HTTP Secure Remote Password (SRP) Authentication Scheme
draft-yusef-httpauth-srp-scheme-01
The information below is for an old version of the document.
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| Authors | Rifaat Shekh-Yusef , Yaron Sheffer | ||
| Last updated | 2015-11-25 | ||
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draft-yusef-httpauth-srp-scheme-01
HTTPAuth R. Shekh-Yusef
Internet-Draft Avaya
Intended status: Standards Track Y. Sheffer
Expires: May 28, 2016 Intuit
November 25, 2015
HTTP Secure Remote Password (SRP) Authentication Scheme
draft-yusef-httpauth-srp-scheme-01
Abstract
This document defines an HTTP Authentication Scheme that is based on
the Secure Remote Password (SRP) protocol. The SRP protocol is an
Augmented Password Authenticated Key Exchange (PAKE) protocol
suitable for authenticating users and exchanging keys over an
untrusted network.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 28, 2016.
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Table of ContentsMelnikov & Leiba Expires August 20, 2021 [Page 28]
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The LOGOUT command informs the server that the client is done with
the connection. The server MUST send a BYE untagged response before
the (tagged) OK response, and then close the network connection.
Example: C: A023 LOGOUT
S: * BYE IMAP4rev2 Server logging out
S: A023 OK LOGOUT completed
(Server and client then close the connection)
6.2. Client Commands - Not Authenticated State
In the not authenticated state, the AUTHENTICATE or LOGIN command
establishes authentication and enters the authenticated state. The
AUTHENTICATE command provides a general mechanism for a variety of
authentication techniques, privacy protection, and integrity
checking; whereas the LOGIN command uses a traditional user name and
plaintext password pair and has no means of establishing privacy
protection or integrity checking.
The STARTTLS command is an alternative form of establishing session
privacy protection and integrity checking, but does not by itself
establish authentication or enter the authenticated state.
Server implementations MAY allow access to certain mailboxes without
establishing authentication. This can be done by means of the
ANONYMOUS [SASL] authenticator described in [ANONYMOUS]. An older
convention is a LOGIN command using the userid "anonymous"; in this
case, a password is required although the server may choose to accept
any password. The restrictions placed on anonymous users are
implementation-dependent.
Once authenticated (including as anonymous), it is not possible to
re-enter not authenticated state.
In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
the following commands are valid in the not authenticated state:
STARTTLS, AUTHENTICATE and LOGIN. See the Security Considerations
(Section 11) for important information about these commands.
6.2.1. STARTTLS Command
Arguments: none
Responses: no specific response for this command
Result: OK - starttls completed, begin TLS negotiation
NO - TLS negotiation can't be initiated, due to server
configuration error
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BAD - STARTTLS received after a successful TLS
negotiation or arguments invalid
Note that STARTTLS command is available only on cleartext ports. The
server MUST always respond with tagged BAD response when STARTTLS
command is received on Implicit TLS port.
A TLS [TLS-1.3] negotiation begins immediately after the CRLF at the
end of the tagged OK response from the server. Once a client issues
a STARTTLS command, it MUST NOT issue further commands until a server
response is seen and the TLS negotiation is complete. Some past
server implementation incorrectly implemented STARTTLS processing and
are known to contain STARTTLS plaintext command injection
vulnerability [CERT-555316]. In order to avoid this vulnerability,
server implementations MUST do one of the following If any data is
received in the same TCP buffer after the CRLF that starts the
STARTTLS command:
1. Extra data from the TCP buffer is interpreted as beginning of the
TLS handshake. (If the data is in cleartext, this will result in
the TLS handshake failing.)
2. Extra data from the TCP buffer is thrown away.
Note that the first option is friendlier to clients that pipeline
beginning of STARTTLS command with TLS handshake data.
After successful TLS negotiation the server remains in the non-
authenticated state, even if client credentials are supplied during
the TLS negotiation. This does not preclude an authentication
mechanism such as EXTERNAL (defined in [SASL]) from using client
identity determined by the TLS negotiation.
Once TLS has been started, the client MUST discard cached information
about server capabilities and SHOULD re-issue the CAPABILITY command.
This is necessary to protect against man-in- the-middle attacks which
alter the capabilities list prior to STARTTLS. The server MAY
advertise different capabilities, and in particular SHOULD NOT
advertise the STARTTLS capability, after a successful STARTTLS
command.
