COSE Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track April 4, 2016
Expires: October 6, 2016

Using RSA Algorithms with COSE Messages


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.

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Table of Contents

1. Introduction

The CBOR Object Signing and Encryption (COSE) [I-D.ietf-cose-msg] specification defines cryptographic message encodings using Concise Binary Object Representation (CBOR) [RFC7049]. This specification defines algorithm encodings and representations enabling RSA algorithms to be used for COSE messages.

1.1. Requirements Notation and Conventions

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 [RFC2119].

2. Signature Algorithms


The RSASSA-PSS signature algorithm is defined in [RFC3447].

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 [RFC3447]. It has been recommended that the same hash function be used for hashing the data as well as in the mask generation function, for this specification we following this recommendation. The salt length is the same length as the hash function output.

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 Table 1.

RSASSA-PSS Algorithm Values
name value hash salt length description
PS256 -26 SHA-256 32 RSASSA-PSS w/ SHA-256
PS384 -27 SHA-384 48 RSASSA-PSS w/ SHA-384
PS512 -28 SHA-512 64 RSASSA-PSS w/ SHA-512

2.1.1. Security Considerations

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. This has the potential to consume resources with potentially bad keys. 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 functions 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 in order to be successful in producing a forgery based on changing the hash function. This is highly unlikely.

3. Recipient Algorithm Classes

3.1. Key Encryption

Key Encryption mode is also called key transport mode in some standards. Key Encryption mode differs from Key Wrap mode in that it uses an asymmetric encryption algorithm rather than a symmetric encryption algorithm to protect the key. This document defines one Key Encryption mode algorithm.

When using a key encryption algorithm, the COSE_encrypt structure for the recipient is organized as follows:


RSAES-OAEP is an asymmetric key encryption algorithm. The definition of RSAEA-OAEP can be find in Section 7.1 of [RFC3447]. 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: Table 2 summarizes the rest of the values.

RSAES-OAEP Algorithm Values
name value hash description
RSAES-OAEP w/SHA-256 -25 SHA-256 RSAES OAEP w/ SHA-256
RSAES-OAEP w/SHA-512 -26 SHA-512 RSAES OAEP w/ SHA-512

The key type MUST be 'RSA'. Security Considerations for RSAES-OAEP

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.

It is highly recommended that checks on the key length be done before starting a decryption operation. One potential denial of service operation is to provide encrypted objects using either abnormally long or oddly sized RSA modulus values. Implementations SHOULD be able to encrypt and decrypt with modulus between 2048 and 16K bits in length. Applications can impose additional restrictions on the length of the modulus.

4. Keys

Key types are identified by the 'kty' member of the COSE_Key object. In this document we define one value for the member.

Key Type Values
name value description
RSA 3 RSA Keys

4.1. RSA Keys

This document defines a key structure for both the public and private halves of RSA keys. Together, an RSA public key and an RSA private key form an RSA key pair. [CREF1]JLS: Looking at the CBOR specification, the bstr that we are looking in our table below should most likely be specified as big numbers rather than as binary strings. This means that we would use the tag 6.2 instead. From my reading of the specification, there is no difference in the encoded size of the resulting output. The specification of bignum does explicitly allow for integers encoded with leading zeros.

The document also provides support for the so-called "multi-prime" RSA where 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 [MultiPrimeRSA].

This document follows the naming convention of [RFC3447] for the naming of the fields of an RSA public or private key. The table Table 4 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 Table 4 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.

RSA Key Parameters
name key type value type description
n 3 -1 bstr Modulus Parameter
e 3 -2 int Exponent Parameter
d 3 -3 bstr Private Exponent Parameter
p 3 -4 bstr First Prime Factor
q 3 -5 bstr Second Prime Factor
dP 3 -6 bstr First Factor CRT Exponent
dQ 3 -7 bstr Second Factor CRT Exponent
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

5. IANA Considerations

5.1. COSE Algorithm Registry

This section registers values in the IANA "COSE Algorithm Registry" registry.

The values in Table 1 are to be added to the registry.

5.2. COSE Key Type Parameter Registry

This section registers values in the IANA "COSE Key Type Parameters" registry.

The values in Table 4 are to be added to the registry.

6. Security Considerations

See the per-algorithm security considerations described in Section 2.1.1 and Section

7. References

7.1. Normative References

[I-D.ietf-cose-msg] Schaad, J., "CBOR Encoded Message Syntax", Internet-Draft draft-ietf-cose-msg-11, March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.

7.2. Informative References

[MultiPrimeRSA] Hinek, M. and D. Cheriton, "On the Security of Multi-prime RSA", June 2006.

Appendix A. Acknowledgements

The initial version of this specification incorporates text from draft-ietf-cose-msg-05 by Jim Schaad.

Appendix B. Document History

[[ to be removed by the RFC Editor before publication as an RFC ]]


  • 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.

Author's Address

Michael B. Jones Microsoft EMail: URI: