Secure Telnet Working Group Russell Housley (SPYRUS)
Todd Horting (SPYRUS)
Internet-Draft Peter Yee (SPYRUS)
Expire in six months March 1998
Telnet Authentication Using DSA
<draft-housley-telnet-auth-dsa-01.txt>
Status of this Memo
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Abstract
This document defines a method to authenticate telnet using the
Digital Signature Algorithm (DSA) [2]. It relies on the Telnet
Authentication Option [1].
1 Introduction
The Telnet protocol provides no protocol security. Telnet servers may
require users to login. This is typically a host level login
consisting of a user name and a password, transmitted in the clear.
The mechanism specified in this document relies on the Telnet
Authentication Option [1].
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2 Telnet Security Extensions
Telnet, as a protocol, has no concept of security. Without
negotiated options, it merely passes characters back and forth
between the NVTs represented by the two Telnet processes. In its
most common usage as a protocol for remote terminal access (TCP port
23), Telnet normally connects to a server that requires user-level
authentication through a user name and password in the clear. The
server does not authenticate itself to the user.
The Telnet Authentication Option provides for user authentication and
server authentication. User authentication replaces or augments the
normal host password mechanism. Server authentication is normally
done in conjunction with user authentication.
In order to support these security services, the two Telnet entities
must first negotiate their willingness to support the Telnet
Authentication Option. Upon agreeing to support these options, the
parties are then able to perform subnegotiations to determine the
authentication protocol to be used, and possibly the remote user name
to be used for authorization checking.
Authentication and parameter negotiation occur within an unbounded
series of exchanges. The server proposes a preference-ordered list
of authentication types (mechanisms) which it supports. In addition
to listing the mechanisms it supports, the server qualifies each
mechanism with a modifier that specifies whether the authentication
is to be one-way or mutual, and in which direction the authentication
is to be performed. The client selects one mechanism from the list
and responds to the server indicating its choice and the first set of
authentication data needed for the selected authentication type. The
server and the client then proceed through whatever number of
iterations are required to arrive at the requested authentication.
3 Use of Digital Signature Algorithm (DSA)
This paper specifies a method in which DSA may be used to achieve
certain security services when used in conjunction with the Telnet
Authentication Option. SHA-1 [3] is used with DSA [2].
DSA may provide either unilateral or mutual authentication. Due to
Telnet's character-by-character nature, it is not well-suited to the
application of integrity-only services, therefore use of the DSA
profile provides authentication but it does not provide session
integrity. This specification follows the token and exchanges
defined in NIST FIPS PUB 196 [4], Standard for Public Key
Cryptographic Entity Authentication Mechanisms, draft of 28 February
1995, including Appendix A on ASN.1 encoding of messages and tokens.
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3.1 Unilateral Authentication with DSA
Unilateral authentication must be done client-to-server. What
follows are the protocol steps necessary to perform DSA
authentication as specified in FIPS PUB 196 under the Telnet
Authentication Option framework. Where failure modes are
encountered, the return codes follow those specified in the Telnet
Authentication Option. They are not enumerated here, as they are
invariant among the mechanisms used. FIPS PUB 196 employs a set of
exchanges that are transferred to provide authentication. Each
exchange employs various fields and tokens, some of which are
optional. In addition, each token has several subfields that are
optional. A conformant subset of the fields and subfields have been
selected. Therefore, the exchanges below do not show the FIPS PUB
196 notations indicating optional fields, as all subfields used are
mandatory. The tokens are ASN.1 encoded as defined in Appendix A of
FIPS PUB 196, and each token is named to indicate the direction in
which it flows (e.g., TokenBA flows from Party B to Party A). In
Figure 1, the client binds the last transmission (token identifier,
certificate, and token) together as an ASN.1 SEQUENCE.
During authentication, the client may provide the user name to the
server by using the authentication name sub-option. If the name sub-
option is not used, the server will generally prompt for a name and
password in the clear. The name sub-option must be sent after the
server sends the list of authentication types supported and before
the client finishes the authentication exchange, this ensures that
the server will not prompt for a user name and password. In figure
1, the name sub-option is sent immediately after the server presents
the list of authentication types supported.
