Network Working Group T. Ts'o, Editor
Internet-Draft Massachusetts Institute of Technology
draft-tso-telnet-krb5-02.txt August 1999
Telnet Authentication: Kerberos Version 5
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 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
time. It is inappropriate to use Internet-Drafts as reference mate-
rial or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
1. Command Names and Codes
Authentication Types
KERBEROS_V5 2
Sub-option Commands
AUTH 0
REJECT 1
ACCEPT 2
RESPONSE 3
FORWARD 4
FORWARD_ACCEPT 5
FORWARD_REJECT 6
2. Command Meanings
Expires February 19100 [Page 1]
Internet-Draft Kerberos Version 5 for Telnet August 1999
IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <Kerberos V5
KRB_AP_REQ message> IAC SE
This is used to pass the Kerberos V5 [1] KRB_AP_REQ message to the
remote side of the connection. The first octet of the <authenti-
cation-type-pair> value is KERBEROS_V5, to indicate that Version 5
of Kerberos is being used. The Kerberos V5 authenticator in the
KRB_AP_REQ message must contain a Kerberos V5 checksum of the
two-byte authentication type pair. This checksum must be verified
by the server to assure that the authentication type pair was cor-
rectly negotiated. The Kerberos V5 authenticator must also in-
clude the optional subkey field, which shall be filled in with a
randomly chosen key. This key shall be used for encryption pur-
poses if encryption is negotiated, and shall be used as the nego-
tiated session key (i.e., used as keyid 0) for the purposes of the
telnet encryption option; if the subkey is not filled in, then the
ticket session key will be used instead.
If the AUTHENTICATION option takes place after the TELNET_OVER_TLS
option is successfully negotiated, the Kerberos V5 checksum shall
consist of the following ASN.1 structure encoded using the Distin-
guished Encoding Rules:
TLS_CHECKSUM_DATA ::= SEQUENCE {
authentication-type-pair OCTET_STRING, -- 2 bytes
SSLversion INTEGER, -- SSL version number
Cipher OCTET_STRING, -- the 3 byte cipher ID
Session_ID OCTET_STRING, -- the Session ID
Master_key OCTET_STRING, -- the master key
}
This allows the TELNET_OVER_TLS to use an anonymous Diffie-Helman
exchange to establish an encryption key, and then to securely ver-
ify the key negotiated by the anonymous Diffie-Helman exchange to
prevent any man-in-the-middle attacks.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE
This command indicates that the authentication was successful.
If the AUTH_HOW_MUTUAL bit is set in the second octet of the au-
thentication-type-pair, the RESPONSE command must be sent before
the ACCEPT command is sent.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT <op-
tional reason for rejection> IAC SE
This command indicates that the authentication was not successful,
and if there is any more data in the sub-option, it is an ASCII
Expires February 19100 [Page 2]
Internet-Draft Kerberos Version 5 for Telnet August 1999
text message of the reason for the rejection.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
<KRB_AP_REP message> IAC SE
This command is used to perform mutual authentication. It is only
used when the AUTH_HOW_MUTUAL bit is set in the second octet of
the authentication-type-pair. After an AUTH command is verified,
a RESPONSE command is sent which contains a Kerberos V5 KRB_AP_REP
message to perform the mutual authentication.
IAC SB AUTHENTICATION <authentication-type-pair> FORWARD <KRB_CRED
message> IAC SE
This command is used to forward kerberos credentials for use by
the remote session. The credentials are passed as a Kerberos V5
KRB_CRED message which includes, among other things, the forwarded
Kerberos ticket and a session key associated with the ticket. Part
of the KRB_CRED message is encrypted in the key previously ex-
changed for the telnet session by the AUTH suboption.
IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_ACCEPT IAC
SE
This command indicates that the credential forwarding was success-
ful.
IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_REJECT <op-
tional reason for rejection> IAC SE
This command indicates that the credential forwarding was not suc-
cessful, and if there is any more data in the sub-option, it is an
ASCII text message of the reason for the rejection.
3. Implementation Rules
If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial
AUTH command, and the server responds with either ACCEPT or REJECT.
In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the serv-
er will send a RESPONSE before it sends the ACCEPT.
If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial
AUTH command, and the client responds with either ACCEPT or REJECT.
In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
client will send a RESPONSE before it sends the ACCEPT.
The Kerberos principal used by the server will generally be of the
Expires February 19100 [Page 3]
Internet-Draft Kerberos Version 5 for Telnet August 1999
form "host/<hostname>@realm". That is, the first component of the
Kerberos principal is "host"; the second component is the fully qual-
ified lower-case hostname of the server; and the realm is the Ker-
beros realm to which the server belongs.
Any Telnet IAC characters that occur in the KRB_AP_REQ or KRB_AP_REP
messages, the KRB_CRED structure, or the optional rejection text
string must be doubled as specified in [2]. Otherwise the following
byte might be mis-interpreted as a Telnet command.
