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Internet Engineering Task Force (IETF) M. Jones Request for Comments: 8230 Microsoft Category: Standards Track September 2017 ISSN: 2070-1721

                     Using RSA Algorithms with
         CBOR Object Signing and Encryption (COSE) Messages

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 are specified for the use of RSA
 Probabilistic Signature Scheme (RSASSA-PSS) signatures, RSA
 Encryption Scheme - Optimal Asymmetric Encryption Padding (RSAES-
 OAEP) encryption, and RSA keys.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8230.

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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Jones Standards Track [Page 1] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
 2.  RSASSA-PSS Signature Algorithm  . . . . . . . . . . . . . . .   3
 3.  RSAES-OAEP Key Encryption Algorithm . . . . . . . . . . . . .   4
 4.  RSA Keys  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.1.  COSE Algorithms Registrations . . . . . . . . . . . . . .   6
   5.2.  COSE Key Type Registrations . . . . . . . . . . . . . . .   7
   5.3.  COSE Key Type Parameters Registrations  . . . . . . . . .   7
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.1.  Key Size Security Considerations  . . . . . . . . . . . .   9
   6.2.  RSASSA-PSS Security Considerations  . . . . . . . . . . .  10
   6.3.  RSAES-OAEP Security Considerations  . . . . . . . . . . .  10
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
   7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

Jones Standards Track [Page 2] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

1. Introduction

 The CBOR Object Signing and Encryption (COSE) [RFC8152] 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 BCP
 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

2. RSASSA-PSS Signature Algorithm

 The RSASSA-PSS signature algorithm is defined in [RFC8017].
 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 [RFC8017].  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 |
   +-------+-------+---------+-------------+-----------------------+
                 Table 1: RSASSA-PSS Algorithm Values

Jones Standards Track [Page 3] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

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 [RFC8017].  The
 algorithm is parameterized using a mask 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 as defined in [RFC8017] 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 8017        | -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
 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

Jones Standards Track [Page 4] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 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 cryptosystems, the reader is
 referred to [MultiPrimeRSA].
 This document follows the naming convention of [RFC8017] for the
 naming of the fields of an RSA public or private key, and the
 corresponding fields have identical semantics.  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
    value 32,768 is represented as the CBOR byte sequence 0b010_00010,
    0x80 0x00 (major type 2, additional information 2 for the length).

Jones Standards Track [Page 5] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 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  | the RSA modulus n                 |
 | 3     | e     | -2    | bstr  | the RSA public exponent e         |
 | 3     | d     | -3    | bstr  | the RSA private exponent d        |
 | 3     | p     | -4    | bstr  | the prime factor p of n           |
 | 3     | q     | -5    | bstr  | the prime factor q of n           |
 | 3     | dP    | -6    | bstr  | dP is d mod (p - 1)               |
 | 3     | dQ    | -7    | bstr  | dQ is d mod (q - 1)               |
 | 3     | qInv  | -8    | bstr  | qInv is the CRT coefficient       |
 |       |       |       |       | q^(-1) mod p                      |
 | 3     | other | -9    | array | other prime infos, an array       |
 | 3     | r_i   | -10   | bstr  | a prime factor r_i of n, where i  |
 |       |       |       |       | >= 3                              |
 | 3     | d_i   | -11   | bstr  | d_i = d mod (r_i - 1)             |
 | 3     | t_i   | -12   | bstr  | the CRT coefficient t_i = (r_1 *  |
 |       |       |       |       | r_2 * ... * r_(i-1))^(-1) mod r_i |
 +-------+-------+-------+-------+-----------------------------------+
                      Table 4: RSA Key Parameters

5. IANA Considerations

5.1. COSE Algorithms Registrations

 IANA has registered 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 document
 o  Recommended: Yes
 o  Name: PS384
 o  Value: -38
 o  Description: RSASSA-PSS w/ SHA-384
 o  Reference: Section 2 of this document
 o  Recommended: Yes

Jones Standards Track [Page 6] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 o  Name: PS512
 o  Value: -39
 o  Description: RSASSA-PSS w/ SHA-512
 o  Reference: Section 2 of this document
 o  Recommended: Yes
 o  Name: RSAES-OAEP w/ RFC 8017 default parameters
 o  Value: -40
 o  Description: RSAES-OAEP w/ SHA-1
 o  Reference: Section 3 of this document
 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 document
 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 document
 o  Recommended: Yes

