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COSE Working Group                                              M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                           June 15, 2017
Expires: December 17, 2017


                Using RSA Algorithms with COSE Messages
                        draft-jones-cose-rsa-04

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 17, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of




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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 Registrations . . . . . . . . . . . . . .   5
     5.2.  COSE Key Type Registrations . . . . . . . . . . . . . . .   6
     5.3.  COSE Key Type Parameters Registrations  . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Key Size Security Considerations  . . . . . . . . . . . .   8
     6.2.  RSASSA-PSS Security Considerations  . . . . . . . . . . .   9
     6.3.  RSAES-OAEP Security Considerations  . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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

                   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 found 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 MGF1 from [RFC3447] and uses the same hash
      function as h.

   o  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        | -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 in the following
   table.

                      +------+-------+-------------+
                      | 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.  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 the following table below 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
      number of octets needed to represent the value.  For instance, the



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      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     | Name  | Label | CBOR     | Description                  |
   | Type    |       |       | Type     |                              |
   +---------+-------+-------+----------+------------------------------+
   | 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                  |
   +---------+-------+-------+----------+------------------------------+

                        Table 4: RSA Key Parameters

5.  IANA Considerations

5.1.  COSE Algorithms Registrations

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

   o  Name: PS256
   o  Value: -37
   o  Description: RSASSA-PSS w/ SHA-256
   o  Reference: Section 2 of [[ this specification ]]
   o  Recommended: Yes

   o  Name: PS384
   o  Value: -38
   o  Description: RSASSA-PSS w/ SHA-384
   o  Reference: Section 2 of [[ this specification ]]
   o  Recommended: Yes




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   o  Name: PS512
   o  Value: -39
   o  Description: RSASSA-PSS w/ SHA-512
   o  Reference: Section 2 of [[ this specification ]]
   o  Recommended: Yes

   o  Name: RSAES-OAEP w/ RFC 3447 default parameters
   o  Value: -40
   o  Description: RSAES-OAEP w/ SHA-1
   o  Reference: Section 3 of [[ this specification ]]
   o  Recommended: Yes

   o  Name: RSAES-OAEP w/ SHA-256
   o  Value: -41
   o  Description: RSAES-OAEP w/ SHA-256
   o  Reference: Section 3 of [[ this specification ]]
   o  Recommended: Yes

   o  Name: RSAES-OAEP w/ SHA-512
   o  Value: -42
   o  Description: RSAES-OAEP w/ SHA-512
   o  Reference: Section 3 of [[ this specification ]]
   o  Recommended: Yes

5.2.  COSE Key Type Registrations

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

   o  Name: RSA
   o  Value: 3
   o  Description: RSA Key
   o  Reference: Section 4 of [[ this specification ]]

5.3.  COSE Key Type Parameters Registrations

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

   o  Key Type: 3
   o  Name: n
   o  Label: -1
   o  CBOR Type: bstr
   o  Description: Modulus Parameter
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: e



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   o  Label: -2
   o  CBOR Type: bstr
   o  Description: Exponent Parameter
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: d
   o  Label: -3
   o  CBOR Type: bstr
   o  Description: Private Exponent Parameter
   o  Reference: Section 4 of [[ this specification ]]

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

   o  Key Type: 3
   o  Name: q
   o  Label: -5
   o  CBOR Type: bstr
   o  Description: Second Prime Factor
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: dP
   o  Label: -6
   o  CBOR Type: bstr
   o  Description: First Factor CRT Exponent
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: dQ
   o  Label: -7
   o  CBOR Type: bstr
   o  Description: Second Factor CRT Exponent
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: qInv
   o  Label: -8
   o  CBOR Type: bstr
   o  Description: First CRT Coefficient
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3



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   o  Name: other
   o  Label: -9
   o  CBOR Type: array
   o  Description: Other Primes Info
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: r_i
   o  Label: -10
   o  CBOR Type: bstr
   o  Description: i-th factor, Prime Factor
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: d_i
   o  Label: -11
   o  CBOR Type: bstr
   o  Description: i-th factor, Factor CRT Exponent
   o  Reference: Section 4 of [[ this specification ]]

   o  Key Type: 3
   o  Name: t_i
   o  Label: -12
   o  CBOR Type: bstr
   o  Description: i-th factor, Factor CRT Coefficient
   o  Reference: Section 4 of [[ this specification ]]

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 party trusted by the recipient.



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   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 [RFC7515].  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 [HASHID].  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.)

   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" [Boneh99], must not be used.

7.  References

7.1.  Normative References

   [Boneh99]  Boneh, D., "Twenty Years of Attacks on the RSA
              Cryptosystem", Notices of the American Mathematical
              Society (AMS), Vol. 46, No. 2, pp. 203-213, 1999,
              <http://crypto.stanford.edu/~dabo/pubs/papers/
              RSA-survey.pdf>.

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






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   [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>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

7.2.  Informative References

   [HASHID]   Kaliski, B., "On Hash Function Firewalls in Signature
              Schemes", Lecture Notes in Computer Science, Volume
              2271, pp. 1-16, DOI 10.1007/3-540-45760-7_1, February
              2002, <https://rd.springer.com/
              chapter/10.1007/3-540-45760-7_1>.

   [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 Roni Even, Steve Kent, 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 ]]

   -04





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   o  Addressed SecDir review comments by Steve Kent and Gen-ART review
      comments by Roni Even.

   -03

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

   o  Acknowledged reviewers.

   -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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