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Example: C: a001 CAPABILITY
S: * CAPABILITY IMAP4rev2 STARTTLS LOGINDISABLED
S: a001 OK CAPABILITY completed
C: a002 STARTTLS
S: a002 OK Begin TLS negotiation now
<TLS negotiation, further commands are under TLS layer>
C: a003 CAPABILITY
S: * CAPABILITY IMAP4rev2 AUTH=PLAIN
S: a003 OK CAPABILITY completed
C: a004 AUTHENTICATE PLAIN dGVzdAB0ZXN0AHRlc3Q=
S: a004 OK Success (tls protection)
6.2.2. AUTHENTICATE Command
Arguments: SASL authentication mechanism name
OPTIONAL initial response
Responses: continuation data can be requested
Result: OK - authenticate completed, now in authenticated state
NO - authenticate failure: unsupported authentication
mechanism, credentials rejected
BAD - command unknown or arguments invalid,
authentication exchange cancelled
The AUTHENTICATE command indicates a [SASL] authentication mechanism
to the server. If the server supports the requested authentication
mechanism, it performs an authentication protocol exchange to
authenticate and identify the client. It MAY also negotiate an
OPTIONAL security layer for subsequent protocol interactions. If the
requested authentication mechanism is not supported, the server
SHOULD reject the AUTHENTICATE command by sending a tagged NO
response.
The AUTHENTICATE command supports the optional "initial response"
feature defined in Section 5.1 of [SASL]. The client doesn't need to
use it. If a SASL mechanism supports "initial response", but it is
not specified by the client, the server handles this as specified in
Section 3 of [SASL].
The service name specified by this protocol's profile of [SASL] is
"imap".
The authentication protocol exchange consists of a series of server
challenges and client responses that are specific to the
authentication mechanism. A server challenge consists of a command
continuation request response with the "+" token followed by a BASE64
encoded (see Section 4 of [RFC4648]) string. The client response
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consists of a single line consisting of a BASE64 encoded string. If
the client wishes to cancel an authentication exchange, it issues a
line consisting of a single "*". If the server receives such a
response, or if it receives an invalid BASE64 string (e.g.
characters outside the BASE64 alphabet, or non-terminal "="), it MUST
reject the AUTHENTICATE command by sending a tagged BAD response.
As with any other client response, the initial response MUST be
encoded as BASE64. It also MUST be transmitted outside of a quoted
string or literal. To send a zero-length initial response, the
client MUST send a single pad character ("="). This indicates that
the response is present, but is a zero-length string.
When decoding the BASE64 data in the initial response, decoding
errors MUST be treated as in any normal SASL client response, i.e.
with a tagged BAD response. In particular, the server should check
for any characters not explicitly allowed by the BASE64 alphabet, as
well as any sequence of BASE64 characters that contains the pad
character ('=') anywhere other than the end of the string (e.g.,
"=AAA" and "AAA=BBB" are not allowed).
If the client uses an initial response with a SASL mechanism that
does not support an initial response, the server MUST reject the
command with a tagged BAD response.
If a security layer is negotiated through the [SASL] authentication
exchange, it takes effect immediately following the CRLF that
concludes the authentication exchange for the client, and the CRLF of
the tagged OK response for the server.
While client and server implementations MUST implement the
AUTHENTICATE command itself, it is not required to implement any
authentication mechanisms other than the PLAIN mechanism described in
[PLAIN]. Also, an authentication mechanism is not required to
support any security layers.
Note: a server implementation MUST implement a configuration in
which it does NOT permit any plaintext password mechanisms, unless
either the STARTTLS command has been negotiated, TLS has been
negotiated on an Implicit TLS port, or some other mechanism that
protects the session from password snooping has been provided.
Server sites SHOULD NOT use any configuration which permits a
plaintext password mechanism without such a protection mechanism
against password snooping. Client and server implementations
SHOULD implement additional [SASL] mechanisms that do not use
plaintext passwords, such the GSSAPI mechanism described in
[RFC4752], the SCRAM-SHA-256/SCRAM-SHA-256-PLUS [SCRAM-SHA-256]
mechanisms and/or EXTERNAL [SASL] mechanism for mutual TLS
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Operations Overview . . . . . . . . . . . . . . . . . . . . . 3
3. Initial Request . . . . . . . . . . . . . . . . . . . . . . . 6
4. Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Response . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Confirmation . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Username Hashing . . . . . . . . . . . . . . . . . . . . . . 10
8. Internationalization Considerations . . . . . . . . . . . . . 10
9. Integration with Other Protocols . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
12. Normative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Some protocols (e.g. HTTP [HTTP-P7], SIP [RFC3261], OAUTH 2.0
[RFC6749], and STUN [RFC5389]) use a general framework for access
control and authentication, via a set of challenge-response
authentication schemes, which can be used by a server to challenge a
client request and by a client to provide authentication information.
Many of these systems that use the challenge-response framework rely
on passwords chosen by users which usually have low entropy and weak
randomness, and as a result cannot be used as cryptographic keys.
