Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 9053 August Cellars
Obsoletes: 8152 August 2022
Category: Informational
ISSN: 2070-1721
CBOR Object Signing and Encryption (COSE): Initial Algorithms
Abstract
Concise Binary Object Representation (CBOR) is a data format designed
for small code size and small message size. There is a need to be
able to define basic security services for this data format. This
document defines a set of algorithms that can be used with the CBOR
Object Signing and Encryption (COSE) protocol (RFC 9052).
This document, along with RFC 9052, obsoletes RFC 8152.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9053.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(
https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Terminology
1.2. Changes from RFC 8152
1.3. Document Terminology
1.4. CDDL Grammar for CBOR Data Structures
1.5. Examples
2. Signature Algorithms
2.1. ECDSA
2.1.1. Security Considerations for ECDSA
2.2. Edwards-Curve Digital Signature Algorithm (EdDSA)
2.2.1. Security Considerations for EdDSA
3. Message Authentication Code (MAC) Algorithms
3.1. Hash-Based Message Authentication Codes (HMACs)
3.1.1. Security Considerations for HMAC
3.2. AES Message Authentication Code (AES-CBC-MAC)
3.2.1. Security Considerations for AES-CBC-MAC
4. Content Encryption Algorithms
4.1. AES-GCM
4.1.1. Security Considerations for AES-GCM
4.2. AES-CCM
4.2.1. Security Considerations for AES-CCM
4.3. ChaCha20 and Poly1305
4.3.1. Security Considerations for ChaCha20/Poly1305
5. Key Derivation Functions (KDFs)
5.1. HMAC-Based Extract-and-Expand Key Derivation Function
(HKDF)
5.2. Context Information Structure
6. Content Key Distribution Methods
6.1. Direct Encryption
6.1.1. Direct Key
6.1.2. Direct Key with KDF
6.2. Key Wrap
6.2.1. AES Key Wrap
6.3. Direct Key Agreement
6.3.1. Direct ECDH
6.4. Key Agreement with Key Wrap
6.4.1. ECDH with Key Wrap
7. Key Object Parameters
7.1. Elliptic Curve Keys
7.1.1. Double Coordinate Curves
7.2. Octet Key Pair
7.3. Symmetric Keys
8. COSE Capabilities
8.1. Assignments for Existing Algorithms
8.2. Assignments for Existing Key Types
8.3. Examples
9. CBOR Encoding Restrictions
10. IANA Considerations
10.1. Changes to the "COSE Key Types" Registry
10.2. Changes to the "COSE Algorithms" Registry
10.3. Changes to the "COSE Key Type Parameters" Registry
10.4. Expert Review Instructions
11. Security Considerations
12. References
12.1. Normative References
12.2. Informative References
Acknowledgments
Author's Address
1. Introduction
There has been an increased focus on small, constrained devices that
make up the Internet of Things (IoT). One of the standards that has
come out of this process is "Concise Binary Object Representation
(CBOR)" [STD94]. CBOR extended the data model of JavaScript Object
Notation (JSON) [STD90] by allowing for binary data, among other
changes. CBOR has been adopted by several of the IETF working groups
dealing with the IoT world as their method of encoding data
structures. CBOR was designed specifically to be small in terms of
both messages transported and implementation size and to have a
schema-free decoder. A need exists to provide message security
services for IoT, and using CBOR as the message-encoding format makes
sense.
The core COSE specification consists of two documents. [RFC9052]
contains the serialization structures and the procedures for using
the different cryptographic algorithms. This document provides an
initial set of algorithms for use with those structures.
1.1. 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Changes from RFC 8152
* Extracted the sections dealing with specific algorithms and placed
them into this document. The sections dealing with structure and
general processing rules are placed in [RFC9052].
* Made text clarifications and changes in terminology.
* Removed all of the details relating to countersignatures and
placed them in [COUNTERSIGN].
1.3. Document Terminology
In this document, we use the following terminology:
Byte: A synonym for octet.
Constrained Application Protocol (CoAP): A specialized web transfer
protocol for use in constrained systems. It is defined in
[RFC7252].
Authenticated Encryption (AE) algorithms [RFC5116]: Encryption
algorithms that provide an authentication check of the contents
along with the encryption service. An example of an AE algorithm
used in COSE is AES Key Wrap [RFC3394]. These algorithms are used
for key encryption, but Authenticated Encryption with Associated
Data (AEAD) algorithms would be preferred.
AEAD algorithms [RFC5116]: Encryption algorithms that provide the
same authentication service of the content as AE algorithms do,
and also allow associated data that is not part of the encrypted
body to be included in the authentication service. An example of
an AEAD algorithm used in COSE is AES-GCM [RFC5116]. These
algorithms are used for content encryption and can be used for key
encryption as well.
The term "byte string" is used for sequences of bytes, while the term
"text string" is used for sequences of characters.
The tables for algorithms contain the following columns:
* A name for the algorithm for use in documents.
* The value used on the wire for the algorithm. One place this is
used is the algorithm header parameter of a message.
* A short description so that the algorithm can be easily identified
when scanning the IANA registry.
Additional columns may be present in a table depending on the
algorithms.
1.4. CDDL Grammar for CBOR Data Structures
When COSE was originally written, the Concise Data Definition
Language (CDDL) [RFC8610] had not yet been published in an RFC, so it
could not be used as the data description language to normatively
describe the CBOR data structures employed by COSE. For that reason,
the CBOR data objects defined here are described in prose.
Additional (non-normative) descriptions of the COSE data objects are
provided in a subset of CDDL, described in [RFC9052].
1.5. Examples
A GitHub project has been created at [GitHub-Examples] that contains
a set of testing examples. Each example is found in a JSON file that
contains the inputs used to create the example, some of the
intermediate values that can be used for debugging, and the output of
the example. The results are encoded using both hexadecimal and CBOR
diagnostic notation format.
Some of the examples are designed to be failure-testing cases; these
are clearly marked as such in the JSON file.
2. Signature Algorithms
Section 8.1 of [RFC9052] contains a generic description of signature
algorithms. This document defines signature algorithm identifiers
for two signature algorithms.
2.1. ECDSA
The Elliptic Curve Digital Signature Algorithm (ECDSA) [DSS] defines
a signature algorithm using Elliptic Curve Cryptography (ECC).
Implementations SHOULD use a deterministic version of ECDSA such as
the one defined in [RFC6979]. The use of a deterministic signature
algorithm allows systems to avoid relying on random number generators
in order to avoid generating the same value of "k" (the per-message
random value). Biased generation of the value "k" can be attacked,
and collisions of this value lead to leaked keys. It additionally
allows performing deterministic tests for the signature algorithm.
The use of deterministic ECDSA does not lessen the need to have good
random number generation when creating the private key.
The ECDSA signature algorithm is parameterized with a hash function
(h). In the event that the length of the hash function output is
greater than the group of the key, the leftmost bytes of the hash
output are used.
The algorithms defined in this document can be found in Table 1.
+=======+=======+=========+==================+
| Name | Value | Hash | Description |
+=======+=======+=========+==================+
| ES256 | -7 | SHA-256 | ECDSA w/ SHA-256 |
+-------+-------+---------+------------------+
| ES384 | -35 | SHA-384 | ECDSA w/ SHA-384 |
+-------+-------+---------+------------------+
| ES512 | -36 | SHA-512 | ECDSA w/ SHA-512 |
+-------+-------+---------+------------------+
Table 1: ECDSA Algorithm Values
This document defines ECDSA as working only with the curves P-256,
P-384, and P-521. This document requires that the curves be encoded
using the "EC2" (two coordinate elliptic curve) key type.
Implementations need to check that the key type and curve are correct
when creating and verifying a signature. Future documents may define
it to work with other curves and key types in the future.
In order to promote interoperability, it is suggested that SHA-256 be
used only with curve P-256, SHA-384 be used only with curve P-384,
and SHA-512 be used only with curve P-521. This is aligned with the
recommendation in Section 4 of [RFC5480].
The signature algorithm results in a pair of integers (R, S). These
integers will be the same length as the length of the key used for
the signature process. The signature is encoded by converting the
integers into byte strings of the same length as the key size. The
length is rounded up to the nearest byte and is left padded with zero
bits to get to the correct length. The two integers are then
concatenated together to form a byte string that is the resulting
signature.
Using the function defined in [RFC8017], the signature is:
Signature = I2OSP(R, n) | I2OSP(S, n)
where n = ceiling(key_length / 8)
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "EC2".
* If the "alg" field is present, it MUST match the ECDSA signature
algorithm being used.
* If the "key_ops" field is present, it MUST include "sign" when
creating an ECDSA signature.
* If the "key_ops" field is present, it MUST include "verify" when
verifying an ECDSA signature.
2.1.1. Security Considerations for ECDSA
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.
Note: Use of a deterministic signature technique is a good idea even
when good random number generation exists. Doing so both reduces the
possibility of having the same value of "k" in two signature
operations and allows for reproducible signature values, which helps
testing. There have been recent attacks involving faulting the
device in order to extract the key. This can be addressed by
combining both randomness and determinism [CFRG-DET-SIGS].
There are two substitution attacks that can theoretically be mounted
against the ECDSA signature algorithm.
* Changing the curve used to validate the signature: If one changes
the curve used to validate the signature, then potentially one
could have two messages with the same signature, each computed
under a different curve. The only requirements on the new curve
are that its order be the same as the old one and that it be
acceptable to the client. An example would be to change from
using the curve secp256r1 (aka P-256) to using secp256k1. (Both
are 256-bit curves.) We currently 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.
