| RFC 9045 | CRMF Algorithm Requirements Update | June 2021 |
| Housley | Standards Track | [Page] |
This document updates the cryptographic algorithm requirements for thePassword-Based Message Authentication Code in the Internet X.509 PublicKey Infrastructure Certificate Request Message Format (CRMF) specified inRFC 4211.¶
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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.¶
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This document updates the cryptographic algorithm requirements for thePassword-Based Message Authentication Code (MAC) in the Internet X.509Public Key Infrastructure Certificate Request Message Format (CRMF)[RFC4211]. The algorithms specified in[RFC4211] were appropriate in2005; however, these algorithms are no longer considered the bestchoices:¶
This update specifies algorithms that are more appropriate today.¶
CRMF is defined using Abstract Syntax Notation One (ASN.1)[X680].¶
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.¶
Section 4.1 of [RFC4211] specifies the proof-of-possession (POP)processing. This section is updated to explicitly allow the useof the PBMAC1 algorithm presented inSection 7.1 of [RFC8018].¶
OLD:¶
algId identifies the algorithm used to compute the MAC value. All implementationsMUST support id-PasswordBasedMAC. The details on this algorithm are presented in section4.4.¶
NEW:¶
algId identifies the algorithm used to compute the MAC value. All implementationsMUST support id-PasswordBasedMAC as presented inSection 4.4 of [RFC4211]. ImplementationsMAY also support PBMAC1 as presented inSection 7.1 of [RFC8018].¶
Section 4.4 of [RFC4211] specifies a Password-Based MAC that relies ona one-way function to compute a symmetric key from the password and a MACalgorithm. This section specifies algorithm requirements for the one-wayfunction and the MAC algorithm.¶
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 MACSHOULD NOT be used for any other purpose.¶
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 implementationsMUST support SHA-1.¶
NEW:¶
owf identifies the algorithm and associated parameters used to compute the key used in the MAC process. All implementationsMUST support SHA-256[SHS].¶
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 iterationCountMUST 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 iterationCountMUST be a minimum of 100; however, the iterationCountSHOULD 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.¶
Change the paragraph describing the "mac" as follows:¶
OLD:¶
mac identifies the algorithm and associated parameters of the MAC function to be used. All implementationsMUST support HMAC-SHA1[HMAC]. All implementationsSHOULD support DES-MAC and Triple-DES-MAC[PKCS11].¶
NEW:¶
mac identifies the algorithm and associated parameters of the MAC function to be used. All implementationsMUST support HMAC-SHA256[HMAC]. All implementationsSHOULD support AES-GMAC[AES][GMAC] with a 128-bit key.¶
For convenience, the identifiers for these two algorithms arerepeated 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, theparametersSHOULD be present. When present, the parametersMUST contain atype of NULL as specified in[RFC4231].¶
The ASN.1 algorithm identifier for AES-GMAC[AES][GMAC] with a 128-bitkey 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, theparametersMUST be present, and the parametersMUST contain theGMACParameters 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. Thenonce may have any number of bits between 8 and (2^64)-1, but itMUST be amultiple of 8 bits. Within the scope of any GMAC key, the nonce valueMUST be unique. A nonce value of 12 octets can be processed moreefficiently, so that length for the nonce value isRECOMMENDED.¶
The GMACParameters length parameter field tells the size of themessage authentication code in octets. GMAC supports lengths between12 and 16 octets, inclusive. However, for use with CRMF, the maximumlength of 16 octetsMUST be used.¶
This document has no IANA actions.¶
The security of the Password-Based MAC relies on the number of times thehash function is applied as well as the entropy of the shared secret (thepassword). Hardware support for hash calculation is available at very lowcost[PHS], which reduces the protection provided by a high iterationCountvalue. Therefore, the entropy of the password is crucial for the security ofthe Password-Based MAC function. In 2010, researchers showed that about halfof the real-world passwords in a leaked corpus can be broken with less than150 million trials, indicating a median entropy of only 27 bits[DMR]. Higherentropy can be achieved by using randomly generated strings. For example,assuming an alphabet of 60 characters, a randomly chosen password with 10 charactersoffers 59 bits of entropy, and 20 characters offers 118 bits of entropy. Usinga one-time password also increases the security of the MAC, assuming that theintegrity-protected transaction will complete before the attacker is able tolearn 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 newcryptanalysis techniques are developed and computing capabilitiesimprove, the work required to break a particular cryptographicalgorithm will reduce, making an attack on the algorithm morefeasible for more attackers. While it is unknown how cryptanalyticattacks will evolve, it is certain that they will get better. It isunknown how much better they will become or when the advances willhappen. For this reason, the algorithm requirements for CRMF areupdated by this specification.¶
When a Password-Based MAC is used, implementations must protect thepassword and the MAC key. Compromise of either the password or the MACkey may result in the ability of an attacker to undermine authentication.¶
Many thanks toHans 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, andSean Turnerfor their careful review and improvements.¶