Network Working Group S. Bellovin
Request for Comments: 4107 Columbia University
BCP: 107 R. Housley
Category: Best Current Practice Vigil Security
June 2005
Guidelines for Cryptographic Key Management
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The question often arises of whether a given security system requires
some form of automated key management, or whether manual keying is
sufficient. This memo provides guidelines for making such decisions.
When symmetric cryptographic mechanisms are used in a protocol, the
presumption is that automated key management is generally but not
always needed. If manual keying is proposed, the burden of proving
that automated key management is not required falls to the proposer.
1. Introduction
The question often arises of whether or not a given security system
requires some form of automated key management, or whether manual
keying is sufficient.
There is not one answer to that question; circumstances differ. In
general, automated key management SHOULD be used. Occasionally,
relying on manual key management is reasonable; we propose some
guidelines for making that judgment.
On the other hand, relying on manual key management has significant
disadvantages, and we outline the security concerns that justify the
preference for automated key management. However, there are
situations in which manual key management is acceptable.
Bellovin & Housley Best Current Practice [Page 1]RFC 4107 Guidelines for Cryptographic Key Management June 2005
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 RFC 2119 [B].
2. Guidelines
These guidelines are for use by IETF working groups and protocol
authors who are determining whether to mandate automated key
management and whether manual key management is acceptable. Informed
judgment is needed.
The term "key management" refers to the establishment of
cryptographic keying material for use with a cryptographic algorithm
to provide protocol security services, especially integrity,
authentication, and confidentiality. Automated key management
derives one or more short-term session keys. The key derivation
function may make use of long-term keys to incorporate authentication
into the process. The manner in which this long-term key is
distributed to the peers and the type of key used (pre-shared
symmetric secret value, RSA public key, DSA public key, and others)
is beyond the scope of this document. However, it is part of the
overall key management solution. Manual key management is used to
distribute such values. Manual key management can also be used to
distribute long-term session keys.
Automated key management and manual key management provide very
different features. In particular, the protocol associated with an
automated key management technique will confirm the liveness of the
peer, protect against replay, authenticate the source of the short-
term session key, associate protocol state information with the
short-term session key, and ensure that a fresh short-term session
key is generated. Further, an automated key management protocol can
improve interoperability by including negotiation mechanisms for
cryptographic algorithms. These valuable features are impossible or
extremely cumbersome to accomplish with manual key management.
For some symmetric cryptographic algorithms, implementations must
prevent overuse of a given key. An implementation of such algorithms
can make use of automated key management when the usage limits are
nearly exhausted, in order to establish replacement keys before the
limits are reached, thereby maintaining secure communications.
Examples of automated key management systems include IPsec IKE and
Kerberos. S/MIME and TLS also include automated key management
functions.
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RFC 4107 Guidelines for Cryptographic Key Management June 2005
Key management schemes should not be designed by amateurs; it is
almost certainly inappropriate for working groups to design their
own. To put it in concrete terms, the very first key management
protocol in the open literature was published in 1978 [NS]. A flaw
and a fix were published in 1981 [DS], and the fix was cracked in
1994 [AN]. In 1995 [L], a new flaw was found in the original 1978
version, in an area not affected by the 1981/1994 issue. All of
these flaws were obvious once described -- yet no one spotted them
earlier. Note that the original protocol (translated to employ
certificates, which had not been invented at that time) was only
three messages.
Key management software is not always large or bloated. Even IKEv1
[HC] can be done in less than 200 Kbytes of object code, and TLS [DA]
in half that space. Note that this TLS estimate includes other
functionality as well.
A session key is used to protect a payload. The nature of the
payload depends on the layer where the symmetric cryptography is
applied.
In general, automated key management SHOULD be used to establish
session keys. Strong justification is needed in the security
considerations section of a proposal that makes use of manual key
management.
2.1. Automated Key Management
Automated key management MUST be used if any of these conditions
hold:
A party will have to manage n^2 static keys, where n may become
large.
Any stream cipher (such as RC4 [TK], AES-CTR [NIST], or AES-CCM
[WHF]) is used.
An initialization vector (IV) might be reused, especially an
implicit IV. Note that random or pseudo-random explicit IVs are
not a problem unless the probability of repetition is high.
