COSE Working Group J. Schaad
Internet-Draft August Cellars
Intended status: Informational July 20, 2015
Expires: January 21, 2016
CBOR Encoded Message Syntax
draft-ietf-cose-msg-02
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 [1]. 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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on January 21, 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 . . . . . . . . . . . . . . . . 4
1.2. Requirements Terminology . . . . . . . . . . . . . . . . 5
1.3. CBOR Grammar . . . . . . . . . . . . . . . . . . . . . . 5
1.4. CBOR Related Terminology . . . . . . . . . . . . . . . . 6
1.5. Document Terminology . . . . . . . . . . . . . . . . . . 6
1.6. Mandatory to Implement Algorithms . . . . . . . . . . . . 6
2. The COSE_MSG structure . . . . . . . . . . . . . . . . . . . 7
3. Header Parameters . . . . . . . . . . . . . . . . . . . . . . 10
3.1. COSE Headers . . . . . . . . . . . . . . . . . . . . . . 11
4. Signing Structure . . . . . . . . . . . . . . . . . . . . . . 14
5. Encryption object . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Key Management Methods . . . . . . . . . . . . . . . . . 18
5.2. Encryption Algorithm for AEAD algorithms . . . . . . . . 19
5.3. Encryption algorithm for AE algorithms . . . . . . . . . 20
6. MAC objects . . . . . . . . . . . . . . . . . . . . . . . . . 21
7. Key Structure . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. COSE Key Map Labels . . . . . . . . . . . . . . . . . . . 23
8. Signature Algorithms . . . . . . . . . . . . . . . . . . . . 26
8.1. ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1.1. Security Considerations . . . . . . . . . . . . . . . 28
8.2. RSASSA-PSS . . . . . . . . . . . . . . . . . . . . . . . 28
8.2.1. Security Considerations . . . . . . . . . . . . . . . 29
9. Message Authentication (MAC) Algorithms . . . . . . . . . . . 29
9.1. Hash-based Message Authentication Codes (HMAC) . . . . . 29
9.1.1. Security Considerations . . . . . . . . . . . . . . . 30
9.2. AES Message Authentication Code (AES-MAC) . . . . . . . . 30
10. Content Encryption Algorithms . . . . . . . . . . . . . . . . 30
10.1. AES GCM . . . . . . . . . . . . . . . . . . . . . . . . 30
10.2. AES CCM . . . . . . . . . . . . . . . . . . . . . . . . 31
10.2.1. Security Considerations . . . . . . . . . . . . . . 33
11. Key Derivation Functions (KDF) . . . . . . . . . . . . . . . 34
11.1. HMAC-based Extract-and-Expand Key Derivation Function
(HKDF) . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.2. Context Information Structure . . . . . . . . . . . . . 35
12. Key Management Algorithms . . . . . . . . . . . . . . . . . . 38
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12.1. Direct Encryption . . . . . . . . . . . . . . . . . . . 39
12.1.1. Direct Key . . . . . . . . . . . . . . . . . . . . . 39
12.2. Key Wrapping . . . . . . . . . . . . . . . . . . . . . . 40
12.2.1. AES Key Wrapping . . . . . . . . . . . . . . . . . . 40
12.3. Key Encryption . . . . . . . . . . . . . . . . . . . . . 41
12.3.1. RSA OAEP . . . . . . . . . . . . . . . . . . . . . . 41
12.4. Direct Key Agreement . . . . . . . . . . . . . . . . . . 42
12.4.1. ECDH . . . . . . . . . . . . . . . . . . . . . . . . 43
12.5. Key Agreement with KDF . . . . . . . . . . . . . . . . . 46
12.5.1. ECDH ES + HKDF . . . . . . . . . . . . . . . . . . . 46
12.6. Password . . . . . . . . . . . . . . . . . . . . . . . . 47
12.6.1. PBES2 . . . . . . . . . . . . . . . . . . . . . . . 47
13. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
13.1. Elliptic Curve Keys . . . . . . . . . . . . . . . . . . 48
13.1.1. Single Coordinate Curves . . . . . . . . . . . . . . 48
13.1.2. Double Coordinate Curves . . . . . . . . . . . . . . 49
13.2. RSA Keys . . . . . . . . . . . . . . . . . . . . . . . . 50
13.3. Symmetric Keys . . . . . . . . . . . . . . . . . . . . . 51
14. CBOR Encoder Restrictions . . . . . . . . . . . . . . . . . . 52
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
15.1. CBOR Tag assignment . . . . . . . . . . . . . . . . . . 52
15.2. COSE Object Labels Registry . . . . . . . . . . . . . . 52
15.3. COSE Header Label Table . . . . . . . . . . . . . . . . 53
15.4. COSE Header Algorithm Label Table . . . . . . . . . . . 53
15.5. COSE Algorithm Registry . . . . . . . . . . . . . . . . 54
15.6. COSE Key Map Registry . . . . . . . . . . . . . . . . . 55
15.7. COSE Key Parameter Registry . . . . . . . . . . . . . . 56
15.8. Media Type Registration . . . . . . . . . . . . . . . . 56
15.8.1. COSE Security Message . . . . . . . . . . . . . . . 56
15.8.2. COSE Key media type . . . . . . . . . . . . . . . . 58
16. Security Considerations . . . . . . . . . . . . . . . . . . . 60
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 60
17.1. Normative References . . . . . . . . . . . . . . . . . . 60
17.2. Informative References . . . . . . . . . . . . . . . . . 61
Appendix A. AEAD and AE algorithms . . . . . . . . . . . . . . . 63
Appendix B. Three Levels of Recipient Information . . . . . . . 64
Appendix C. Examples . . . . . . . . . . . . . . . . . . . . . . 65
C.1. Examples of MAC messages . . . . . . . . . . . . . . . . 66
C.1.1. Shared Secret Direct MAC . . . . . . . . . . . . . . 66
C.1.2. ECDH Direct MAC . . . . . . . . . . . . . . . . . . . 66
C.1.3. Wrapped MAC . . . . . . . . . . . . . . . . . . . . . 67
C.1.4. Multi-recipient MAC message . . . . . . . . . . . . . 68
C.2. Examples of Encrypted Messages . . . . . . . . . . . . . 69
C.2.1. Direct ECDH . . . . . . . . . . . . . . . . . . . . . 69
C.2.2. Direct plus Key Derivation . . . . . . . . . . . . . 70
C.3. Examples of Signed Message . . . . . . . . . . . . . . . 71
C.3.1. Single Signature . . . . . . . . . . . . . . . . . . 71
C.3.2. Multiple Signers . . . . . . . . . . . . . . . . . . 72
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Appendix D. COSE Header Algorithm Label Table . . . . . . . . . 72
Appendix E. Document Updates . . . . . . . . . . . . . . . . . . 73
E.1. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 73
E.2. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 73
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 76
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 of 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 JavaScript Object Notation (JSON) documents and then to encode
the results using the JSON format [RFC7159]. This document does the
same work for use with the Concise Binary Object Representation
(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.
o The author did not always agree with some of the decisions made by
the JOSE working group. Many of these decisions have been re-
examined, and where it seems to the author to be superior or
simpler, replaced.
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.
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o Signed messages separate the concept of protected and unprotected
attributes that are for the content and the signature.
o Key management has been made to be more uniform. All key
management techniques are represented as a recipient rather than
only have some of them be so.
o MAC messages are separated from signed messages.
o MAC messages have the ability to do key management 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.
1.3. CBOR Grammar
There currently is no standard CBOR grammar available for use by
specifications. While we describe the CBOR structures in prose, they
are agumented in the text by the use of the CBOR Data Definition
Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl]. The use of
CDDL is intended to be explanitory. In the event of a conflict
between the text and the CDDL grammar, the text is authorative.
(Problems may be introduced at a later point because the CDDL grammar
is not yet fixed.)
