Internet DRAFT - draft-altman-rfc2944bis
Network Working Group T. Wu
Internet-Draft: draft-altman-rfc2944bis-02 Standford University
Obsoletes: 2944 J. Altman
Telnet Authentication: SRP
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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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.
This document specifies an authentication scheme for the Telnet
protocol under the framework described in , using the Secure
Remote Password Protocol (SRP) authentication mechanism. The
specific mechanism, SRP-SHA1, is described in [RFC2945].
This document updates a previous specification of the Telnet
Authentication SRP method, RFC 2944, to allow SRP Telnet authentication
to be used in conjunction with the START_TLS option .
0. Changes since RFC 2944
. Consolidates and expands the security considerations section
. Describes integration with START_TLS
1. Command Names and Codes
2. Command Meanings
IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH IAC SE
This command indicates that the client has supplied the
username and is ready to receive that user's field parameters.
There is no authentication information to be sent to the remote
side of the connection yet. This should only be sent after the
IAC SB AUTHENTICATION NAME command has been issued. If the
modifier byte (second byte of the authentication-type-pair)
has any bits other than AUTH_WHO_MASK or AUTH_HOW_MASK set,
both bytes are included in the session key hash described later.
This ensures that the authentication type pair was correctly
negotiated, while maintaining backward-compatibility with existing
If the ENCRYPT_START_TLS bit mask is in use, the checksum must
concatenate the TLS Client Finished Message and TLS Server Finished
Message to the authentication-type-pair in the session key hash.
This ensure that the TLS negotiation was not susceptible to a man
in the middle attack if the TLS credentials are not verified.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> PARAMS <values
of modulus, generator, and salt> IAC SE
This command is used to pass the three parameter values used
in the exponentiation to the client. These values are often
called n, g, and s.
IAC SB AUTHENTICATION IS <authentication-type-pair> EXP <client's
exponential residue> IAC SE
This command is used to pass the client's exponential residue,
otherwise known as A, computed against the parameters exchanged
IAC SB AUTHENTICATION REPLY <authentication-type-pair> CHALLENGE
<server's exponential residue> IAC SE
This command is used to pass the server's exponential residue,
computed against the same parameters. This quantity is actually
the sum of two residues, i.e. g^x + g^b. For details see [SRP]
IAC SB AUTHENTICATION IS <authentication-type-pair> RESPONSE
<response from client> IAC SE
This command gives the server proof of the client's authenticity
with a 160-bit (20 byte) response.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT
<server's response> IAC SE
This command indicates that the authentication was successful.
The server will construct its own proof of authenticity and
include it as sub-option data.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT
<optional 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
text message of the reason for the rejection.
For the PARAMS command, since three pieces of data are being
transmitted, each parameter is preceded by a 16-bit (two byte) length
specifier in network byte order. The EXP commands do not have a
count in front of the data because there is only one piece of data in
that suboption. The CHALLENGE, RESPONSE, and ACCEPT data also do not
have a count because they are all fixed in size.
3. Implementation Rules
Currently, only AUTH_CLIENT_TO_SERVER mode is supported. Although
the SRP protocol effectively performs implicit mutual authentication
as a result of the two-way proofs, only the AUTH_HOW_ONE_WAY
authentication mode is currently defined. The AUTH_HOW_MUTUAL
setting is being reserved for an explicit mutual-authentication
variant of the SRP protocol to be defined in future specifications.
All large number data sent in the arguments of the PARAMS and EXP
commands must be in network byte order, i.e. most significant byte
first. No padding is used.
The SRP-SHA1 mechanism, as described in [RFC2945] generates a 40-byte
session key, which allows implementations to use different keys for
incoming and outgoing traffic, increasing the security of the
encrypted session. It is recommended that the Telnet ENCRYPT method,
if it is used, be able to take advantage of the longer session keys.
User "tjw" may wish to log in on machine "foo". The client would
send IAC SB AUTHENTICATION NAME "tjw" IAC SE IAC SB AUTHENTICATION IS
SRP AUTH IAC SE. The server would look up the field and salt
parameters for "tjw" from its password file and send them back to the
client. Client and server would then exchange exponential residues
and calculate their session keys (after the client prompted "tjw" for
his password). Then, the client would send the server its proof that
it knows the session key. The server would either send back an
ACCEPT or a REJECT. If the server accepts authentication, it also
sends its own proof that it knows the session key to the client.
