Internet Engineering Task Force Y. Oiwa
Internet-Draft H. Watanabe
Intended status: Standards Track H. Takagi
Expires: May 22, 2008 RCIS, AIST
H. Suzuki
Yahoo! Japan
November 19, 2007
Mutual Authentication Protocol for HTTP
draft-oiwa-http-mutualauth-01
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies the "Mutual authentication protocol for
Hyper-Text Transport Protocol". This protocol provides true mutual
authentication between HTTP clients and servers using simple
password-based authentication. Unlike Basic and Digest HTTP access
authentication protocol, the protocol ensures that server knows the
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user's entity (encrypted password) upon successful authentication.
This prevents common phishing attacks: phishing attackers cannot
convince users that the user has been authenticated to the genuine
website. Furthermore, even when a user has been authenticated
against an illegitimate server, the server cannot gain any bit of
information about user's passwords. The protocol is designed as an
extension to the HTTP protocol, and the protocol design intends to
replace existing authentication mechanism such as Basic/Digest access
authentications and form-based authentications.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
3. Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Tokens and Extensive-tokens . . . . . . . . . . . . . . . 6
3.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Strings . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. 401-B0 . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. 401-B0-stale . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. req-A1 . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. 401-B1 . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5. req-A3 . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.6. 200-B4 . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Decision procedure for the client . . . . . . . . . . . . . . 12
6. Authentication Algorithms . . . . . . . . . . . . . . . . . . 14
7. Validation Methods . . . . . . . . . . . . . . . . . . . . . . 16
8. Session Management . . . . . . . . . . . . . . . . . . . . . . 17
9. Extension 1: Optional Mutual Authentication . . . . . . . . . 17
10. Methods to extend this protocol . . . . . . . . . . . . . . . 18
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
12. Security Considerations . . . . . . . . . . . . . . . . . . . 19
12.1. General Assumptions . . . . . . . . . . . . . . . . . . . 19
12.2. Implementation Considerations . . . . . . . . . . . . . . 19
13. Notice on intellectual properties . . . . . . . . . . . . . . 20
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
14.1. Normative References . . . . . . . . . . . . . . . . . . . 20
14.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
This document specifies the "Mutual authentication protocol for
Hyper-Text Transport Protocol". This protocol provides true mutual
authentication between HTTP clients and servers using simple
password-based authentication. Unlike Basic and Digest HTTP access
authentication protocol [RFC2617], the protocol ensures that server
knows the user's entity (encrypted password) upon successful
authentication. This prevents common phishing attacks: phishing
attackers cannot convince users that the user has been authenticated
to the genuine website. Furthermore, even when a user has been
authenticated against an illegitimate server, the server cannot gain
any bit of information about user's passwords.
Recently, phishing attacks are getting more and more sophisticated.
Phishers not only steal user's password directly, but imitate
successful authentication to steal user's sensitive information,
check the password validity by forwarding the password to the
legitimate server, or employ a man-in-the-middle attack to hijack
user's login session. Existing countermeasures such as one-time
passwords cannot completely solve these problems.
The protocol prevents such attacks by providing users a way to
discriminate between true and fake web servers using their own
passwords. Even when a user inputs his/her password to a fake
website, using this authentication method, any information about the
password does not leak to the phisher, and the user certainly notices
that the mutual authentication has failed. Phishers cannot make such
authentication attempt succeed, even if they forward received data
from a user to the legitimate server or vice versa. Users can safely
input sensitive data to the web forms after confirming that the
mutual authentication has succeeded.
To achieve this goal, this protocol uses a mechanism in ISO/IEC
11770-4 [ISO.11770-4.2006], a kind of PAKE (Password-Authenticated
Key Exchange) authentication algorithms as a basis. The use of PAKE
mechanism allows users to use familiar ID/password based accesses,
without fear of leaking any password information to the communication
peer. The protocol, as a whole, is designed as a natural extension
to the HTTP protocol [RFC2616].
The design also considers to replace current form-based Web
authentication, which is very vulnerable against phishing attacks.
To this purpose, several extensions to current HTTP authentication
mechanism [RFC2617] are introduced.
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1.1. Requirements Language
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 [RFC2119].
