As per Relevance of the word extended, we have this rfc below:
Network Working Group C.
Request for Comments: 2243 The Inner
Category: Standards Track November 1997
OTP Extended
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1997). All Rights Reserved
This document provides a specification for a type of response to
OTP [RFC 1938] challenge that carries explicit indication of
response's encoding. Codings for the two mandatory OTP data
using this new type of response are presented
This document also provides a specification for a response
allows an OTP generator to request that a server re-initialize
sequence and change parameters such as the secret pass phrase
1. Conventions, Terms, and
This document specifies the data formats and software
needed to use OTP extended responses. The data formats are
three ways: using an ad-hoc UNIX manual page style syntax,
augmented BNF described in sections two and three of RFC 822, and
examples. Should there be any conflict between these descriptions
the augmented BNF takes precedence. The software behaviors
described in words, and specific behavior compliance requirements
itemized using the requirements terminology (specifically, the
MUST, SHOULD, and MAY) defined in RFC 2119.
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RFC 2243 OTP Extended Responses November 1997
2. Extended Challenges and Extended
This document builds on the protocol and terminology specified in
1938 and assumes that you have already read this document
understand its contents
An extended challenge is a single line of printable text
by either a new line sequence appropriate for the context of its
(e.g., ASCII CR followed by ASCII LF) or a whitespace character.
contains a standard OTP challenge, a whitespace character, and a
that generators use to determine which extended responses
supported by a server
An extended response is a single line of printable text terminated
a new line sequence appropriate for the context of its use.
contains two or more tokens that are separated with a single
(':') character. The first token contains a type specifier
indicates the format of the rest of the response. The tokens
follow are argument data for the OTP extended response. At least
token of data MUST be present
2.1.
In UNIX manual page like syntax, the general form of an
challenge could be described as
<standard OTP challenge> ext[,<extension set id>[, ...]]
And the general form of an extended response could be described as
:[:[:...]]
In augmented BNF syntax, the syntax of the general form of
extended challenge and an extended response is
extended-challenge = otp-challenge 1*LWSP-char capability-
(NL / *LWSP-char
otp-challenge = standard OTP challenge
capability-list = "ext" *("," extension-set-id
extension-set-id = *
extended-response = type 1*(":" argument)
type =
argument =
token = 1*
NL = sequence appropriate for the
in which OTP is being used
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RFC 2243 OTP Extended Responses November 1997
An example of an extended challenge indicating support for
extended responses and for a mythical response set "foo" is
otp-md5 123 mi1234 ext,
An example of an extended response using a mythical type named "foo
is
foo:some data:some more data:12345
2.2.
A server compliant with this specification
1. MUST be able to receive and parse the general form of
extended
2. MUST be able to receive, parse, and correctly process
extended responses specified in this
3. MUST process the type field in a case-insensitive
4. MUST reject any authentication attempt using an
response if it does not support that type of
5. SHOULD provide an appropriate indication to the
if the response was rejected because of (4)
6. MUST limit the length of the input
7. MUST accept otherwise arbitrary amounts of
wherever a response allows
8. MUST be able to receive and correctly process standard
A generator compliant with this specification
1. MUST be able to generate standard OTP
2. MUST use standard responses unless an extended
has been received for the particular server AND
3. MUST generate the type field in lower
4. MUST NOT send a response type for which the server has
indicated support through an extended
Extension set identifiers and extension type identifiers named
the prefix "x-" are reserved for private use among
consenting implementations. Implementations that do not recognise
particular "x-" extension MUST ignore that extension. This means
all "x-" extensions are likely to be non-interoperable with
extensions. Careful consideration should be given to the
of a server interacting with with a generator implementation which
although it recognizes a given "x-" extension, uses it for
different purpose. All of the remaining extension namespace
reserved to IANA, which will only officially assign the
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RFC 2243 OTP Extended Responses November 1997
into this namespace after the IESG approves of such an assignment
During the lifetime of the OTP WG, it is recommended that the
consult with the OTP WG prior to approving such an assignment
3. The "hex" and "word"
There exists a very rare case in which a standard OTP response
be a valid coding in both the hexadecimal and six-word formats.
example of this is the response "ABE ACE ADA ADD BAD A."
solution to this problem mandated by the OTP specification is
compliant servers MUST attempt to parse and verify a
response in both hexadecimal and six-word formats and must
the authentication successful if either succeeds
This problem can be solved easily using extended responses. The "hex
response and the "word" response are two response types that
an OTP in an extended response that explicitly describes
encoding. These responses start with a type label of "hex" for
hexadecimal OTP and "word" for a six-word coded OTP. These
contain one argument field that contains a standard OTP
coded in the indicated format
3.1.