For one-way DSA authentication, the two-octet authentication type
pair is "DSA CLIENT-TO-SERVER ONE-WAY ENCRYPT_OFF INI_CRED_FWD_OFF."
This is encoded as two-octets: '0B01' in hexadecimal. This indicates
that the DSA authentication mechanism will be used to authenticate
the client to the server, and that no encryption will be performed.
Within the unbounded authentication exchange, implementation is
greatly simplified if each portion of the exchange carries a unique
identifier. For this reason, a single octet suboption identifier is
carried immediately after the two-octet authentication type pair.
The exchanges detailed below, in Figure 1, presume knowledge of FIPS
PUB 196 and the Telnet Authentication Option. The client is Party A,
while the server is Party B. At the end of the exchanges, the client
is authenticated to the server.
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---------------------------------------------------------------------
Client (Party A) Server (Party B)
<-- IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION -->
<-- IAC SB AUTHENTICATION SEND
<list of authentication options>
IAC SE
IAC SB AUTHENTICATION
NAME <user name> -->
IAC SB AUTHENTICATION IS
'0B01' '1' IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
'0B01' '2'
Sequence( TokenID, TokenBA )
IAC SE
IAC SB AUTHENTICATION IS
'0B01' '3'
Sequence( TokenID, CertA, TokenAB )
IAC SE -->
---------------------------------------------------------------------
Figure 1
3.2 Mutual Authentication with DSA
Mutual authentication is slightly more complex. Figure 2 illustrates
the exchanges.
For mutual DSA authentication, the two-octet authentication type pair
is "DSA CLIENT-TO-SERVER MUTUAL ENCRYPT_OFF INI_CRED_FWD_OFF." This
is encoded as two-octets: '0B03' in hexadecimal. This indicates that
the DSA authentication mechanism will be used to mutually
authenticate the client and the server, and that no encryption will
be performed.
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---------------------------------------------------------------------
Client (Party A) Server (Party B)
IAC WILL AUTHENTICATION -->
<-- IAC DO AUTHENTICATION
<-- IAC SB AUTHENTICATION SEND
<list of authentication options>
IAC SE
IAC SB AUTHENTICATION
NAME <user name> -->
IAC SB AUTHENTICATION IS
'0B03' '1' IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
'0B03' '2'
Sequence( TokenID, TokenBA )
IAC SE
IAC SB AUTHENTICATION IS
'0B03' '3'
Sequence( TokenID, CertA, TokenAB )
IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
'0B03' '4'
Sequence( TokenID, CertB, TokenBA2 )
IAC SE
---------------------------------------------------------------------
Figure 2
4.0 Security Considerations
This entire memo is about security mechanisms. For DSA to provide
the authentication discussed, the implementation must protect the
private key from disclosure.
5.0 Acknowledgements
We would like to thank William Nace for support during implementation
of this specification.
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6.0 References
[1] - Borman, David A. "Telnet Authentication Option".
RFC 1416. February 1993.
[2] - Digital Signature Standard (DSS). FIPS Pub 186.
May 19, 1994.
[3] - Secure Hash Standard. FIPS Pub 180-1. April 17, 1995.
[4] - Standard for Entity Authentication Using Public Key
Cryptography. FIPS Pub 196. February 18, 1997.
7.0 Author's Address
Russell Housley
SPYRUS
PO Box 1198
Herndon, VA 20172
USA
Phone: +1 703 435-7344
Email: housley@spyrus.com
Todd Horting
SPYRUS
PO Box 1198
Herndon, VA 20172
USA
Phone: +1 703 435-4711
Email: thorthing@spyrus.com
Peter Yee
SPYRUS
2460 N. First Street
Suite 100
San Jose, CA 95131-1023
USA
Phone: +1 408 432-8180
Email: yee@spyrus.com
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