Internet DRAFT - draft-jones-cose-rsa
draft-jones-cose-rsa
COSE Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track June 22, 2017
Expires: December 24, 2017
Using RSA Algorithms with COSE Messages
draft-jones-cose-rsa-05
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 24, 2017.
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Copyright (c) 2017 IETF Trust and the persons identified as the
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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 [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
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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 [RFC8017]. 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 [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
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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 [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
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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,
0x80 0x00 (major type 2, additional information 2 for the length).
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
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 8017 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: the RSA modulus n
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: the RSA public exponent e
o Reference: Section 4 of [[ this specification ]]
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 specification ]]
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 specification ]]
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 specification ]]
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 specification ]]
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 specification ]]
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 specification ]]
o Key Type: 3
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o Name: other
o Label: -9
o CBOR Type: array
o Description: other prime infos, an array
o Reference: Section 4 of [[ this specification ]]
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 specification ]]
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 specification ]]
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 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
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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 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 8017 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 8017. 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://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>.
[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>.
[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,
<http://www.rfc-editor.org/info/rfc8017>.
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 Ben Campbell, Roni Even, Steve Kent,
Kathleen Moriarty, Eric Rescorla, Adam Roach, 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 ]]
-05
o Addressed IESG review comments.
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o Updated the RFC 3447 reference to RFC 8017.
o Updated the field descriptions to use the wording from
Section A.1.2 of RFC 8017.
o Corrected an error in the RSAES-OAEP security considerations.
-04
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
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Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
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