* Changing the hash function used to validate the signature: If one
either has two different hash functions of the same length or can
truncate a hash function, then one could potentially find
collisions between the hash functions rather than within a single
hash function. For example, truncating SHA-512 to 256 bits might
collide with a SHA-256 bit hash value. As the hash algorithm is
part of the signature algorithm identifier, this attack is
mitigated by including a signature algorithm identifier in the
protected-header bucket.
2.2. Edwards-Curve Digital Signature Algorithm (EdDSA)
[RFC8032] describes the elliptic curve signature scheme Edwards-curve
Digital Signature Algorithm (EdDSA). In that document, the signature
algorithm is instantiated using parameters for the edwards25519 and
edwards448 curves. The document additionally describes two variants
of the EdDSA algorithm: Pure EdDSA, where no hash function is applied
to the content before signing, and HashEdDSA, where a hash function
is applied to the content before signing and the result of that hash
function is signed. For EdDSA, the content to be signed (either the
message or the prehash value) is processed twice inside of the
signature algorithm. For use with COSE, only the pure EdDSA version
is used. This is because it is not expected that extremely large
contents are going to be needed and, based on the arrangement of the
message structure, the entire message is going to need to be held in
memory in order to create or verify a signature. Therefore, there
does not appear to be a need to be able to do block updates of the
hash, followed by eliminating the message from memory. Applications
can provide the same features by defining the content of the message
as a hash value and transporting the COSE object (with the hash
value) and the content as separate items.
The algorithm defined in this document can be found in Table 2. A
single signature algorithm is defined, which can be used for multiple
curves.
+=======+=======+=============+
| Name | Value | Description |
+=======+=======+=============+
| EdDSA | -8 | EdDSA |
+-------+-------+-------------+
Table 2: EdDSA Algorithm Value
[RFC8032] describes the method of encoding the signature value.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "OKP" (Octet Key
Pair).
* The "crv" field MUST be present, and it MUST be a curve defined
for this signature algorithm.
* If the "alg" field is present, it MUST match "EdDSA".
* If the "key_ops" field is present, it MUST include "sign" when
creating an EdDSA signature.
* If the "key_ops" field is present, it MUST include "verify" when
verifying an EdDSA signature.
2.2.1. Security Considerations for EdDSA
Public values are computed differently in EdDSA and Elliptic Curve
Diffie-Hellman (ECDH); for this reason, the public key from one
should not be used with the other algorithm.
If batch signature verification is performed, a well-seeded
cryptographic random number generator is REQUIRED (Section 8.2 of
[RFC8032]). Signing and nonbatch signature verification are
deterministic operations and do not need random numbers of any kind.
3. Message Authentication Code (MAC) Algorithms
Section 8.2 of [RFC9052] contains a generic description of MAC
algorithms. This section defines the conventions for two MAC
algorithms.
3.1. Hash-Based Message Authentication Codes (HMACs)
HMAC [RFC2104] [RFC4231] was designed to deal with length extension
attacks. The HMAC algorithm was also designed to allow new hash
functions to be directly plugged in without changes to the hash
function. The HMAC design process has been shown to be solid;
although the security of hash functions such as MD5 has decreased
over time, the security of HMAC combined with MD5 has not yet been
shown to be compromised [RFC6151].
The HMAC algorithm is parameterized by an inner and outer padding, a
hash function (h), and an authentication tag value length. For this
specification, the inner and outer padding are fixed to the values
set in [RFC2104]. The length of the authentication tag corresponds
to the difficulty of producing a forgery. For use in constrained
environments, we define one HMAC algorithm that is truncated. There
are currently no known issues with truncation; however, the security
strength of the message tag is correspondingly reduced in strength.
When truncating, the leftmost tag-length bits are kept and
transmitted.
The algorithms defined in this document can be found in Table 3.
+=============+=======+=========+============+======================+
| Name | Value | Hash | Tag Length | Description |
+=============+=======+=========+============+======================+
| HMAC | 4 | SHA-256 | 64 | HMAC w/ SHA-256 |
| 256/64 | | | | truncated to 64 bits |
+-------------+-------+---------+------------+----------------------+
| HMAC | 5 | SHA-256 | 256 | HMAC w/ SHA-256 |
| 256/256 | | | | |
+-------------+-------+---------+------------+----------------------+
| HMAC | 6 | SHA-384 | 384 | HMAC w/ SHA-384 |
| 384/384 | | | | |
+-------------+-------+---------+------------+----------------------+
| HMAC | 7 | SHA-512 | 512 | HMAC w/ SHA-512 |
| 512/512 | | | | |
+-------------+-------+---------+------------+----------------------+
Table 3: HMAC Algorithm Values
Some recipient algorithms transport the key, while others derive a
key from secret data. For those algorithms that transport the key
(such as AES Key Wrap), the size of the HMAC key SHOULD be the same
size as the output of the underlying hash function. For those
algorithms that derive the key (such as ECDH), the derived key MUST
be the same size as the output of the underlying hash function.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the HMAC algorithm
being used.
* If the "key_ops" field is present, it MUST include "MAC create"
when creating an HMAC authentication tag.
* If the "key_ops" field is present, it MUST include "MAC verify"
when verifying an HMAC authentication tag.
Implementations creating and validating MAC values MUST validate that
the key type, key length, and algorithm are correct and appropriate
for the entities involved.
3.1.1. Security Considerations for HMAC
HMAC has proved to be resistant to attack even when used with
weakened hash algorithms. The current best known attack is to brute
force the key. This means that key size is going to be directly
related to the security of an HMAC operation.
3.2. AES Message Authentication Code (AES-CBC-MAC)
AES-CBC-MAC is the instantiation of the CBC-MAC construction (defined
in [MAC]) using AES as the block cipher. For brevity, we also use
"AES-MAC" to refer to AES-CBC-MAC. (Note that this is not the same
algorithm as AES Cipher-Based Message Authentication Code (AES-CMAC)
[RFC4493].)
AES-CBC-MAC is parameterized by the key length, the authentication
tag length, and the Initialization Vector (IV) used. For all of
these algorithms, the IV is fixed to all zeros. We provide an array
of algorithms for various key and tag lengths. The algorithms
defined in this document are found in Table 4.
+=========+=======+============+============+==================+
| Name | Value | Key Length | Tag Length | Description |
+=========+=======+============+============+==================+
| AES-MAC | 14 | 128 | 64 | AES-MAC 128-bit |
| 128/64 | | | | key, 64-bit tag |
+---------+-------+------------+------------+------------------+
| AES-MAC | 15 | 256 | 64 | AES-MAC 256-bit |
| 256/64 | | | | key, 64-bit tag |
+---------+-------+------------+------------+------------------+
| AES-MAC | 25 | 128 | 128 | AES-MAC 128-bit |
| 128/128 | | | | key, 128-bit tag |
+---------+-------+------------+------------+------------------+
| AES-MAC | 26 | 256 | 128 | AES-MAC 256-bit |
| 256/128 | | | | key, 128-bit tag |
+---------+-------+------------+------------+------------------+
Table 4: AES-MAC Algorithm Values
Keys may be obtained from either a key structure or a recipient
structure. Implementations creating and validating MAC values MUST
validate that the key type, key length, and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the AES-MAC algorithm
being used.
* If the "key_ops" field is present, it MUST include "MAC create"
when creating an AES-MAC authentication tag.
* If the "key_ops" field is present, it MUST include "MAC verify"
when verifying an AES-MAC authentication tag.
3.2.1. Security Considerations for AES-CBC-MAC
A number of attacks exist against Cipher Block Chaining Message
Authentication Code (CBC-MAC) that need to be considered.
* A single key must only be used for messages of a fixed or known
length. If this is not the case, an attacker will be able to
generate a message with a valid tag given two message and tag
pairs. This can be addressed by using different keys for messages
of different lengths. The current structure mitigates this
problem, as a specific encoding structure that includes lengths is
built and signed. (CMAC also addresses this issue.)
* In Cipher Block Chaining (CBC) mode, if the same key is used for
both encryption and authentication operations, an attacker can
produce messages with a valid authentication code.
* If the IV can be modified, then messages can be forged. This is
addressed by fixing the IV to all zeros.
4. Content Encryption Algorithms
Section 8.3 of [RFC9052] contains a generic description of content
encryption algorithms. This document defines the identifier and
usages for three content encryption algorithms.
4.1. AES-GCM
The Galois/Counter Mode (GCM) mode is a generic AEAD block cipher
mode defined in [AES-GCM]. The GCM mode is combined with the AES
block encryption algorithm to define an AEAD cipher.
The GCM mode is parameterized by the size of the authentication tag
and the size of the nonce. This document fixes the size of the nonce
at 96 bits. The size of the authentication tag is limited to a small
set of values. For this document, however, the size of the
authentication tag is fixed at 128 bits.
The set of algorithms defined in this document is in Table 5.
+=========+=======+==========================================+
| Name | Value | Description |
+=========+=======+==========================================+
| A128GCM | 1 | AES-GCM mode w/ 128-bit key, 128-bit tag |
+---------+-------+------------------------------------------+
| A192GCM | 2 | AES-GCM mode w/ 192-bit key, 128-bit tag |
+---------+-------+------------------------------------------+
| A256GCM | 3 | AES-GCM mode w/ 256-bit key, 128-bit tag |
+---------+-------+------------------------------------------+
Table 5: Algorithm Values for AES-GCM
Keys may be obtained from either a key structure or a recipient
structure. Implementations that are encrypting or decrypting MUST
validate that the key type, key length, and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the AES-GCM algorithm
being used.
* If the "key_ops" field is present, it MUST include "encrypt" or
"wrap key" when encrypting.
* If the "key_ops" field is present, it MUST include "decrypt" or
"unwrap key" when decrypting.
4.1.1. Security Considerations for AES-GCM
When using AES-GCM, the following restrictions MUST be enforced:
* The key and nonce pair MUST be unique for every message encrypted.