2. Protocol Overview
The following sequence is a typical sequence for the first access to
the resource.
o If the server (S) has received a request for mutual-authentication
protected resources from the Client (C) (which is not a req-A1 nor
a req-A3 message), it sends a 401-B0 message to C.
When C has received a 401-B0 message, C SHOULD check validity of
the message. If succeed, C processes the body of the message, and
enables the password entry field.
o If the user has input the username and password as a response to
the 401-B0 message, C creates a value s_A, calculates the value
w_A, and construct and send a req-A1 message.
o If S has received an req-A1 message, S should record the received
w_A value, validate w_A using T(w_A), and then look up the
username from the user table. if the user is found, S prepares a
new session id (sid), record it into a session table, and then
construct s_B, calculate w_B, and then send an 401-B1 message.
If there is no matching user found, the server SHOULD construct a
fake w_B value, and let the protocol going on by sending an 401-B1
message.
o When C has received an 401-B1 message as a response for a req-A1
message, C should compute z and K_i, compute o_A, and send an
req-A3 message.
If C receives any messages other than 401-B1, C MUST NOT process
the message body and treat it as a fatal communication error
condition. This case includes the reception of HTTP OK (200-
status) message.
o If S has received an req-A3 message, S should look up the received
sid from the session table. If no matching sid message is
received, or if S has not received the corresponding req-A1
message beforehand, S SHOULD send an 401-B0-stale message.
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Otherwise, S should computes o_A' and check its value. If the
validation has failed, the server SHOULD send an 401-B0 message.
If the validation has succeeded, the server SHOULD calculate o_B,
and send a 200-B4 message.
o When C has received a 401-B0-stale message as a response to req-A3
message, and it is the first time that it has received a 401-B0-
stale message, C SHOULD redo constructing req-A1 message.
Instead, when C has received an 401-B0 message, it means the
authentication has been failed, possibly due to that the wrong
password has been given. C MAY ignore the body of the 401-B0
message in this case.
When C has received an 200-B4 message, C MUST first compute the
value of o_B' and validate the value o_B sent from the server. If
it has not verified successfully, C MUST ignore the body of the
message, and treat it as a fatal communication error condition.
If it has succeed, C will process the body of the message.
If C receives any messages other than 401-B0-stale or 401-B1, C
MUST NOT process the message body and other headers and treat it
as a fatal communication error condition. This case includes the
reception of usual HTTP OK (200-status) messages.
For the second or later request to the server, if the client knows
that the resource is likely to require the authentication, the client
MAY omit first unauthenticated request and send req-A1 message
immediately. Furthermore, if client owns a valid session ID (sid),
the client MAY send a req-A3 message using existing sid. In either
case, the first (and only the first) response from the server MAY be
a normal, unauthenticated message, and client MAY accept such
messages. For more detail, see Section 5.
3. Message Syntax
The Mutual authentication protocol uses four headers:
WWW-Authenticate (in responses with status code 401),
Optional-WWW-Authenticate (in responses with positive status codes),
Authorization (in requests), and Authentication-info (in positive
responses). These three headers share the common syntax described in
Figure 1. The syntax is denoted in the augmented BNF syntax defined
in [RFC2616], plus that "a"..."b" means any ASCII characters between
"a" and "b" inclusive. The syntax is a subset of the one described
in [RFC2617].
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header = header-name ":" spaces "Mutual" spaces fields
header-name = "WWW-Authenticate" | "Optional-WWW-Authenticate"
| "Authorization" | "Authentication-info"
spaces = 1*(SP | HT | CR LF 1*(SP | HT))
fields = field 0*("," spaces fields)
field = key "=" value
key = extensive-token
extensive-token = token | extension-token
extension-token = token "@" token
token = 1*("0"..."9" | "A"..."Z" | "a"..."z"
| "-" | "_" | ".")
value = extensive-token | integer | hex-integer
| hex-fixed-number
| base64-fixed-number | string
integer = "0" | ("1"..."9") 0*("0"..."9")
hex-integer = "0"
| ("1"..."9" | "A"..."F" | "a"..."f")
0*("0"..."9" | "A"..."F" | "a"..."f")
hex-fixed-number = 1*("0"..."9" | "A"..."F" | "a"..."f")
base64-fixed-number = string
string = <"> *(<TEXT except <"> and "\"> | "\\"
| "\" <"> | "\,")* <">
Figure 1: the BNF syntax for the headers used in the protocol
3.1. Tokens and Extensive-tokens
The tokens MUST be interpreted case-insensitive, and SHOULD be sent
in the same case as shown in the specification. When these are used
as (partial) inputs to any hash or other mathematical functions, it
MUST be used in lower-case. All hex-fixed-number or hex-integer
numbers are also case-insensitive, and SHOULD be sent in lower-case.
