INTERNET DRAFT R. Housley
Internet Engineering Task Force (IETF) Vigil Security
Intended Status: Standards Track T. Polk
NIST
Expires: 14 May 2010 14 November 2010
Database of Long-Lived Symmetric Cryptographic Keys
<draft-ietf-karp-crypto-key-table-00.txt>
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Abstract
This document specifies the information contained in a database of
long-lived cryptographic keys used by many different security
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protocols. The database design supports both manual and automated
key management. In many instances, the security protocols do not
directly use the long-lived key, but rather a key derivation function
is used to derive a short-lived key from a long-lived key.
1. Introduction
This document specifies the information that needs to be included in
a database of long-lived cryptographic keys. This conceptual
database is designed to support both manual key management and
automated key management. The intent is to allow many different
implementation approaches to the specified cryptographic key
database.
Security protocols such as TCP-AO [RFC5925] are expected to use an
application program interface (API) to select a long-lived key from
the database. In many instances, the long-lived keys are not used
directly in security protocols, but rather a key derivation function
is used to derive short-lived key from the long-lived keys in the
database. In other instances, security protocols will directly use
the long-lived key from the database. The database design supports
both use cases.
2. Conceptual Database Structure
The database is characterized as a table, where each row represents a
single long-lived symmetric cryptographic key. Each key should only
have one row; however, in the (hopefully) very rare cases where the
same key is used for more than one purpose, multiple rows will
contain the same key value. The columns in the table represent the
key value and attributes of the key.
To accommodate manual key management, then formatting of the fields
has been purposefully chosen to allow updates with a plain text
editor.
The table has the following columns:
LocalKeyID
LocalKeyID is a 16-bit integer in hexadecimal. The LocalKeyID
can be used by a peer to identify this entry in the database.
For pairwise keys, the most significant bit in LocalKeyID is
set to zero, and the integer value must be unique among all the
pairwise keys in the database. For group keys, the most
significant bit in LocalKeyID is set to one, but collisions
among group key identifiers must be accommodated.
PeerKeyID
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For pairwise keys, the PeerKeyID field is a 16 bit integer in
hexadecimal provided by the peer. If the peer has not yet
provided this value, the PeerKeyID is set to "unknown". For
group keying, the PeerKeyID field is set to "group", which
easily accommodates group keys generated by a third party.
Peers
The Peers field identifies the peer system or set of systems
that have this key configured in their own database of long-
lived keys. For pairwise keys, the database on the peer system
LocalKeyID field will contain the value specified in the
PeerKeyID field in the local database. For group keying, the
Peers field names the group, not the individual systems that
comprise the group.
Interfaces
The Interfaces field identifies the set of interfaces for which
it is appropriate to use this key. When the long-lived value
in the Key field is intended for use on any interface, the
Interfaces field is set to "all".
Protocol
The Protocol field identifies a single security protocol where
this key may be used to provide cryptographic protection.
KDF
The KDF field indicates which key derivation function is used
to generate short-lived keys from the long-lived value in the
Key field. When the long-lived value in the Key field is
intended for direct use, the KDF field is set to "none".
KDFInputs
The KDFInputs field is used when supplementary public or
private data is supplied to the KDF. For protocols that do not
require additional information for the KDF, the KDFInputs field
is set to "none". The Protocol field will determine the format
of this field if it is not "none".
AlgID
The AlgID field indicates which cryptographic algorithm to be
used with the security protocol for the specified peer. The
algorithm may be an encryption algorithm and mode (such as
AES-128-CBC), an authentication algorithm (such as HMAC-SHA1-96
or AES-128-CMAC), or any other symmetric cryptographic
algorithm needed by a security protocol. If the KDF field
contains "none", then the long-lived key is used directly with
this algorithm, otherwise the derived short-lived key is used
with this algorithm. When the long-lived key is used to
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generate a set of short-lived keys for use with the security
protocol, the AlgID field identifies a ciphersuite rather than
a single cryptographic algorithm.
Key
The Key is a hexadecimal string representing a long-lived
symmetric cryptographic key. The size of the Key depends on
the KDF and the AlgID. For example, a KDF=none and
AlgID=AES128 requires a 128-bit key, which is represented by 32
hexadecimal digits.
Direction
The Direction field indicates whether this key may be used for
inbound traffic, outbound traffic, or both. The supported
values are "in", "out", and "both", respectively. The Protocol
field will determine which of these values are valid.
NotBefore
The NotBefore field specifies the earliest date and time in
Universal Coordinated Time (UTC) at which this key should be
considered for use. The format is YYYYMMDDHHSSZ, where four
digits specify the year, two digits specify the month, two
digits specify the day, two digits specify the hour, and two
digits specify the minute. The "Z" is included as a clear
indication that the time is in UTC.
NotAfter
The NotAfter field specifies the latest date and time at which
this key should be considered for use. The format is the same
as the NotBefore field.
Note that some security protocols use a KeyID value of zero for
special purposes, so care is needed if this KeyID value is included
in the table.