* The total number of messages encrypted for a single key MUST NOT
exceed 2^32 [SP800-38D]. An explicit check is required only in
environments where it is expected that this limit might be
exceeded.
* [RFC8446] contains an analysis on the use of AES-CGM for its
environment. Based on that recommendation, one should restrict
the number of messages encrypted to 2^24.5.
* A more recent analysis in [ROBUST] indicates that the number of
failed decryptions needs to be taken into account as part of
determining when a key rollover is to be done. Following the
recommendation in DTLS (Section 4.5.3 of [RFC9147]), the number of
failed message decryptions should be limited to 2^36.
Consideration was given to supporting smaller tag values; the
constrained community would desire tag sizes in the 64-bit range.
Such use drastically changes both the maximum message size (generally
not an issue) and the number of times that a key can be used. Given
that Counter with CBC-MAC (CCM) is the usual mode for constrained
environments, restricted modes are not supported.
4.2. AES-CCM
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 an AEAD cipher that is commonly used in
constrained devices.
The CCM mode has two parameter choices. The first choice is M, the
size of the authentication field. The choice of the value for M
involves a trade-off between message growth (from the tag) and the
probability that an attacker can undetectably modify a message. The
second choice is L, the size of the length field. This value
requires a trade-off between the maximum message size and the size of
the nonce.
It is unfortunate that the specification for CCM specified L and M as
a count of bytes rather than a count of bits. This leads to possible
misunderstandings where AES-CCM-8 is frequently used to refer to a
version of CCM mode where the size of the authentication is 64 bits
and not 8 bits. In most cryptographic algorithm specifications,
these values have traditionally been specified as bit counts rather
than byte counts. This document will follow the convention of using
bit counts so that it is easier to compare the different algorithms
presented in this document.
We define a matrix of algorithms in this document over the values of
L and M. Constrained devices are usually operating in situations
where they use short messages and want to avoid doing recipient-
specific cryptographic operations. This favors smaller values of
both L and M. Less-constrained devices will want to be able to use
larger messages and are more willing to generate new keys for every
operation. This favors larger values of L and M.
The following values are used for L:
16 bits (2): This limits messages to 2^16 bytes (64 KiB) in length.
This is sufficiently long for messages in the constrained world.
The nonce length is 13 bytes allowing for 2^104 possible values of
the nonce without repeating.
64 bits (8): This limits messages to 2^64 bytes in length. The
nonce length is 7 bytes, allowing for 2^56 possible values of the
nonce without repeating.
The following values are used for M:
64 bits (8): This produces a 64-bit authentication tag. This
implies that there is a 1 in 2^64 chance that a modified message
will authenticate.
128 bits (16): This produces a 128-bit authentication tag. This
implies that there is a 1 in 2^128 chance that a modified message
will authenticate.
+====================+=======+====+=====+========+===============+
| Name | Value | L | M | Key | Description |
| | | | | Length | |
+====================+=======+====+=====+========+===============+
| AES-CCM-16-64-128 | 10 | 16 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 13-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-16-64-256 | 11 | 16 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 13-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-64-64-128 | 12 | 64 | 64 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 7-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-64-64-256 | 13 | 64 | 64 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 64-bit tag, |
| | | | | | 7-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-16-128-128 | 30 | 16 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-16-128-256 | 31 | 16 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 13-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-64-128-128 | 32 | 64 | 128 | 128 | AES-CCM mode |
| | | | | | 128-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 7-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
| AES-CCM-64-128-256 | 33 | 64 | 128 | 256 | AES-CCM mode |
| | | | | | 256-bit key, |
| | | | | | 128-bit tag, |
| | | | | | 7-byte nonce |
+--------------------+-------+----+-----+--------+---------------+
Table 6: Algorithm Values for AES-CCM
Keys may be obtained from either a key structure or a recipient
structure. Implementations that are encrypting or decrypting MUST
validate that the key type, key length, and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the AES-CCM algorithm
being used.
* If the "key_ops" field is present, it MUST include "encrypt" or
"wrap key" when encrypting.
* If the "key_ops" field is present, it MUST include "decrypt" or
"unwrap key" when decrypting.
4.2.1. Security Considerations for AES-CCM
When using AES-CCM, the following restrictions MUST be enforced:
* The key and nonce pair MUST be unique for every message encrypted.
Note that the value of L influences the number of unique nonces.
* The total number of times the AES block cipher is used MUST NOT
exceed 2^61 operations. This limit is the sum of times the block
cipher is used in computing the MAC value and performing stream
encryption operations. An explicit check is required only in
environments where it is expected that this limit might be
exceeded.
* [RFC9147] contains an analysis on the use of AES-CCM for its
environment. Based on that recommendation, one should restrict
the number of messages encrypted to 2^23.
* In addition to the number of messages successfully decrypted, the
number of failed decryptions needs to be tracked as well.
Following the recommendation in DTLS (Section 4.5.3 of [RFC9147]),
the number of failed message decryptions should be limited to
2^23.5. If one is using the 64-bit tag, then the limits are
significantly smaller if one wants to keep the same integrity
limits. A protocol recommending this needs to analyze what level
of integrity is acceptable for the smaller tag size. It may be
that, to keep the desired level of integrity, one needs to rekey
as often as every 2^7 messages.
[RFC3610] additionally calls out one other consideration of note. It
is possible to do a precomputation attack against the algorithm in
cases where portions of the plaintext are 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.
4.3. ChaCha20 and Poly1305
ChaCha20 and Poly1305 combined together is an AEAD mode that is
defined in [RFC8439]. This is an algorithm defined using a cipher
that is not AES and thus would not suffer from any future weaknesses
found in AES. These cryptographic functions are designed to be fast
in software-only implementations.
The ChaCha20/Poly1305 AEAD construction defined in [RFC8439] has no
parameterization. It takes as inputs a 256-bit key and a 96-bit
nonce, as well as the plaintext and additional data, and produces the
ciphertext as an output. We define one algorithm identifier for this
algorithm in Table 7.
+===================+=======+==========================+
| Name | Value | Description |
+===================+=======+==========================+
| ChaCha20/Poly1305 | 24 | ChaCha20/Poly1305 w/ |
| | | 256-bit key, 128-bit tag |
+-------------------+-------+--------------------------+
Table 7: Algorithm Value for ChaCha20/Poly1305
Keys may be obtained from either a key structure or a recipient
structure. Implementations that are encrypting or decrypting MUST
validate that the key type, key length, and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the ChaCha20/Poly1305
algorithm being used.
* If the "key_ops" field is present, it MUST include "encrypt" or
"wrap key" when encrypting.
* If the "key_ops" field is present, it MUST include "decrypt" or
"unwrap key" when decrypting.
4.3.1. Security Considerations for ChaCha20/Poly1305
The key and nonce values MUST be a unique pair for every invocation
of the algorithm. Nonce counters are considered to be an acceptable
way of ensuring that they are unique.
A more recent analysis in [ROBUST] indicates that the number of
failed decryptions needs to be taken into account as part of
determining when a key rollover is to be done. Following the
recommendation in DTLS (Section 4.5.3 of [RFC9147]), the number of
failed message decryptions should be limited to 2^36.
[RFC8446] notes that the (64-bit) record sequence number would wrap
before the safety limit is reached for ChaCha20/Poly1305. COSE
implementations should not send more than 2^64 messages encrypted
using a single ChaCha20/Poly1305 key.
5. Key Derivation Functions (KDFs)
Section 8.4 of [RFC9052] contains a generic description of key
derivation functions. This document defines a single context
structure and a single KDF. These elements are used for all of the
recipient algorithms defined in this document that require a KDF
process. These algorithms are defined in Sections 6.1.2, 6.3.1, and
6.4.1.
5.1. HMAC-Based Extract-and-Expand Key Derivation Function (HKDF)
The HKDF key derivation algorithm is defined in [RFC5869] and [HKDF].
The HKDF algorithm takes these inputs:
secret: A shared value that is secret. Secrets may be either
previously shared or derived from operations like a Diffie-Hellman
(DH) key agreement.
salt: An optional value that is used to change the generation
process. The salt value can be either public or private. If the
salt is public and carried in the message, then the "salt"
algorithm header parameter defined in Table 9 is used. While
[RFC5869] suggests that the length of the salt be the same as the
length of the underlying hash value, any positive salt length will
improve the security, as different key values will be generated.
This parameter is protected by being included in the key
computation and does not need to be separately authenticated. The
salt value does not need to be unique for every message sent.
length: The number of bytes of output that need to be generated.
context information: Information that describes the context in which
the resulting value will be used. Making this information
specific to the context in which the material is going to be used
ensures that the resulting material will always be tied to that
usage. The context structure defined in Section 5.2 is used by
the KDFs in this document.
PRF: The underlying pseudorandom function to be used in the HKDF
algorithm. The PRF is encoded into the HKDF algorithm selection.
HKDF is defined to use HMAC as the underlying PRF. However, it is
possible to use other functions in the same construct to provide a
different KDF that is more appropriate in the constrained world.
Specifically, one can use AES-CBC-MAC as the PRF for the expand step,
but not for the extract step. When using a good random shared secret
of the correct length, the extract step can be skipped. For the AES
algorithm versions, the extract step is always skipped.
The extract step cannot be skipped if the secret is not uniformly
random -- for example, if it is the result of an ECDH key agreement
step. This implies that the AES HKDF version cannot be used with
ECDH. If the extract step is skipped, the "salt" value is not used
as part of the HKDF functionality.
The algorithms defined in this document are found in Table 8.