Extensive-tokens are used where the set of acceptable tokens are
extensible. Any non-standard extensions of this protocol MUST use
the extension-tokens of format "<token>@<domain-name>", where domain-
name is the valid registered (sub-)domain name on the Internet owned
by the party who defines extensions.
3.2. Numbers
The syntax definitions of integer and hex-integer only allow
representations which do not contain extra leading 0s.
The numbers represented as a hex-fixed-number SHALL have even
characters (i.e. multiple of eight bits). When these are generated
from the cryptographic values, those SHOULD have the natural length:
if these are generated from a hash function, these lengths SHOULD
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correspond to the hash size; if these are representing elements of a
mathematical group, its lengths SHOULD be the shortest which can
represent all elements in the group. Other values such as session-id
are represented in any (even) length determined by the side who
generates it first, and the same length SHALL be used throughout the
whole communications by both peers.
The numbers represented as a base64-fixed-number SHALL be generated
as follows: first, the number is converted to a big-endian octet-
string representation. The length of the representation is
determined in the same way as above. Then, the string is encoded by
the Base 64 encoding [RFC3548], and then enclosed by two double-
quotations.
3.3. Strings
All strings outside ASCII or equivalent character sets SHOULD be
encoded using UTF-8 encoding [RFC3629] of the ISO 10646-1 character
set [ISO.10646-1.1993]. Both peers SHOULD reject any invalid UTF-8
sequences which causes decoding ambiguities (e.g. containing <"> in
the second or later byte of the UTF-8 encoded characters). To encode
character strings, these will first be encoded according to UTF-8
without leading BOM, then all occurrences of characters <"> and "\"
will be escaped by prepending "\", and two <">s will be put around
the string. If the contents of the strings are comma-separated
values, the commas in the values are also quoted by "\".
If strings are representing a domain name or URI which contains non-
ASCII characters, the host parts SHOULD be encoded using puny-code
defined in [RFC3492] instead of UTF-8, and SHOULD use lower-case
ASCII characters.
For Base64-fixed-numbers, which use the string syntax, see the
previous section.
4. Messages
In this section, formats and requirements of the headers for each
message are presented. The allowed type for values for each header
field is shown in parenthesis after the key names.
Note: The term "optional" here means that omitting the field is
allowed and has specific meanings in communications (i.e. it is not
generally "OPTIONAL" defined in [RFC2119]).
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4.1. 401-B0
Every 401-B0 message SHALL be a valid HTTP 401 (Authentication
Required) message containing one (and only one: hereafter not
explicitly noticed) "WWW-Authenticate" header of the following
format.
WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx, realm=xxxx,
stale=0
The header SHALL contain the fields with the following keys:
algorithm: (extensive-token) specifies the authentication
algorithm to be used. The value MUST be one of the
tokens described in Section 6, or the tokens specified
in other supplemental specification documentations.
validation: (extensive-token) specifies the method of host
validation. The value MUST be one of the tokens
described in Section 7, or the tokens specified in
other supplemental specification documentations.
realm: (string) is a UTF-8 encoded name of the authentication
domain inside the server.
pwd-hash: (optional, extensive-token) specifies the hash
algorithm (referred to by ph) used for additionally
hashing the password. The valid tokens are
* none: ph(p) = p
* md5: ph(p) = MD5(p)
* digest-md5: ph(p) defined as A1 in [RFC2617].
* sha1: ph(p) = SHA1(p)
If omitted, the value "none" is assumed.
auth-domain: (optional, string) MUST currently be equal to the
host-part of the requested URI, and assumed to have
that value if omitted. The triple of auth-domain,
algorithm, and realm determines the "authentication
realm" which defines the area where the same user-name
and passwords are applicable.