CDDL productions that together define the grammar are interspersed in
the document like this:
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start = COSE_MSG
The collected CDDL can be extracted from the XML version of this
document via the following XPath expression below. (Depending on the
XPath evaluator one is using, it may be necessary to deal with >
as an entity.)
//artwork[@type='CDDL']/text()
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
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 choice data items.
The CDLL grammar that defines a type that represents a label is given
below:
label = int / tstr
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.
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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 used 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].
o Advertising in the certificate (capabilities extension) [RFC4262]
o Minimum requirements for the S/MIME which have been updated over
time [RFC2633][RFC5751]
2. The COSE_MSG structure
The COSE_MSG structure is a top level CBOR object that corresponds to
the DataContent type in the Cryptographic Message Syntax (CMS)
[RFC5652]. This structure allows for a top level message to be sent
that could be any of the different security services. The security
service is identified within the message.
The COSE_Tagged_MSG CBOR type takes the COSE_MSG and prepends a CBOR
tag of TBD1 to the encoding of COSE_MSG. By having both a tagged and
untagged version of the COSE_MSG structure, it becomes easy to either
use COSE_MSG as a top level object or embedded in another object.
The tagged version allows for a method of placing the COSE_MSG
structure into a choice, using a consistent tag value to determine
that this is a COSE object.
The existence of the COSE_MSG and COSE_Tagged_MSG CBOR data types are
not intended to prevent protocols from using the individual security
primitives directly. Where only a single service is required, that
structure can be used directly.
Each of the top-level security objects use a CBOR map as the base
structure. Items in the map at the top level are identified by a
label. The type of the value associated with the label is determined
by the definition of the label.
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The set of labels present in a security object is not restricted to
those defined in this document. However, it is not recommended that
additional fields be added to a structure unless this is going to be
done in a closed environment. When new fields need to be added, it
is recommended that a new message type be created so that processing
of the field can be ensured. Using an older structure with a new
field means that any security properties of the new field will not be
enforced. Before a new field is added at the outer level, strong
consideration needs to be given to defining a new header field and
placing it into the protected headers. Applications should make a
determination if non-standardized fields are going to be permitted.
It is suggested that libraries allow for an option to fail parsing if
non-standardized fields exist, this is especially true if they do not
allow for access to the fields in other ways.
A label 'msg_type' is defined to distinguish between the different
structures when they appear as part of a COSE_MSG object. [CREF4]
[CREF5]
0 - Reserved.
1 - Signed Message.
2 - Encrypted Message
3 - Authenticated Message (MACed message)
Implementations MUST be prepared to find an integer under this label
that does not correspond to the values 1 to 3. If this is found then
the client MUST stop attempting to parse the structure and fail. The
value of 0 is reserved and not to be used. If the value of 0 is
found, then clients MUST fail processing the structure.
Implementations need to recognize that the set of values might be
extended at a later date, but they should not provide a security
service based on guesses of what is there.
NOTE: Is there any reason to allow for a marker of a COSE_Key
structure and allow it to be a COSE_MSG? Doing so does allow for a
security risk, but may simplify the code. [CREF6]
The CDDL grammar that corresponds to the above is:
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COSE_MSG = COSE_Sign /
COSE_encrypt /
COSE_mac
COSE_Tagged_MSG = #6.999(COSE_MSG) ; Replace 999 with TBD1
; msg_type values
msg_type_reserved=0
msg_type_signed=1
msg_type_encrypted=2
msg_type_mac=3
The top level of each of the COSE message structures are encoded as
maps. We use an integer to distinguish between the different
security message types. By searching for the integer under the label
identified by msg_type (which is in turn an integer), one can
determine which security message is being used and thus what syntax
is for the rest of the elements in the map.
+-------------+--------+--------------------------------------------+
| name | number | comments |
+-------------+--------+--------------------------------------------+
| msg_type | 1 | Occurs only in top level messages |
| | | |
| protected | 2 | Occurs in all structures |
| | | |
| unprotected | 3 | Occurs in all structures |
| | | |
| payload | 4 | Contains the content of the structure |
| | | |
| signatures | 5 | For COSE_Sign - array of signatures |
| | | |
| signature | 6 | For COSE_signature only |
| | | |
| ciphertext | 4 | TODO: Should we reuse the same as payload |
| | | or not? |
| | | |
| recipients | 9 | For COSE_encrypt and COSE_mac |
| | | |
| tag | 10 | For COSE_mac only |
+-------------+--------+--------------------------------------------+
Table 1: COSE Map Labels
The CDDL grammar that provides the label values is:
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; message_labels
msg_type=1
protected=2
unprotected=3
payload=4
signatures=5
signature=6
ciphertext=4
recipients=9
tag=10
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 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 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 present for 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 element
is not 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 range 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 Labels' IANA registry (Section 15.3.
Two buckets are provided for each layer: [CREF7]
protected contains attributes about the layer that are to be
cryptographically protected. This bucket MUST NOT be used 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. This 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 connical encoding.
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unprotected contains attributes about the layer that are not
cryptographically protected.
Both of the buckets are optional and are omitted if there are no
items contained in the map. The CDDL fragment that describes the two
buckets is:
header_map = {+ label => any }
Headers = (
? protected => bstr,
? unprotected => header_map
)
3.1. COSE Headers
The set of header fields defined in this document are:
alg This field is used to indicate the algorithm used for the
security processing. This field MUST be present at each level of
a signed, encrypted or authenticated message. This field using
the integer '1' for the label. The value is taken from the 'COSE
Algorithm Registry' (see Section 15.4).
crit This field is used to ensure that applications will take
appropriate action based on the values found. The field is used
to indicate which protected header labels an application that is
processing a message is required to understand. This field uses
the integer '2' for the label. The value is an array of COSE
Header Labels. When present, this 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.)
The header 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 field is processed.
cty This field is used to indicate the content type of the data in
the payload or ciphertext fields. The field uses the integer of
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'3' for the label. The value can be either an integer or a
string. [CREF8] Integers are from the XXXXX[CREF9] IANA registry
table. Strings are from the IANA 'mime-content types' registry.
Applications SHOULD provide this field if the content structure is
potentially ambiguous.
kid This field one of the ways that can be used to find the key to
be used. This value can be matched against the 'kid' field 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. This field uses the integer
value of '4' for the label. The value of field is the CBOR 'bstr'
type. The internal structure of 'kid' 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 normally be
placed in the unprotected headers bucket.
nonce This field holds either a nonce or Initialization Vector
value. This value can be used either as a counter value for a
protocol or as an IV for an algorithm. TODO: Talk about zero
extending the value in some cases. [CREF10]
This table contains a list of all of the generic header parameters
defined in document. In the table is the data value type to be used
for CBOR as well as the integer value that can be used as a
replacement for the name in order to further decrease the size of the
sent item.
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+----------+-------+----------+-------------+-----------------------+
| name | label | value | registry | description |
+----------+-------+----------+-------------+-----------------------+
| alg | 1 | int / | COSE | Integers are taken |
| | | tstr | Algorithm | from table --POINT TO |
| | | | Registry | REGISTRY-- |
| | | | | |
| crit | 2 | [+ | COSE Header | integer values are |
| | | label] | Label | from -- POINT TO |
| | | | Registry | REGISTRY -- |
| | | | | |
| cty | 3 | tstr / | media-types | Value is either a |
| | | int | registry | media-type or an |
| | | | | integer from the |
| | | | | media-type registry |
| | | | | |
| jku | * | tstr | | URL to COSE key |
| | | | | object |
| | | | | |
| jwk | * | COSE_Key | | contains a COSE key |
| | | | | not a JWK key |
| | | | | |
| kid | 4 | bstr | | key identifier |
| | | | | |
| nonce | 5 | bstr | | Nonce or |
| | | | | Initialization Vector |
| | | | | (IV) |
| | | | | |
| x5c | * | bstr* | | X.509 Certificate |
| | | | | Chain |
| | | | | |
| x5t | * | bstr | | SHA-1 thumbprint of |
| | | | | key |
| | | | | |
| x5t#S256 | * | bstr | | SHA-256 thumbprint of |
| | | | | key |
| | | | | |
| x5u | * | tstr | | URL for X.509 |
| | | | | certificate |
| | | | | |
| zip | * | int / | | Integers are taken |
| | | tstr | | from the table |
| | | | | --POINT TO REGISTRY-- |
+----------+-------+----------+-------------+-----------------------+
Table 2: Header Labels
OPEN ISSUES:
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1. Which of the following items do we want to have standardized in
this document: jku, jwk, x5c, x5t, x5t#S256, x5u, zip
2. 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.