+==============+===================+========================+
| Name | PRF | Description |
+==============+===================+========================+
| HKDF SHA-256 | HMAC with SHA-256 | HKDF using HMAC |
| | | SHA-256 as the PRF |
+--------------+-------------------+------------------------+
| HKDF SHA-512 | HMAC with SHA-512 | HKDF using HMAC |
| | | SHA-512 as the PRF |
+--------------+-------------------+------------------------+
| HKDF AES- | AES-CBC-MAC-128 | HKDF using AES-MAC as |
| MAC-128 | | the PRF w/ 128-bit key |
+--------------+-------------------+------------------------+
| HKDF AES- | AES-CBC-MAC-256 | HKDF using AES-MAC as |
| MAC-256 | | the PRF w/ 256-bit key |
+--------------+-------------------+------------------------+
Table 8: HKDF Algorithms
+======+=======+======+============================+=============+
| Name | Label | Type | Algorithm | Description |
+======+=======+======+============================+=============+
| salt | -20 | bstr | direct+HKDF-SHA-256, | Random salt |
| | | | direct+HKDF-SHA-512, | |
| | | | direct+HKDF-AES-128, | |
| | | | direct+HKDF-AES-256, ECDH- | |
| | | | ES+HKDF-256, ECDH-ES+HKDF- | |
| | | | 512, ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, ECDH- | |
| | | | ES+A128KW, ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, ECDH- | |
| | | | SS+A128KW, ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+------+-------+------+----------------------------+-------------+
Table 9: HKDF Algorithm Parameters
5.2. 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 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 encoding used in JSON Object Signing
and Encryption (JOSE) (Section 4.6.2 of [RFC7518]).
The context information structure refers to PartyU and PartyV as the
two parties that are doing the key derivation. Unless the
application protocol defines differently, we assign PartyU to the
entity that is creating the message and PartyV to the entity that is
receiving the message. By defining this association, different keys
will be derived for each direction, as the context information is
different in each direction.
The context structure is built from information that is known to both
entities. This information can be obtained from a variety of
sources:
* Fields can be defined by the application. This is commonly used
to assign fixed names to parties, but it can be used for other
items such as nonces.
* Fields can be defined by usage of the output. Examples of this
are the algorithm and key size that are being generated.
* Fields can be defined by parameters from the message. We define a
set of header parameters in Table 10 that can be used to carry the
values associated with the context structure. Examples of this
are identities and nonce values. These header parameters are
designed to be placed in the unprotected bucket of the recipient
structure; they do not need to be in the protected bucket, since
they are already included in the cryptographic computation by
virtue of being included in the context structure.
+==========+=======+======+===========================+=============+
| Name | Label | Type | Algorithm | Description |
+==========+=======+======+===========================+=============+
| PartyU | -21 | bstr | direct+HKDF-SHA-256, | PartyU |
| identity | | | direct+HKDF-SHA-512, | identity |
| | | | direct+HKDF-AES-128, | information |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, | |
| | | | ECDH-ES+HKDF-512, | |
| | | | ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, | |
| | | | ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, | |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
| PartyU | -22 | bstr | direct+HKDF-SHA-256, | PartyU |
| nonce | | / | direct+HKDF-SHA-512, | provided |
| | | int | direct+HKDF-AES-128, | nonce |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, | |
| | | | ECDH-ES+HKDF-512, | |
| | | | ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, | |
| | | | ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, | |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
| PartyU | -23 | bstr | direct+HKDF-SHA-256, | PartyU |
| other | | | direct+HKDF-SHA-512, | other |
| | | | direct+HKDF-AES-128, | provided |
| | | | direct+HKDF-AES-256, | information |
| | | | ECDH-ES+HKDF-256, | |
| | | | ECDH-ES+HKDF-512, | |
| | | | ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, | |
| | | | ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, | |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
| PartyV | -24 | bstr | direct+HKDF-SHA-256, | PartyV |
| identity | | | direct+HKDF-SHA-512, | identity |
| | | | direct+HKDF-AES-128, | information |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, | |
| | | | ECDH-ES+HKDF-512, | |
| | | | ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, | |
| | | | ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, | |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
| PartyV | -25 | bstr | direct+HKDF-SHA-256, | PartyV |
| nonce | | / | direct+HKDF-SHA-512, | provided |
| | | int | direct+HKDF-AES-128, | nonce |
| | | | direct+HKDF-AES-256, | |
| | | | ECDH-ES+HKDF-256, | |
| | | | ECDH-ES+HKDF-512, | |
| | | | ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, | |
| | | | ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, | |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
| PartyV | -26 | bstr | direct+HKDF-SHA-256, | PartyV |
| other | | | direct+HKDF-SHA-512, | other |
| | | | direct+HKDF-AES-128, | provided |
| | | | direct+HKDF-AES-256, | information |
| | | | ECDH-ES+HKDF-256, | |
| | | | ECDH-ES+HKDF-512, | |
| | | | ECDH-SS+HKDF-256, | |
| | | | ECDH-SS+HKDF-512, | |
| | | | ECDH-ES+A128KW, | |
| | | | ECDH-ES+A192KW, | |
| | | | ECDH-ES+A256KW, | |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH-SS+A192KW, | |
| | | | ECDH-SS+A256KW | |
+----------+-------+------+---------------------------+-------------+
Table 10: Context Algorithm Parameters
We define a CBOR object to hold the context information. This object
is referred to as COSE_KDF_Context. The object is based on 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 normally is either a key wrap
algorithm identifier or a content encryption algorithm identifier.
The values are from the "COSE Algorithms" registry. This field is
required to be present. The field exists in the context
information so that a different key is generated for each
algorithm even if all of the other context information is the
same. In practice, this means if algorithm A is broken and thus
finding the key is relatively easy, the key derived for algorithm
B will not be the same as the key derived for algorithm A.
PartyUInfo: This field holds information about PartyU. The
PartyUInfo is encoded as a CBOR array. The elements of PartyUInfo
are encoded in the order presented below. The elements of the
PartyUInfo array are:
identity: This contains the identity information for PartyU. The
identities can be assigned in one of two manners. First, 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 it sends or receives. The second
way for a protocol to assign identities is to use a name based
on a naming system (i.e., DNS or 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 is set
to nil in that case.
nonce: This contains a nonce value. The nonce can be either
implicit from the protocol or carried as a value in the
unprotected header bucket.
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 its value
obtained in a different manner.
This option does not need to be specified; if not needed, it is
set to nil.
other: This contains other information that is defined by the
protocol. This option does not need to be specified; if not
needed, it is set to nil.
PartyVInfo: This field holds information about PartyV. The content
of the structure is the same as for the PartyUInfo but for PartyV.
SuppPubInfo: This field contains public information that is mutually
known to both parties, and is encoded as a CBOR array.
keyDataLength: This is set to the number of bits of the desired
output value. This practice means if algorithm A can use two
different key lengths, the key derived for the longer key size
will not contain the key for the shorter key size as a prefix.
protected: This field contains the protected parameter field. If
there are no elements in the "protected" field, then use a
zero-length bstr.
other: This field is for free-form data defined by the
application. For example, an application could define two
different byte strings to be placed here to generate different
keys for a data stream versus 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 included.
SuppPrivInfo: This field contains private information that is
mutually known private information. An example of this
information would be a pre-existing shared secret. (This could,
for example, be used in combination with an ECDH key agreement to
provide a secondary proof of identity.) 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
included.
The following CDDL fragment corresponds to the text above.
PartyInfo = (
identity : bstr / nil,
nonce : bstr / int / nil,
other : bstr / nil
)
COSE_KDF_Context = [
AlgorithmID : int / tstr,
PartyUInfo : [ PartyInfo ],
PartyVInfo : [ PartyInfo ],
SuppPubInfo : [
keyDataLength : uint,
protected : empty_or_serialized_map,
? other : bstr
],
? SuppPrivInfo : bstr
]
6. Content Key Distribution Methods
Section 8.5 of [RFC9052] contains a generic description of content
key distribution methods. This document defines the identifiers and
usage for a number of content key distribution methods.
6.1. Direct Encryption
A direct encryption algorithm is defined in Section 8.5.1 of
[RFC9052]. Information about how to fill in the COSE_Recipient
structure is detailed there.
6.1.1. Direct Key
This recipient algorithm is the simplest; the identified key is
directly used as the key for the next layer down in the message.
There are no algorithm parameters defined for this algorithm. The
algorithm identifier value is assigned in Table 11.
When this algorithm is used, the "protected" field MUST be zero
length. The key type MUST be "Symmetric".
+========+=======+============================================+
| Name | Value | Description |
+========+=======+============================================+
| direct | -6 | Direct use of content encryption key (CEK) |
+--------+-------+--------------------------------------------+
Table 11: Direct Key
6.1.1.1. Security Considerations for Direct Key
This recipient algorithm has several potential problems that need to
be considered:
* These keys need to have some method of being regularly updated
over time. All of the content encryption algorithms specified in
this document have limits on how many times a key can be used
without significant loss of security.
* These keys need to be dedicated to a single algorithm. There have
been a number of attacks developed over time when a single key is
used for multiple different algorithms. One example of this is
the use of a single key for both the CBC encryption mode and the
CBC-MAC authentication mode.
* Breaking one message means all messages are broken. If an
adversary succeeds in determining the key for a single message,
then the key for all messages is also determined.
6.1.2. Direct Key with KDF
These recipient algorithms take a common shared secret between the
two parties and apply the HKDF function (Section 5.1), using the
context structure defined in Section 5.2 to transform the shared
secret into the CEK. The "protected" field can be of nonzero length.
Either the "salt" parameter for HKDF (Table 9) or the "PartyU nonce"
parameter for the context structure (Table 10) MUST be present (both
can be present if desired). The value in the "salt"/"nonce"
parameter can be generated either randomly or deterministically. The
requirement is that it be a unique value for the shared secret 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 output
of the hash function underlying HKDF. While there is no way to
guarantee that it will be unique, there is a high probability that it
will be unique. If the salt/nonce value is generated
deterministically, it can be guaranteed to be unique, and thus there
is no length requirement.
