



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.

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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