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stale: (token) MUST be "0".
Any additional fields SHOULD NOT be contained in the header, except
those explicitly specified in supplement specifications of the
"authentication algorithm".
The algorithm will determine the types and the values for w_A, w_B,
o_A and o_B.
4.2. 401-B0-stale
A 401-B0-stale message is a variant of 401-B0 message, which means
that the client has sent a request message which is not for any
active session.
WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx, realm=xxxx,
stale=1
The header MUST contain the same fields as in 401-B0, except that
stale field holds the integer 1.
4.3. req-A1
Every req-A1 message SHALL be a valid HTTP request message containing
a "Authorization" header of the following format.
Authorization: Mutual algorithm=xxxx, validation=xxxx, user=xxxx,
realm=xxxx, wa=xxxx
The header SHALL contain the fields with the following keys:
algorithm, validation, auth-domain, realm: MUST be the same value as
it is received from S.
user: (string) is the UTF-8 encoded name of the user.
wa: (algorithm-determined) is the value of w_A specified
by the used algorithm.
4.4. 401-B1
Every 401-B1 message SHALL be a valid HTTP 401 (Authentication
Required) message containing a "WWW-Authenticate" header of the
following format.
WWW-Authenticate: Mutual sid=xxxx, wb=xxxx, nc-max=x, nc-window=x,
time=x, path=xxxx
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The header SHALL contain the fields with the following keys:
sid: (hex-fixed-number) MUST be a session id, which is a
random integer. The sid SHOULD have uniqueness of at
least 80 bits or the square of the maximal estimated
transactions concurrently available in the session
table, whichever is larger. Sids are local to each
authentication realm concerned: the same sids for
different authentication realms SHOULD be treated as
independent ones.
wb: (algorithm-determined) is the value of w_B specified
by the algorithm.
nc-max: (hex-integer) is the maximal value of nonce counts
which S accepts.
nc-window: (hex-integer) the number of available nonce slots
which S will accept. The value of nc-window is
RECOMMENDED to be thirty-two ("20" in hex-integer) or
more.
time: (integer) represents the suggested time (in seconds)
which C can reuse the session key represented by sid.
It is RECOMMENDED to be at least 60. The value of
this field is not directly linked to the duration that
S keeps track of the session represented by sid.
path: (optional, string) specifies for which path in the URI
space the same authentication is expected to apply.
The value is in the same format as it is specified in
[RFC2617] for the Digest authentications, and clients
are RECOMMENDED to recognize it. The all path
elements contained in the field MUST be inside the
specified auth-domain: if not, client SHOULD ignore
such elements.
4.5. req-A3
Every req-A3 message SHALL be a valid HTTP request message containing
a "Authorization" header of the following format.
Authorization: Mutual sid=xxxx, nc=x, oa=xxxx
The fields contained in the header is as follows:
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sid: (hex-fixed-number) MUST be one of the sid values which
has been received from S.
nc: (hex-integer) is a nonce value which is unique among
the requests sharing the same sid. The value of nc
SHOULD satisfy the following properties:
* It is not larger than the nc-max value which has
been sent from S in the session represented by the
sid.
* C have not sent the same value in the same session.
* It is not smaller than (largest-nc - nc-window),
where largest-nc is the maximal value of nc which
has previously been sent in the session, and nc-
window is the value of the nc-window field which
has been sent from S in the session.
oa: (algorithm-determined) is the value of o_A specified
by the algorithm.
4.6. 200-B4
Every 200-B1 message SHALL be a valid HTTP message which is not 401
(Authentication Required) type, containing an "Authentication-Info"
header of the following format.
Authentication-Info: Mutual sid=xxxx, ob=xxxx
The fields contained in the header is as follows:
sid: (hex-fixed-number) MUST be the value received from C.
ob: (algorithm-determined) is the value of o_B specified
by the algorithm.
logout-timeout: (optional, integer) is a number of seconds after
which the client should re-validate the user's
password for the current authentication realm. As a
special case, the value 0 means that the client SHOULD
automatically forget the user-inputed password to the
current authentication realm and revert to the
unauthenticated state (i.e.~server-initiated logout).