A new IV must be used for each message if the same key is used. The
IV can be modified in a predictable manner, a random manner, or an
unpredictable manner (e.g., encrypting a counter).
The IV used for a key can also be generated using the same HKDF
functionality used to generate the key. If HKDF is used for
generating the IV, the algorithm identifier is set to 34 ("IV-
GENERATION").
The set of algorithms defined in this document can be found in
Table 12.
+=====================+=======+==============+=====================+
| Name | Value | KDF | Description |
+=====================+=======+==============+=====================+
| direct+HKDF-SHA-256 | -10 | HKDF SHA-256 | Shared secret w/ |
| | | | HKDF and SHA-256 |
+---------------------+-------+--------------+---------------------+
| direct+HKDF-SHA-512 | -11 | HKDF SHA-512 | Shared secret w/ |
| | | | HKDF and SHA-512 |
+---------------------+-------+--------------+---------------------+
| direct+HKDF-AES-128 | -12 | HKDF AES- | Shared secret w/ |
| | | MAC-128 | AES-MAC 128-bit key |
+---------------------+-------+--------------+---------------------+
| direct+HKDF-AES-256 | -13 | HKDF AES- | Shared secret w/ |
| | | MAC-256 | AES-MAC 256-bit key |
+---------------------+-------+--------------+---------------------+
Table 12: Direct Key with KDF
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the algorithm being
used.
* If the "key_ops" field is present, it MUST include "derive key" or
"derive bits".
6.1.2.1. Security Considerations for Direct Key with KDF
The shared secret needs to have some method of being regularly
updated over time. The shared secret forms the basis of trust.
Although not used directly, it should still be subject to scheduled
rotation.
These methods do not provide for perfect forward secrecy, as the same
shared secret is used for all of the keys generated; however, if the
key for any single message is discovered, only the message or series
of messages using that derived key are compromised. A new key
derivation step will generate a new key that requires the same amount
of work to get the key.
6.2. Key Wrap
Key wrap is defined in Section 8.5.2 of [RFC9052]. Information about
how to fill in the COSE_Recipient structure is detailed there.
6.2.1. AES Key Wrap
The AES Key Wrap algorithm is defined in [RFC3394]. This algorithm
uses an AES key to wrap a value that is a multiple of 64 bits. As
such, it can be used to wrap a key for any of the content encryption
algorithms defined in this document. The algorithm requires a single
fixed parameter, the initial value. This is fixed to the value
specified in Section 2.2.3.1 of [RFC3394]. There are no public key
parameters that vary on a per-invocation basis. The protected header
bucket MUST be empty.
Keys may be obtained from either a key structure or a recipient
structure. Implementations that are encrypting or decrypting MUST
validate that the key type, key length, and algorithm are correct and
appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "Symmetric".
* If the "alg" field is present, it MUST match the AES Key Wrap
algorithm being used.
* If the "key_ops" field is present, it MUST include "encrypt" or
"wrap key" when encrypting.
* If the "key_ops" field is present, it MUST include "decrypt" or
"unwrap key" when decrypting.
+========+=======+==========+=============================+
| 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 13: AES Key Wrap Algorithm Values
6.2.1.1. Security Considerations for AES Key Wrap
The shared secret needs to have some method of being regularly
updated over time. The shared secret is the basis of trust.
6.3. Direct Key Agreement
Direct Key Agreement is defined in Section 8.5.4 of [RFC9052].
Information about how to fill in the COSE_Recipient structure is
detailed there.
6.3.1. Direct ECDH
The mathematics for ECDH can be found in [RFC6090]. In this
document, the algorithm is extended to be used with the two curves
defined in [RFC7748].
ECDH is parameterized by the following:
Curve Type/Curve: The curve selected controls not only the size of
the shared secret, but the mathematics for computing the shared
secret. The curve selected also controls how a point in the curve
is represented and what happens for the identity points on the
curve. In this specification, we allow for a number of different
curves to be used. A set of curves is defined in Table 18.
The math used to obtain the computed secret is based on the curve
selected and not on the ECDH algorithm. For this reason, a new
algorithm does not need to be defined for each of the curves.
Computed Secret to Shared Secret: Once the computed secret is known,
the resulting value needs to be converted to a byte string to run
the KDF. The x-coordinate is used for all of the curves defined
in this document. For curves X25519 and X448, the resulting value
is used directly, as it is a byte string of a known length. For
the P-256, P-384, and P-521 curves, the x-coordinate is run
through the Integer-to-Octet-String primitive (I2OSP) function
defined in [RFC8017], using the same computation for n as is
defined in Section 2.1.
Ephemeral-Static or Static-Static: The key agreement process may be
done using either a static or an ephemeral key for the sender's
side. When using ephemeral keys, the sender MUST generate a new
ephemeral key for every key agreement operation. The ephemeral
key is placed in the "ephemeral key" parameter and MUST be present
for all algorithm identifiers that use ephemeral keys. When using
static keys, the sender MUST either generate a new random value or
create a unique value for use as a KDF input. For the KDFs used,
this means that either the "salt" parameter for HKDF (Table 9) or
the "PartyU nonce" parameter for the context structure (Table 10)
MUST be present (both can be present if desired). The value in
the parameter MUST be unique for the pair of keys being used. It
is acceptable to use a global counter that is incremented for
every Static-Static operation and use the resulting value. Care
must be taken that the counter is saved to permanent storage in a
way that avoids reuse of that counter value. When using static
keys, the static key should be identified to the recipient. The
static key can be identified by providing either the key ("static
key") or a key identifier for the static key ("static key id").
Both of these header parameters are defined in Table 15.
Key Derivation Algorithm: The result of an ECDH key agreement
process does not provide a uniformly random secret. As such, it
needs to be run through a KDF in order to produce a usable key.
Processing the secret through a KDF also allows for the
introduction of context material: how the key is going to be used
and one-time material for Static-Static key agreement. All of the
algorithms defined in this document use one of the HKDF algorithms
defined in Section 5.1 with the context structure defined in
Section 5.2.
Key Wrap Algorithm: No key wrap algorithm is used. This is
represented in Table 14 as "none". The key size for the context
structure is the content layer encryption algorithm size.
COSE does not have an Ephemeral-Ephemeral version defined. The
reason for this is that COSE is not an online protocol by itself and
thus does not have a method of establishing ephemeral secrets on both
sides. The expectation is that a protocol would establish the
secrets for both sides, and then they would be used as Static-Static
for the purposes of COSE, or that the protocol would generate a
shared secret and a direct encryption would be used.
The set of direct ECDH algorithms defined in this document is found
in Table 14.
+==========+=======+=========+==================+=====+=============+
|Name | Value | KDF | Ephemeral-Static |Key |Description |
| | | | |Wrap | |
+==========+=======+=========+==================+=====+=============+
|ECDH-ES + | -25 | HKDF -- | yes |none |ECDH ES w/ |
|HKDF-256 | | SHA-256 | | |HKDF -- |
| | | | | |generate key |
| | | | | |directly |
+----------+-------+---------+------------------+-----+-------------+
|ECDH-ES + | -26 | HKDF -- | yes |none |ECDH ES w/ |
|HKDF-512 | | SHA-512 | | |HKDF -- |
| | | | | |generate key |
| | | | | |directly |
+----------+-------+---------+------------------+-----+-------------+
|ECDH-SS + | -27 | HKDF -- | no |none |ECDH SS w/ |
|HKDF-256 | | SHA-256 | | |HKDF -- |
| | | | | |generate key |
| | | | | |directly |
+----------+-------+---------+------------------+-----+-------------+
|ECDH-SS + | -28 | HKDF -- | no |none |ECDH SS w/ |
|HKDF-512 | | SHA-512 | | |HKDF -- |
| | | | | |generate key |
| | | | | |directly |
+----------+-------+---------+------------------+-----+-------------+
Table 14: ECDH Algorithm Values
+===========+=======+==========+===================+=============+
| Name | Label | Type | Algorithm | Description |
+===========+=======+==========+===================+=============+
| ephemeral | -1 | COSE_Key | ECDH-ES+HKDF-256, | Ephemeral |
| key | | | ECDH-ES+HKDF-512, | public key |
| | | | ECDH-ES+A128KW, | for the |
| | | | ECDH-ES+A192KW, | sender |
| | | | ECDH-ES+A256KW | |
+-----------+-------+----------+-------------------+-------------+
| static | -2 | COSE_Key | ECDH-SS+HKDF-256, | Static |
| key | | | ECDH-SS+HKDF-512, | public key |
| | | | ECDH-SS+A128KW, | for the |
| | | | ECDH-SS+A192KW, | sender |
| | | | ECDH-SS+A256KW | |
+-----------+-------+----------+-------------------+-------------+
| static | -3 | bstr | ECDH-SS+HKDF-256, | Static |
| key id | | | ECDH-SS+HKDF-512, | public key |
| | | | ECDH-SS+A128KW, | identifier |
| | | | ECDH-SS+A192KW, | for the |
| | | | ECDH-SS+A256KW | sender |
+-----------+-------+----------+-------------------+-------------+
Table 15: ECDH Algorithm Parameters
This document defines these algorithms to be used with the curves
P-256, P-384, P-521, X25519, and X448. Implementations MUST verify
that the key type and curve are correct. Different curves are
restricted to different key types. Implementations MUST verify that
the curve and algorithm are appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "EC2" or "OKP".
* If the "alg" field is present, it MUST match the key agreement
algorithm being used.
* If the "key_ops" field is present, it MUST include "derive key" or
"derive bits" for the private key.