In UNIX manual page like syntax, the format of these responses
be described as
hex:<hexadecimal number
word:dictionary words
In augmented BNF syntax and with the definitions already provided
the syntax of these responses is
hex-response = "hex:" hex-64bit
hex-64bit = 16(hex-char *LWSP-char
hex-char = ("A" / "B" / "C" / "D" / "E" / "F" /
"a" / "b" / "c" / "d" / "e" / "f" /
"0" / "1" / "2" / "3" / "4" / "5" /
"6" / "7" / "8" / "9")
word-response = "word:" word-64bit
word-64bit = 6(otp-word 1*LWSP-char
otp-word = standard OTP
dictionary
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RFC 2243 OTP Extended Responses November 1997
Examples of these responses are
hex:8720 33d4 6202 9172
word:VAST SAUL TAKE SODA SUCH
3.2.
A server compliant with this specification
1. MUST process all arguments in a case-insensitive
A generator compliant with this specification
1. SHOULD generate otp-word tokens in upper case with
spaces separating
2. SHOULD generate hexadecimal numbers using only lower
for
4. The "init-hex" and "init-word"
The OTP specification requires that implementations provide a
for a client to re-initialize or change its OTP information with
server but does not require any specific protocol for doing it
Implementations that support the OTP extended responses described
this document MUST support the response with the "init-hex"
"init-word" type specifiers, which provide a standard way for
client to re-initialize its OTP information with a server.
response is intended to be used only by automated clients. Because
this, the recommended form of this response uses the
encoding for binary data. It is possible for a user to type an "init
hex" or "init-word" response
4.1.
In UNIX manual page like syntax, the format of these responses
be described as
init-hex:::
init-word:::
In augmented BNF syntax and with the definitions already provided
the syntax of the "init-hex" response is
init-hex-response = "init-hex:" current-OTP ":" new-params ":"
new-OTP
current-OTP = hex-64
new-OTP = hex-64
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RFC 2243 OTP Extended Responses November 1997
new-params = algorithm SPACE sequence-number SPACE
algorithm = "md4" / "md5" / "sha1"
sequence-number = 4*3
seed = 16*1(ALPHA / DIGIT
In augmented BNF syntax and with the definitions already provided
the syntax of the "init-word" response is
init-word-response = "init-word:" current-OTP ":" new-params ":"
new-OTP
current-OTP = word-64
new-OTP = word-64
new-params = algorithm SPACE sequence-number SPACE
algorithm = "md4" / "md5" / "sha1"
sequence-number = 4*3
seed = 16*1(ALPHA / DIGIT
Note that all appropriate fields for the "init-hex" response MUST
hexadecimally coded and that all appropriate fields for the "init
word" response MUST be six-word coded
Examples of these responses are
init-hex:f6bd 6b33 89b8 7203:md5 499 ke6118:23d1 b253 5ae0 2b7
init-hex:c9b2 12bb 6425 5a0f:md5 499 ke0986:fd17 cef1 b4df 093
init-word:MOOD SOFT POP COMB BOLO LIFE:md5 499 ke1235:
ARTY WEAR TAD RUG HALO
init-word:END KERN BALM NICK EROS WAVY:md5 499 ke1235:
BABY FAIN OILY NIL TIDY
(Note that all of these responses are one line. Due to their length
they had to be split into multiple lines in order to be
here. These responses MUST NOT span more than one line in actual use
4.2. Description of
The current-OTP field contains the (RFC 1938) response to the
challenge. The new-params field contains the parameters for
client's new requested challenge and the new-OTP field contains
response to that challenge. If the re-initialization is successful,
server MUST store the new OTP in its database as the last
OTP received and the sequence number in the next challenge
by the server MUST be one less than the sequence number specified
the new-params field
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RFC 2243 OTP Extended Responses November 1997
The new-params field is hashed as a string the same way that a
or secret pass phrase would be. All other field values are hashed
their uncoded binary forms, in network byte order and without
padding
4.3.