This does not, however, mean that the long-term
memories for the passwords (such as password reminders
and auto fill-ins) should be removed. If a new value
of timeout is received for the same authentication
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realm, it overrides the previous timeout.
5. Decision procedure for the client
To securely implement the protocol, the user client must be careful
to accepting authenticated responses from the server.
Clients SHOULD implement the decision procedure equivalent to the one
shown below. (Unless implementers understand what is required for
the security, they should not alter this.)
Step 1 If the client software needs to get a new Web resource,
check whether the resource is expected to be inside some
authentication realm for which the user has already
authenticated. If yes, go to Step 2. Otherwise, go to Step
5.
Step 2 Check whether there is an available sid for the
authentication realm you expects. If there is one, go to
Step 3. Otherwise, go to Step 4.
Step 3 Send a req-A3 request.
* If you receive a 401-B0 message with a different
authentication realm than expected, go to Step 6.
* If you receive a 401-B0-stale message, go to Step 9.
* If you receive a 401-B0 message, go to Step 13.
* If you receive a valid 200-B4 message, go to Step 14.
* If you receive a normal response (without Mutual-specific
headers), go to Step 11.
Step 4 Send a req-A1 request.
* If you receive a 401-B0 message with a different
authentication realm than expected, go to Step 6.
* If you receive a 401-B0-stale message, go to Step 9.
* If you receive a 401-B1 message, go to Step 10.
* If you receive a normal response (without Mutual-specific
headers), go to Step 10.
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Step 5 Send a request without any authentication headers.
* If you receive a 401-B0 message, go to Step 6.
* If you receive a normal response (without Mutual-specific
headers), go to Step 11.
Step 6 Check whether you know the user's password for the requested
authentication realm. If yes, go to Step 7. Otherwise, go
to Step 12.
Step 7 Check whether there is an available sid for the
authentication realm you expects. If there is one, go to
Step 8. Otherwise, go to Step 9.
Step 8 Send a req-A3 request.
* If you receive a 401-B0 message with a different
authentication realm than expected, go to Step 6.
* If you receive a 401-B0-stale message, go to Step 9.
* If you receive a 401-B0 message, go to Step 13.
* If you receive a valid 200-B4 message, go to Step 14.
Step 9 Send a req-A1 request.
* If you receive a 401-B1 message, go to Step 10.
Step 10 Send a req-A3 request.
* If you receive a 401-B0 message, go to Step 13.
* If you receive a valid 200-B4 message, go to Step 14.
Step 11 This case means that the resource requested is out of the
authenticated area. The client will be in "UNAUTHENTICATED"
status.
Step 12 This case means that the resource requested requires Mutual
authentication, and the user is not authenticated yet. The
client will be in "AUTH_REQUESTED" status, is RECOMMENDED to
process the content sent from the server and ask user a
username and password. If the user has input those, go to
Step 9.
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Step 13 This case means that in some reason the authentication
failed: possibly the password or the username is invalid for
the authenticated resource. Forget the password for the
authentication realm and go to Step 12.
Step 14 This case means that the mutual authentication has been
succeeded. The client will be in "AUTH_SUCCEEDED" status.
All other kind of responses than shown in above procedure SHOULD be
interpreted as fatal communication error, and in such cases user
clients MUST NOT process any data (contents and other content-related
headers) sent from the server.
The client software SHOULD show the three client status to the end-
user.
6. Authentication Algorithms
This document specifies only one family of the authentication
algorithm. The family consists of four authentication algorithms,
which only differ in underlying mathematical groups and security
parameters. The algorithms do not add any additional fields. The
tokens for algorithms are
o "iso11770-4-ec-p256" for the 256-bit prime-field elliptic-curve
setting.
o "iso11770-4-ec-p521" for the 521-bit prime-field elliptic-curve
setting.
o "iso11770-4-dl-2048" for the 2048-bit discrete-logarithm setting.
o "iso11770-4-dl-4096" for the 4096-bit discrete-logarithm setting.
The clients SHOULD support at least "iso11770-4-dl-2048" algorithm,
and are advised to support all of the above four algorithms whenever
possible. The server software implementations SHOULD support at
least "iso11770-4-dl-2048" algorithm, unless it is known that users
will not use it.