* If the "key_ops" field is present, it MUST be empty for the public
key.
6.3.1.1. Security Considerations for ECDH
There is a method of checking that points provided from external
entities are valid. For the "EC2" key format, this can be done by
checking that the x and y values form a point on the curve. For the
"OKP" format, there is no simple way to perform point validation.
Consideration was given to requiring that the public keys of both
entities be provided as part of the key derivation process (as
recommended in Section 6.1 of [RFC7748]). This was not done, because
COSE is used in a store-and-forward format rather than in online key
exchange. In order for this to be a problem, either the receiver
public key has to be chosen maliciously or the sender has to be
malicious. In either case, all security evaporates anyway.
A proof of possession of the private key associated with the public
key is recommended when a key is moved from untrusted to trusted
(either by the end user or by the entity that is responsible for
making trust statements on keys).
6.4. Key Agreement with Key Wrap
Key Agreement with Key Wrap is defined in Section 8.5.5 of [RFC9052].
Information about how to fill in the COSE_Recipient structure is
detailed there.
6.4.1. ECDH with Key Wrap
These algorithms are defined in Table 16.
ECDH with Key Agreement is parameterized by the same header
parameters as for ECDH; see Section 6.3.1, with the following
modifications:
Key Wrap Algorithm: Any of the key wrap algorithms defined in
Section 6.2 are supported. The size of the key used for the key
wrap algorithm is fed into the KDF. The set of identifiers is
found in Table 16.
+=========+=====+=========+==================+========+=============+
|Name |Value| KDF | Ephemeral-Static |Key Wrap|Description |
+=========+=====+=========+==================+========+=============+
|ECDH-ES +|-29 | HKDF -- | yes |A128KW |ECDH ES w/ |
|A128KW | | SHA-256 | | |HKDF and AES |
| | | | | |Key Wrap w/ |
| | | | | |128-bit key |
+---------+-----+---------+------------------+--------+-------------+
|ECDH-ES +|-30 | HKDF -- | yes |A192KW |ECDH ES w/ |
|A192KW | | SHA-256 | | |HKDF and AES |
| | | | | |Key Wrap w/ |
| | | | | |192-bit key |
+---------+-----+---------+------------------+--------+-------------+
|ECDH-ES +|-31 | HKDF -- | yes |A256KW |ECDH ES w/ |
|A256KW | | SHA-256 | | |HKDF and AES |
| | | | | |Key Wrap w/ |
| | | | | |256-bit key |
+---------+-----+---------+------------------+--------+-------------+
|ECDH-SS +|-32 | HKDF -- | no |A128KW |ECDH SS w/ |
|A128KW | | SHA-256 | | |HKDF and AES |
| | | | | |Key Wrap w/ |
| | | | | |128-bit key |
+---------+-----+---------+------------------+--------+-------------+
|ECDH-SS +|-33 | HKDF -- | no |A192KW |ECDH SS w/ |
|A192KW | | SHA-256 | | |HKDF and AES |
| | | | | |Key Wrap w/ |
| | | | | |192-bit key |
+---------+-----+---------+------------------+--------+-------------+
|ECDH-SS +|-34 | HKDF -- | no |A256KW |ECDH SS w/ |
|A256KW | | SHA-256 | | |HKDF and AES |
| | | | | |Key Wrap w/ |
| | | | | |256-bit key |
+---------+-----+---------+------------------+--------+-------------+
Table 16: ECDH Algorithm Values with Key Wrap
When using a COSE key for this algorithm, the following checks are
made:
* The "kty" field MUST be present, and it MUST be "EC2" or "OKP".
* If the "alg" field is present, it MUST match the key agreement
algorithm being used.
* If the "key_ops" field is present, it MUST include "derive key" or
"derive bits" for the private key.
* If the "key_ops" field is present, it MUST be empty for the public
key.
7. Key Object Parameters
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.
Private members allow individuals to archive keys. However, there
are some circumstances in which private keys may be distributed to
entities in a protocol. Examples include: entities that have poor
random number generation, centralized key creation for multicast-type
operations, and protocols in which a shared secret is used as a
bearer token for authorization purposes.
Key types are identified by the "kty" member of the COSE_Key object.
In this document, we define four values for the member:
+===========+=======+==========================+
| Name | Value | Description |
+===========+=======+==========================+
| OKP | 1 | Octet Key Pair |
+-----------+-------+--------------------------+
| EC2 | 2 | Elliptic Curve Keys w/ |
| | | x- and y-coordinate pair |
+-----------+-------+--------------------------+
| Symmetric | 4 | Symmetric Keys |
+-----------+-------+--------------------------+
| Reserved | 0 | This value is reserved |
+-----------+-------+--------------------------+
Table 17: Key Type Values
7.1. Elliptic Curve Keys
Two different key structures are defined for elliptic curve keys.
One version uses both an x-coordinate and a y-coordinate, potentially
with point compression ("EC2"). This is the conventional elliptic
curve (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 ("OKP").
Applications MUST check that the curve and the key type are
consistent and reject a key if they are not.
+=========+=======+==========+=====================================+
| Name | Value | Key Type | Description |
+=========+=======+==========+=====================================+
| P-256 | 1 | EC2 | NIST P-256, also known as secp256r1 |
+---------+-------+----------+-------------------------------------+
| P-384 | 2 | EC2 | NIST P-384, also known as secp384r1 |
+---------+-------+----------+-------------------------------------+
| P-521 | 3 | EC2 | NIST P-521, also known as secp521r1 |
+---------+-------+----------+-------------------------------------+
| X25519 | 4 | OKP | X25519 for use w/ ECDH only |
+---------+-------+----------+-------------------------------------+
| X448 | 5 | OKP | X448 for use w/ ECDH only |
+---------+-------+----------+-------------------------------------+
| Ed25519 | 6 | OKP | Ed25519 for use w/ EdDSA only |
+---------+-------+----------+-------------------------------------+
| Ed448 | 7 | OKP | Ed448 for use w/ EdDSA only |
+---------+-------+----------+-------------------------------------+
Table 18: Elliptic Curves
7.1.1. Double Coordinate Curves
Generally, protocols transmit elliptic-curve points as either the
x-coordinate and y-coordinate or the x-coordinate and a sign bit for
the y-coordinate. The latter encoding has not been recommended by
the IETF due to potential IPR issues. However, for operations in
constrained environments, the ability to shrink a message by not
sending the y-coordinate is potentially useful.
For EC keys with both coordinates, the "kty" member is set to 2
(EC2). The key parameters defined in this section are summarized in
Table 19. The members that are defined for this key type are:
crv: This 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: This contains the x-coordinate for the EC point. The integer
is converted to a byte string as defined in [SEC1]. Leading-
zero octets MUST be preserved.
y: This contains either the sign bit or the value of the
y-coordinate for the EC point. When encoding the value y, the
integer is converted to a byte string (as defined in [SEC1])
and encoded as a CBOR bstr. Leading-zero octets MUST be
preserved. Compressed point encoding is also supported.
Compute the sign bit as laid out in the Elliptic-Curve-Point-
to-Octet-String Conversion function of [SEC1]. If the sign bit
is zero, then encode y as a CBOR false value; otherwise, encode
y as a CBOR true value. The encoding of the infinity point is
not supported.
d: This contains the private key.
For public keys, it is REQUIRED that "crv", "x", and "y" be present
in the structure. For private keys, it is REQUIRED that "crv" and
"d" be present in the structure. For private keys, it is RECOMMENDED
that "x" and "y" also be present, but they can be recomputed from the
required elements, and omitting them saves on space.
+======+======+=======+========+=================================+
| Key | Name | Label | CBOR | Description |
| Type | | | Type | |
+======+======+=======+========+=================================+
| 2 | crv | -1 | int / | EC identifier -- Taken from the |
| | | | tstr | "COSE Elliptic Curves" registry |
+------+------+-------+--------+---------------------------------+
| 2 | x | -2 | bstr | x-coordinate |
+------+------+-------+--------+---------------------------------+
| 2 | y | -3 | bstr / | y-coordinate |
| | | | bool | |
+------+------+-------+--------+---------------------------------+
| 2 | d | -4 | bstr | Private key |
+------+------+-------+--------+---------------------------------+
Table 19: EC Key Parameters
7.2. Octet Key Pair
A new key type is defined for Octet Key Pairs (OKPs). Do not assume
that keys using this type are elliptic curves. This key type could
be used for other curve types (for example, mathematics based on
hyper-elliptic surfaces).
The key parameters defined in this section are summarized in
Table 20. The members that are defined for this key type are:
crv: This 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: This contains the public key. The byte string contains the
public key as defined by the algorithm. (For X25519,
internally it is a little-endian integer.)
d: This 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. For private keys, 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 | Label | Type | Description |
| | Type | | | |
+======+==========+=======+=======+=================================+
| crv | 1 | -1 | int / | EC identifier -- Taken from the |
| | | | tstr | "COSE Elliptic Curves" registry |
+------+----------+-------+-------+---------------------------------+
| x | 1 | -2 | bstr | Public Key |
+------+----------+-------+-------+---------------------------------+
| d | 1 | -4 | bstr | Private key |
+------+----------+-------+-------+---------------------------------+
Table 20: Octet Key Pair Parameters
7.3. 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 4 ("Symmetric"). The
member that is defined for this key type is:
k: This contains the value of the key.
This key structure does not have a form that contains only public
members. As it is expected that this key structure is going to be
transmitted, care must be taken that it is never transmitted
accidentally or insecurely. For symmetric keys, it is REQUIRED that
"k" be present in the structure.
