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Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 9045 Vigil Security
Updates: 4211 June 2021
Category: Standards Track
ISSN: 2070-1721
Algorithm Requirements Update to the Internet X.509 Public Key
Infrastructure Certificate Request Message Format (CRMF)
Abstract
This document updates the cryptographic algorithm requirements for
the Password-Based Message Authentication Code in the Internet X.509
Public Key Infrastructure Certificate Request Message Format (CRMF)
specified in RFC 4211.
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
https://www.rfc-editor.org/info/rfc9045.
Copyright Notice
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Table of Contents
1. Introduction
2. Terminology
3. Signature Key POP
4. Password-Based Message Authentication Code
4.1. Introduction Paragraph
4.2. One-Way Function
4.3. Iteration Count
4.4. MAC Algorithm
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Author's Address
1. Introduction
This document updates the cryptographic algorithm requirements for
the Password-Based Message Authentication Code (MAC) in the Internet
X.509 Public Key Infrastructure Certificate Request Message Format
(CRMF) [RFC4211]. The algorithms specified in [RFC4211] were
appropriate in 2005; however, these algorithms are no longer
considered the best choices:
* HMAC-SHA1 [HMAC] [SHS] is not broken yet, but there are much
stronger alternatives [RFC6194].
* DES-MAC [PKCS11] provides 56 bits of security, which is no longer
considered secure [WITHDRAW].
* Triple-DES-MAC [PKCS11] provides 112 bits of security, which is
now deprecated [TRANSIT].
This update specifies algorithms that are more appropriate today.
CRMF is defined using Abstract Syntax Notation One (ASN.1) [X680].
2. Terminology
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.
3. Signature Key POP
Section 4.1 of [RFC4211] specifies the proof-of-possession (POP)
processing. This section is updated to explicitly allow the use of
the PBMAC1 algorithm presented in Section 7.1 of [RFC8018].
OLD:
| algId identifies the algorithm used to compute the MAC value. All
| implementations MUST support id-PasswordBasedMAC. The details on
| this algorithm are presented in section 4.4.
NEW:
| algId identifies the algorithm used to compute the MAC value. All
| implementations MUST support id-PasswordBasedMAC as presented in
| Section 4.4 of [RFC4211]. Implementations MAY also support PBMAC1
| as presented in Section 7.1 of [RFC8018].
4. Password-Based Message Authentication Code
Section 4.4 of [RFC4211] specifies a Password-Based MAC that relies
on a one-way function to compute a symmetric key from the password
and a MAC algorithm. This section specifies algorithm requirements
for the one-way function and the MAC algorithm.
4.1. Introduction Paragraph
Add guidance about limiting the use of the password as follows:
OLD:
| This MAC algorithm was designed to take a shared secret (a
| password) and use it to compute a check value over a piece of
| information. The assumption is that, without the password, the
| correct check value cannot be computed. The algorithm computes
| the one-way function multiple times in order to slow down any
| dictionary attacks against the password value.
NEW:
| This MAC algorithm was designed to take a shared secret (a
| password) and use it to compute a check value over a piece of
| information. The assumption is that, without the password, the
| correct check value cannot be computed. The algorithm computes
| the one-way function multiple times in order to slow down any
| dictionary attacks against the password value. The password used
| to compute this MAC SHOULD NOT be used for any other purpose.
4.2. One-Way Function
Change the paragraph describing the "owf" as follows:
OLD:
| owf identifies the algorithm and associated parameters used to
| compute the key used in the MAC process. All implementations MUST
| support SHA-1.
NEW:
| owf identifies the algorithm and associated parameters used to
| compute the key used in the MAC process. All implementations MUST
| support SHA-256 [SHS].
4.3. Iteration Count
Update the guidance on appropriate iteration count values as follows:
OLD:
| iterationCount identifies the number of times the hash is applied
| during the key computation process. The iterationCount MUST be a
| minimum of 100. Many people suggest using values as high as 1000
| iterations as the minimum value. The trade off here is between
| protection of the password from attacks and the time spent by the
| server processing all of the different iterations in deriving
| passwords. Hashing is generally considered a cheap operation but
| this may not be true with all hash functions in the future.
