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 RSASSA-PSS algorithms specified in this document are in the following table.
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 found 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 MGF1 from and uses the same hash function as h.
P is always set to the empty octet string.

The following table 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 in the following table.
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.
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 the following table below 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.

The following table provides a summary of the label values and the types associated with each of those labels.
Key Type
Name
Label
CBOR Type
Description
3
n
-1
bstr
Modulus Parameter
3
e
-2
bstr
Exponent Parameter
3
d
-3
bstr
Private Exponent Parameter
3
p
-4
bstr
First Prime Factor
3
q
-5
bstr
Second Prime Factor
3
dP
-6
bstr
First Factor CRT Exponent
3
dQ
-7
bstr
Second Factor CRT Exponent
3
qInv
-8
bstr
First CRT Coefficient
3
other
-9
array
Other Primes Info
3
r_i
-10
bstr
i-th factor, Prime Factor
3
d_i
-11
bstr
i-th factor, Factor CRT Exponent
3
t_i
-12
bstr
i-th factor, Factor CRT Coefficient
This section registers the following values in the IANA "COSE Algorithms" registry .
Name: PS256
Value: -37
Description: RSASSA-PSS w/ SHA-256
Reference: of [[ this specification ]]
Recommended: Yes

Name: PS384
Value: -38
Description: RSASSA-PSS w/ SHA-384
Reference: of [[ this specification ]]
Recommended: Yes

Name: PS512
Value: -39
Description: RSASSA-PSS w/ SHA-512
Reference: of [[ this specification ]]
Recommended: Yes

Name: RSAES-OAEP w/ RFC 3447 default parameters
Value: -40
Description: RSAES-OAEP w/ SHA-1
Reference: of [[ this specification ]]
Recommended: Yes

Name: RSAES-OAEP w/ SHA-256
Value: -41
Description: RSAES-OAEP w/ SHA-256
Reference: of [[ this specification ]]
Recommended: Yes

Name: RSAES-OAEP w/ SHA-512
Value: -42
Description: RSAES-OAEP w/ SHA-512
Reference: of [[ this specification ]]
Recommended: Yes

This section registers the following values in the IANA "COSE Key Type" registry .
Name: RSA
Value: 3
Description: RSA Key
Reference: of [[ this specification ]]

This section registers the following values in the IANA "COSE Key Type Parameters" registry .
Key Type: 3
Name: n
Label: -1
CBOR Type: bstr
Description: Modulus Parameter
Reference: of [[ this specification ]]

Key Type: 3
Name: e
Label: -2
CBOR Type: bstr
Description: Exponent Parameter
Reference: of [[ this specification ]]

Key Type: 3
Name: d
Label: -3
CBOR Type: bstr
Description: Private Exponent Parameter
Reference: of [[ this specification ]]

Key Type: 3
Name: p
Label: -4
CBOR Type: bstr
Description: First Prime Factor
Reference: of [[ this specification ]]

Key Type: 3
Name: q
Label: -5
CBOR Type: bstr
Description: Second Prime Factor
Reference: of [[ this specification ]]

Key Type: 3
Name: dP
Label: -6
CBOR Type: bstr
Description: First Factor CRT Exponent
Reference: of [[ this specification ]]

Key Type: 3
Name: dQ
Label: -7
CBOR Type: bstr
Description: Second Factor CRT Exponent
Reference: of [[ this specification ]]

Key Type: 3
Name: qInv
Label: -8
CBOR Type: bstr
Description: First CRT Coefficient
Reference: of [[ this specification ]]

Key Type: 3
Name: other
Label: -9
CBOR Type: array
Description: Other Primes Info
Reference: of [[ this specification ]]

Key Type: 3
Name: r_i
Label: -10
CBOR Type: bstr
Description: i-th factor, Prime Factor
Reference: of [[ this specification ]]

Key Type: 3
Name: d_i
Label: -11
CBOR Type: bstr
Description: i-th factor, Factor CRT Exponent
Reference: of [[ this specification ]]

Key Type: 3
Name: t_i
Label: -12
CBOR Type: bstr
Description: i-th factor, Factor CRT Coefficient
Reference: of [[ this specification ]]

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 verified that it is controlled by a party trusted by the recipient.
This approach means that no cryptography will be done until a trust decision about the key has been made,
a process described in Appendix D, Item 4 of .
Second, applications can impose maximum as well as minimum length requirements on keys.
This limits the resources that would otherwise be consumed by the use of overly large keys.
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.)
Keys used with RSAES-OAEP must follow the constraints in
Section 7.1 of RFC 3447.
Also, keys with a low public key exponent value,
as described in Section 3 of
"Twenty Years of Attacks on the RSA Cryptosystem",
must not be used.
Twenty Years of Attacks on the RSA Cryptosystem
On the Security of Multi-prime RSAUniversity of WaterlooUniversity of Waterloo
CBOR Object Signing and Encryption (COSE)
IANA
On Hash Function Firewalls in Signature Schemes
This specification incorporates text from draft-ietf-cose-msg-05 by Jim Schaad.
Thanks are due to
Roni Even,
Steve Kent,
Kathleen Moriarty,
Rich Salz,
and
Jim Schaad
for their reviews of the specification.
[[ to be removed by the RFC Editor before publication as an RFC ]]
-04
Addressed SecDir review comments by Steve Kent and Gen-ART review comments by Roni Even.

-03
Clarified the Security Considerations in ways suggested by Kathleen Moriarty.
Acknowledged reviewers.

-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.