RFC 2943
TELNET Authentication Using DSA
Pages: 12
Proposed Standard
Proposed Standard
Network Working Group R. Housley Request for Comments: 2943 T. Horting Category: Standards Track P. Yee SPYRUS September 2000 TELNET Authentication Using DSA Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved.Abstract
This document defines a telnet authentication mechanism using the Digital Signature Algorithm (DSA) [FIPS186]. It relies on the Telnet Authentication Option [RFC2941].1. Command Names and Codes
AUTHENTICATION 37 Authentication Commands: IS 0 SEND 1 REPLY 2 NAME 3 Authentication Types: DSS 14 Modifiers: AUTH_WHO_MASK 1 AUTH_CLIENT_TO_SERVER 0 AUTH_SERVER_TO CLIENT 1
AUTH_HOW_MASK 2
AUTH_HOW_ONE_WAY 0
AUTH_HOW_MUTUAL 2
ENCRYPT_MASK 20
ENCRYPT_OFF 0
ENCRYPT_USING_TELOPT 4
ENCRYPT_AFTER_EXCHANGE 16
ENCRYPT_RESERVED 20
INI_CRED_FWD_MASK 8
INI_CRED_FWD_OFF 0
INI_CRED_FWD_ON 8
Sub-option Commands:
DSS_INITIALIZE 1
DSS_TOKENBA 2
DSS_CERTA_TOKENAB 3
DSS_CERTB_TOKENBA2 4
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 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 this option, the
parties are then able to perform sub-option negotiations to 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)
DSA is also known as the Digital Signature Standard (DSS), and the names are used interchangeably. 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 [FIPS180-1] is used with DSA [FIPS186]. 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 [FIPS196], Standard for Public Key Cryptographic Entity Authentication Mechanisms including Appendix A on ASN.1 encoding of messages and tokens. All data that is covered by a digital signature must be encoded using the Distinguished Encoding Rules (DER). However, other data may use either the Basic Encoding Rules (BER) or DER [X.208].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. 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). All data that is covered by a digital signature must be encoded using the
Distinguished Encoding Rules (DER). Data that is not covered by a digital signature may use either the Basic Encoding Rules (BER) or DER [X.208]. Figure 1 illustrates the exchanges for unilateral authentication. 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 DSS authentication, the two-octet authentication type pair is DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_ONE_WAY | ENCRYPT_OFF | INI_CRED_FWD_OFF. This indicates that the DSS authentication mechanism will be used to authenticate the client to the server and that no encryption will be performed. CertA is the clients certificate. Both certificates are X.509 certificates that contain DSS public keys[RFC2459]. The client must validate the server's certificate before using the DSA public key it contains. 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 sub-option identifier is carried immediately after the two-octet authentication type pair. The exchanges detailed in Figure 1 below 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.
------------------------------------------------------------------
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
DSS
AUTH_CLIENT_TO_SERVER |
AUTH_HOW_ONE_WAY |
ENCRYPT_OFF |
INI_CRED_FWD_OFF
DSS_INITIALIZE
IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
DSS
AUTH_CLIENT_TO_SERVER |
AUTH_HOW_ONE_WAY |
ENCRYPT_OFF |
INI_CRED_FWD_OFF
DSS_TOKENBA
Sequence( TokenID, TokenBA )
IAC SE
IAC SB AUTHENTICATION IS
DSS
AUTH_CLIENT_TO_SERVER |
AUTH_HOW_ONE_WAY |
ENCRYPT_OFF |
INI_CRED_FWD_OFF
DSS_CERTA_TOKENAB
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 DSS authentication, the two-octet authentication type pair is DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL | ENCRYPT_OFF | INI_CRED_FWD_OFF. This indicates that the DSS authentication mechanism will be used to mutually authenticate the client and the server and that no encryption will be performed. --------------------------------------------------------------------- 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 DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL | ENCRYPT_OFF | INI_CRED_FWD_OFF DSS_INITIALIZE IAC SE --> <-- IAC SB AUTHENTICATION REPLY DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL | ENCRYPT_OFF | INI_CRED_FWD_OFF DSS_TOKENBA Sequence( TokenID, TokenBA ) IAC SE
Client (Party A) Server (Party B)
IAC SB AUTHENTICATION IS
DSS
AUTH_CLIENT_TO_SERVER |
AUTH_HOW_MUTUAL |
ENCRYPT_OFF |
INI_CRED_FWD_OFF
DSS_CERTA_TOKENAB
Sequence( TokenID, CertA, TokenAB )
IAC SE -->
<-- IAC SB AUTHENTICATION REPLY
DSS
AUTH_CLIENT_TO_SERVER |
AUTH_HOW_MUTUAL |
ENCRYPT_OFF |
INI_CRED_FWD_OFF
DSS_CERTB_TOKENBA2
Sequence( TokenID, CertB,
TokenBA2 )
IAC SE
---------------------------------------------------------------------
Figure 2
4. ASN.1 Syntax
As stated earlier, a conformant subset of the defined fields and
subfields from FIPS PUB 196 have been selected. This section
provides the ASN.1 syntax for that conformant subset.