This algorithm uses Key Agreement Mechanism 3 (KAM3) defined in
Section 6.3 of ISO/IEC-11770-4 [ISO.11770-4.2006] as a basis.
For the elliptic-curve settings, the underlying fields and the curves
used for elliptic-curve cryptography are the prime field and the
Curve P-256 and P-511, respectively, specified in the appendix of
FIPS PUB 186-2 [FIPS.186-2.2000] specification. The hash functions H
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are SHA-256 for P-256 curve and SHA-512 for P-521 curve,
respectively, defined in FIPS PUB 180-2 [FIPS.180-2.2002]. The
representation of fields wa, wb, oa, and ob is hex-fixed-number.
For discrete-logarithm settings, the underlying groups are 2048-bit
and 4096-bit MODP groups defined in [RFC3526] respectively. The hash
functions H are SHA-256 for the 2048-bit field and SHA-512 for the
4096-bit field, respectively. The representation of fields wa, wb,
oa, and ob is base64-fixed-number.
The password-based string pi used by this authentication is derived
in the following manner:
pi = H(VS(algorithm) | VS(auth-domain) | VS(realm) | VS(username) |
VS(ph(password)).
The values of algorithm, realm and auth-domain are taken from the
values contained in the 401-B0 message. When pi is used in the
context of an octet string, it SHALL have the natural length derived
from the size of the output of function H (e.g. 32 octets for SHA-
256). The function ph is defined by the value of the pwd-hash field
given in a 401-B0 message.
The function VI encodes natural numbers into octet strings in the
following manner: integers are represented in big-endian radix-128
string, where each digit is represented by a octet 0x80-0xff except
the last digit represented by 0x00-0x7f. The first octet MUST NOT be
0x80. For example, VI(i) = octet(i) for i < 128, and VI(i) =
octet(0x80 | (i >> 7)) | octet(i & 127) for 128 <= i < 16384. This
encoding is the same as the one used in the length field in the ASN.1
encoding [ITU.X690.1994].
The function VS encodes variable-length octet string into decodable
octet string, as in the following manner:
VS(s) = VI(length(s)) | s
where length(s) is a number of octets (not characters) in s.
The equations for J, w_A, T, z, K_i, and w_B are those specified for
KAM3 in [ISO.11770-4.2006], given that pi is derived in the above
equation.
The values o_A (o_A') and o_B (o_B') are derived by the following
equation, instead of ones specified in [ISO.11770-4.2006].
o_A = H(hex(04) | GEtoOS_x(w_A) | GEtoOS_x(W_B) | GEtoOS_x(z) |
GEtoOS_x(g_1) | VI(nc) | VS(v))
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o_B = H(hex(03) | GEtoOS_x(w_A) | GEtoOS_x(W_B) | GEtoOS_x(z) |
GEtoOS_x(g_1) | VI(nc) | VS(v))
7. Validation Methods
The "validation method" specifies a method to "relate" the mutual
authentication processed by this protocol with other authentications
already performed in the underlying layers and to prevent man-in-the-
middle attacks. It decides the value of v which is an input to
authentication protocols.
The valid tokens for the validation field and corresponding values of
v are as follows:
host: hostname validation: v will be the ASCII string in the
following format: "scheme://host:port". The scheme
and host are lower-case, and the port is in a shortest
decimal representation. Even if the request-URI does
not have a port part, v will include the one.
tls-cert: TLS certificate validation: v will be the octet string
of the fingerprint of the public key certificate used
in underlying TLS/SSL [RFC4346] connection.
tls-key: TLS shared-key validation: v will be the octet string
of the shared master secret negotiated in underlying
TLS/SSL connection.
If the HTTP protocol is used on unencrypted channel, the validation
type MUST be "host". If HTTP/TLS [RFC2818] (https) protocol is used
with server certificates, the validation type MUST be either "tls-
cert" or "tls-key". If HTTP/TLS protocol is used with anonymous
Diffie-Hellman key exchange, the validation type MUST be "tls-key"
(but see the note below).
The client MUST validate this field upon reception of 401-B0
messages.