3. 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 attributes
about the content and attributes about the signature to be carried
along with the signature itself. These attributes may be
authenticated by the signature, or just present. Examples of
attributes about the content would be the type of content, when the
content was created, and who created the content. Examples of
attributes 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
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is [RFC5652].) More detailed information on multiple signature
evaluation can be found in [RFC5752].
The CDDL grammar for a signature message is:
COSE_Sign = {
msg_type => msg_type_signed,
Headers,
? payload => bstr,
signatures => [+ COSE_signature]
}
The fields is the structure have the following semantics:
msg_type identifies this as providing the signed security service.
The value MUST be msg_type_signed (1).
protected contains attributes about the payload that are to be
protected by the signature. An example of such an attribute would
be the content type ('cty') attribute. The content is a CBOR map
of attributes that is encoded to a byte stream. This field MUST
NOT contain attributes about the signature, even if those
attributes are common across multiple signatures. The labels in
this map are typically taken from Table 2. [CREF11]
unprotected contains attributes about the payload that are not
protected by the signature. An example of such an attribute would
be the content type ('cty') attribute. This field MUST NOT
contain attributes about a signature, even if the attributes are
common across multiple signatures. The labels in this map are
typically taken from Table 2. [CREF12]
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, 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.
We use the values in Table 1 as the labels in the COSE_Sign map.
While other labels can be present in the map, it is not generally a
recommended practice. The other labels can be either of integer or
string type, use of other types SHOULD be treated as an error.
The CDDL grammar structure for a signature is:
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COSE_signature = {
Headers,
signature => bstr
}
The fields in the structure have the following semantics:
protected contains additional information to be authenticated by the
signature. The field holds data about the signature operation.
The field MUST NOT hold attributes about the payload being signed.
The content is a CBOR map of attributes that is encoded to a byte
stream. At least one of protected and unprotected MUST be
present.
unprotected contains attributes about the signature that are not
protected by the signature. This field MUST NOT contain
attributes about the payload being signed. At least one of
protected and unprotected MUST be present.
signature contains the computed signature value.
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,
payload: bstr
]
How to compute 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, an bstr
of length zero is used.
2. Create the value ToBeSigned by encoding the Sig_structure to a
byte string.
3. 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).
4. Place the resulting signature value in the 'signature' field of
the map.
How to verify a signature:
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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, an bstr
of length zero is used.
2. Create the value ToBeSigned by encoding the Sig_structure to a
byte string.
3. 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).
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.
5. Encryption object
In this section we describe the structure and methods to be used when
doing an encryption in COSE. In COSE, we use 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
plain text and for the different key management techniques.
One of the byproducts of using the same technique for encrypting and
encoding both the content and the keys using the various key
management techniques, is a requirement that all of the key
management techniques use an Authenticated Encryption (AE) algorithm.
(For the purpose of this document we use a slightly loose definition
of AE algorithms.) When encrypting the plain text, it is normal to
use an Authenticated Encryption with Additional Data (AEAD)
algorithm. For key management, either AE or AEAD algorithms can be
used. See Appendix A for more details about the different types of
algorithms. [CREF13]
The CDDL grammar structure for encryption is:
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COSE_encrypt = {
msg_type=>msg_type_encrypted,
COSE_encrypt_fields
}
COSE_encrypt_fields = (
Headers,
? ciphertext => bstr,
? recipients => [+{COSE_encrypt_fields}]
)
Description of the fields:
msg_type identifies this as providing the encrypted security
service. The value MUST be msg_type_encrypted (2).
protected contains the information about the plain text or
encryption process that is to be integrity protected. The field
is encoded in CBOR as a 'bstr'. The contents of the protected
field is a CBOR map of the protected data names and values. The
map is CBOR encoded before placing it into the bstr. Only values
associated with the current cipher text are to be placed in this
location even if the value would apply to multiple recipient
structures.
unprotected contains information about the plain text that is not
integrity protected. Only values associated with the current
cipher text are to be placed in this location even if the value
would apply to multiple recipient structures.
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
omitted.
recipients contains the recipient information. It is required that
at least one recipient MUST be present for the content encryption
layer.
5.1. Key Management Methods
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. The details of this encryption
depends on the key management technique used, but the six generally
techniques are:
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none: The CEK is the same as as the identified previously
distributed symmetric key.
symmetric key-encryption keys: The CEK is encrypted using a
previously distributed symmetric key-encryption key.
key agreement: the recipient's public key and a sender's private key
are used to generate a pairwise symmetric key, 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 or other shared secret value.
Section 12 provides details on a number of different key management
algorithms and discusses which elements need to be present for each
of the key management techniques.
5.2. 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.
Enc_structure = [
protected: bstr,
external_aad: bstr
]
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
'external_aad' field. If no data was supplied, then a zero
length binary value is used.
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 key
management method being used: For:
No Recipients: The key to be used is determined by the algorithm
and key at the current level.
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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.
5.3. 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 key
management method 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.
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.
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6. MAC objects
In this section we describe the structure and methods to be used when
doing MAC authentication in COSE. JOSE used a variant of the
signature structure for doing MAC operations and it is restricted to
using a single pre-shared secret to do the authentication. [CREF14]
This document allows for the use of all of the same methods of key
management 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 of the key management
methods 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 key management to verify who sent it. The key
management modes that support this are ones that either use a pre-
shared secret, or do static-static key agreement. 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.)
COSE_mac = {
msg_type=>msg_type_mac,
Headers,
? payload => bstr,
tag => bstr,
recipients => [+{COSE_encrypt_fields}]
}
Field descriptions:
msg_type identifies this as providing the encrypted security
service. The value MUST be msg_type_mac (3).
protected contains attributes about the payload that are to be
protected by the MAC. An example of such an attribute would be
the content type ('cty') attribute. The content is a CBOR map of
attributes that is encoded to a byte stream. This field MUST NOT
contain attributes about the recipient, even if those attributes
are common across multiple recipients. At least one of protected
and unprotected MUST be present.
unprotected contains attributes about the payload that are not
protected by the MAC. An example of such an attribute would be
the content type ('cty') attribute. This field MUST NOT contain
attributes about a recipient, even if the attributes are common
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across multiple recipients. At least one of protected and
unprotected MUST be present.
payload contains the serialized content to be MACed. 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 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.
MAC_structure = [
protected: bstr,
external_aad: bstr,
payload: bstr
]
How to compute a MAC:
1. Create a MAC_structure and copy the protected and payload
elements 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'.
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.
7. Key Structure
There are only a few changes between JOSE and COSE for how keys are
formatted. As with JOSE, COSE uses a map to contain the elements of
a key. Those values, which in JOSE are base64url encoded because
they are binary values, are encoded as bstr values in COSE.
For COSE we use the same set of fields that were defined in
[RFC7517]. [CREF15] [CREF16]
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COSE_Key = {
kty => tstr / int,
? key_ops => [+ tstr / int ],
? alg => tstr / int,
? kid => bstr,
* label => values
}
COSE_KeySet = [+COSE_Key]
The element "kty" is a required element in a COSE_Key map. All other
elements are optional and not all of the elements listed in [RFC7517]
or [RFC7518] have been listed here even though they can all appear in
a COSE_Key map.
