draft-ietf-cat-kerberos-extra-tgt-02.txt [plain text]
INTERNET-DRAFT Jonathan Trostle
draft-ietf-cat-kerberos-extra-tgt-02.txt Cisco Systems
Updates: RFC 1510 Michael M. Swift
expires January 30, 2000 University of WA
Extension to Kerberos V5 For Additional Initial Encryption
0. Status Of This Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This document defines an extension to the Kerberos protocol
specification (RFC 1510)  to enable a preauthentication field in
the AS_REQ message to carry a ticket granting ticket. The session
key from this ticket granting ticket will be used to
cryptographically strengthen the initial exchange in either the
conventional Kerberos V5 case or in the case the user stores their
encrypted private key on the KDC .
In Kerberos V5, the initial exchange with the KDC consists of the
AS_REQ and AS_REP messages. For users, the encrypted part of the
AS_REP message is encrypted in a key derived from a password.
Although a password policy may be in place to prevent dictionary
attacks, brute force attacks may still be a concern due to
insufficient key length.
This draft specifies an extension to the Kerberos V5 protocol to
allow a ticket granting ticket to be included in an AS_REQ message
preauthentication field. The session key from this ticket granting
ticket will be used to cryptographically strengthen the initial
exchange in either the conventional Kerberos V5 case or in the case
the user stores their encrypted private key on the KDC . The
session key from the ticket granting ticket is combined with the
user password key (key K2 in the encrypted private key on KDC
option) using HMAC to obtain a new triple des key that is used in
place of the user key in the initial exchange. The ticket granting
ticket could be obtained by the workstation using its host key.
3. The Extension
The following new preauthentication type is proposed:
The preauthentication-data field contains a ticket granting ticket
encoded as an ASN.1 octet string. The server realm of the ticket
granting ticket must be equal to the realm in the KDC-REQ-BODY of
the AS_REQ message. In the absence of a trust relationship, the
local Kerberos client should send the AS_REQ message without this
In the conventional (non-pkinit) case, we require the RFC 1510
PA-ENC-TIMESTAMP preauthentication field in the AS_REQ message.
If neither it or the PA-PK-KEY-REQ preauthentication field is
included in the AS_REQ message, the KDC will reply with a
KDC_ERR_PREAUTH_FAILED error message.
We propose the following new etypes:
The encryption key is obtained by:
(1) Obtaining an output M from the HMAC-SHA1 function  using
the user password key (the key K2 in the encrypted private
key on KDC option of pkinit) as the text and the triple des
session key as the K input in HMAC:
M = H(K XOR opad, H(K XOR ipad, text)) where H = SHA1.
The session key from the accompanying ticket granting ticket
must be a triple des key when one of the triple des xor
encryption types is used.
(2) Concatenate the output M (20 bytes) with the first 8 non-parity
bits of the triple-des ticket granting ticket session key to
get 168 bits that will be used for the new triple-des encryption
(3) Set the parity bits of the resulting key.
The resulting triple des key is used to encrypt the timestamp
for the PA-ENC-TIMESTAMP preauthentication value (or in the
encrypted private key on KDC option of pkinit, it is used in
place of the key K2 to both sign in the PA-PK-KEY-REQ and for
encryption in the PA-PK-KEY-REP preauthentication types).
If the KDC decrypts the encrypted timestamp and it is not within
the appropriate clock skew period, the KDC will reply with the
KDC_ERR_PREAUTH_FAILED error. The same error will also be sent if
the above ticket granting ticket fails to decrypt properly, or if
it is not a valid ticket.
The KDC will create the shared triple des key from the ticket
granting ticket session key and the user password key (the key K2
in the encrypted private key on KDC case) using HMAC as specified
above and use it to validate the AS_REQ message and then to
encrypt the encrypted part of the AS_REP message (use it in place
of the key K2 for encryption in the PA-PK-KEY-REP preauthentication
Local workstation policy will determine the exact behaviour of
the Kerberos client with respect to the extension protocol. For
example, the client should consult policy to decide when to use
use the extension. This policy could be dependent on the user
identity, or whether the workstation is in the same realm as the
user. One possibility is for the workstation logon to fail if
the extension is not used. Another possibility is for the KDC
to set a flag in tickets issued when this extension is used.
A similar idea was proposed in OSF DCE RFC 26.0 ; there a
preauthentication field containing a ticket granting ticket,
a randomly generated subkey encrypted in the session key from
the ticket, and a timestamp structure encrypted in the user
password and then the randomly generated subkey was proposed.
Some advantages of the current proposal are that the KDC has two
fewer decryptions to perform per request and the client does not
have to generate a random key.
 J. Kohl, C. Neuman. The Kerberos Network Authentication
Service (V5). Request for Comments 1510.
 B. Tung, C. Neuman, J. Wray, A. Medvinsky, M. Hur, J. Trostle.
Public Key Cryptography for Initial Authentication in Kerberos.
 H. Krawczyk, M. Bellare, R. Canetti. HMAC: Keyed-Hashing for
Message Authentication. Request for Comments 2104.
 J. Pato. Using Pre-authentication to Avoid Password Guessing
Attacks. OSF DCE SIG Request for Comments 26.0.
5. Acknowledgement: We thank Ken Hornstein for some helpful comments.
6. Expires January 30, 2000.
7. Authors' Addresses
170 W. Tasman Dr.
San Jose, CA 95134, U.S.A.
Phone: (408) 527-6201