NEW:
| iterationCount identifies the number of times the hash is applied
| during the key computation process. The iterationCount MUST be a
| minimum of 100; however, the iterationCount SHOULD be as large as
| server performance will allow, typically at least 10,000 [DIGALM].
| There is a trade-off between protection of the password from
| attacks and the time spent by the server processing the
| iterations. As part of that trade-off, an iteration count smaller
| than 10,000 can be used when automated generation produces shared
| secrets with high entropy.
4.4. MAC Algorithm
Change the paragraph describing the "mac" as follows:
OLD:
| mac identifies the algorithm and associated parameters of the MAC
| function to be used. All implementations MUST support HMAC-SHA1
| [HMAC]. All implementations SHOULD support DES-MAC and Triple-
| DES-MAC [PKCS11].
NEW:
| mac identifies the algorithm and associated parameters of the MAC
| function to be used. All implementations MUST support HMAC-SHA256
| [HMAC]. All implementations SHOULD support AES-GMAC [AES] [GMAC]
| with a 128-bit key.
For convenience, the identifiers for these two algorithms are
repeated here.
The ASN.1 algorithm identifier for HMAC-SHA256 is defined in
[RFC4231]:
id-hmacWithSHA256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) digestAlgorithm(2) 9 }
When this object identifier is used in the ASN.1 algorithm
identifier, the parameters SHOULD be present. When present, the
parameters MUST contain a type of NULL as specified in [RFC4231].
The ASN.1 algorithm identifier for AES-GMAC [AES] [GMAC] with a
128-bit key is defined in [RFC9044]:
id-aes128-GMAC OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101) csor(3)
nistAlgorithm(4) aes(1) 9 }
When this object identifier is used in the ASN.1 algorithm
identifier, the parameters MUST be present, and the parameters MUST
contain the GMACParameters structure as follows:
GMACParameters ::= SEQUENCE {
nonce OCTET STRING,
length MACLength DEFAULT 12 }
MACLength ::= INTEGER (12 | 13 | 14 | 15 | 16)
The GMACParameters nonce parameter is the GMAC initialization vector.
The nonce may have any number of bits between 8 and (2^64)-1, but it
MUST be a multiple of 8 bits. Within the scope of any GMAC key, the
nonce value MUST be unique. A nonce value of 12 octets can be
processed more efficiently, so that length for the nonce value is
RECOMMENDED.
The GMACParameters length parameter field tells the size of the
message authentication code in octets. GMAC supports lengths between
12 and 16 octets, inclusive. However, for use with CRMF, the maximum
length of 16 octets MUST be used.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
The security of the Password-Based MAC relies on the number of times
the hash function is applied as well as the entropy of the shared
secret (the password). Hardware support for hash calculation is
available at very low cost [PHS], which reduces the protection
provided by a high iterationCount value. Therefore, the entropy of
the password is crucial for the security of the Password-Based MAC
function. In 2010, researchers showed that about half of the real-
world passwords in a leaked corpus can be broken with less than 150
million trials, indicating a median entropy of only 27 bits [DMR].
Higher entropy can be achieved by using randomly generated strings.
For example, assuming an alphabet of 60 characters, a randomly chosen
password with 10 characters offers 59 bits of entropy, and 20
characters offers 118 bits of entropy. Using a one-time password
also increases the security of the MAC, assuming that the integrity-
protected transaction will complete before the attacker is able to
learn the password with an offline attack.
Please see [RFC8018] for security considerations related to PBMAC1.
Please see [HMAC] and [SHS] for security considerations related to
HMAC-SHA256.
Please see [AES] and [GMAC] for security considerations related to
AES-GMAC.
