COSE Working Group M. Jones Internet-Draft Microsoft Intended status: Standards Track December 31, 2016 Expires: July 4, 2017 Using RSA Algorithms with COSE Messages draft-jones-cose-rsa-01 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. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 4, 2017. Copyright Notice Copyright (c) 2016 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 Expires July 4, 2017 [Page 1] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Notation and Conventions . . . . . . . . . . 2 2. Signature Algorithms . . . . . . . . . . . . . . . . . . . . 2 2.1. RSASSA-PSS . . . . . . . . . . . . . . . . . . . . . . . 2 2.1.1. Security Considerations . . . . . . . . . . . . . . . 3 3. Recipient Algorithm Classes . . . . . . . . . . . . . . . . . 4 3.1. Key Encryption . . . . . . . . . . . . . . . . . . . . . 4 3.1.1. RSAES-OAEP . . . . . . . . . . . . . . . . . . . . . 4 3.1.1.1. Security Considerations for RSAES-OAEP . . . . . 5 4. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. RSA Keys . . . . . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5.1. COSE Algorithms Registry . . . . . . . . . . . . . . . . 7 5.2. COSE Key Types Registry . . . . . . . . . . . . . . . . . 7 5.3. COSE Key Type Parameters Registry . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 8 7.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8 Appendix B. Document History . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 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 2.1. RSASSA-PSS 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 Jones Expires July 4, 2017 [Page 2] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 (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. 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 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 to be successful in producing a forgery based on changing the hash function. This is highly unlikely. Jones Expires July 4, 2017 [Page 3] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 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: o The 'protected' field MUST be absent. o The plain text to be encrypted is the key from next layer down (usually the content layer). o At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the asymmetric key. 3.1.1. RSAES-OAEP 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. Jones Expires July 4, 2017 [Page 4] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 +----------------------------+-------+---------+--------------------+ | Name | Value | Hash | Description | +----------------------------+-------+---------+--------------------+ | RSAES-OAEP w/ default | -40 | SHA-1 | RSAES OAEP w/ | | 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'. 3.1.1.1. 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. 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. 4. Keys Key types are identified by the 'kty' member of the COSE_Key object. This specification defines one value for this member. Jones Expires July 4, 2017 [Page 5] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 +------+-------+-------------+ | Name | Value | Description | +------+-------+-------------+ | RSA | 3 | RSA Key | +------+-------+-------------+ Table 3: Key Type Values 4.1. RSA Keys 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 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. Jones Expires July 4, 2017 [Page 6] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 +-------+----------+-------+-------+--------------------------------+ | 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. The values in Table 1 and Table 2 are to be added to the registry. 5.2. COSE Key Types Registry This section registers values in the IANA "COSE Key Types" registry. 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. 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 3.1.1.1. Jones Expires July 4, 2017 [Page 7] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 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, . [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 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 ]] -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 Jones Expires July 4, 2017 [Page 8] Internet-Draft Using RSA Algorithms with COSE Messages December 2016 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/ Jones Expires July 4, 2017 [Page 9]