While cannot be used directly as cryptographic keys, the passwords
can still be used to derive cryptographic keys.
This document defines an HTTP Authentication Scheme that is based on
the Secure Remote Password (SRP) protocol. The SRP protocol is an
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Augmented Password Authenticated Key Exchange (PAKE) protocol
suitable for authenticating users and exchanging keys over an
untrusted network, based on a shared password, without requiring a
Public Key Infrastructure (PKI) or any trusted third party.
The SRP protocol provides many security benefits: it resists
dictionary attacks mounted by either passive or active network
intruders, allowing even weak passphrases to be used safely. It also
offers perfect forward secrecy, which protects past sessions and
passwords against future compromises. Finally, user passwords are
stored in a form that is not plaintext-equivalent to the password
itself, so an attacker who captures the password database cannot use
it directly to compromise security and gain immediate access to the
host.
1.1. Terminology
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. Operations Overview
During its setup, the server must choose a large-prime, and as a
result the integers from 1 to (large-prime - 1) form a group under
multiplication mod large-prime, and a generator for this group is
also chosen.
When a user account is created, the server selects a hash function
and a user salt, and uses a realm and the user password to create a
password-verifier as follows:
derived-private-key = H(username:realm:password:salt)
password-verifier = generator ^ derived-private-key
Note that all values in this document are computed modulo large-
prime.
The server then stores the following information in the database:
o username
o hash-algorithm
o salt
o password-verifier
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The following flow describes at a high-level the flow of messages
based on the challenge-response framework:
Client Server
-------------------------------------------------------------------
| |
Discovery of the protection space stage (optional) |
| |
| Authorization: SRP |
|------------------------------------------------------------->|
| |
| WWW-Authenticate: SRP |
| realm="realm" |
|<-------------------------------------------------------------|
| |
| |
Mutual AuthN and establishment of session-key stage |
| |
| Authorization: SRP |
| username="username" |
|------------------------------------------------------------->|
| |
| WWW-Authenticate: SRP |
| large-prime="large-prime" |
| generator="generator" |
| hash-algorithm="hash-algorithm" |
| salt="salt", |
| server-public-key="server-public-key" |
|<-------------------------------------------------------------|
| |
| Authorization: SRP |
| server-public-key="server-public-key" |
| client-public-key="client-public-key" |
| client-pop="client-pop" |
|------------------------------------------------------------->|
| |
| WWW-Authenticate: SRP |
| server-pop="server-pop" |
|<-------------------------------------------------------------|
| |
| |
| |
| |
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The HTTP Authentication Framework [RFC7235] defines "protection
space" as a combination of the canonical root URI of the server being
accessed and the "realm" value. The "realm" values allow the
partitioning of the server resources into a set of protection spaces,
each with its own authentication scheme and/or authorization
database.
A protection space determines the scope of protection covered by a
set of credentials that can be applied automatically. If a prior
request has been authorized, then the client may reuse the same
credentials for all other requests within that protection space, for
a period of time determined by the authentication scheme.
If the client is aware of the realm associated with the protection
space it is trying to access, then the client initiates the
communication to the server by sending an initial request with an
Authorization header with SRP scheme which includes the username
parameter associated with that protection space.
Otherwise, If the client is not aware of the realm associated with
the resources it is trying to access, then the initial request will
include the SRP scheme with no parameters. This will allow the
server to challenge the request and provide the client with the realm
associated with the resource. The client is then expected to retry
the request with the username parameter.
The server generates its private key and calculates its associated
public key, then challenges the request and includes the WWW-
Authenticate with the large-prime, generator, hash-algorithm, salt,
and server-public-key.
The client calculates the session-key and re-tries the request and
includes an Authorization header with client-pop to prove to the
server that it is in possession of the session-key.
The server verifies the client-pop and calculates the server-pop to
prove to the client that it is in possession of the same session-key.
At the end of the above process, the client and the server would have
established a communication channel after completing a mutual
authentication, and each side would be in possession of the same
session-key.
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3. Initial Request
The initial request from the client MUST include an Authorization
header field with the SRP scheme.
If the client is aware of the realm associated with the resource it
is trying to access, then the client MUST include the following
parameter:
username
The user's name in a specific realm.
Otherwise, the initial request MUST be sent without any parameters,
to allow the server to challenge the request and send the realm to
the client. The client is then expected to retry the request with
the username associated with the realm of the resource being
accessed.
4. Challenge
If the initial request received from the client does not have a
username parameter, then the server MUST challenge the request by
responding with 401 and MUST include the realm parameter. The client
is expected to retry the request and include the username parameter.
When the server receives the request with the username parameter, the
server looks up the hash-algorithm, salt, and password-verifier
associated with the username provided by the client in the initial
request.