Figure 1 and Figure 2 include representations of the structures
defined in this section. Implementors should refer to the following
table to determine the ASN.1 definitions that match the figure
references:
Figure 1 Sequence( TokenID, TokenBA ) MessageBA
Sequence( TokenID, CertA, TokenAB ) MessageAB
Figure 2 Sequence( TokenID, TokenBA ) MessageBA
Sequence( TokenID, CertA, TokenAB ) MessageAB
Sequence( TokenID, CertB, TokenBA2 ) MessageBA2
The following ASN.1 definitions specify the conformant subset of FIPS
196. For simplicity, no optional fields or subfields are included.
The ASN.1 definition for CertificationPath is imported from CCITT
Recommendation X.509 [X.509], and The ASN.1 definition for Name is
imported from CCITT Recommendation X.501 [X.501]. These ASN.1
definitions are not repeated here. All DSA signature values are encoded as a sequence of two integers, employing the same conventions specified in RFC 2459, section 7.2.2. MessageBA ::= SEQUENCE { tokenId [0] TokenId, tokenBA TokenBA } TokenBA ::= SEQUENCE { ranB RandomNumber, timestampB TimeStamp } MessageAB ::= SEQUENCE { tokenId [0] TokenId, certA [1] CertData, tokenAB TokenAB } TokenAB ::= SEQUENCE { ranA RandomNumber, ranB RandomNumber, entityB EntityName, timestampB TimeStamp, absigValue OCTET STRING } MessageBA2 ::= SEQUENCE { tokenId [0] TokenId, certB [1] CertData, tokenBA2 TokenBA2 } TokenBA2 ::= SEQUENCE { ranB [0] RandomNumber, ranA [1] RandomNumber, entityA EntityName, timestampB2 TimeStamp, ba2sigValue OCTET STRING } CertData ::= SEQUENCE { certPath [0] CertificationPath } -- see X.509 EntityName ::= SEQUENCE OF CHOICE { -- only allow one! directoryName [4] Name } -- see X.501 RandomNumber ::= INTEGER -- 20 octets
TokenId ::= SEQUENCE {
tokenType INTEGER, -- see table below
protoVerNo INTEGER } -- always 0x0001
TimeStamp ::= GeneralizedTime
The TokenId.TokenType is used to distinguish the message type and the
authentication type (either unilateral or mutual). The following
table provides the values needed to implement this specification:
Message Type Authentication Type TokenId.TokenType
MessageBA Unilateral 0x0001
Mutual 0x0011
MessageAB Unilateral 0x0002
Mutual 0x0012
MessageBA Mutual 0x0013
5. 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.
Implementations must randomly generate DSS private keys, 'k' values
used in DSS signatures, and nonces. The use of inadequate pseudo-
random number generators (PRNGs) to generate cryptographic values can
result in little or no security. An attacker may find it much easier
to reproduce the PRNG environment that produced the values, searching
the resulting small set of possibilities, rather than using a brute
force search. The generation of quality random numbers is difficult.
RFC 1750 [RFC1750] offers important guidance in this area, and
Appendix 3 of FIPS PUB 186 [FIPS186] provides one quality PRNG
technique.
6. Acknowledgements
We would like to thank William Nace for support during implementation
of this specification.
7. IANA Considerations
The authentication type DSS and its associated suboption values are registered with IANA. Any suboption values used to extend the protocol as described in this document must be registered with IANA before use. IANA is instructed not to issue new suboption values without submission of documentation of their use.8. References
FIPS180-1 Secure Hash Standard. FIPS Pub 180-1. April 17, 1995. <http://csrc.nist.gov/fips/fips180-1.pdf> FIPS186 Digital Signature Standard (DSS). FIPS Pub 186. May 19, 1994. <http://csrc.nist.gov/fips/fips186.pdf> FIPS196 Standard for Entity Authentication Using Public Key Cryptography. FIPS Pub 196. February 18, 1997. <http://csrc.nist.gov/fips/fips196.pdf> RFC1750 Eastlake, 3rd, D., Crocker, S. and J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994. RFC2459 Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509 Public Key Infrastructure: X.509 Certificate and CRL Profile", RFC 2459, January 1999. RFC2941 T'so, T. and J. Altman, "Telnet Authentication Option", RFC 2941, September 2000. X.208 CCITT. Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1). 1988. X.501 CCITT. Recommendation X.501: The Directory - Models. 1988. X.509 CCITT. Recommendation X.509: The Directory - Authentication Framework. 1988.
9. Authors' Addresses
Russell Housley SPYRUS 381 Elden Street, Suite 1120 Herndon, VA 20172 USA EMail: housley@spyrus.com Todd Horting SPYRUS 381 Elden Street, Suite 1120 Herndon, VA 20172 USA EMail: thorting@spyrus.com Peter Yee SPYRUS 5303 Betsy Ross Drive Santa Clara, CA 95054 USA EMail: yee@spyrus.com
10. Full Copyright Statement
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