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draft-ietf-cat-kerb-key-derivation-00.txt
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Network Working Group M. Horowitz
<draft-ietf-cat-kerb-key-derivation-00.txt> Cygnus Solutions
Internet-Draft November, 1996
Key Derivation for Kerberos V5
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
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(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Distribution of this memo is unlimited. Please send comments to the
<cat-ietf@mit.edu> mailing list.
Abstract
In the Kerberos protocol [RFC1510], cryptographic keys are used in a
number of places. In order to minimize the effect of compromising a
key, it is desirable to use a different key for each of these places.
Key derivation [Horowitz96] can be used to construct different keys
for each operation from the keys transported on the network. For
this to be possible, a small change to the specification is
necessary.
Overview
Under RFC1510 as stated, key derivation could be specified as a set
of encryption types which share the same key type. The constant for
each derivation would be a function of the encryption type. However,
it is generally accepted that, for interoperability, key types and
encryption types must map one-to-one onto each other. (RFC 1510 is
being revised to address this issue.) Therefore, to use key
derivcation with Kerberos V5 requires a small change to the
specification.
For each place where a key is used in Kerberos, a ``key usage'' must
be specified for that purpose. The key, key usage, and
encryption/checksum type together describe the transformation from
plaintext to ciphertext, or plaintext to checksum. For backward
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Internet Draft Key Derivation for Kerberos V5 November, 1996
compatibility, old encryption types would be defined independently of
the key usage.
Key Usage Values
This is a complete list of places keys are used in the kerberos
protocol, with key usage values and RFC 1510 section numbers:
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
client key (section 5.4.1)
2. AS-REP Ticket and TGS-REP Ticket (includes tgs session key or
application session key), encrypted with the service key
(section 5.4.2)
3. AS-REP encrypted part (includes tgs session key or application
session key), encrypted with the client key (section 5.4.2)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs
session key (section 5.4.1)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs
authenticator subkey (section 5.4.1)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed
with the tgs session key (sections 5.3.2, 5.4.1)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes tgs
authenticator subkey), encrypted with the tgs session key
(section 5.3.2)
8. TGS-REP encrypted part (includes application session key),
encrypted with the tgs session key (section 5.4.2)
9. TGS-REP encrypted part (includes application session key),
encrypted with the tgs authenticator subkey (section 5.4.2)
10. AP-REQ Authenticator cksum, keyed with the application session
key (section 5.3.2)
11. AP-REQ Authenticator (includes application authenticator
subkey), encrypted with the application session key (section
5.3.2)
12. AP-REP encrypted part (includes application session subkey),
encrypted with the application session key (section 5.5.2)
13. KRB-PRIV encrypted part, encrypted with a key chosen by the
application (section 5.7.1)
14. KRB-CRED encrypted part, encrypted with a key chosen by the
application (section 5.6.1)
15. KRB-SAVE cksum, keyed with a key chosen by the application
(section 5.8.1)
16. Data which is defined in some specification outside of
Kerberos to be encrypted using an RFC1510 encryption type.
17. Data which is defined in some specification outside of
Kerberos to be checksummed using an RFC1510 checksum type.
A few of these key usages need a little clarification. A service
which receives an AP-REQ has no way to know if the enclosed Ticket
was part of an AS-REP or TGS-REP. Therefore, key usage 2 must always
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Internet Draft Key Derivation for Kerberos V5 November, 1996
be used for generating a Ticket, whether it is in response to an AS-
REQ or TGS-REQ.
There might exist other documents which define protocols in terms of
the RFC1510 encryption types or checksum types. Such documents would
not know about key usages. In order that these documents continue to
be meaningful until they are updated, key usages 16 and 17 must be
used to derive keys for encryption and checksums, respectively. New
protocols defined in terms of the Kerberos encryption and checksum
types should use their own key usages. Key usages may be registered
with IANA to avoid conflicts. Key usages shall be unsigned 32 bit
integers. Zero is not permitted.
Defining Cryptosystems Using Key Derivation
Kerberos requires that the ciphertext component of EncryptedData be
tamper-resistant as well as confidential. This implies encryption
and integrity functions, which must each use their own separate keys.
So, for each key usage, two keys must be generated, one for
encryption (Ke), and one for integrity (Ki):
Ke = DK(protocol key, key usage | 0xAA)
Ki = DK(protocol key, key usage | 0x55)
where the key usage is represented as a 32 bit integer in network
byte order. The ciphertest must be generated from the plaintext as
follows:
ciphertext = E(Ke, confounder | length | plaintext | padding) |
H(Ki, confounder | length | plaintext | padding)
The confounder and padding are specific to the encryption algorithm
E.
When generating a checksum only, there is no need for a confounder or
padding. Again, a new key (Kc) must be used. Checksums must be
generated from the plaintext as follows:
Kc = DK(protocol key, key usage | 0x99)
MAC = H(Kc, length | plaintext)
Note that each enctype is described by an encryption algorithm E and
a keyed hash algorithm H, and each checksum type is described by a
keyed hash algorithm H. HMAC, with an appropriate hash, is
recommended for use as H.
Security Considerations
This entire document addresses shortcomings in the use of
cryptographic keys in Kerberos V5.
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Internet Draft Key Derivation for Kerberos V5 November, 1996
Acknowledgements
I would like to thank Uri Blumenthal, Sam Hartman, and Bill
Sommerfeld for their contributions to this document.
References
[Horowitz96] Horowitz, M., "Key Derivation for Authentication,
Integrity, and Privacy", draft-horowitz-key-derivation-00.txt,
November 1996. [RFC1510] Kohl, J. and Neuman, C., "The Kerberos
Network Authentication Service (V5)", RFC 1510, September 1993.
Author's Address
Marc Horowitz
Cygnus Solutions
955 Massachusetts Avenue
Cambridge, MA 02139
Phone: +1 617 354 7688
Email: marc@cygnus.com
Horowitz [Page 4]