COSE Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track April 4, 2016
Expires: October 6, 2016
Using RSA Algorithms with COSE Messages
draft-jones-cose-rsa-00
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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. COSE Algorithm Registry . . . . . . . . . . . . . . . . . 6
5.2. COSE Key Type Parameter Registry . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Appendix B. Document History . . . . . . . . . . . . . . . . . . 8
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. 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
(sLen). For this specification, the mask generation function is
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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, for this specification we following
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 | -26 | SHA-256 | 32 | RSASSA-PSS w/ SHA-256 |
| PS384 | -27 | SHA-384 | 48 | RSASSA-PSS w/ SHA-384 |
| PS512 | -28 | 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 in order to be successful
in producing a forgery based on changing the hash function. This is
highly unlikely.
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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.
+----------------------+-------+---------+-----------------------+
| name | value | hash | description |
+----------------------+-------+---------+-----------------------+
| RSAES-OAEP w/SHA-256 | -25 | SHA-256 | RSAES OAEP w/ SHA-256 |
| RSAES-OAEP w/SHA-512 | -26 | SHA-512 | RSAES OAEP w/ SHA-512 |
+----------------------+-------+---------+-----------------------+
Table 2: RSAES-OAEP Algorithm Values
The key type MUST be 'RSA'.
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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.
4. Keys
Key types are identified by the 'kty' member of the COSE_Key object.
In this document we define one value for the member.
+------+-------+-------------+
| name | value | description |
+------+-------+-------------+
| RSA | 3 | RSA Keys |
+------+-------+-------------+
Table 3: Key Type Values
4.1. RSA Keys
This document defines a key structure for both the public and private
halves of RSA keys. Together, an RSA public key and an RSA private
key form an RSA key pair. [[CREF1: Looking at the CBOR
specification, the bstr that we are looking in our table below should
most likely be specified as big numbers rather than as binary
strings. This means that we would use the tag 6.2 instead. From my
reading of the specification, there is no difference in the encoded
size of the resulting output. The specification of bignum does
explicitly allow for integers encoded with leading zeros. --JLS]]
The document also provides support for the so-called "multi-prime"
RSA where 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 table
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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.
+-------+----------+-------+-------+--------------------------------+
| name | key type | value | type | description |
+-------+----------+-------+-------+--------------------------------+
| n | 3 | -1 | bstr | Modulus Parameter |
| e | 3 | -2 | int | 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 Algorithm Registry
This section registers values in the IANA "COSE Algorithm Registry"
registry.
The values in Table 1 are to be added to the registry.
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5.2. COSE Key Type Parameter 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.
7. References
7.1. Normative References
[I-D.ietf-cose-msg]
Schaad, J., "CBOR Encoded Message Syntax", draft-ietf-
cose-msg-11 (work in progress), March 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
[MultiPrimeRSA]
Hinek, M. and D. Cheriton, "On the Security of Multi-prime
RSA", June 2006.
Appendix A. Acknowledgements
The initial version of this specification incorporates text from
draft-ietf-cose-msg-05 by Jim Schaad.
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Appendix B. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-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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