7.1. COSE Key Map Labels
This document defines a set of common map elements for a COSE Key
object. Table 3 provides a summary of the elements defined in this
section. There are also a set of map elements that are defined for a
specific key type. Key specific elements 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 |
| | | | | |
| x5u | * | tstr | | |
| | | | | |
| x5c | * | bstr* | | |
| | | | | |
| x5t | * | bstr | | |
| | | | | |
| x5t#S256 | * | bstr | | |
| | | | | |
| use | * | tstr | | deprecated - don't |
| | | | | use |
+----------+-------+-------------+------------+---------------------+
Table 3: Key Map Labels
kty: This field is used to identify the family of keys for this
structure, and thus the set of fields to be found. The set of
values can be found in Table 19.
alg: This field is used to restrict the algorithms that are to be
used with this key. If this field 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 algorthms
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.
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kid: This field 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' field in
a message in order to filter down the set of keys that need to be
checked.
key_ops: This field 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 4.
Only the 'kty' field MUST be present in a key object. All other
members may be omitted if their behavior is not needed.
+---------+-------+-------------------------------------------------+
| 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 4: Key Operation Values
The following provides a CDDL fragment which duplicates the
assignment labels from Table 3 and Table 4.
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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 two
of the letters in RSASSA-PSS are signature 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 [TBD].) In this structure, the message content is
processed, but part of is included in the siguature. 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 algoritms 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
because the same as doing a singature with appendix. Thirdly, in the
event that multple 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 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. When a hash function
is used that has greater security than is provided by the length of
the key, the signature algorithm uses the leftmost keyLength bits of
the hash function output.
+-------+-------+---------+------------------+
| 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 5: ECDSA Algorithm Values
In order to promote interoperability, it is suggested that SHA-256 be
used only with keys of length 256, SHA-384 be used only with keys of
length 384 and SHA-512 be used only with keys of length 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
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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
On of the issues that needs to be discussed is substitution attacks.
There are two different things that can potentially be substituted in
this algorithm. Both of these attacks are current theoretical only.
The first substitution attack is changing the curve used to validate
the signature, the only requirement is that the order of the key
match the length of R and S. It is theoretically possible to use a
different curve and get a different result. 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.
The second substitution attack is to change the hash function that is
used to verify the signature. This attack can be mitigated by
including the signature algorithm identifier in the data to be
signed.
8.2. RSASSA-PSS
The RSASSA-PSS signature algorithm is defined in [RFC3447].
The RSASSA-PSS signature algorithm is parametized with a hash
function, a mask generation function and a salt length (sLen). For
this specification, the mask generation function is fixed to be MGF1
as defined in [RFC3447]. It has been recommended that the same hash
function be used for hashing the data as well as in the mask
generation function, for this specification we following this
recommendation. The salt length is the same length as the hash
function output.
Three algorithms are defined in this document. These algorithms are:
PS256: This uses the hash algorithm SHA-256 for signature
processing. The value used for this algorithm is -10. The key
type used for this algorithm is 'RSA'.
PS384: This uses the hash algorithm SHA-384 for signature
processing. The value used for this algorithm is "PS384". The
key type used for this algorithm is 'RSA'.
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PS512: This uses the hash algorithm SHA-512 for signature
processing. The value used for this algorithm is -11. The key
type used for this algorithm is 'RSA'.
There are no algorithm parameters defined for these signature
algorithms. A summary of the algorithm definitions can be found in
Table 6.
+-------+-------+---------+-------------+-----------------------+
| name | value | hash | salt length | description |
+-------+-------+---------+-------------+-----------------------+
| PS256 | -10 | SHA-256 | 32 | RSASSA-PSS w/ SHA-256 |
| | | | | |
| PS384 | * | SHA-384 | 48 | RSASSA-PSS w/ SHA-384 |
| | | | | |
| PS512 | -11 | SHA-512 | 64 | RSASSA-PSS w/ SHA-512 |
+-------+-------+---------+-------------+-----------------------+
Table 6: RSA Algorithm Values
8.2.1. Security Considerations
Key size. is there a MUST for 2048? or do we need to specify a
minimum here?
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.)
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, in part, to deal with the birthday
attacks on straight hash functions. The algorithm was also designed
to all 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].
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For use in constrained environments, we define a set of HMAC
algorithms that are truncated. 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.
+-----------+-------+---------+--------+----------------------------+
| name | value | Hash | Length | description |
+-----------+-------+---------+--------+----------------------------+
| HMAC | * | SHA-256 | 64 | HMAC w/ SHA-256 truncated |
| 256/64 | | | | to 8 bytes |
| | | | | |
| 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 7: HMAC Algorithm Values
9.1.1. Security Considerations
TBD.
9.2. AES Message Authentication Code (AES-MAC)
There are a set of different algorithms that we can specify here.
Which should it be?
AES-MAC - Use standard CBC mode
AES-CMAC - RFC 4493 - has improved security over AES-CBC. The
padding is different from CBC mode and requires one extra AES
block encryption step plus and xor operation.
10. Content Encryption Algorithms
10.1. AES GCM
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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 8: Algorithm Value for AES-GCM
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 constrainted 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 undetecably 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
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 key management
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:
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16-bits (2) limits messages to 2^16 bytes in length. 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 byes 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 an 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 an modified message will
authenticate.
+--------------------+--------+----+-----+-----+--------------------+
| name | value | L | M | k | description |
+--------------------+--------+----+-----+-----+--------------------+
| AES-CCM-16-64-128 | A281C | 16 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-16-64-192 | A282C | 16 | 64 | 192 | AES-CCM mode |
| | | | | | 192-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-16-64-256 | A283C | 16 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-64-64-128 | A881C | 64 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 64-bit tag, 7-byte |
| | | | | | nonce |
| | | | | | |
| AES-CCM-64-64-192 | A882C | 64 | 64 | 192 | AES-CCM mode |
| | | | | | 192-bit key, |
| | | | | | 64-bit tag, 7-byte |
| | | | | | nonce |
| | | | | | |
| AES-CCM-64-64-256 | A883C | 64 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
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| | | | | | 64-bit tag, 7-byte |
| | | | | | nonce |
| | | | | | |
| AES-CCM-16-128-128 | A2161C | 16 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-16-128-192 | A2162C | 16 | 128 | 192 | AES-CCM mode |
| | | | | | 192-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-16-128-256 | A2163C | 16 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
| | | | | | |
| AES-CCM-64-128-128 | A8161C | 64 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 7-byte nonce |
| | | | | | |
| AES-CCM-64-128-192 | A8162C | 64 | 128 | 192 | AES-CCM mode |
| | | | | | 192-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 7-byte nonce |
| | | | | | |
| AES-CCM-64-128-256 | A8163C | 64 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 7-byte nonce |
+--------------------+--------+----+-----+-----+--------------------+
Table 9: Algorithm Values for AES-CCM
M00TODO: Make a determination of which ones get 1-, 2- or 3-byte
identifiers. I.e. which ones are going to be popular.
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
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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.
11. Key Derivation Functions (KDF)
11.1. HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
See [RFC5869].
Inputs:
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. The 'salt' parameter is
encoded as a binary string. 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
hash function - The underlying hash function to be used in the
HKDF algorithm. The hash function is encoded into the HKDF
algorithm selection.
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+----------+---------+---------+
| name | hash | context |
+----------+---------+---------+
| HKDF-256 | SHA-256 | XXX |
| | | |
| HKDF-512 | SHA-512 | XXX |
+----------+---------+---------+
Table 10: HKDF algorithms
+------+-------+------+-------------+
| name | label | type | description |
+------+-------+------+-------------+
| salt | -20 | bstr | Random salt |
+------+-------+------+-------------+
Table 11: HKDF 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 CBOR encoding.
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. This is because we are assuming a set of
stand alone store and forward messaging processes.
Application protocols are free to define the roles differently. For
example, they could assign the PartyU role to the entity that
initiates the connection and allow directly sending multiple messages
over the line without changing the role information.