However, when the protocol is used on web browsers with any scripting
capabilities, the anonymous Diffie-Hellman family of TLS/SSL cipher-
suite MUST NOT be used even if "tls-key" validated Mutual
authentication has been employed, and the certificate shown in TLS/
SSL negotiation MUST be verified using PKI. For other systems, if
the "tls-key" validation is used on TLS/SSL protocol without
certificate verification using PKI, those systems MUST ensure that
all transactions with authenticated peer servers MUST use and be
validated by the Mutual authentication protocol, regardless of the
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existence of the 401-B0 responses.
8. Session Management
By the first 4 messages (first request, 401-B0, req-A1 and 401-B1), a
session represented by a sid is generated. This session can be used
for 1 or more requests for resources protected by the same realm in
the same server.
The server SHOULD accept at least one req-A3 request for each
session, given that the request reaches the server in a time window
specified by the timeout field in the 401-B1 message, and that there
are no emergent reasons (such as flooding attacks) to forget the
sessions. After that, the server MAY discard any session at any time
and MAY send 401-B0-stale messages for any req-A3 requests.
The client MAY send more than one requests using a single session
specified by the sid. However, for all such requests, the values of
the nonce-counter (nc field) MUST be different from each other. The
server MUST check for duplication of the received nonces, and if any
duplication is detected, the server MUST discard the session and
respond by a 401-B1-stale message.
In addition, for each sessions, if the client has already sent a
request with nonce value x, it SHOULD NOT send requests with a nonce
value not larger than (x - nc-window). The server MAY reject any
requests with nonces violating this rule with 401-B1-stale responses.
Values of nonces and nonce-related values MUST always be treated as
natural numbers within infinite range. Implementations using fixed-
width integers or fixed-precision floating numbers MUST handle
integer overflow correctly and carefully. Such implementations are
RECOMMENDED to accept any larger values which cannot be represented
in the fixed-width integer representations, as long as other limits
such as internal header-length restrictions are not involved. The
protocol is designed carefully so that both clients and servers can
implement the protocol only with fixed-width integers, by rounding
any overflowed values to the maximum possible value.
9. Extension 1: Optional Mutual Authentication
In several Web applications, users can access the same contents both
as a guest user and as a authenticated users. In usual Web
applications, it is implemented using Cookies and custom form-based
authentications. The extension described in this section provides a
replacement for those authentication systems. The support for this
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extension is RECOMMENDED.
Servers MAY send HTTP successful responses (response code 200, 206
and others) containing the Optional-WWW-Authenticate header, when it
is allowed to send 401-B0 responses and the requests do not contain
Authentication-Info: headers. Such responses are hereafter called
200-Optional-B0 responses.
HTTP/1.1 200 OK
Optional-WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx,
realm=xxxx, stale=0
The fields contained in the Optional-WWW-Authenticate header is the
same as the 401-B0 message described in Section 4.1. The client
software supporting the mutual authentication protocol receiving a
200-Optional-B0 message will process the contents of the message and
enables an authentication input field.
When the user input the username and password, the client resends the
request with req-A1 header. The server MUST respond with a 401-B1
message. In terms of the state management in Section 5, 200-
Optional-B0 responses are treated as if it is 401-B0 response: these
messages SHOULD NOT be sent as a response to req-A1 and req-A3
messages, unless the authentication realm sent from the client or
indicated by sid is different from the one which the server expects.
Servers requesting optional mutual authentication SHOULD send the
path field in 401-B1 messages with an appropriate value. Client
software supporting optional mutual authentication MUST recognize the
field, and MUST send either req-A1 or req-A3 request for the URI
space inside the specified paths, instead of unauthenticated
requests.
10. Methods to extend this protocol
If a non-standard extension to the this protocol is implemented, it
MUST use the extension-tokens defined in Section 3 to avoid conflicts
with this protocol and other extensions.
Authentication algorithms other than those defined in this document
MAY use other representations for keys "wa", "wb", "oa" and "ob",
replace those keys, and/or add fields to the messages containing
those fields by supplemental specifications. If those specifications
use keys other than shown above, it is RECOMMENDED to use extension-
tokens to avoid any key-name conflict with the future extension of
this protocol.
