Internet-Draft R. Housley
Intended Status: Best Current Practice Vigil Security
Expires: 2 July 2015 29 December 2014
Guidelines for Cryptographic Algorithm Agility
<draft-iab-crypto-alg-agility-02.txt>
Abstract
Many IETF protocols use cryptographic algorithms to provide
confidentiality, integrity, or non-repudiation. Communicating peers
must support the same set of cryptographic algorithms for these
mechanisms to work properly. This memo provides guidelines to ensure
that protocols have the ability to migrate from one algorithm suite
to another over time.
Status of this Memo
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Copyright and License Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Guidelines for Cryptographic Algorithm Agility December 2014
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1. Introduction
Many IETF protocols use cryptographic algorithms to provide
confidentiality, integrity, authentication, or digital signature.
For interoperability, communicating peers must support the same set
of cryptographic algorithms. In most cases, a combination of
compatible cryptographic algorithms will be used to provide the
desired security services. The set of cryptographic algorithms being
used at a particular time is often referred to as a cryptographic
algorithm suite.
Cryptographic algorithms age; they become weaker with time. As new
cryptanalysis techniques are developed and computing capabilities
improve, the work factor to break a particular cryptographic
algorithm will reduce. Algorithm agility is achieved when a protocol
can easily support more that one cryptographic algorithm suite, which
ensures that protocols can migrate from one algorithm suite to
another over time. For the protocol implementer, this means that
implementations should be modular to easily accommodate the insertion
of new algorithms. For the protocol designer, this means that one or
more algorithm identifier must be carried, the set of mandatory-to-
implement algorithms will change over time, and an IANA registry of
algorithm identifiers will be needed.
1.1. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
2. Algorithm Agility Guidelines
These guidelines are for use by IETF working groups and protocol
authors for IETF protocols that make use of cryptographic algorithms.
2.1. Algorithm Identifiers
IETF protocols that make use of cryptographic algorithms MUST carry
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Guidelines for Cryptographic Algorithm Agility December 2014
one or more algorithm identifier.
Some approaches carry one identifier for each algorithm that is used.
Other approaches carry one identifier for a suite of algorithms.
Both approaches are used in IETF protocols. Designers are encouraged
to pick one of these approaches and use it consistently throughout
the protocol or family of protocols. However, suite identifiers make
it easier for the protocol designer to ensure that the algorithm
selections are complete and compatible for future assignments.
Regardless of the approach used, protocols MUST NOT negotiate
symmetric ciphers and cipher modes separately.
In the IPsec protocol suite, IKE [RFC2409][RFC4306] carries the
algorithm identifiers for AH and ESP [RFC4302][RFC4303]. Such
separation is completely fine design choice.
An IANA registry SHOULD be used for these algorithm identifiers.
2.2. Mandatory-to-Implement Algorithms
For interoperability, communicating peers must support the
cryptographic algorithm suite. For this reason, the protocol SHOULD
specify one or more mandatory-to-implement algorithm. This is not
done for protocols that are embedded in other protocols. For
example, S/MIME [RFC5751] makes use of the cryptographic message
Syntax (CMS) [RFC5652]. Other protocols also make use of CMS.
S/MIME specifies the mandatory-to-implement algorithms, not CMS.
The IETF needs to be able to change the mandatory-to-implement
algorithms over time. It is highly desirable to make this change
without updating the base protocol specification. Therefore the base
protocol specification SHOULD reference a companion algorithms
document, allowing the update of one document without necessarily
requiring an update to the other. This division also facilitates the
advancement of the base protocol specification on the standards
maturity ladder even if the algorithm document changes frequently.
Some cryptographic algorithms are inherently tied to a specific key
size, but others allow many different key sizes. Likewise, some
algorithms support parameters of different sizes, such as integrity
check values or nonces. The algorithm specification MUST identify
the specific key sizes and parameter sizes that are to be supported.
When more than one key size is available, expect the mandatory-to-
implement key size to increase over time.
Guidance on cryptographic key size for asymmetric keys can be found
in BCP 86 [RFC3766].
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Symmetric keys used for protection of long-term values SHOULD be at
least 128 bits.
2.3. Transition from Weak Algorithms
Transition from an algorithm that is found to be weak can be tricky.
It is straightforward to specify an alternative algorithm. When the
alternative algorithm is widely deployed, then the weak algorithm
should no longer be used. However, knowledge about the
implementation and deployment of the alternative algorithm is
imperfect, so one cannot be completely assured of interoperability
with alternative algorithm.
To facilitate transition, protocols MUST be able to advertise which
algorithms are supported. This may naturally occur as part of an
algorithm selection or negotiation mechanism.
In the worst case, the algorithm may be found to be tragically
flawed, permitting a casual attacker to download a simple script to
break it. Sadly, this has happened when a secure algorithm is used
incorrectly or used with poor key management. In such situations,
the protection offered by the algorithm is severely compromised,
perhaps to the point that one wants to stop using the weak algorithm
altogether, rejecting offers to use the weak algorithm well before
the alternative algorithm is widely deployed.
In any case, there comes a point in time where one refuses to use the
weak algorithm. This can happen on a flag day, or each installation
can select a date on their own.
