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IETF RFC 8230
Last modified on Tuesday, September 5th, 2017
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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/RFC 8230.
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
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RFC 8230 Using RSA Algorithms with COSE Messages September 2017
1. Introduction
The CBOR Object Signing and Encryption (COSE) [RFC 8152] specification
defines cryptographic message encodings using Concise Binary Object
Representation (CBOR) [RFC 7049]. 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 [RFC 2119] [RFC 8174] 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 [RFC 8017].
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 [RFC 8017]. 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
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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 [RFC 8017]. 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 [RFC 8017] 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
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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 [RFC 8017] 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).
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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
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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
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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
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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.
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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 [RFC 7515]. 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 [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://www.ams.org/notices/199902/boneh.pdf>.
Jones Standards Track PAGE 10
RFC 8230 Using RSA Algorithms with COSE Messages September 2017
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC 7049,
October 2013, <https://www.rfc-editor.org/info/RFC 7049>.
[RFC 7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC 7515, May
2015, <https://www.rfc-editor.org/info/RFC 7515>.
[RFC 8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A.
Rusch, "PKCS #1: RSA Cryptography Specifications Version
2.2", RFC 8017, DOI 10.17487/RFC 8017, November 2016,
<https://www.rfc-editor.org/info/RFC 8017>.
[RFC 8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC 8152, July 2017,
<https://www.rfc-editor.org/info/RFC 8152>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
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
RFC TOTAL SIZE: 21655 bytes
PUBLICATION DATE: Tuesday, September 5th, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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