Large amounts of data might need to be encrypted in a short time,
causing frequent change of the short-term session key.
Long-term session keys are used by more than two parties.
Multicast is a necessary exception, but multicast key management
standards are emerging in order to avoid this in the future.
Sharing long-term session keys should generally be discouraged.
Bellovin & Housley Best Current Practice [Page 3]RFC 4107 Guidelines for Cryptographic Key Management June 2005
The likely operational environment is one where personnel (or
device) turnover is frequent, causing frequent change of the
short-term session key.
2.2. Manual Key Management
Manual key management may be a reasonable approach in any of these
situations:
The environment has very limited available bandwidth or very high
round-trip times. Public key systems tend to require long
messages and lots of computation; symmetric key alternatives, such
as Kerberos, often require several round trips and interaction
with third parties.
The information being protected has low value.
The total volume of traffic over the entire lifetime of the long-
term session key will be very low.
The scale of each deployment is very limited.
Note that assertions about such things should often be viewed with
skepticism. The burden of demonstrating that manual key management
is appropriate falls to the proponents -- and it is a fairly high
hurdle.
Systems that employ manual key management need provisions for key
changes. There MUST be some way to indicate which key is in use to
avoid problems during transition. Designs SHOULD sketch plausible
mechanisms for deploying new keys and replacing old ones that might
have been compromised. If done well, such mechanisms can later be
used by an add-on key management scheme.
Lack of clarity about the parties involved in authentication is not a
valid reason for avoiding key management. Rather, it tends to
indicate a deeper problem with the underlying security model.
2.3. Key Size and Random Values
Guidance on cryptographic key size for public keys that are used for
exchanging symmetric keys can be found in BCP 86 [OH].
When manual key management is used, long-term shared secret values
SHOULD be at least 128 bits.
Guidance on random number generation can be found in BCP 106 [ESC].
Bellovin & Housley Best Current Practice [Page 4]RFC 4107 Guidelines for Cryptographic Key Management June 2005
When manual key management is used, long-term shared secrets MUST be
unpredictable "random" values, ensuring that an adversary will have
no greater expectation than 50% of finding the value after searching
half the key search space.
3. Security Considerations
This document provides guidance to working groups and protocol
designers. The security of the Internet is improved when automated
key management is employed.
The inclusion of automated key management does not mean that an
interface for manual key management is prohibited. In fact, manual
key management is very helpful for debugging. Therefore,
implementations ought to provide a manual key management interface
for such purposes, even if it is not specified by the protocol.
4. References
This section contains normative and informative references.
4.1. Normative References
[B] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[ESC] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[OH] Orman, H. and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
April 2004
4.2. Informative References
[AN] M. Abadi and R. Needham, "Prudent Engineering Practice for
Cryptographic Protocols", Proc. IEEE Computer Society
Symposium on Research in Security and Privacy, May 1994.
[DA] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[DS] D. Denning and G. Sacco. "Timestamps in key distributed
protocols", Communication of the ACM, 24(8):533--535, 1981.
[HC] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
Bellovin & Housley Best Current Practice [Page 5]RFC 4107 Guidelines for Cryptographic Key Management June 2005
[L] G. Lowe. "An attack on the Needham-Schroeder public key
authentication protocol", Information Processing Letters,
56(3):131--136, November 1995.
[NIST] National Institute of Standards and Technology.
"Recommendation for Block Cipher Modes of Operation -- Methods
and Techniques," NIST Special Publication SP 800-38A, December
2001.
[NS] R. Needham and M. Schroeder. "Using encryption for
authentication in large networks of computers", Communications
of the ACM, 21(12), December 1978.
[TK] Thayer, R. and K. Kaukonen. "A Stream Cipher Encryption
Algorithm", Work in Progress.
[WHF] Whiting, D., Housley, R., and N. Ferguson , "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
Authors' Addresses
Steven M. Bellovin
Department of Computer Science
Columbia University
1214 Amsterdam Avenue, M.C. 0401
New York, NY 10027-7003
Phone: +1 212-939-7149
EMail: bellovin@acm.org
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
Phone: +1 703-435-1775
EMail: housley@vigilsec.com
Bellovin & Housley Best Current Practice [Page 6]RFC 4107 Guidelines for Cryptographic Key Management June 2005
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