3. Key Selection and Rollover
When a system desires to protect a unicast protocol data unit for a
remote system H using security protocol P via interface I, the local
system selects a long-lived key at time T from the database, any key
that satisfies the following conditions may be used:
(1) the Peer field includes H;
(2) the PeerKeyID field is not "unknown";
(3) the Protocol field matches P;
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(4) the Interfaces field includes I;
(5) the Direction field is either "out" or "both"; and
(6) NotBefore <= T <= NotAfter.
The value in the PeerKeyID field is used to identify the selected key
to the remote system H.
Group key selection is different than pairwise key selection. When a
system desires to protect a multicast protocol data unit for a group
of systems G using security protocol P via interface I, the local
system selects a long-lived key at time T from the database, any key
that satisfies the following conditions may be used:
(1) the Peer field includes the multicast group G;
(2) the PeerKeyID field is "group";
(3) the Protocol field matches P;
(4) the Interfaces field includes I;
(5) the Direction field is either "out" or "both"; and
(6) NotBefore <= T <= NotAfter.
The value in the LocalKeyID field is used to identify the selected
key since all of the systems in the group G use the same identifier.
During algorithm transition, multiple entries may exist associated
with different cryptographic algorithms or ciphersuites. Systems
should support selection of keys based on algorithm preference.
In addition, multiple entries with overlapping use periods are
expected to be employed to provide orderly key rollover. In these
cases, the expectation is that systems will transition to the newest
key available. To meet this requirement, this specification
recommends supplementing the key selection algorithm with the
following differentiation: select the long-lived key specifying the
most recent time in the NotBefore field.
When a system participates in a security protocol, a sending peer
system H has selected a long-lived key and the LocalKeyID is included
in the protocol control information. When retrieving the long-lived
key (for direct use or for key derivation), the local system should
confirm the following conditions are satisfied before use:
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(1) the Peer field includes H;
(2) the Protocol field matches P;
(3) the Interface field includes I;
(4) the Direction field is either "in" or "both"; and
(5) NotBefore <= T <= NotAfter.
Note that the key usage is loosely bound by the times specified in
the NotBefore and NotAfter fields. New security associations should
not be established except within the period of use specified by these
fields, while allowing some grace time for clock skew. However, if a
security association has already been established based on a
particular long-lived key, exceeding the lifetime does not have any
direct impact. Implementations of protocols that involve long-lived
security association should be designed to periodically interrogate
the database and rollover to new keys without tearing down the
security association.
For group keying, the local system should confirm the following
conditions are satisfied before use:
(1) the Peer field includes the multicast group G;
(2) the PeerKeyID field is "group";
(3) the Protocol field matches P;
(4) the Interface field includes I;
(5) the Direction field is either "in" or "both"; and
(6) NotBefore <= T <= NotAfter.
As long as a key remains in the database, the key may be used for
received traffic. Any key that is unacceptable for received traffic
needs to be removed from the database.
4. Operational Considerations
If usage periods for long-lived keys do not overlap and system clocks
are inconsistent, it is possible to construct scenarios where systems
cannot agree upon a long-lived key. When installing a series of keys
to be used one after the other (sometimes called a key chain),
operators should configure the NotAfter field of the preceding key to
be several days after the NotBefore field of the subsequent key to
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ensure that clock skew is not a concern.
For group keys, the most significant bit in LocalKeyID must be set to
one. Collisions among group key identifiers can be avoided by
subdividing the remaining 15 bits of the LocalKeyID field into an
identifier of the group key generator and an identifier assigned by
that generator.
5. Security Considerations
Management of encryption and authentication keys has been a
significant operational problem, both in terms of key synchronization
and key selection. For example, current guidance [RFC3562] warns
against sharing TCP MD5 keying material between systems, and
recommends changing keys according to a schedule. The same general
operational issues are relevant for the management of other
cryptographic keys.
It is recognized in [RFC4107] that automated key management is not
viable in some situations. The conceptual database specified in this
document is intended to accommodate both manual key management and
automated key management. A future specification to automatically
populate rows in the database is envisioned.
Designers should recognize the warning provided in [RFC4107]:
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.
6. IANA Considerations
No IANA actions are required.
{{{ RFC Editor: Please remove this section prior to publication. }}}
7. Acknowledgments
This document reflects many discussions with many different people
over many years. In particular, the authors thank Jari Arkko, Ran
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Atkinson, Ron Bonica, Ross Callon, Lars Eggert, Pasi Eronen, Adrian
Farrel, Sam Hartman, Gregory Lebovitz, Sandy Murphy, Eric Rescorla,
Mike Shand, Dave Ward, and Brian Weis for their insights.
8. Informational References
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", RFC 4107, BCP 107, June 2005.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
Authors' Addresses
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Tim Polk
National Institute of Standards and Technology
100 Bureau Drive, Mail Stop 8930
Gaithersburg, MD 20899-8930
USA
EMail: tim.polk@nist.gov
Housley & Polk [Page 8]