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11. IANA Considerations
The tokens used for authentication-algorithm, pwd-hash, and
validation fields MUST be allocated by IANA. To acquire registered
token, IESG Approval outlined in [RFC2434] is required. Extension-
tokens MAY be freely used for any non-standard, private and/or
experimental uses for those fields provided that the domain part in
the token is appropriately used.
12. Security Considerations
12.1. General Assumptions
o The protocol, by itself, does not assure any secrecy and relies on
transport security including DNS security. HTTP/TLS SHOULD be
used where transport security is not assured and data secrecy is
important. The protocol is secure against passive eavesdropping
and replay attacks, though.
o When used with HTTP/TLS, the protocol gives true protection
against active man-in-the-middle attacks for each HTTP request/
response pair, even when the server certificate is not used or is
unreliable. However, in such cases, JavaScript or similar
scripting facilities can be used to affect Mutually-authenticated
contents from those not protected by this authentication
mechanism. This is why this memo requires that sane TLS server
certificates MUST be presented.
12.2. Implementation Considerations
o To securely implement the protocol, the Authentication-Info
headers in the 200-B4 messages MUST always be validated by the
client. If the validation is failed, the client MUST NOT process
any content sent with the message, including the body part. Non-
compliance to this will enable phishing attacks.
o The authentication status on the client-side SHOULD be visible to
the users of the client. In addition, the method for asking
user's name and passwords SHOULD be carefully designed so that (1)
the user can easily distinguish request of this authentication
methods from other existing authentication methods such as Basic
and Digest methods, and (2) the Web contents cannot imitate the
user-interfaces of this protocol.
An informational memo regarding user-interface considerations and
recommendations for implementing this protocol will be separately
published.
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o For HTTP/TLS communications, when a web form is submitted from
Mutually-authenticated pages with the validation methods of "tls-
cert" to a URI which is protected by the same realm (so indicated
by the path field), if server certificate has been changed since
the pages has been received, the peer is RECOMMENDED to be
revalidated using a req-A1 message with an "Expect: continue"
header. The same applies when the page is received with the
validation methods of "tls-key", and when the TLS session has been
expired.
o Server-side storages of user passwords are advised to have the
values encrypted by one-way function J(pi), instead of the real
passwords, those hashed by ph, or pi.
13. Notice on intellectual properties
The National Institute of Advanced Industrial Science and Technology
(AIST) and Yahoo! Japan, Inc. has jointly submitted a patent
application about the protocol proposed in this documentation to the
Patent Office of Japan. The patent is intended to be open to any
implementors of this protocol and its variants under non-exclusive
royalty-free manner once the protocol is accepted as an Internet
standard. For the detail of the patent application, contact the
author of this document.
The elliptic-curve based authentication algorithms might involve
several existing patents of third-parties. The authors of the
document take no position regarding the validity or scope of such
patents, and other patents as well.
14. References
14.1. Normative References
[FIPS.180-2.2002]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[FIPS.186-2.2000]
National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", FIPS PUB 186-2, January 2000, <
http://csrc.nist.gov/publications/fips/fips186-2/
fips186-2-change1.pdf>.
[ISO.10646-1.1993]
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International Organization for Standardization,
"Information Technology - Universal Multiple-octet coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO Standard 10646-1, May 1993.
[ISO.11770-4.2006]
International Organization for Standardization,
"Information technology - Security techniques - Key
management - Part 4: Mechanisms based on weak secrets",
ISO Standard 11770-4, May 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
14.2. Informative References
[ITU.X690.1994]
International Telecommunications Union, "Information
Technology - ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, 1994.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
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Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
Authors' Addresses
Yutaka Oiwa
National Institute of Advanced Industrial Science and Technology
Research Center for Information Security
Akihabara Daibiru #1102
1-18-13 Sotokanda
Chiyoda-ku, Tokyo
JP
Phone: +81 3-5298-4722
Email: mutual-auth-contact@m.aist.go.jp
Hajime Watanabe
National Institute of Advanced Industrial Science and Technology
Hiromitsu Takagi
National Institute of Advanced Industrial Science and Technology
Hirofumi Suzuki
Yahoo! Japan, Inc.
Roppongi Hills Mori Tower
6-10-1 Roppongi
Minato-ku, Tokyo
JP
Phone: +81 3-6440-6290
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