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information. ]
IAC SB AUTHENTICATION SEND
[ The server has requested SRP authentication. It has indicated
a preference for ENCRYPT_USING_TELOPT, which requires the
Telnet ENCRYPT option to be negotiated once authentication
succeeds. If the client does not support this, the server
is willing to fall back to an encryption-optional mode.
The client will now respond with the name of the
user that it wants to log in as. ]
IAC SB AUTHENTICATION NAME
"tjw" IAC SE
IAC SB AUTHENTICATION IS
SRP CLIENT|ONE_WAY|ENCRYPT_USING_TELOPT AUTH
[ The server looks up the appropriate information for "tjw" and
sends back the parameters in a PARAMS command. The parameters
consist of the values N, g, and s, each preceded with a two-
byte size parameter. ]
IAC SB AUTHENTICATION REPLY
ss ss nn nn nn nn ...
ss ss gg gg gg gg ...
ss ss tt tt tt tt ...
[ Both sides send their exponential residues. The client
sends its value A and the server sends its value B. In SRP,
the CHALLENGE message may be computed but not sent before
the EXP command. ]
IAC SB AUTHENTICATION IS
SRP CLIENT|ONE_WAY|ENCRYPT_USING_TELOPT EXP
aa aa aa aa aa aa aa aa ...
IAC SB AUTHENTICATION REPLY
bb bb bb bb bb bb bb bb ...
[ The client sends its response to the server. This is the
message M in the SRP protocol, which proves possession of
the session key by the client.
Since ENCRYPT_USING_TELOPT is specified, the two octets
of the authentication-type-pair are appended to the
session key K before the hash for M is computed. If
the client and server had agreed upon a mode without
the encryption flag set, nothing would be appended to K.
Both this message and the server's response are as long as
the output of the hash; the length is 20 bytes for SHA-1. ]
IAC SB AUTHENTICATION IS
SRP CLIENT|ONE_WAY|ENCRYPT_USING_TELOPT RESPONSE
xx xx xx xx xx xx xx xx ...
[ The server accepts the response and sends its own proof. ]
IAC SB AUTHENTICATION REPLY
yy yy yy yy yy yy yy yy ...
5. Security Considerations
The ability to negotiate a common authentication mechanism between
client and server is a feature of the authentication option that
should be used with caution. When the negotiation is performed, no
authentication has yet occurred. Therefore, each system has no way
of knowing whether or not it is talking to the system it intends. An
intruder could attempt to negotiate the use of an authentication
system which is either weak, or already compromised by the intruder.
Since SRP relies on the security of the underlying public-key
cryptosystem, the modulus "n" should be large enough to resist
brute-force attack. A length of at least 1024 bits is recommended,
and implementations should reject attempts to use moduli that are
shorter than 512 bits, or attempts to use invalid moduli and
generator parameters (non-safe-prime "n" or non-primitive "g").
As an implementation of the TELNET AUTH option  all of the
Security Considerations from that RFC MUST be considered
applicable to this sub-option.
This mechanism does not include all of the telnet authentication
negotiation exchanges in the integrity checksum as recommended in .
This means that the selection of this option is vulnerable to downgrade
attacks when multiple authentication type pairs are offered by the
6. IANA Considerations
The authentication type SRP 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.
 Ts'o, T. and J. Altman, "Telnet Authentication Option",
 Altman, J. and Boe, M., "TLS-based Telnet Security",
[SRP] T. Wu, "The Secure Remote Password Protocol", In
Proceedings of the 1998 ISOC Network and Distributed
System Security Symposium, San Diego, CA, pp. 97-111.
[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
RFC 2945, September 2000.
8. Author's Address
Stanford, CA 94305
Watson Hall Room 716
612 West 115th Street
New York NY 10025
Phone: +1 (212) 854-1344
Mailing List: telnet-wg@BSDI.COM
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