011eb209c016416b4247b4d1475c398d35c4ac24d1c9fadda7eefe2
857e25a500d29aea539e58e8ca7737fe450d4e87ed3f78e637c12bbd213e78ba8
3a55f7e89934'
]
]
])
C.4. COSE Keys
C.4.1. Public Keys
This is an example of a COSE Key set. This example includes the
public keys for all of the previous examples.
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In order the keys are:
o An EC key with a kid of "meriadoc.brandybuck@buckland.example"
o An EC key with a kid of "peregrin.took@tuckborough.example"
o An EC key with a kid of "bilbo.baggins@hobbiton.example"
o An EC key with a kid of "11"
Size of binary file is 481 bytes
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[
{
-1: 1,
-2: h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de4
39c08551d',
-3: h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eec
d0084d19c',
1: 2,
2: h'6d65726961646f632e6272616e64796275636b406275636b6c616e64
2e6578616d706c65'
},
{
-1: 3,
-2: h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737b
f5de7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620
085e5c8f42ad',
-3: h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e
247e60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f
3fe1ea1d9475',
1: 2,
2: h'62696c626f2e62616767696e7340686f626269746f6e2e6578616d70
6c65'
},
{
-1: 1,
-2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b
4d91d6280',
-3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e
03bf822bb',
1: 2,
2: h'706572656772696e2e746f6f6b407475636b626f726f7567682e6578
616d706c65'
},
{
-1: 1,
-2: h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a8
6d6a09eff',
-3: h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed2
8bbfc117e',
1: 2,
2: h'3131'
}
]
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C.4.2. Private Keys
This is an example of a COSE Key set. This example includes the
private keys for all of the previous examples.
In order the keys are:
o An EC key with a kid of "meriadoc.brandybuck@buckland.example"
o A shared-secret key with a kid of "our-secret"
o An EC key with a kid of "peregrin.took@tuckborough.example"
o A shared-secret key with a kid of "018c0ae5-4d9b-471b-
bfd6-eef314bc7037"
o An EC key with a kid of "bilbo.baggins@hobbiton.example"
o An EC key with a kid of "11"
Size of binary file is 782 bytes
[
{
1: 2,
2: h'6d65726961646f632e6272616e64796275636b406275636b6c616e64
2e6578616d706c65',
-1: 1,
-2: h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de4
39c08551d',
-3: h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eec
d0084d19c',
-4: h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad9118
40fa208cf'
},
{
1: 4,
2: h'6f75722d736563726574',
-1: h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dce
a6c427188'
},
{
1: 2,
2: h'62696c626f2e62616767696e7340686f626269746f6e2e6578616d70
6c65',
-1: 3,
-2: h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737b
f5de7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620
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085e5c8f42ad',
-3: h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e
247e60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f
3fe1ea1d9475',
-4: h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476
680b55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609
fdf177feb26d'
},
{
1: 2,
-1: 1,
2: h'706572656772696e2e746f6f6b407475636b626f726f7567682e6578
616d706c65',
-2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b
4d91d6280',
-3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e
03bf822bb',
-4: h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522
a848df1c3'
},
{
1: 4,
2: h'30313863306165352d346439622d343731622d626664362d65656633
3134626337303337',
-1: h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dce
a6c427188'
},
{
1: 2,
2: h'3131',
-1: 1,
-2: h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a8
6d6a09eff',
-3: h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed2
8bbfc117e',
-4: h'57c92077664146e876760c9520d054aa93c3afb04e306705db60903
08507b4d3'
}
]
Appendix D. Document Updates
D.1. Version -05 to -06
o Remove new CFRG Elliptical Curve key agreement algorithms.
o Remove RSA algorithms
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o Define a creation time and sequence number for discussions.
o Remove message type field from all structures.
o Define CBOR tagging for all structures with IANA registrations.
D.2. Version -04 to -05
o Removed the jku, x5c, x5t, x5t#S256, x5u, and jwk headers.
o Add enveloped data vs encrypted data structures.
o Add counter signature parameter.
D.3. Version -03 to -04
o Change top level from map to array.
o Eliminate the term "key management" from the document.
o Point to content registries for the 'content type' attribute
o Push protected field into the KDF functions for recipients.
o Remove password based recipient information.
o Create EC Curve Registry.
D.4. Version -02 to -03
o Make a pass over all of the algorithm text.
o Alter the CDDL so that Keys and KeySets are top level items and
the key examples validate.
o Add sample key structures.
o Expand text on dealing with Externally Supplied Data.
o Update the examples to match some of the renumbering of fields.
D.5. Version -02 to -03
o Add a set of straw man proposals for algorithms. It is possible/
expected that this text will be moved to a new document.
o Add a set of straw man proposals for key structures. It is
possible/expected that this text will be moved to a new document.
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o Provide guidance on use of externally supplied authenticated data.
o Add external authenticated data to signing structure.
D.6. Version -01 to -2
o Add first pass of algorithm information
o Add direct key derivation example.
D.7. Version -00 to -01
o Add note on where the document is being maintained and
contributing notes.
o Put in proposal on MTI algorithms.
o Changed to use labels rather than keys when talking about what
indexes a map.
o Moved nonce/IV to be a common header item.
o Expand section to discuss the common set of labels used in
COSE_Key maps.
o Start marking element 0 in registries as reserved.
o Update examples.
Editorial Comments
[CREF1] JLS: Need to check this list for correctness before publishing.
[CREF2] JLS: I have not gone through the document to determine what
needs to be here yet. We mostly want to grab terms which are
used in unusual ways or are not generally understood.
[CREF3] JLS: It would be possible to extend this section to talk about
those decisions which an application needs to think about rather
than just talking about MTI algorithms.
[CREF4] JLS: A completest version of this grammar would list the options
available in the protected and unprotected headers. Do we want
to head that direction?
[CREF5] Hannes: Ensure that the list of examples only includes items
which are implemented in this specification. Check the other
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places where such lists occur and ensure that they also follow
this rule.
[CREF6] JLS: We can really simplify the grammar for COSE_Key to be just
the kty (the one required field) and the generic item. The
reason to do this is that it makes things simpler. The reason
not to do this says that we really need to add a lot more items
so that a grammar check can be done that is more tightly
enforced.
[CREF7] Ilari: Look to see if we need to be clearer about how the fields
defined in the table are transported and thus why they have
labels.
[CREF8] JLS: Should we register both or just the cose+cbor one?
[CREF9] JLS: Should we create the equivalent of the smime-type parameter
to identify the inner content type?
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
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