Using RSA Algorithms with COSE Messages
Microsoft
mbj@microsoft.com
http://self-issued.info/
Security
COSE Working Group
The CBOR Object Signing and Encryption (COSE) specification
defines cryptographic message encodings using
Concise Binary Object Representation (CBOR).
This specification defines algorithm encodings and representations
enabling RSA algorithms to be used for COSE messages.
Encodings for the use of RSASSA-PSS signatures, RSAES-OAEP encryption,
and RSA keys are specified.
The CBOR Object Signing and Encryption (COSE) specification
defines cryptographic message encodings using
Concise Binary Object Representation (CBOR) .
This specification defines algorithm encodings and representations
enabling RSA algorithms to be used for COSE messages.
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
RFC 2119 .
The RSASSA-PSS signature algorithm is defined in .
The RSASSA-PSS signature algorithm is parameterized with a hash function (h), a mask generation function (mgf) and a salt length (sLen). For this specification, the mask generation function is fixed to be MGF1 as defined in . It has been recommended that the same hash function be used for hashing the data as well as in the mask generation function.
This specification follows this recommendation.
The salt length is the same length as the hash function output.
Implementations need to check that the key type is 'RSA' when creating or verifying a signature.
The algorithms defined in this document can be found in .
Name
Value
Hash
Salt Length
Description
PS256
-37
SHA-256
32
RSASSA-PSS w/ SHA-256
PS384
-38
SHA-384
48
RSASSA-PSS w/ SHA-384
PS512
-39
SHA-512
64
RSASSA-PSS w/ SHA-512
RSAES-OAEP is an asymmetric key encryption algorithm.
The definition of RSAEA-OAEP can be find in Section 7.1 of .
The algorithm is parameterized using a masking generation function (mgf), a hash function (h) and encoding parameters (P).
For the algorithm identifiers defined in this section:
mgf is always set to MFG1 from and uses the same hash function as h.
P is always set to the empty octet string.

summarizes the rest of the values.
Name
Value
Hash
Description
RSAES-OAEP w/ RFC 3447 default parameters
-40
SHA-1
RSAES OAEP w/ SHA-1
RSAES-OAEP w/ SHA-256
-41
SHA-256
RSAES OAEP w/ SHA-256
RSAES-OAEP w/ SHA-512
-42
SHA-512
RSAES OAEP w/ SHA-512
The key type MUST be 'RSA'.
Key types are identified by the 'kty' member of the COSE_Key object.
This specification defines one value for this member.
Name
Value
Description
RSA
3
RSA Key
This document defines a key structure for both the public and private parts of RSA keys.
Together, an RSA public key and an RSA private key form an RSA key pair.
The document also provides support for the so-called "multi-prime" RSA keys, in which the modulus may have more than two prime factors.
The benefit of multi-prime RSA is lower computational cost for the decryption and signature primitives.
For a discussion on how multi-prime affects the security of RSA crypto-systems, the reader is referred to .
This document follows the naming convention of for the naming of the fields of an RSA public or private key.
provides a summary of the label values and the types associated with each of those labels.
The requirements for fields for RSA keys are as follows:
For all keys, 'kty' MUST be present and MUST have a value of 3.
For public keys, the fields 'n' and 'e' MUST be present. All other fields defined in MUST be absent.
For private keys with two primes, the fields 'other', 'r_i', 'd_i' and 't_i' MUST be absent; all other fields MUST be present.
For private keys with more than two primes, all fields MUST be present.
For the third to nth primes, each of the primes is represented as a map containing the fields 'r_i', 'd_i' and 't_i'.
The field 'other' is an array of those maps.
All numeric key parameters are encoded in an unsigned big-endian representation as an octet sequence using the CBOR byte string type (major type 2).
The octet sequence MUST utilize the minimum number of octets needed to represent the value.
For instance, the value 32,768 is represented as the CBOR byte sequence 0b010_00010 (major type 2, additional information 2 for the length), 0x80 0x00.

Name
Key Type
Value
Type
Description
n
3
-1
bstr
Modulus Parameter
e
3
-2
bstr
Exponent Parameter
d
3
-3
bstr
Private Exponent Parameter
p
3
-4
bstr
First Prime Factor
q
3
-5
bstr
Second Prime Factor
dP
3
-6
bstr
First Factor CRT Exponent
dQ
3
-7
bstr
Second Factor CRT Exponent
qInv
3
-8
bstr
First CRT Coefficient
other
3
-9
array
Other Primes Info
r_i
3
-10
bstr
i-th factor, Prime Factor
d_i
3
-11
bstr
i-th factor, Factor CRT Exponent
t_i
3
-12
bstr
i-th factor, Factor CRT Coefficient
This section registers values in the IANA "COSE Algorithms" registry.
The values in and
are to be added to the registry.
This section registers values in the IANA "COSE Key Types" registry.
The values in are to be added to the registry.
This section registers values in the IANA "COSE Key Type Parameters" registry.
The values in are to be added to the registry.
A key size of 2048 bits or larger MUST be used with these algorithms.
This key size corresponds roughly to the same strength as provided by a 128-bit symmetric encryption algorithm.
Implementations SHOULD be able to encrypt and decrypt with modulus between 2048 and 16K bits in length.
Applications can impose additional restrictions on the length of the modulus.
In addition to needing to worry about keys that are too small to provide the required security, there are issues with keys that are too large.
Denial of service attacks have been mounted with overly large keys or oddly sized keys.
This has the potential to consume resources with these keys.
It is highly recommended that checks on the key length be done before starting a cryptographic operation.
There are two reasonable ways to address this attack.
First, a key should not be used for a cryptographic operation until it has been matched back to an authorized user.
This approach means that no cryptography would be done except for authorized users.
Second, applications can impose maximum as well as minimum length requirements on keys.
This limits the resources consumed even if the matching is not performed until the cryptography has been done.
There is a theoretical hash substitution attack that can be mounted against RSASSA-PSS.
However, the requirement that the same hash function be used consistently for all operations is an effective mitigation against it.
Unlike ECDSA, hash function outputs are not truncated so that the full hash value is always signed.
The internal padding structure of RSASSA-PSS means that one needs to have multiple collisions between the two hash functions to be successful in producing a forgery based on changing the hash function.
This is highly unlikely.
A version of RSAES-OAEP using the default parameters specified in Appendix A.2.1 of RFC 3447
is included because this is the most widely implemented set of OAEP parameter choices.
(Those default parameters are the SHA-1 hash function and
the MGF1 with SHA-1 mask generation function.)
While SHA-1 is deprecated as a general-purpose hash function,
no known practical attacks are enabled by its use in this context.
On the Security of Multi-prime RSAUniversity of WaterlooUniversity of Waterloo
This specification incorporates text from draft-ietf-cose-msg-05 by Jim Schaad.
[[ to be removed by the RFC Editor before publication as an RFC ]]
-02
Reorganized the security considerations.
Flattened the section structure.
Applied wording improvements suggested by Jim Schaad.

-01
Completed the sets of IANA registration requests.
Revised the algorithm assignments based on those in draft-ietf-cose-msg-24.

-00
This specification addresses COSE issue #21: Restore RSA-PSS and the "RSA" key type.
The initial version of this specification incorporates text from draft-ietf-cose-msg-05 --
the last COSE message specification version before the RSA algorithms were removed.