Cryptographic algorithms age; they become weaker with time. As new
cryptanalysis techniques are developed and computing capabilities
improve, the work required to break a particular cryptographic
algorithm will reduce, making an attack on the algorithm more
feasible for more attackers. While it is unknown how cryptanalytic
attacks will evolve, it is certain that they will get better. It is
unknown how much better they will become or when the advances will
happen. For this reason, the algorithm requirements for CRMF are
updated by this specification.
When a Password-Based MAC is used, implementations must protect the
password and the MAC key. Compromise of either the password or the
MAC key may result in the ability of an attacker to undermine
authentication.
7. References
7.1. Normative References
[AES] National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", FIPS PUB 197,
DOI 10.6028/NIST.FIPS.197, November 2001,
<https://doi.org/10.6028/NIST.FIPS.197>.
[GMAC] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D,
November 2007, <https://doi.org/10.6028/NIST.SP.800-38D>.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[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>.
[RFC4211] Schaad, J., "Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)", RFC 4211,
DOI 10.17487/RFC4211, September 2005,
<https://www.rfc-editor.org/info/rfc4211>.
[RFC8018] Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
Password-Based Cryptography Specification Version 2.1",
RFC 8018, DOI 10.17487/RFC8018, January 2017,
<https://www.rfc-editor.org/info/rfc8018>.
[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>.
[RFC9044] Housley, R., "Using the AES-GMAC Algorithm with the
Cryptographic Message Syntax (CMS)", RFC 9044,
DOI 10.17487/RFC9044, May 2021,
<https://www.rfc-editor.org/info/rfc9044>.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://doi.org/10.6028/NIST.FIPS.180-4>.
[X680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, August 2015.
7.2. Informative References
[DIGALM] National Institute of Standards and Technology, "Digital
Identity Guidelines: Authentication and Lifecycle
Management", NIST Special Publication 800-63B,
DOI 10.6028/NIST.SP.800-63B, June 2017,
<https://doi.org/10.6028/NIST.SP.800-63B>.
[DMR] Dell'Amico, M., Michiardi, P., and Y. Roudier, "Password
Strength: An Empirical Analysis",
DOI 10.1109/INFCOM.2010.5461951, March 2010,
<https://doi.org/10.1109/INFCOM.2010.5461951>.
[PHS] Pathirana, A., Halgamuge, M., and A. Syed, "Energy
Efficient Bitcoin Mining to Maximize the Mining Profit:
Using Data from 119 Bitcoin Mining Hardware Setups",
International Conference on Advances in Business
Management and Information Technology, pp. 1-14, November
2019.
[PKCS11] RSA Laboratories, "PKCS #11 v2.11: Cryptographic Token
Interface Standard", November 2001.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, DOI 10.17487/RFC4231, December 2005,
<https://www.rfc-editor.org/info/rfc4231>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[TRANSIT] National Institute of Standards and Technology,
"Transitioning the Use of Cryptographic Algorithms and Key
Lengths", NIST Special Publication 800-131Ar2,
DOI 10.6028/NIST.SP.800-131Ar2, March 2019,
<https://doi.org/10.6028/NIST.SP.800-131Ar2>.
[WITHDRAW] National Institute of Standards and Technology, "NIST
Withdraws Outdated Data Encryption Standard", June 2005,
<https://www.nist.gov/news-events/news/2005/06/nist-
withdraws-outdated-data-encryption-standard>.
Acknowledgements
Many thanks to Hans Aschauer, Hendrik Brockhaus, Quynh Dang, Roman
Danyliw, Lars Eggert, Tomas Gustavsson, Jonathan Hammell, Tim
Hollebeek, Ben Kaduk, Erik Kline, Lijun Liao, Mike Ounsworth,
Francesca Palombini, Tim Polk, Ines Robles, Mike StJohns, and Sean
Turner for their careful review and improvements.
Author's Address
Russ Housley
Vigil Security, LLC
516 Dranesville Road
Herndon, VA 20170
United States of America
Email: housley@vigilsec.com