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
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is used for different algorithms, then completely different keys
will be generated each of those algorithms. (This practice means
if algorithm A uses a shorter key than algorithm B and thus can be
found easier, the key derived for algorithm B will not contain the
key for algorithm A as a prefix.) [CREF17]
PartyUInfo This field holds information about party U. The
ParytUInfo structure is divided into three pieces:
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 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).
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 validated 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. [CREF18]
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.
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keyDataLength This is set to the number of bits of the desired
output value.
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.
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.
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,
? other : bstr
],
? SuppPrivInfo : bstr
]
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+---------------+-------+-----------+-------------------------------+
| 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
12. Key Management Algorithms
There are a number of different key management methods that can be
used in the COSE encryption system. In this section we will discuss
each of the key management methods, what fields need to be specified,
and which algorithms are defined in this document to deal with each
of them.
The names of the key management methods used here are the same as are
defined in [RFC7517]. Other specifications use different terms for
the key management methods or do not support some of the key
management methods.
At the moment we do not have any key management methods that allow
for the use of protected headers. This may be changed in the future
if, for example, the AES-GCM Key wrap method defined in [RFC7518]
were extended to allow for authenticated data. In that event, the
use of the 'protected' field, which is current forbidden below, would
be permitted.
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12.1. Direct Encryption
In direct encryption mode, a shared secret between the sender and the
recipient is used as the key. [CREF19] When direct encryption mode
is used, it MUST be the only mode used on the message. It is a
massive security leak to have both direct encryption and a different
key management mode on the same message.
For JOSE, direct encryption key management is the only key management
method allowed for doing MACed messages. In COSE, all of the key
management methods can be used for MACed messages.
The COSE_encrypt structure for the recipient is organized as follows:
o The 'protected', 'ciphertext' and 'recipients' fields MUST be
absent.
o At a minimum, the 'unprotected' field MUST contain the 'alg'
parameter and SHOULD contain a parameter identifying the shared
secret.
12.1.1. Direct Key
We define two key agreement algorithms that function as direct key
algorithms. These algorithms are:
Direct: This key management technique is the simplest method, 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 key management methods.
Direct KDF: This key managment takes 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 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, i.e 256-bits. 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.
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+------------+-------+--------------+----------------------+
| name | value | KDF | description |
+------------+-------+--------------+----------------------+
| direct | -6 | N/A | Direct use of CEK |
| | | | |
| direct+KDF | * | HKDF SHA-256 | Shared secret w/ KDF |
+------------+-------+--------------+----------------------+
Table 13: Direct Key
12.1.1.1. Security Considerations
Lifetime, Length, Compromise
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
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encryption algorithms defined in this document. [CREF20] 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.
+--------+-------+----------+-----------------------------+
| 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 14: AES Key Wrap Algorithm Values
12.2.1.1. Security Considerations for AES-KW
There are no specific security considerations for this algorithm.
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. The only current Key
Encryption mode algorithm supported is RSAES-OAEP.
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.
12.3.1. RSA OAEP
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+----------+-------+-----------------------+
| name | value | description |
+----------+-------+-----------------------+
| RSA-OAEP | -2 | RSAES OAEP w/ SHA-256 |
+----------+-------+-----------------------+
Table 15: RSA OAEP Algorithm Values
12.3.1.1. Security Considerations for RSA OAEP
A key size of 2048 bits or larger MUST be used with this algorithm.
This key size corresponds roughly to the same strength as provided by
a 128-bit symmetric encryption algorithm.
It is highly recommended that checks on the key length be done before
starting a decryption operation. One potential denial of service
operation is to provide encrypted objects using either abnormally
long or oddly sized RSA modulus values. Implementations SHOULD be
able to encrypt and decrypt with modulus between 2048 and 16K bits in
length.[CREF21] Applications can impose additional restrictions on
the length of the modulus.
12.4. Direct Key Agreement
When using the 'Direct Key Agreement' key managment method, the two
parties use 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
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
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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, it MUST be the only key
management mode used on the message and there MUST be only one
recipient. 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 (Section 12.5.1) 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
NOTE: Curves 25519 and Goldilocks are elements at risk.
We define one set of key agreement algorithms structured around
Elliptic Curves Diffie-Hellman problem. [CREF22] We define both an
ephemeral-static and a static-static version of these algorithms. We
allow for multiple curves to be used, it needs to be noted that the
math required for the curves as well as the point representation is
going to be different. [CREF23]
We setup to use two different curve structures for the ECDH
algorithms.
Weierstrass Curves: These are the ones one is used to seeing from
NIST. We define three NIST curves for use with this document.
These curves are P-256, P-384 and P-512. (The mathematics can be
found in [RFC6090].) For these curves, the key type 'EC2' is used
(Section 13.1.2).
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Montgomery Curves: These curves are Curve25519 and Goldilocks.
(The mathematics can be found in [I-D.irtf-cfrg-curves].) For
these curves, the key type 'EC1' is used (Section 13.1.1).
As shown in Table 16 we define two ECDH algorithm identifiers for EC
direct key agreement. These identifiers are:
ECDH-ES: This algorithm does a key agreement operation using a
static key for the recipient and an ephemeral key for the sender.
The ephemeral key MUST be generated fresh for every message. The
HKDF function (Section 11.1) is used with the context structure in
Section 11.2 to transform the key agreement secret into the
necessary key. Since the ephemeral key is generated freshly, the
'salt' parameter of HKDF is not needed and can be absent.
One new algorithm parameter is defined for use with this
algorithm. This parameter is:
ephemeral key: This parameter is used to hold and transport the
ephemeral key generated by the sender of the message. This
parameter has a label of -1 and a type of COSE_Key. This
parameter can be placed in the unprotected bucket, if it is
changed then the correct key will not be able to be generated.
The parameter is summarized in Table 17.
ECDH-SS: This algorithm does a key agreement operation using two
static keys, one for the recipient and one for the sender. The
HKDF function (Section 11.1) is used with the context structure in
Section 11.2 to transform the key agreement 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 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, i.e 256-bits. 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.
Two new algorithm parameters are defined for use with this
algorithm. These parameters are:
static key: This parameter is used to hold and transport the
static key used by the sender of the message. This parameter
has the label of -2 and a type of COSE_Key. The parameter can
be placed in the unprotected bucket, if it is changed then the
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correct key will not be able to be generated. If the data
origination service is desired, then the message recipient
needs to validate that the key in this field is associated with
the sender.
static key identifier: This parameter is used to hold a reference
to the static key used by the sender of the message. The value
is expected to match the 'kid' member of a COSE_Key structure
published by the sender. The value in this field cannot be
assumed to uniquely identify a single key, multiple keys may
need to be found and tested. Not all of the keys identified by
a kid value may be associated with the sender of the message.
If the data origination service is desired, then the message
recipient needs to validate that the key in this field is
associated with the sender.
These parameters are summarized in Table 17.
+---------+---------+--------------+--------------------------------+
| name | value | KDF | description |
+---------+---------+--------------+--------------------------------+
| ECDH-ES | ECDH-ES | HKDF - | ECDH ES w/ HKDF - generate key |
| | | SHA-256 | directly |
| | | | |
| ECDH-SS | ECDH-SS | HKDF - | ECDH SS w/ HKDF - generate key |
| | | SHA-256 | directly |
+---------+---------+--------------+--------------------------------+
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
M00TODO: Talk about curves and point formats.
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+------------+----------+-------+-------------------------------+
| name | key type | value | description |
+------------+----------+-------+-------------------------------+
| P-256 | EC2 | 1 | NIST P-256 also known as .... |
| | | | |
| P-384 | EC2 | 2 | NIST P-384 also known as .... |
| | | | |
| P-521 | EC2 | 3 | NIST P-512 also known as .... |
| | | | |
| Curve25519 | EC1 | 1 | Provide reference |
| | | | |
| Goldilocks | EC1 | 2 | Provide reference |
+------------+----------+-------+-------------------------------+
Table 18: EC Curves
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 MUST be absent if the key wrap algorithm is
an AE algorithm. [CREF24]
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, a parameter identifying the recipient asymmetric key,
and a parameter with the sender's asymmetric public key.
