Network Working Group                                 Steven M. Bellovin
Internet Draft                                        AT&T Labs Research

Expiration Date: October 2003                                 April 2003

           Guidelines for Mandating Automated Key Management


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   The question often arises of whether or not a given security system
   requires some form of automated key management, or whether manual
   keying would suffice.  This memo proposes guidelines for making such
   decisions; the presumption is that automated key management is
   generally but not always needed; if manual keying is proposed, the
   burden of proof is on the proposer.

Bellovin                                                        [Page 1]

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1. Introduction

   The question often arises of whther or not a given security system
   requires some form of automated key management, or whether manual
   keying would suffice.

   There is no 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 its
   disadvantages.  We thus outline concerns that would suggest that
   manual key management would be a bad idea.

1.1. Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

2. Requirements

   These are a set of guidelines, not rules, for evaluating when
   automated key management should or shouldn't be used.  Informed
   judgment is necessary when applying them.

   In this context, "key management" is automatic derivation of session
   key(s), as opposite to long-term keys used to authenticate the
   derived key(s). How this long-term key gets to the talking entities
   and what kind of a key it is (pre-shared secret, RSA public key, DSA,
   you-name-it) is beyond the scope of this document.  Examples of key
   management systems include IKE and Kerberos; S/MIME and TLS include
   key management functions.

   A session key is used to protect application data.

   In general, automated key management SHOULD be used.  This is a very
   strong "SHOULD".

   Key management MUST be used if:

        A central party will have to manage n^2 static keys.

        A stream cipher such as RC4 or AES counter mode [AESMODE] is

Bellovin                                                        [Page 2]

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        Long-lived session keys are used by more than two parties.
        (Except for multicast, this is a dubious situation in the first
        place, and should generally be discouraged no matter what.)

        The likely operational environment is one where personnel (or
        device) turnover is reasonably frequent, thus creating a
        requirement for frequent rekeying.

Even manually-keyed systems need some provision 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/or revoking compromised keys.  If done well, such
mechanisms can later be used by an add-on key management scheme.

Lack of automated key management can lead to vulnerabilities, including
(but not limited to) cryptographic weaknesses or loss of some
functionality, such as replay protection.

Key management software is not always large or bloated; even IKEv1 can
be done in <200K, and TLS in half that much space.  (TLS includes other
functionality as well.)

Lack of clarity about who the principals are is not a valid reason for
avoiding key management.  Rather, it tends to indicate a deeper problem
with the underlying security model.

Key management schemes should not be designed by amateurs; it is almost
certainly inappropriate for WGs to design their own.  To put it in
concrete terms, the very first key management protocol in the open
literature was published in 1978.  A flaw was published in 1983.  The
fix proposed in 1983 was cracked in 1994.  In 1996, a new flaw was found
in the original 1978 version, in an area not affected by the 1983/1994
issue.  All of these flaws were blindingly obvious once described -- but
no one spotted them earlier.  Note that the original protocol
(translated to know about certificates, which hadn't been invented at
the time) was only 3 messages.

Situations where a desire to avoid key mangement may be reasonable

     Very limited available bandwidth or very high round-trip times.
     There are interactions here -- 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.

Bellovin                                                        [Page 3]

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     Low value of the information

     Very limited scale of each deployment

Note that assertions about such things should often be viewed with the
skepticism afforded to claims that "this will only be used on a LAN or
two".  In other words, the burden of demonstrating that manual key
management is appropriate should be on the proponents --- and it's a
fairly high barrier that they need to overcome.

3. References

   [AESMODE]   "Recommendation for Block Cipher Modes of Operation -
               Methods and Techniques", NIST Special Publication SP
               800-38A, December 2001.

   [RFC2119]   "Key words for use in RFCs to Indicate Requirement
               Levels", S. Bradner.  RFC 2119.  March 1997.

4. Author Information

Steven M. Bellovin
AT&T Labs Research
Shannon Laboratory
180 Park Avenue
Florham Park, NJ 07932
Phone: +1 973-360-8656

Bellovin                                                        [Page 4]