5.2. COSE Key Type Registrations

 IANA has registered the following value in the IANA "COSE Key Types"
 registry [IANA.COSE].
 o  Name: RSA
 o  Value: 3
 o  Description: RSA Key
 o  Reference: Section 4 of this document

5.3. COSE Key Type Parameters Registrations

 IANA has registered 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: the RSA modulus n
 o  Reference: Section 4 of this document

Jones Standards Track [Page 7] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 o  Key Type: 3
 o  Name: e
 o  Label: -2
 o  CBOR Type: bstr
 o  Description: the RSA public exponent e
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: d
 o  Label: -3
 o  CBOR Type: bstr
 o  Description: the RSA private exponent d
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: p
 o  Label: -4
 o  CBOR Type: bstr
 o  Description: the prime factor p of n
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: q
 o  Label: -5
 o  CBOR Type: bstr
 o  Description: the prime factor q of n
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: dP
 o  Label: -6
 o  CBOR Type: bstr
 o  Description: dP is d mod (p - 1)
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: dQ
 o  Label: -7
 o  CBOR Type: bstr
 o  Description: dQ is d mod (q - 1)
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: qInv
 o  Label: -8
 o  CBOR Type: bstr
 o  Description: qInv is the CRT coefficient q^(-1) mod p
 o  Reference: Section 4 of this document

Jones Standards Track [Page 8] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 o  Key Type: 3
 o  Name: other
 o  Label: -9
 o  CBOR Type: array
 o  Description: other prime infos, an array
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: r_i
 o  Label: -10
 o  CBOR Type: bstr
 o  Description: a prime factor r_i of n, where i >= 3
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: d_i
 o  Label: -11
 o  CBOR Type: bstr
 o  Description: d_i = d mod (r_i - 1)
 o  Reference: Section 4 of this document
 o  Key Type: 3
 o  Name: t_i
 o  Label: -12
 o  CBOR Type: bstr
 o  Description: the CRT coefficient t_i = (r_1 * r_2 * ... *
    r_(i-1))^(-1) mod r_i
 o  Reference: Section 4 of this document

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.

Jones Standards Track [Page 9] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 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 [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 an Elliptic Curve Digital Signature
 Algorithm (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 [RFC8017] 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 [RFC8017].  Also, keys with a low private 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://www.ams.org/notices/199902/boneh.pdf>.

Jones Standards Track [Page 10] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <https://www.rfc-editor.org/info/rfc2119>.
 [RFC7049]   Bormann, C. and P. Hoffman, "Concise Binary Object
             Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
             October 2013, <https://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, <https://www.rfc-editor.org/info/rfc7515>.
 [RFC8017]   Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A.
             Rusch, "PKCS #1: RSA Cryptography Specifications Version
             2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016,
             <https://www.rfc-editor.org/info/rfc8017>.
 [RFC8152]   Schaad, J., "CBOR Object Signing and Encryption (COSE)",
             RFC 8152, DOI 10.17487/RFC8152, July 2017,
             <https://www.rfc-editor.org/info/rfc8152>.
 [RFC8174]   Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2. Informative References

 [HASHID]    Kaliski, B., "On Hash Function Firewalls in Signature
             Schemes", Lecture Notes in Computer Science (LNCS),
             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,
             <http://cacr.uwaterloo.ca/techreports/
             2006/cacr2006-16.pdf>.

Jones Standards Track [Page 11] RFC 8230 Using RSA Algorithms with COSE Messages September 2017

Acknowledgements

 This specification incorporates text from "CBOR Encoded Message
 Syntax" (September 2015) authored by Jim Schaad and Brian Campbell.
 Thanks are due to Ben Campbell, Roni Even, Steve Kent, Kathleen
 Moriarty, Eric Rescorla, Adam Roach, Rich Salz, and Jim Schaad for
 their reviews of the specification.

Author's Address

 Michael B. Jones
 Microsoft
 Email: mbj@microsoft.com
 URI:   http://self-issued.info/

Jones Standards Track [Page 12]

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