Algorithm Requirements Update to the Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)
Vigil Security, LLC
516 Dranesville Road
Herndon
VA
20170
United States of America
housley@vigilsec.com
Security
Authentication
Message Authentication Code
Password-Based Message Authentication Code
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.
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)
. The algorithms specified in were appropriate in
2005; however, these algorithms are no longer considered the best
choices:
- HMAC-SHA1 is not broken yet, but there are much stronger alternatives .
- DES-MAC provides 56 bits of security, which is no longer considered secure .
- Triple-DES-MAC provides 112 bits of security, which is now deprecated .
This update specifies algorithms that are more appropriate today.
CRMF is defined using Abstract Syntax Notation One (ASN.1) .
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
when, and only when, they appear in all capitals, as shown here.
Signature Key POP
specifies the proof-of-possession (POP)
processing. This section is updated to explicitly allow the use
of the PBMAC1 algorithm presented in .
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 .
NEW:
algId identifies the algorithm used to compute the MAC value. All
implementations MUST support id-PasswordBasedMAC as presented in
. Implementations MAY also support
PBMAC1 as presented in .
Password-Based Message Authentication Code
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.
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.
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 .
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
. 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.
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
. All implementations SHOULD support DES-MAC and Triple-DES-MAC .
NEW:
mac identifies the algorithm and associated parameters of the MAC
function to be used. All implementations MUST support HMAC-SHA256
. All implementations SHOULD support 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 :
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 .
The ASN.1 algorithm identifier for AES-GMAC with a 128-bit
key is defined in :
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:
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.
IANA Considerations
This document has no IANA actions.
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 , 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 . 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 for security considerations related to PBMAC1.
Please see and for security considerations related to HMAC-SHA256.
Please see and 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.
References
Normative References
Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)
Advanced Encryption Standard (AES)
National Institute of Standards and Technology
Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC
HMAC: Keyed-Hashing for Message Authentication
Secure Hash Standard (SHS)
National Institute of Standards and Technology
Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation
ITU-T
Informative References
Password Strength: An Empirical Analysis
Digital Identity Guidelines: Authentication and Lifecycle Management
National Institute of Standards and Technology
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
PKCS #11 v2.11: Cryptographic Token Interface Standard
RSA Laboratories
Transitioning the Use of Cryptographic Algorithms and Key Lengths
National Institute of Standards and Technology
NIST Withdraws Outdated Data Encryption Standard
National Institute of Standards and Technology
Acknowledgements
Many thanks to
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
, and
for their careful review and improvements.