Telnet DSA: TELNET Authentication Using DSA
How a Telnet client proves who it is, and optionally checks the server, using DSA digital signatures and certificates.
Housley, R., Horting, T., and P. Yee, "TELNET Authentication Using DSA", RFC 2943, DOI 10.17487/RFC2943, September 2000, <https://www.rfc-editor.org/info/rfc2943>.
@misc{rfc2943,
series = {Request for Comments},
number = 2943,
howpublished = {RFC 2943},
publisher = {RFC Editor},
doi = {10.17487/RFC2943},
url = {https://www.rfc-editor.org/info/rfc2943},
author = {R. Housley and T. Horting and P. Yee},
title = {{TELNET Authentication Using DSA}},
pagetotal = 12,
year = 2000,
month = sep,
} RFC text
Contents
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
Copyright (C) The Internet Society (2000). All Rights Reserved.
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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English.
The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§2Why Telnet Needs an Authentication Mechanism
TELNET on its own has no concept of security. Without negotiated options it just passes characters between the two network virtual terminals. In its most common use, remote terminal access on TCP port 23, the server asks for a user name and password in the clear, and the server does not authenticate itself to the user.
The TELNET Authentication Option (RFC 2941) adds user authentication and server authentication. The two sides first agree to support the option. Then the server offers a preference-ordered list of authentication types, each with a modifier saying whether the authentication is one-way or mutual and in which direction. The client picks one and replies with the first set of authentication data. They then run as many exchanges as the chosen type needs.
RFC 2943 defines one such type, built on the Digital Signature Algorithm.
Plain Telnet is like shouting your password across a crowded room. The authentication option lets the two computers agree on a safer way to show who they are.
§3Using DSA with the Authentication Option
The Digital Signature Algorithm (DSA), also called the Digital Signature Standard (DSS), is used with SHA-1. The RFC uses the two names interchangeably. DSA can give unilateral or mutual authentication.
The specification follows the tokens and exchanges of NIST FIPS PUB 196, which defines entity authentication with public key cryptography, including the ASN.1 encoding of messages and tokens. The RFC picks a conformant subset of the fields and subfields.
Because Telnet works character by character, it is not well suited to integrity-only services. This profile therefore provides authentication but not session integrity.
All data covered by a digital signature MUST be encoded with the Distinguished Encoding Rules (DER). Other data may use either BER or DER.
A digital signature is like a stamp only you own. The RFC says how to use the stamp to prove who you are when you log in, but it does not promise nobody can tamper with what you type afterwards.
§3.1One-Way Authentication of the Client
Unilateral authentication must be done client-to-server. The client is Party A and the server is Party B. After the usual IAC DO/WILL AUTHENTICATION exchange and the server's SEND list, the client may send its user name with the NAME sub-option. The name must be sent after the server's list and before the client finishes the exchange, which stops the server from prompting for a name and password in the clear.
The two-octet authentication type pair for one-way DSS is DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_ONE_WAY | ENCRYPT_OFF | INI_CRED_FWD_OFF. It means the client is authenticated to the server and no encryption is done. A single octet sub-option identifier follows the pair so each step of the open-ended exchange is easy to recognise.
- The client sends
DSS_INITIALIZE. - The server replies with
DSS_TOKENBA, carrying TokenBA. - The client sends
DSS_CERTA_TOKENAB, carrying its certificate CertA and TokenAB.
Both certificates are X.509 certificates that contain DSS public keys. The client must validate the server's certificate before using the DSA public key in it. At the end, the client is authenticated to the server.
The client says hello, the server hands over a fresh challenge, and the client signs it and sends back its ID card. The server checks the signature and knows who it is talking to.
§3.2Mutual Authentication
Mutual authentication is slightly more complex. The type pair is DSS AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL | ENCRYPT_OFF | INI_CRED_FWD_OFF, meaning the client and server authenticate each other and no encryption is done.
The first three steps match the one-way case: DSS_INITIALIZE, DSS_TOKENBA and DSS_CERTA_TOKENAB. The mutual case adds a final server message, DSS_CERTB_TOKENBA2, which carries the server's certificate CertB and a second signed token, TokenBA2. That last step is what lets the client check the server.
For mutual checking, the server also signs something and shows its own ID card at the end, so both sides know who the other is.
- IAC WILL AUTHENTICATION
- IAC DO AUTHENTICATION
- SEND (list of authentication options)
- NAME (user name)
- IS DSS ... DSS_INITIALIZEType pair says mutual, encryption off.
- REPLY DSS_TOKENBATokenID and TokenBA.
- IS DSS_CERTA_TOKENABTokenID, CertA and TokenAB.
- REPLY DSS_CERTB_TOKENBA2TokenID, CertB and TokenBA2.
§4ASN.1 Messages and Token Types
The RFC gives the ASN.1 syntax for its subset of FIPS 196 and includes no optional fields or subfields. The structures are MessageBA (TokenBA), MessageAB (CertA and TokenAB) and MessageBA2 (CertB and TokenBA2).
Each token holds random numbers and a timestamp. A RandomNumber is an integer of 20 octets. TokenAB and TokenBA2 also carry the name of the other party and a signature value. All DSA signature values are encoded as a sequence of two integers, following RFC 2459, section 7.2.2.
Every message starts with a TokenId holding a tokenType and a protocol version number that is always 0x0001. The token type tells the receiver which message it is and whether the run is unilateral or mutual.
Each message is a form with fixed boxes: a random number, the time, and a signature. The first box says which kind of form it is.
§5Protecting Keys and Random Values
The whole memo is about security mechanisms. For DSA to authenticate anyone, the implementation must protect the private key from disclosure.
Implementations must generate DSS private keys, the k values used in DSS signatures, and nonces randomly. A weak pseudo-random number generator can leave little or no security, because an attacker may reproduce its environment and search the small set of possible values instead of brute-forcing. The RFC points to RFC 1750 and to Appendix 3 of FIPS PUB 186 for guidance.
Keep your stamp safe, and pick your secret numbers by really rolling the dice. If your dice are rigged, a sneaky person can guess your numbers.
§7IANA Registrations
The authentication type DSS and its sub-option values are registered with IANA. Any sub-option values used to extend the protocol must be registered before use, and IANA is told not to issue new sub-option values without documentation of their use.
The RFC lists the codes it uses. The option AUTHENTICATION is 37, the type DSS is 14, and the sub-option commands are 1 to 4.
The numbers used for each kind of message are written in an official list, so everyone uses the same numbers for the same things.
- 37AUTHENTICATION — The Telnet option.
- 14DSS — The authentication type defined here.
- 1DSS_INITIALIZE — Client starts the exchange.
- 2DSS_TOKENBA — Server's first token.
- 3DSS_CERTA_TOKENAB — Client's certificate and token.
- 4DSS_CERTB_TOKENBA2 — Server's certificate and second token (mutual only).