A server compliant with this specification
1. SHOULD NOT allow a user to use the same value for
seed and secret pass phrase
2. MUST disable all OTP access to any principal
sequence number would be less than
3. MUST decrement the sequence number if a
response includes a valid current-OTP, but the server
unable to successfully process the new-params or new-OTP
any reason
A generator compliant with this specification
1. SHOULD NOT allow a user to use the same value for
seed and secret pass
2. MUST take specific steps to prevent infinite loops
re-initialization attempts in case of
3. SHOULD provide the user with some indication that
re-initialization is taking
4. SHOULD NOT do a re-initialization without the user'
permission, either for that specific instance or as
configuration
5. SHOULD NOT retry a failed re-initialization without a user'
6. SHOULD warn the user if the sequence number falls below
7. MUST refuse to generate OTPs with a sequence number below
5. Security
All of the security considerations for the OTP system also apply
the OTP system with extended responses
These extended responses, like OTP itself, do not protect the
against active attacks. The IPsec Authentication Header (RFC-1826)
(or another technique with at least as much strength as IPsec AH
SHOULD be used to protect against such attacks
The consequences of a successful active attack on the re
initialization response may be more severe than simply hijacking
single session. An attacker could substitute his own response
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RFC 2243 OTP Extended Responses November 1997
that of a legitimate user. The attacker may then be able to use
OTP system to authenticate himself as the user at will (at
until detected).
Failure to implement server requirement 3 in section 4.3 opens
implementation to an attack based on replay of the current-OTP
of the response
6.
Like RFC 1938, the protocol described in this document was created
contributors in the IETF OTP working group. Specific
were made by Neil Haller, who provided input on the overall
requirements of a re-initialization protocol, Denis Pinkas,
suggested several modifications to the originally proposed re
initialization protocol, and Phil Servita, who opened the debate
the first real protocol proposal and provided lots of specific
on the design of this and earlier protocols. The extensions to
OTP challenge were suggested by Chris Newman and John Valdes
Randall Atkinson and Ted T'so also contributed their views
discussions about details of the protocol extensions in
document
[RFC 822] Crocker, D., "Standard for the Format of ARPA
Text Messages," RFC 822, August 1982.
[RFC 1825] Atkinson, R., "Security Architecture for the
Protocol," RFC 1825, August 1995.
[RFC 1938] Haller, N. and C. Metz, "A One-Time Password System,"
RFC 1938, May 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs
Indicate Requirement Level," RFC 2119,
March 1997.
Author's
Craig
The Inner
Box 10314-1936
Blacksburg, VA 24062-0314
(DSN) 354-8590
cmetz@inner.
Metz Standards Track [Page 8]
RFC 2243 OTP Extended Responses November 1997
Appendix: Reference
The following responses were generated by a development version
the One-Time Passwords in Everything (OPIE) implementation of
specification
All of these are responses to the challenge
otp-md5 499 ke1234
Note that the re-initialization responses use the same secret
phrase for new and current and a new seed of "ke1235". Also,
responses have been split for formatting purposes into
lines; they MUST NOT be multiple lines in actual use
The secret pass phrase for these responses is
This is a test
The OTP standard hexadecimal response is
5bf0 75d9 959d 036
The OTP standard six-word response is
BOND FOGY DRAB NE RISE
The OTP extended "hex" response is
hex:5Bf0 75d9 959d 036
The OTP extended "word" response is
word:BOND FOGY DRAB NE RISE
The OTP extended "init-hex" response is
init-hex:5bf0 75d9 959d 036f:md5 499 ke1235:3712 dcb4 aa53 16c
The OTP extended "init-word" response is
init-word:BOND FOGY DRAB NE RISE MART:md5 499 ke1235: RED
NOW BEAN PA
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RFC 2243 OTP Extended Responses November 1997
Full Copyright
Copyright (C) The Internet Society (1997). All Rights Reserved
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Metz Standards Track [Page 10]
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