12.5.1. ECDH ES + HKDF
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+----------------+-------+----------+-------------------------------+
| name | value | KDF | description |
+----------------+-------+----------+-------------------------------+
| ECDH-ES+A128KW | * | HKDF - | ECDH ES w/ Concat KDF and AES |
| | | SHA-256 | Key wrap w/ 128 bit key |
| | | | |
| ECDH-ES+A192KW | * | HKDF - | ECDH ES w/ Concat KDF and AES |
| | | SHA-256 | Key wrap w/ 192 bit key |
| | | | |
| ECDH-ES+A256KW | * | HKDF - | ECDH ES w/ Concat KDF and AES |
| | | SHA-256 | Key wrap w/ 256 bit key |
+----------------+-------+----------+-------------------------------+
12.6. Password
[CREF25]
12.6.1. PBES2
+--------------------+-------+--------------------------------------+
| name | value | description |
+--------------------+-------+--------------------------------------+
| PBES2-HS256+A128KW | * | PBES2 w/ HMAC SHA-256 and AES Key |
| | | wrap w/ 128 bit key |
| | | |
| PBES2-HS384+A192KW | * | PBES2 w/ HMAC SHA-384 and AES Key |
| | | wrap w/ 192 bit key |
| | | |
| PBES2-HS512+A256KW | * | PBES2 w/ HMAC SHA-512 and AES Key |
| | | wrap w/ 256 bit key |
+--------------------+-------+--------------------------------------+
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.
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Keys are identified by the 'kty' member of the COSE_Key object. In
this document we define four values for the member.
+-----------+-------+--------------------------------------------+
| name | value | description |
+-----------+-------+--------------------------------------------+
| EC1 | 1 | Elliptic Curve Keys w/ X Coordinate only |
| | | |
| EC2 | 2 | Elliptic Curve Keys w/ X,Y Coordinate pair |
| | | |
| RSA | 3 | RSA Keys |
| | | |
| Symmetric | 4 | Symmetric Keys |
+-----------+-------+--------------------------------------------+
Table 19: Key Type Values
13.1. Elliptic Curve Keys
Two different key structures are being 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. An example of this is
Curve25519 [I-D.irtf-cfrg-curves].
13.1.1. Single Coordinate Curves
NOTE: This section represents at risk work depending on the ability
to get good references for Curve25519 and Goldilocks.
New versions of ECC have been targeted at variants where only a
single value of the EC Point need to be transmitted. This work is
currently going on in the IRTF CFRG group.
For EC keys with both coordinates, the 'kty' member is set to 1
(EC1). The members that are defined for this key type are:
crv contains an identifier of the curve to be used with the key.
[CREF26] The curves defined in this document for this key type can
be found in Table 18. 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 use ???. Note that the octet string
represents a little-endian encoding of x. [CREF27]
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d contains the private key.
For public keys, it is REQUIRED that 'crv' and 'x' be present in the
structure. For private keys, it is REQUIRED that 'crv' and 'd' be
present in the structure. It is RECOMMENDED that 'x' also be
present, but it can be recomputed from the required elements and
omitting it saves on space.
+------+-------+-------+--------+-----------------------------------+
| name | key | value | type | description |
| | type | | | |
+------+-------+-------+--------+-----------------------------------+
| crv | 1 | -1 | int / | EC Curve identifier - Taken from |
| | | | tstr | the COSE General Registry |
| | | | | |
| x | 1 | -2 | bstr | X Coordinate |
| | | | | |
| d | 1 | -4 | bstr | Private key |
+------+-------+-------+--------+-----------------------------------+
Table 20: EC Key Parameters
13.1.2. 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 recommend in the IETF
due to potential IPR issues with Certicom. 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 members that are defined for this key type are:
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 18. 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. [CREF28]
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.
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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. 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 21: EC Key Parameters
13.2. RSA Keys
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+-------+----------+-------+-------+----------------------------+
| name | key type | value | type | description |
+-------+----------+-------+-------+----------------------------+
| n | 3 | -1 | bstr | Modulus Parameter |
| | | | | |
| e | 3 | -2 | int | Exponent Parameter |
| | | | | |
| d | 3 | -3 | bstr | Private Exponent Parameter |
| | | | | |
| p | 3 | -4 | bstr | First Prime Factor |
| | | | | |
| q | 3 | -5 | bstr | Second Prime Factor |
| | | | | |
| dp | 3 | -6 | bstr | First Factor CRT Exponent |
| | | | | |
| dq | 3 | -7 | bstr | Second Factor CRT Exponent |
| | | | | |
| qi | 3 | -8 | bstr | First CRT Coefficient |
| | | | | |
| other | 3 | -9 | array | Other Primes Info |
| | | | | |
| r | 3 | -10 | bstr | Prime Factor |
| | | | | |
| d | 3 | -11 | bstr | Factor CRT Exponent |
| | | | | |
| t | 3 | -12 | bstr | Factor CRT Coefficient |
+-------+----------+-------+-------+----------------------------+
Table 22: RSA Key Parameters
13.3. 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:
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.
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+------+----------+-------+------+-------------+
| name | key type | value | type | description |
+------+----------+-------+------+-------------+
| k | 4 | -1 | bstr | Key Value |
+------+----------+-------+------+-------------+
Table 23: Symmetric Key Parameters
14. CBOR Encoder Restrictions
There as 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 a new tag from the "Concise Binary
Object Representation (CBOR) Tags" registry. It is requested that
the tag be assigned in the 0 to 23 value range.
Tag Value: TBD1
Data Item: COSE_Msg
Semantics: COSE security message.
15.2. COSE Object Labels Registry
It is requested that IANA create a new registry entitled "COSE Object
Labels Registry". [CREF29]
This table is initially populated by the table in Table 1.
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15.3. COSE Header Label Table
It is requested that IANA create a new registry entitled "COSE Header
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.
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).
The initial contents of the registry can be found in Table 2. The
specification column for all rows in that table should be this
document.
Additionally, the label of 0 is to be marked as 'Reserved'.
15.4. 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:
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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, and Appendix D. The specification column for all
rows in that table should be this document.
15.5. 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
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.
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specification A document where the algorithm is defined (if publicly
available).
The initial contents of the registry can be found in the following:
Table 9, Table 8, Table 5, Table 7, Table 13, Table 14, Table 15.
The specification column for all rows in that table should be this
document.
15.6. COSE Key Map Registry
It is requested that IANA create a new registry entitled "COSE Key
Map 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
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.
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15.7. COSE Key Parameter Registry
It is requested that IANA create a new registry "COSE Key
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,
Table 21, Table 22, and Table 23. The specification column for all
of these entries will be this document.
15.8. Media Type Registration
15.8.1. COSE Security Message
This section registers the "application/cose" and "application/
cose+cbor" media types in the "Media Types" registry. [CREF30] These
media types are used to indicate that the content is a COSE_MSG.
Type name: application
Subtype name: cose
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
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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
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
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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
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
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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
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:
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* 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
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.
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.
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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.
[I-D.irtf-cfrg-curves]
Langley, A. and R. Salz, "Elliptic Curves for Security",
draft-irtf-cfrg-curves-02 (work in progress), March 2015.
[I-D.mcgrew-aead-aes-cbc-hmac-sha2]
McGrew, D., Foley, J., and K. Paterson, "Authenticated
Encryption with AES-CBC and HMAC-SHA", draft-mcgrew-aead-
aes-cbc-hmac-sha2-05 (work in progress), July 2014.
[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.
[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.
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[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.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[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.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", May 2009.
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[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. AEAD and AE algorithms
The set of encryption algorithms that can be used with this
specification is restricted to authenticated encryption (AE) and
authenticated encryption with additional data (AEAD) algorithms.