2.4. Balance Security Strength
When selecting a suite of cryptographic algorithms, the strength of
each algorithm MUST be considered.
In CMS [RFC5652], a previously distributed symmetric key-encryption
key can be used to encrypt a content-encryption key, which is in turn
used to encrypt the content. The key-encryption and content-
encryption algorithms are often different. If, for example, a
message content is encrypted with 128-bit AES key and the content-
encryption key is wrapped with a 256-bit AES key, then at most 128
bits of protection is provided. In this situation, the algorithm and
key size selections should ensure that the key encryption is at least
as strong as the content encryption. In general, wrapping one key
with another key of a different size yields the security strength of
the shorter key.
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3. Algorithm Agility in Protocol Design
Some attempts at algorithm agility have not been completely
successful. This section provides some of the insights based on
protocol designs and deployments.
3.1. Algorithm Identifiers
If a protocol does not carry an algorithm identifier, then the
protocol version number or some other major change is needed to
transition from one algorithm to another. The inclusion of an
algorithm identifier is a minimal step toward cryptographic algorithm
agility. In addition, an IANA registry is needed to pair the
identifier with an algorithm specification.
Sometimes application layer protocols can make use of transport layer
security protocols, such as TLS or DTLS. This insulates the
application layer protocol from the cryptography altogether, but it
may still be necessary to handle the transition from unprotected
traffic to protected traffic in the application layer protocol.
3.2. Migration Mechanisms
Cryptographic algorithm selection or negotiation SHOULD be integrity
protected. If selection is not integrity-protected, then the
protocol will be subject to a downgrade attack. Without integrity
protection of algorithm selection, transition to a new cryptographic
algorithm suite will not be smooth.
When a protocol specifies a single mandatory-to-implement integrity
algorithm, eventually that algorithm will be found to be weak.
Perhaps there will be a flaw found in the integrity algorithm that
greatly shortens its expected life.
Extra care is needed when a mandatory-to-implement algorithm is used
to provide integrity protection for the negotiation of other
cryptographic algorithms. In this situation, a flaw in the
mandatory-to-implement algorithm may allow an attacker to influence
the choices of the other algorithms.
Performance is always a factor is selecting cryptographic algorithms.
In many algorithms, shorter keys offer higher performance, but less
security. Performance and security need to be balanced. Yet, all
algorithms age, and the advances in computing power available to the
attacker will eventually make them obsolete. For this reason,
protocols need mechanisms to migrate from one algorithm suite to
another over time, not just the integrity algorithm, but all
cryptographic algorithms.
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3.3. Cryptographic Key Management
Traditionally, protocol designers have avoided a more than one
approach to key management because it makes the security analysis of
the overall protocol more difficult. However, with the increasing
deployment of frameworks such as EAP and GSSAPI, the key management
is very flexible, often hiding many of the details from the
application. As a result, more and more protocols support multiple
key management approaches. In fact, the key management approach may
be negotiable, which creates a design challenge to protect the
negotiation of the key management approach before it is used to
produce cryptographic keys.
Protocols can negotiate a key management approach, derive an initial
cryptographic key, and then authenticate the negotiation. However,
if the authentication fails, the only recourse is to start the
negotiation over from the beginning.
Some environments will restrict the key management approaches by
policy. Such policies tend to improve interoperability within a
particular environment, but they cause problems for individuals that
need to work in multiple incompatible environments.
4. Security Considerations
This document provides guidance to working groups and protocol
designers. The security of the Internet is improved when broken or
weak cryptographic algorithms can be easily replaced with strong
ones.
The ability to use a algorithm of one's own choosing is very
desirable; however, this does not mean that any and all cryptographic
algorithms ought to be available in every implementation. Mandatory-
to-implement algorithms ought to be well studied, giving rise to
significant confidence. In addition, inclusion of too many
alternatives may add complexity to algorithm selection or
negotiation.
Some protocols are used to protected stored data. For example,
S/MIME [RFC5751] can protect a message kept in a mailbox. To recover
the protected stored data, protocol implementations need to support
older algorithms, even when they no longer use the older algorithms
for the protection of new stored data.
Support for too many algorithms can lead to implementation
vulnerabilities. When many algorithms are supported, some of them
will be rarely used. Any code that is rarely used can contain
undetected bugs, and algorithm implementations are no different.
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Section 2.3 talks about algorithm transition without considering any
other aspects of the protocol design. In practice, there are
dependencies between the cryptographic algorithm and other aspects of
the protocol. For example, the BEAST attack [BEAST] against TLS
[RFC5246] caused many sites to turn off modern cryptographic
algorithms in favor of older and clearly weaker algorithms.
5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
April 2004.
6. Informative References
[BEAST] http://en.wikipedia.org/wiki/
Transport_Layer_Security#BEAST_attack.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
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Acknowledgements
Thanks to Bernard Aboba, David Black, Jon Callas, Tony Finch, Ian
Grigg, Wes Hardaker, Joe Hildebrand, Christian Huitema, Paul Lambert,
Ben Laurie, Eliot Lear, Kristof Teichel, and Nico Williams for their
review and insightful comments. While some of these people do not
agree with some aspects of this document, the discussion that
resulted for their comments has certainly resulted in a better
document.
Author's Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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