+======+==========+=======+======+=============+
| Name | Key Type | Label | Type | Description |
+======+==========+=======+======+=============+
| k | 4 | -1 | bstr | Key Value |
+------+----------+-------+------+-------------+
Table 21: Symmetric Key Parameters
8. COSE Capabilities
The capabilities of an algorithm or key type need to be specified in
some situations. This has a counterpart in the S/MIME
specifications, where SMIMECapabilities is defined in Section 2.5.2
of [RFC8551]. This document defines the same concept for COSE.
The algorithm identifier is not included in the capabilities data, as
it should be encoded elsewhere in the message. The key type
identifier is included in the capabilities data, as it is not
expected to be encoded elsewhere.
Two different types of capabilities are defined: capabilities for
algorithms and capabilities for key type. Once defined by
registration with IANA, the list of capabilities for an algorithm or
key type is immutable. If it is later found that a new capability is
needed for a key type or algorithm, it will require that a new code
point be assigned to deal with that. As a general rule, the
capabilities are going to map to algorithm-specific header parameters
or key parameters, but they do not need to do so. An example of this
is the HSS-LMS key type capabilities defined below, where the hash
algorithm used is included.
The capability structure is an array of values; the values included
in the structure are dependent on a specific algorithm or key type.
For algorithm capabilities, the first element should always be a key
type value if applicable, but the items that are specific to a key
(for example, a curve) should not be included in the algorithm
capabilities. This means that if one wishes to enumerate all of the
capabilities for a device that implements ECDH, it requires that all
of the combinations of algorithms and key pairs be specified. The
last example of Section 8.3 provides a case where this is done by
allowing for a cross product to be specified between an array of
algorithm capabilities and key type capabilities (see the ECDH-
ES+A25KW element). For a key, the first element should be the key
type value. While this means that the key type value will be
duplicated if both an algorithm and key capability are used, the key
type is needed in order to understand the rest of the values.
8.1. Assignments for Existing Algorithms
For the current set of algorithms in the registry other than IV-
GENERATION (those in this document as well as those in [RFC8230],
[RFC8778], and [RFC9021]), the capabilities list is an array with one
element, the key type (from the "COSE Key Types" Registry). It is
expected that future registered algorithms could have zero, one, or
multiple elements.
8.2. Assignments for Existing Key Types
There are a number of pre-existing key types; the following deals
with creating the capability definition for those structures:
* OKP, EC2: The list of capabilities is:
- The key type value. (1 for OKP or 2 for EC2.)
- One curve for that key type from the "COSE Elliptic Curves"
registry.
* RSA: The list of capabilities is:
- The key type value (3).
* Symmetric: The list of capabilities is:
- The key type value (4).
* HSS-LMS: The list of capabilities is:
- The key type value (5).
- Algorithm identifier for the underlying hash function from the
"COSE Algorithms" registry.
* WalnutDSA: The list of capabilities is:
- The key type value (6).
- The N value (group and matrix size) for the key, a uint.
- The q value (finite field order) for the key, a uint.
8.3. Examples
Capabilities can be used in a key derivation process to make sure
that both sides are using the same parameters. The three examples
below show different ways that one might utilize parameters in
specifying an application protocol:
* Only an algorithm capability: This is useful if the protocol wants
to require a specific algorithm, such as ES256, but it is agnostic
about which curve is being used. This requires that the algorithm
identifier be specified in the protocol. See the first example.
* Only a key type capability: This is useful if the protocol wants
to require a specific key type and curve, such as P-256, but will
accept any algorithm using that curve (e.g., both ECDSA and ECDH).
See the second example.
* Both algorithm and key type capabilities: This is used if the
protocol needs to nail down all of the options surrounding an
algorithm -- e.g., EdDSA with the curve Ed25519. As with the
first example, the algorithm identifier needs to be specified in
the protocol. See the third example, which just concatenates the
two capabilities together.
Algorithm ES256
0x8102 / [2 \ EC2 \ ] /
Key type EC2 with P-256 curve:
0x820201 / [2 \ EC2 \, 1 \ P-256 \] /
ECDH-ES + A256KW with an X25519 curve:
0x8101820104 / [1 \ OKP \],[1 \ OKP \, 4 \ X25519 \] /
The capabilities can also be used by an entity to advertise what it
is capable of doing. The decoded example below is one of many
encodings that could be used for that purpose. Each array element
includes three fields: the algorithm identifier, one or more
algorithm capabilities, and one or more key type capabilities.
[
[-8 / EdDSA /,
[1 / OKP key type /],
[
[1 / OKP /, 6 / Ed25519 / ],
[1 /OKP/, 7 /Ed448 /]
]
],
[-7 / ECDSA with SHA-256/,
[2 /EC2 key type/],
[
[2 /EC2/, 1 /P-256/],
[2 /EC2/, 3 /P-521/]
]
],
[ -31 / ECDH-ES+A256KW/,
[
[ 2 /EC2/],
[1 /OKP/ ]
],
[
[2 /EC2/, 1 /P-256/],
[1 /OKP/, 4 / X25519/ ]
]
],
[ 1 / A128GCM /,
[ 4 / Symmetric / ],
[ 4 / Symmetric /]
]
]
Examining the above:
* The first element indicates that the entity supports EdDSA with
curves Ed25519 and Ed448.
* The second element indicates that the entity supports ECDSA with
SHA-256 with curves P-256 and P-521.
* The third element indicates that the entity supports Ephemeral-
Static ECDH using AES256 key wrap. The entity can support the
P-256 curve with an EC2 key type and the X25519 curve with an OKP
key type.
* The last element indicates that the entity supports AES-GCM of 128
bits for content encryption.
The entity does not advertise that it supports any MAC algorithms.
9. CBOR Encoding Restrictions
This document limits the restrictions it imposes on how the CBOR
Encoder needs to work. The new encoding restrictions are aligned
with the Core Deterministic Encoding Requirements specified in
Section 4.2.1 of RFC 8949 [STD94]. It has been narrowed down to the
following restrictions:
* The restriction applies to the encoding of the COSE_KDF_Context.
* Encoding MUST be done using definite lengths, and the length of
the (encoded) argument MUST be the minimum possible length. This
means that the integer 1 is encoded as "0x01" and not "0x1801".
* Applications MUST NOT generate messages with the same label used
twice as a key in a single map. Applications MUST NOT parse and
process messages with the same label used twice as a key in a
single map. Applications can enforce the parse-and-process
requirement by using parsers that will fail the parse step or by
using parsers that will pass all keys to the application, and the
application can perform the check for duplicate keys.
10. IANA Considerations
IANA has updated all COSE registries except for "COSE Header
Parameters" and "COSE Key Common Parameters" to point to this
document instead of [RFC8152].
10.1. Changes to the "COSE Key Types" Registry
IANA has added a new column in the "COSE Key Types" registry. The
new column is labeled "Capabilities" and has been populated according
to the entries in Table 22.
+=======+===========+============================+
| Value | Name | Capabilities |
+=======+===========+============================+
| 1 | OKP | [kty(1), crv] |
+-------+-----------+----------------------------+
| 2 | EC2 | [kty(2), crv] |
+-------+-----------+----------------------------+
| 3 | RSA | [kty(3)] |
+-------+-----------+----------------------------+
| 4 | Symmetric | [kty(4)] |
+-------+-----------+----------------------------+
| 5 | HSS-LMS | [kty(5), hash algorithm] |
+-------+-----------+----------------------------+
| 6 | WalnutDSA | [kty(6), N value, q value] |
+-------+-----------+----------------------------+
Table 22: Key Type Capabilities
10.2. Changes to the "COSE Algorithms" Registry
IANA has added a new column in the "COSE Algorithms" registry. The
new column is labeled "Capabilities" and has been populated with
"[kty]" for all current, nonprovisional registrations.
IANA has updated the Reference column in the "COSE Algorithms"
registry to include this document as a reference for all rows where
it was not already present.
IANA has added a new row to the "COSE Algorithms" registry.
+===============+=======+===============+===========+=============+
| Name | Value | Description | Reference | Recommended |
+===============+=======+===============+===========+=============+
| IV-GENERATION | 34 | For doing IV | RFC 9053 | No |
| | | generation | | |
| | | for symmetric | | |
| | | algorithms. | | |
+---------------+-------+---------------+-----------+-------------+
Table 23: New entry in the COSE Algorithms registry
The Capabilities column for this registration is to be empty.
10.3. Changes to the "COSE Key Type Parameters" Registry
IANA has modified the description to "Public Key" for the line with
"Key Type" of 1 and the "Name" of "x". See Table 20, which has been
modified with this change.
10.4. Expert Review Instructions
All of the IANA registries established by [RFC8152] are, at least in
part, defined as Expert Review [RFC8126]. This section gives some
general guidelines for what the experts should be looking for, but
they are being designated as experts for a reason, so they should be
given substantial latitude.
Expert reviewers should take the following into consideration:
* Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate an existing registration
and that the code point is likely to be used in deployments. The
ranges tagged as private use are intended for testing purposes and
closed environments; code points in other ranges should not be
assigned for testing.
* Standards Track or BCP RFCs are required to register a code point
in the Standards Action range. Specifications should exist for
Specification Required ranges, but early assignment before an RFC
is available is considered to be permissible. Specifications are
needed for the first-come, first-served range if the points are
expected to be used outside of closed environments in an
interoperable way. When specifications are not provided, the
description provided needs to have sufficient information to
identify what the point is being used for.
* Experts should take into account the expected usage of fields when
approving code point assignment. The fact that the Standards
Action range is only available to Standards Track documents does
not mean that a Standards Track document cannot have points
assigned outside of that range. The length of the encoded value
should be weighed against how many code points of that length are
left and the size of device it will be used on.