OPEN ISSUE:
{
To prevent an attacker from identifying the usernames in the DB, if a
username does not exist in the DB, the server should still go through
the motion of attempting to authenticate the user and fail it only
during the last step, to prevent the attacker from recognizing the
username existence by analyzing how fast the server responds to the
initial request.
}
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The server generates a random number, [1, large-prime - 1], as a
server ephemeral private key (server-private-key), and computes the
associated server ephemeral public key. The server MUST generate a
fresh ephemeral private key for each authentication session, even if
the request is coming from the same user.
The server calculates the server-public-key as follows:
server-public-key = H( 3 * password-verifier + generator ^ server-
private-key )
The server then challenges the initial request from the client by
responding with a "401 Unauthorized" status code and a WWW-
Authenticate header field with and SRP scheme. The header field MUST
include the following parameters:
large-prime
The large prime used to form the finite field GF(n) group,
selected by the server during setup, formatted as a decimal
integer.
generator
A finite field GF(n) group generator selected by the server
during setup, formatted as a decimal integer.
hash-algorithm
The hash algorithm used to create the session-key, e.g SHA256.
salt
A random string used as user's salt.
server-public-key
The server ephemeral public key associated with the server
ephemeral private key.
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5. Response
The client generates a random number, [1, large-prime - 1], as a
client ephemeral private key (client-private-key), and computes the
associated client ephemeral public key as follows:
client-public-key = generator ^ client-private-key
The client calculates the derived-private-key, random nonce, session-
tag, session-key, and client-pop as follows:
derived-private-key = H( username:realm:password:salt )
nonce = H( client-public-key : server-public-key )
session-tag = ( server-public-key - 3 * generator ^ derived-
private-key) ^ (client-private-key + nonce * derived-private-key )
session-key = H ( session-tag )
client-pop = H( client-public-key : server-public-key : session-
tag )
The client is expected to retry the request passing an Authorization
header field with SRP scheme. The header field MUST include the
following parameters:
server-public-key
The server ephemeral public key that the client received from
the server with the challenge request.
client-public-key
The client ephemeral public key associated with the client
ephemeral private key.
client-pop
A client proof-of-possession to prove to the server that the
client is possession of the session-key.
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6. Confirmation
The server MAY use the server-public-key received from the client to
correlate this request with the previous response it sent to the
client. This especially important in the case that this request is
received on a different connection than the one that delivered the
initial request.
The server calculates a random nonce and the session-tag as follows:
nonce = H( client-public-key : server-public-key )
session-tag = ( client-public-key * password-verifier ^ nonce ) ^
server-private-key
At this stage, the server has enough information to verify the
client-pop by locally calculating the expected-client-pop and
comparing it to the one received from the client. The server
calculates the expected-client-pop as follows:
expected-client-pop = H( client-public-key : server-public-key :
session )
The server then compares the expected-client-pop to the client-pop.
If they are different, then the server MUST fail the request by
responding with a "401 Unauthorized" status code and follow the same
procedure used with the initial request.
If the expected-client-pop is the same as the client-pop received
from the client, the server continues the process and calculates its
server-pop and session-key as follows:
server-pop = H( client-public-key : client-pop : session-tag )
session-key = H ( session-tag )
The server then confirms the client's request by responding with 200
OK request. The 200 OK message MUST include an Authentication-Info
header field with the SRP scheme. The header field MUST include the
following parameter:
server-pop
A server proof-of-possession to prove to the client that the
server is possession of the session-key.
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When the client receives the confirmation from the server it verifies
the server-pop by calculating the expected-server-pop and comparing
it to the server-pop. If these values are different, then the client
MUST consider the authentication process to have failed; otherwise,
the authentication process is complete, and both sides would be in
possession of the same session-key.
7. Username Hashing
TBD: https://tools.ietf.org/html/draft-ietf-httpauth-digest-
19#section-3.4.4
8. Internationalization Considerations
TBD: https://tools.ietf.org/html/draft-ietf-httpauth-digest-
19#section-4
9. Integration with Other Protocols
10. Security Considerations
11. IANA Considerations
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12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SRP6] Wu, T., "SRP-6: Improvements and Refinements to the Secure
Remote Password Protocol", IEEE P1363 Working
Group http://srp.stanford.edu/srp6.ps, October 2002.
Authors' Addresses
Rifaat Shekh-Yusef
Avaya
250 Sidney Street
Belleville, Ontario
Canada
Phone: +1-613-967-5267
EMail: rifaat.ietf@gmail.com
Yaron Sheffer
Intuit
4 HaHarash St.
Hod HaSharon 4524075
Israel
EMail: yaronf.ietf@gmail.com
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