4. Examples
User "joe" may wish to log in as user "pete" on machine "foo". If
"pete" has set things up on "foo" to allow "joe" access to his ac-
count, then the client would send IAC SB AUTHENTICATION NAME "pete"
IAC SE IAC SB AUTHENTICATION IS KERBEROS_V5 AUTH <KRB_AP_REQ_MESSAGE>
IAC SE
The server would then authenticate the user as "joe" from the
KRB_AP_REQ_MESSAGE, and if the KRB_AP_REQ_MESSAGE was accepted by
Kerberos, and if "pete" has allowed "joe" to use his account, the
server would then continue the authentication sequence by sending a
RESPONSE (to do mutual authentication, if it was requested) followed
by the ACCEPT.
If forwarding has been requested, the client then sends IAC SB AU-
THENTICATION IS KERBEROS_V5 CLIENT|MUTUAL FORWARD <KRB_CRED structure
with credentials to be forwarded> IAC SE. If the server succeeds in
reading the forwarded credentials, the server sends FORWARD_ACCEPT
else, a FORWARD_REJECT is sent back.
Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V5 CLIENT|MUTUAL
KERBEROS_V5 CLIENT|ONE_WAY IAC
SE
[ The server has requested mutual Version 5 Kerberos
authentication. If mutual authentication is not supported,
then the server is willing to do one-way authentication.
The client will now respond with the name of the user that it
wants to log in as, and the Kerberos ticket. ]
Expires February 19100 [Page 4]
Internet-Draft Kerberos Version 5 for Telnet August 1999
IAC SB AUTHENTICATION NAME
"pete" IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V5 CLIENT|MUTUAL AUTH
<KRB_AP_REQ message> IAC SE
[ Since mutual authentication is desired, the server sends across
a RESPONSE to prove that it really is the right server. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V5 CLIENT|MUTUAL
RESPONSE <KRB_AP_REP message>
IAC SE
[ The server responds with an ACCEPT command to state that the
authentication was successful. ]
IAC SB AUTHENTICATION REPLY KER-
BEROS_V5 CLIENT|MUTUAL ACCEPT
IAC SE
[ If so requested, the client now sends the FORWARD command to
forward credentials to the remote site. ]
IAC SB AUTHENTICATION IS KER-
BEROS_V5 CLIENT|MUTUAL
FORWARD <KRB_CRED message> IAC
SE
[ The server responds with a FORWARD_ACCEPT command to state that
the credential forwarding was successful. ]
Expires February 19100 [Page 5]
Internet-Draft Kerberos Version 5 for Telnet August 1999
IAC SB AUTHENTICATION REPLY KER-
BEROS_V5 CLIENT|MUTUAL FOR-
WARD_ACCEPT IAC SE
5. Security Considerations
The selection of the random session key in the Kerberos V5 authenti-
cator is critical, since this key will be used for encrypting the
telnet data stream if encryption is enabled. It is strongly advised
that the random key selection be done using cryptographic techniques
that involve the Kerberos ticket's session key. For example, using
the current time, encrypting it with the ticket session key, and then
correcting for key parity is a strong way to generate a subsession
key, since the ticket session key is assumed to be never disclosed to
an attacker.
Care should be taken before forwarding a user's Kerberos credentials
to the remote server. If the remote server is not trustworthy, this
could result in the user's credentials being compromised. Hence, the
user interface should not forward credentials by default; it would be
far safer to either require the user to explicitly request creden-
tials forwarding for each connection, or to have a trusted list of
hosts for which credentials forwarding is enabled, but to not enable
credentials forwarding by default for all machines.
6. Acknowledgments
This document was originally written by Dave Borman of Cray Research,
Inc. Theodore Ts'o of MIT revised it to reflect the latest implemen-
tation experience. Cliff Neuman and Prasad Upasani of USC's Informa-
tion Sciences Institute developed the credential forwarding support.
In addition, the contributions of the Telnet Working Group are also
gratefully acknowledged.
7. References
[1] Kohl, J. and B. Neuman, "The Kerberos Network Authentication Sys-
tem (V5)", RFC 1510, USC/Information Sciences Institute, Septem-
ber 1993.
[2] Postel, J.B. and J. Reynolds, "Telnet Option Specifications", RFC
855, STD 8, USC/Information Sciences Institute, May 1983.
Editor's Address
Theodore Ts'o
Massachusetts Institute of Technology
MIT Room E40-343
Expires February 19100 [Page 6]
Internet-Draft Kerberos Version 5 for Telnet August 1999
77 Massachusetts Avenue
Cambridge, MA 02139
Phone: (617) 253-8091
EMail: tytso@mit.edu
Expires February 19100 [Page 7]