* When algorithms are registered, vanity registrations should be
discouraged. One way to do this is to require registrations to
provide additional documentation on security analysis of the
algorithm. Another thing that should be considered is requesting
an opinion on the algorithm from the Crypto Forum Research Group
(CFRG). Algorithms are expected to meet the security requirements
of the community and the requirements of the message structures in
order to be suitable for registration.
11. Security Considerations
There are a number of security considerations that need to be taken
into account by implementers of this specification. The security
considerations that are specific to an individual algorithm are
placed next to the description of the algorithm. While some
considerations have been highlighted here, additional considerations
may be found in the documents listed in the references.
Implementations need to protect the private key material for all
individuals. Some cases in this document need to be highlighted with
regard to this issue.
* Use of the same key for two different algorithms can leak
information about the key. It is therefore recommended that keys
be restricted to a single algorithm.
* Use of "direct" as a recipient algorithm combined with a second
recipient algorithm exposes the direct key to the second
recipient; Section 8.5 of [RFC9052] forbids combining "direct"
recipient algorithms with other modes.
* Several of the algorithms in this document have limits on the
number of times that a key can be used without leaking information
about the key.
The use of ECDH and direct plus KDF (with no key wrap) will not
directly lead to the private key being leaked; the one-way function
of the KDF will prevent that. There is, however, a different issue
that needs to be addressed. Having two recipients requires that the
CEK be shared between two recipients. The second recipient therefore
has a CEK that was derived from material that can be used for the
weak proof of origin. The second recipient could create a message
using the same CEK and send it to the first recipient; the first
recipient would, for either Static-Static ECDH or direct plus KDF,
make an assumption that the CEK could be used for proof of origin,
even though it is from the wrong entity. If the key wrap step is
added, then no proof of origin is implied and this is not an issue.
Although it has been mentioned before, it bears repeating that the
use of a single key for multiple algorithms has been demonstrated in
some cases to leak information about a key, providing the opportunity
for attackers to forge integrity tags or gain information about
encrypted content. Binding a key to a single algorithm prevents
these problems. Key creators and key consumers are strongly
encouraged to not only create new keys for each different algorithm,
but to include that selection of algorithm in any distribution of key
material and strictly enforce the matching of algorithms in the key
structure to algorithms in the message structure. In addition to
checking that algorithms are correct, the key form needs to be
checked as well. Do not use an "EC2" key where an "OKP" key is
expected.
Before using a key for transmission, or before acting on information
received, a trust decision on a key needs to be made. Is the data or
action something that the entity associated with the key has a right
to see or a right to request? A number of factors are associated
with this trust decision. Some highlighted here are:
* What are the permissions associated with the key owner?
* Is the cryptographic algorithm acceptable in the current context?
* Have the restrictions associated with the key, such as algorithm
or freshness, been checked, and are they correct?
* Is the request something that is reasonable, given the current
state of the application?
* Have any security considerations that are part of the message been
enforced (as specified by the application or "crit" header
parameter)?
There are a large number of algorithms presented in this document
that use nonce values. For all of the nonces defined in this
document, there is some type of restriction on the nonce being a
unique value for either a key or some other conditions. In all of
these cases, there is no known requirement on the nonce being both
unique and unpredictable; under these circumstances, it's reasonable
to use a counter for creation of the nonce. In cases where one wants
the pattern of the nonce to be unpredictable as well as unique, one
can use a key created for that purpose and encrypt the counter to
produce the nonce value.
One area that has been getting exposure is traffic analysis of
encrypted messages based on the length of the message. This
specification does not provide a uniform method for providing padding
as part of the message structure. An observer can distinguish
between two different messages (for example, "YES" and "NO") based on
the length for all of the content encryption algorithms that are
defined in this document. This means that it is up to the
applications to document how content padding is to be done in order
to prevent or discourage such analysis. (For example, the text
strings could be defined as "YES" and "NO ".)
The analysis done in [RFC9147] is based on the number of records that
are sent. This should map well to the number of messages sent when
using COSE, so that analysis should hold here as well, under the
assumption that the COSE messages are roughly the same size as DTLS
records. It needs to be noted that the limits are based on the
number of messages, but QUIC and DTLS are always pairwise-based
endpoints. In contrast, [OSCORE-GROUPCOMM] uses COSE in a group
communication scenario. Under these circumstances, it may be that no
one single entity will see all of the messages that are encrypted,
and therefore no single entity can trigger the rekey operation.
12. References
12.1. Normative References
[AES-GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D,
November 2007, <
https://csrc.nist.gov/publications/
nistpubs/800-38D/SP-800-38D.pdf>.
[DSS] National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", FIPS PUB 186-4,
DOI 10.6028/NIST.FIPS.186-4, July 2013,
<
https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-4.pdf>.
[MAC] Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
of Applied Cryptography", CRC Press, Boca Raton, 1996,
<
https://cacr.uwaterloo.ca/hac/>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<
https://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<
https://www.rfc-editor.org/info/rfc2119>.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
September 2002, <
https://www.rfc-editor.org/info/rfc3394>.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, <
https://www.rfc-editor.org/info/rfc3610>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<
https://www.rfc-editor.org/info/rfc5869>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<
https://www.rfc-editor.org/info/rfc6090>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <
https://www.rfc-editor.org/info/rfc6979>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <
https://www.rfc-editor.org/info/rfc7748>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<
https://www.rfc-editor.org/info/rfc8017>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<
https://www.rfc-editor.org/info/rfc8032>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <
https://www.rfc-editor.org/info/rfc8174>.
[RFC8439] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
<
https://www.rfc-editor.org/info/rfc8439>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<
https://www.rfc-editor.org/info/rfc9052>.
[SEC1] Certicom Research, "SEC 1: Elliptic Curve Cryptography",
Standards for Efficient Cryptography, May 2009,
<
https://www.secg.org/sec1-v2.pdf>.
[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, December 2020,
<
https://www.rfc-editor.org/info/std94>.
12.2. Informative References
[CFRG-DET-SIGS]
Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", Work in Progress, Internet-Draft, draft-
mattsson-cfrg-det-sigs-with-noise-04, 15 February 2022,
<
https://datatracker.ietf.org/doc/html/draft-mattsson-
cfrg-det-sigs-with-noise-04>.
[COUNTERSIGN]
Schaad, J. and R. Housley, "CBOR Object Signing and
Encryption (COSE): Countersignatures", Work in Progress,
Internet-Draft, draft-ietf-cose-countersign-08, 22 August
2022, <
https://datatracker.ietf.org/doc/html/draft-ietf-
cose-countersign-08>.
[GitHub-Examples]
"GitHub Examples of COSE", commit 3221310, 3 June 2020,
<
https://github.com/cose-wg/Examples>.
[HKDF] Krawczyk, H., "Cryptographic Extraction and Key
Derivation: The HKDF Scheme", 2010,
<
https://eprint.iacr.org/2010/264.pdf>.
[OSCORE-GROUPCOMM]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group OSCORE - Secure Group Communication
for CoAP", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-groupcomm-14, 7 March 2022,
<
https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-groupcomm-14>.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, DOI 10.17487/RFC4231, December 2005,
<
https://www.rfc-editor.org/info/rfc4231>.
[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <
https://www.rfc-editor.org/info/rfc4493>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<
https://www.rfc-editor.org/info/rfc5116>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<
https://www.rfc-editor.org/info/rfc5480>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<
https://www.rfc-editor.org/info/rfc6151>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<
https://www.rfc-editor.org/info/rfc7252>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<
https://www.rfc-editor.org/info/rfc7518>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<
https://www.rfc-editor.org/info/rfc8126>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<
https://www.rfc-editor.org/info/rfc8152>.
[RFC8230] Jones, M., "Using RSA Algorithms with CBOR Object Signing
and Encryption (COSE) Messages", RFC 8230,
DOI 10.17487/RFC8230, September 2017,
<
https://www.rfc-editor.org/info/rfc8230>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<
https://www.rfc-editor.org/info/rfc8446>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <
https://www.rfc-editor.org/info/rfc8551>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <
https://www.rfc-editor.org/info/rfc8610>.
[RFC8778] Housley, R., "Use of the HSS/LMS Hash-Based Signature
Algorithm with CBOR Object Signing and Encryption (COSE)",
RFC 8778, DOI 10.17487/RFC8778, April 2020,
<
https://www.rfc-editor.org/info/rfc8778>.
[RFC9021] Atkins, D., "Use of the Walnut Digital Signature Algorithm
with CBOR Object Signing and Encryption (COSE)", RFC 9021,
DOI 10.17487/RFC9021, May 2021,
<
https://www.rfc-editor.org/info/rfc9021>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<
https://www.rfc-editor.org/info/rfc9147>.
[ROBUST] Fischlin, M., Günther, F., and C. Janson, "Robust
Channels: Handling Unreliable Networks in the Record
Layers of QUIC and DTLS", February 2020,
<
https://eprint.iacr.org/2020/718.pdf>.
[SP800-38D]
Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-38D, November 2007,
<
https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
nistspecialpublication800-38d.pdf>.
[SP800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key Establishment
Schemes Using Discrete Logarithm Cryptography", NIST
Special Publication 800-56A, Revision 3,
DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
<
https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar2.pdf>.
[STD90] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259, December 2017,
<
https://www.rfc-editor.org/info/std90>.
Acknowledgments
This document is a product of the COSE Working Group of the IETF.
The following individuals are to blame for getting me started on this
project in the first place: Richard Barnes, Matt Miller, and Martin
Thomson.
The initial draft version of the specification was based to some
degree on the outputs of the JOSE and S/MIME Working Groups.
The following individuals provided input into the final form of the
document: Carsten Bormann, John Bradley, Brian Campbell, Michael
B. Jones, Ilari Liusvaara, Francesca Palombini, Ludwig Seitz, and
Göran Selander.
Author's Address
Jim Schaad
August Cellars