This means that there is a strong check that the data decrypted by
the recipient is the same as what was encrypted by the sender.
Encryption modes such as counter have no check on this at all. The
CBC encryption mode had a weak check that the data is correct, given
a random key and random data, the CBC padding check will pass one out
of 256 times. There have been several times that a normal encryption
mode has been combined with an integrity check to provide a content
encryption mode that does provide the necessary authentication. AES-
GCM [AES-GCM], AES-CCM [RFC3610], AES-CBC-HMAC
[I-D.mcgrew-aead-aes-cbc-hmac-sha2] are examples of these composite
modes.
PKCS v1.5 RSA key transport does not qualify as an AE algorithm.
There are only three bytes in the encoding that can be checked as
having decrypted correctly, the rest of the content can only be
probabilistically checked as having decrypted correctly. For this
reason, PKCS v1.5 RSA key transport MUST NOT be used with this
specification. RSA-OAEP was designed to have the necessary checks
that that content correctly decrypted and does qualify as an AE
algorithm.
When dealing with authenticated encryption algorithms, there is
always some type of value that needs to be checked to see if the
authentication level has passed. This authentication value may be:
o A separately generated tag computed by both the encrypter and
decrypter and then compared by the decryptor. This tag value may
be either placed at the end of the cipher text (the decision we
made) or kept separately (the decision made by the JOSE working
group). This is the approach followed by AES-GCM [AES-GCM] and
AES-CCM [RFC3610].
o A fixed value that is part of the encoded plain text. This is the
approach followed by the AES key wrap algorithm [RFC3394].
o A computed value is included as part of the encoded plain text.
The computed value is then checked by the decryptor using the same
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computation path. This is the approach followed by RSAES-OAEP
[RFC3447].
Appendix B. Three Levels of Recipient Information
All of the currently defined Key Management methods 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 key management technique 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 3: 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.
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{
1: 2,
2: h'a10101',
3: {
-1: h'02d1f7e6f26c43d4868d87ce'
},
4: h'64f84d913ba60a76070a9a48f26e97e863e285295a44320878caceb076
3a334806857c67',
9: [
{
3: {
1: -3
},
4: h'5a15dbf5b282ecb31a6074ee3815c252405dd7583e078188',
9: [
{
3: {
1: "ECDH-ES",
5: h'6d65726961646f632e6272616e64796275636b406275636b
6c616e642e6578616d706c65',
4: {
1: 1,
-1: 4,
-2: h'b2add44368ea6d641f9ca9af308b4079aeb519f11e9b8
a55a600b21233e86e68',
-3: h'1a2cf118b9ee6895c8f415b686d4ca1cef362d4a7630a
31ef6019c0c56d33de0'
}
}
}
]
}
]
}
Appendix C. Examples
The examples can be found at https://github.com/cose-wg/Examples. 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.
To make it easier to read, the examples are presented using the
CBOR's diagnostic notation rather than a binary dump. [CREF31] Using
the Ruby based CBOR diagnostic tools at ????, the diagnostic notation
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can be converted into binary files using the following command line:
(install command is?...)
diag2cbor < inputfile > outputfile
The examples can be extracted from the XML version of this docuent
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, trucated to 64 bits
o Key management: direct shared secret
o File name: Mac-04
{
1: 3,
2: h'a1016f4145532d434d41432d3235362f3634',
4: h'546869732069732074686520636f6e74656e742e',
10: h'd9afa663dd740848',
9: [
{
3: {
1: -6,
5: h'6f75722d736563726574'
}
}
]
}
C.1.2. ECDH Direct MAC
This example uses the following:
o MAC: HMAC w/SHA-256, 256-bit key [CREF32]
o Key management: ECDH key agreement, two static keys, HKDF w/
context structure
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{
1: 3,
2: h'a10104',
4: h'546869732069732074686520636f6e74656e742e',
10: h'2ba937ca03d76c3dbad30cfcbaeef586f9c0f9ba616ad67e9205d3857
6ad9930',
9: [
{
3: {
1: "ECDH-SS",
5: h'6d65726961646f632e6272616e64796275636b406275636b6c61
6e642e6578616d706c65',
"spk": {
"kid": "peregrin.took@tuckborough.example"
},
"apu": h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d19558ccf
ec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a58368b01
7e7f2a9e5ce4db5'
}
}
]
}
C.1.3. Wrapped MAC
This example uses the following:
o MAC: AES-MAC, 128-bit key, truncated to 64 bits
o Key management: AES keywrap w/ a pre-shared 256-bit key
{
1: 3,
2: h'a1016e4145532d3132382d4d41432d3634',
4: h'546869732069732074686520636f6e74656e742e',
10: h'6d1fa77b2dd9146a',
9: [
{
3: {
1: -5,
5: h'30313863306165352d346439622d343731622d626664362d6565
66333134626337303337'
},
4: h'711ab0dc2fc4585dce27effa6781c8093eba906f227b6eb0'
}
]
}
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C.1.4. Multi-recipient MAC message
This example uses the following:
o MAC: HMAC w/ SHA-256, 128-bit key
o Key management: Uses three different methods
1. ECDH Ephemeral-Static, Curve P-521, AES-Key Wrap w/ 128-bit
key
2. RSA-OAEP w/ SHA-256
3. AES-Key Wrap w/ 256-bit key
{
1: 3,
2: h'a10104',
4: h'546869732069732074686520636f6e74656e742e',
10: h'7aaa6e74546873061f0a7de21ff0c0658d401a68da738dd8937486519
83ce1d0',
9: [
{
3: {
1: "ECDH-ES+A128KW",
5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65',
4: {
1: 1,
-1: 5,
-2: h'43b12669acac3fd27898ffba0bcd2e6c366d53bc4db71f909
a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2d61
3574e7dc242f79c3',
-3: h'812dd694f4ef32b11014d74010a954689c6b6e8785b333d1a
b44f22b9d1091ae8fc8ae40b687e5cfbe7ee6f8b47918a07bb04e9f5b1a51a334
a16bc09777434113'
}
},
4: h'1b120c848c7f2f8943e402cbdbdb58efb281753af4169c70d0126c
0d16436277160821790ef4fe3f'
},
{
3: {
1: -2,
5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65'
},
4: h'46c4f88069b650909a891e84013614cd58a3668f88fa18f3852940
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a20b35098591d3aacf91c125a2595cda7bee75a490579f0e2f20fd6bc956623bf
de3029c318f82c426dac3463b261c981ab18b72fe9409412e5c7f2d8f2b5abaf7
80df6a282db033b3a863fa957408b81741878f466dcc437006ca21407181a016c
a608ca8208bd3c5a1ddc828531e30b89a67ec6bb97b0c3c3c92036c0cb84aa0f0
ce8c3e4a215d173bfa668f116ca9f1177505afb7629a9b0b5e096e81d37900e06
f561a32b6bc993fc6d0cb5d4bb81b74e6ffb0958dac7227c2eb8856303d989f93
b4a051830706a4c44e8314ec846022eab727e16ada628f12ee7978855550249cc
b58'
},
{
3: {
1: -5,
5: h'30313863306165352d346439622d343731622d626664362d6565
66333134626337303337'
},
4: h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a518e7736549
e998370695e6d6a83b4ae507bb'
}
]
}
C.2. Examples of Encrypted Messages
C.2.1. Direct ECDH
This example uses the following:
o CEK: AES-GCM w/ 128-bit key
o Key managment: ECDH Ephemeral-Static, Curve P-256
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{
1: 2,
2: h'a10101',
3: {
-1: h'c9cf4df2fe6c632bf7886413'
},
4: h'45fce2814311024d3a479e7d3eed063850f3f0b9f3f948677e3ae9869b
cf9ff4e1763812',
9: [
{
3: {
1: "ECDH-ES",
5: h'6d65726961646f632e6272616e64796275636b406275636b6c61
6e642e6578616d706c65',
4: {
1: 1,
-1: 4,
-2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf05
4e1c7b4d91d6280',
-3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d
924b7e03bf822bb'
}
}
}
]
}
C.2.2. Direct plus Key Derivation
This example uses the following:
o CEK: AES-CCM w/128-bit key, trucate the tag to 64-bits
o Key managment: Use HKDF on a shared secret with the following
implicit fields as part of the context.
