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Versions: 00 01 02 03 04 05 RFC 8230

COSE Working Group                                              M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                            May 18, 2017
Expires: November 19, 2017


                Using RSA Algorithms with COSE Messages
                        draft-jones-cose-rsa-03

Abstract

   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.

Status of This Memo

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   This Internet-Draft will expire on November 19, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   2
   2.  RSASSA-PSS Signature Algorithm  . . . . . . . . . . . . . . .   2
   3.  RSAES-OAEP Key Encryption Algorithm . . . . . . . . . . . . .   3
   4.  RSA Keys  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  COSE Algorithms Registry  . . . . . . . . . . . . . . . .   5
     5.2.  COSE Key Type Registry  . . . . . . . . . . . . . . . . .   5
     5.3.  COSE Key Type Parameters Registry . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     6.1.  Key Size Security Considerations  . . . . . . . . . . . .   6
     6.2.  RSASSA-PSS Security Considerations  . . . . . . . . . . .   6
     6.3.  RSAES-OAEP Security Considerations  . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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.  RSASSA-PSS Signature Algorithm

   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



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   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 Table 1.

     +-------+-------+---------+-------------+-----------------------+
     | 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 |
     +-------+-------+---------+-------------+-----------------------+

                   Table 1: RSASSA-PSS Algorithm Values

3.  RSAES-OAEP Key Encryption Algorithm

   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:

   o  mgf is always set to MFG1 from [RFC3447] and uses the same hash
      function as h.

   o  P is always set to the empty octet string.

   Table 2 summarizes the rest of the values.

   +-------------------------------+-------+---------+-----------------+
   | Name                          | Value | Hash    | Description     |
   +-------------------------------+-------+---------+-----------------+
   | RSAES-OAEP w/ RFC 3447        | -40   | SHA-1   | RSAES OAEP w/   |
   | default parameters            |       |         | 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         |
   +-------------------------------+-------+---------+-----------------+

                   Table 2: RSAES-OAEP Algorithm Values




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   The key type MUST be 'RSA'.

4.  RSA Keys

   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     |
                      +------+-------+-------------+

                         Table 3: Key Type Values

   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 [MultiPrimeRSA].

   This document follows the naming convention of [RFC3447] for the
   naming of the fields of an RSA public or private key.  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:

   o  For all keys, 'kty' MUST be present and MUST have a value of 3.

   o  For public keys, the fields 'n' and 'e' MUST be present.  All
      other fields defined in Table 4 MUST be absent.

   o  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.

   o  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.

   o  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



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      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                    |
   +-------+----------+-------+-------+--------------------------------+

                        Table 4: RSA Key Parameters

5.  IANA Considerations

5.1.  COSE Algorithms Registry

   This section registers values in the IANA "COSE Algorithms" registry
   [IANA.COSE].

   The values in Table 1 and Table 2 are to be added to the registry.

5.2.  COSE Key Type Registry

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

   The values in Table 3 are to be added to the registry.

5.3.  COSE Key Type Parameters Registry

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

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



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6.  Security Considerations

6.1.  Key Size Security Considerations

   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 legitimate participant.  This
   approach means that no cryptography would be done except with keys of
   legitimate parties.  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.

6.2.  RSASSA-PSS Security Considerations

   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.

6.3.  RSAES-OAEP Security Considerations

   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.




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7.  References

7.1.  Normative References

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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, <http://www.rfc-editor.org/info/rfc3447>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://www.rfc-editor.org/info/rfc7049>.

7.2.  Informative References

   [IANA.COSE]
              IANA, "CBOR Object Signing and Encryption (COSE)",
              <http://www.iana.org/assignments/cose>.

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

Appendix A.  Acknowledgements

   This specification incorporates text from draft-ietf-cose-msg-05 by
   Jim Schaad.  Thanks are due to Kathleen Moriarty, Rich Salz, and Jim
   Schaad for their reviews of the specification.

Appendix B.  Document History

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

   -03

   o  Clarified the Security Considerations in ways suggested by
      Kathleen Moriarty.

   o  Acknowledged reviewers.



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   -02

   o  Reorganized the security considerations.

   o  Flattened the section structure.

   o  Applied wording improvements suggested by Jim Schaad.

   -01

   o  Completed the sets of IANA registration requests.

   o  Revised the algorithm assignments based on those in draft-ietf-
      cose-msg-24.

   -00

   o  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: mbj@microsoft.com
   URI:   http://self-issued.info/





















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