* APU identity: "lighting-client"
* APV identity: "lighting-server"
* Supplimentary Public Other: "Encryption Example 02"
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{
1: 2,
2: h'a1016e4145532d43434d2d3132382f3634',
3: {
-1: h'8f2720f78dce2737ae61a4fa'
},
4: h'0159973c5d790041cf54be80412b3d12a7be30f6b64193d3bb51dfec',
9: [
{
3: {
1: "dir+kdf",
5: h'6f75722d736563726574',
-10: h'61616262636364646565666667676868'
}
}
]
}
C.3. Examples of Signed Message
C.3.1. Single Signature
This example uses the following:
o Signature Algorithm: RSA-PSS w/ SHA-384, MGF-1
{
1: 1,
4: h'546869732069732074686520636f6e74656e742e',
5: [
{
2: h'a20165505333383405581e62696c626f2e62616767696e7340686f
626269746f6e2e6578616d706c65',
6: h'1b22515f96fd798a331c7b156e90bfea7f558ec6de840e05a8e5f4
b7be44ea1451c48517da7fd216c6143898673c232a96937ebcfb88264a58f5995
82d89cf8a4f20ef35fbfcfd2aad46ad8b99ea6425367afd898de1b712d558b0d2
49d6d180d0b1fb7256140ec3553556f3b5b95a49931a75998dfc23ca905efc7d8
e04deeb92d5936c0824e535aa344396f73913d8a65de0010600270ae5df7f5c8d
52ae525a7642d4c4ff9e219acaa52fd933df003be36b9e3c77ced37129d66745e
2a42baa3d0b3f2675cd51ae8a64fd024d126be5396c91b9236fb5f8548d09881b
b5d40a61c0d342bed9fe8058f36b8722b9e8465dc3b8bfa4f2fd138ce186b73e4
082'
}
]
}
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C.3.2. Multiple Signers
This example uses the following:
o Signature Algorithm: RSA-PSS w/ SHA-256, MGF-1
o Signature Algorithm: ECDSA w/ SHA-512, Curve P-521
{
1: 1,
4: h'546869732069732074686520636f6e74656e742e',
5: [
{
2: h'a10129',
3: {
5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65'
},
6: h'028947ac3521f66f2506013007e2cd7b0cb09a209e76ab5b95f751
eb63f5730f1672a282419c49b9653d742577fb6a6cea9ab2e1d4d5d9e786e2240
4760663cc74a1c2c90160af92628e1ebbc3eeba552f757054b691ab17271396b7
ff2d86c100b94a2fce0438c0b50ca70bcdd3074a0f8dc40c2e44e9b26e9093287
b7245ee13171b28ea0f3e291c2cca64aa17f7094aee2be02b5fe5cd2cf343e18c
eec0c763cb76a128df9a9cbfc37b835f6467d98d74505eee1dccc9e6ebf2405ea
1329b41a33eeb13f1bbef3a272e42b3df96cdaea9016663e31ddff4603eb66a88
5c583b53977c1fb9707550717d7387f84616a6670e27d4007b08879109aaf3720
f33'
},
{
3: {
1: -9,
5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
6d706c65'
},
6: h'0195345953742c6725352a13cdc55402c895133525c9a3b16bb47d
02ca5f57f8a34aebf47298c602a8feb1dd71d1936886f21029a4142abf38c3aa3
94b3597c2f35c01987c801edc7022c8fddacbf25bc8794b9ffb7cb27f9f346ba4
4db6f5c9b60406530f62b378c5da3e7e2259327f4e55f48271873496497724492
d90ba67a4b65112'
}
]
}
Appendix D. COSE Header Algorithm Label Table
This section disappears when we make a decision on password based key
management.
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+------+-----------+-------+-----------+-------------+
| name | algorithm | label | CBOR type | description |
+------+-----------+-------+-----------+-------------+
| p2c | PBE | -1 | int | |
| | | | | |
| p2s | PBE | -2 | bstr | |
+------+-----------+-------+-----------+-------------+
Appendix E. Document Updates
E.1. Version -01 to -02
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.
E.2. 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 algoithms.
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[CREF4] JLS: I have moved msg_type into the individual structures.
However, they would not be necessary in the cases where a) the
security service is known and b) security libraries can setup to
take individual structures. Should they be moved back to just
appearing if used in a COSE_MSG rather than on the individual
structure?
[CREF5] JLS: Should we create an IANA registries for the values of
msg_type?
[CREF6] JLS: OPEN ISSUE
[CREF7] 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?
[CREF8] JLS: After looking at this, I am wondering if the type for this
should be: [int int]/[int tstr] so that we can keep the major/
minor difference of media-types. This does cost a couple of
bytes in the message.
[CREF9] JLS: Need to figure out how we are going to go about creating
this registry -or are we going to modify the current mime-
content table?
[CREF10] JLS: Open to do.
[CREF11] JLS: Should be able to move much of this text into the headers
section and just do a refer to that text from here.
[CREF12] JLS: Should be able to move much of this text into the headers
section and just do a refer to that text from here.
[CREF13] Ilari: I don't follow/understand this text
[CREF14] JLS: Should this sentence be removed?
[CREF15] JLS: Do we remove this line and just define them ourselves?
[CREF16] 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.
[CREF17] JLS: Unless key material is being derived for multiple items
(i.e both a key and an IV) this will be the COSE algorithm
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value. Even then it might still be the COSE algorithm value,
it is just a requirement for a new algorithm. Do we want to
have the ability to derive both the key and a partial IV for
CCM?
[CREF18] JLS: I need to get a better justification for this item. It
has to do with generating new keys for each message in a series
of messages that have the same salt value.
[CREF19] JLS: It would be reasonable to support a shared-secret + KDF
that is not PBE for when one has good randomness in the shared-
secret.
[CREF20] JLS: Do we also want to document the use of RFC 5649 as well?
It allows for other sizes of keys that might be used for HMAC -
i.e. a 200 bit key. The algorithm exists, but I do not
personally know of any standard uses of it.
[CREF21] JLS: Is this range we want to specify?
[CREF22] JLS: Does anybody need pure DH?
[CREF23] JLS: This could just as easily be done by specifying two
different set of algorithm identifiers, one for each of the key
formats. I don't believe that we need to set things up by
having two different sets of algorithm identifiers for the
different keys as the structure of what is represented is going
to be the same, just the math and point formats are going to be
different. The other "difference" is the question of how the
octet string of the shared secret is defined. However, since
we don't need to specify either in this document we can defer
both of them into their respective documents.
[CREF24] JLS: It would be possible to include the protected field in the
KDF rather than the key wrap algorithm if we wanted to. This
would provide the same level of security, it would not be
possible to get the same key if they are different.
[CREF25] JLS: Do we want/need to support this? JOSE did it mainly to
support the encryption of private keys.
[CREF26] JLS: Do we create a registry for curves? Is is the same
registry for both EC1 and EC2?
[CREF27] JLS: Should we use the integer encoding for x and d instead of
bstr?
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[CREF28] JLS: Should we use the integer encoding for x, y and d instead
of bstr?
[CREF29] JLS: Finish the registration process.
[CREF30] JLS: Should we register both or just the cose+cbor one?
[CREF31] JLS: Do we want to keep this as diagnostic notation or should
we switch to having "binary" examples instead?
[CREF32] JLS: Need to examine how this is worked out. In this case the
length of the key to be used is implicit rather than explicit.
This needs to be the case because a direct key could be any
length, however it means that when the key is derived, there is
currently nothing to state how long the derived key needs to
be.
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
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