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INTERNET-DRAFT                                      Daniel Simon
Transport Layer Security Working Group              Microsoft Corp.
Draft-ietf-tls-passauth-00.txt                      November 20, 1996
                                          Expires:  May 25, 1997




Addition of Shared Key Authentication to Transport Layer Security (TLS)


0. Status Of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups.  Note that other groups may also
distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
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documents at any time.  It is inappropriate to use Internet-
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To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the Internet-
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1.  Abstract

This document presents a shared-key authentication mechanism for the TLS
protocol.  It is intended to allow TLS clients to authenticate using a
secret key (such as a password) shared with either the server or a
third-party authentication service.  The security of the secret
authentication key is augmented by its integration into the normal
SSL/TLS server authentication/key exchange mechanism.


2.  Introduction

Recent transport-layer security protocols for the Internet, such as SSL
versions 2.0 and 3.0 [1, 2] and PCT version 1 [3], have effected
challenge-response authentication using strictly public-key (asymmetric)
cryptographic methods, with no use of out-of-band shared secrets.  This
choice has both benefits and drawbacks.  The primary benefit is improved
security:  an asymmetric private key used for authentication is only
stored in one location, and the out-of-band identification necessary for
public key certification need only be reliable, not secret (as an out-
of-band shared key exchange must be).  In addition, the difficult task
of out-of-band shared-key exchange in shared-key authentication systems
often leads implementers to resort to human-friendly shared keys
(manually typed passwords, for instance), which may be vulnerable to
discovery by brute force search or "social engineering".

However, shared-key authentication has certain advantages as well.
These are, chiefly:

- Portability:  Precisely because shared keys are often human-remembered
passwords or passphrases, they can be transported from (trusted) machine
to (trusted) machine with ease--unlike asymmetric private keys, which
must be transported using some physical medium, such as a diskette or
"smart card", to be available for use on any machine.

- Backward Compatibility:  Shared-key authentication is in very wide use
today, and the cost of conversion to its public-key counterpart may not
be worth the extra security, to some installations.

- Established Practice:  Shared-key authentication has been in use for
quite a while, and a valuable body of tools, techniques and expertise
has grown up around it.  In contrast, public-key authentication is very
new, its associated tools and methods are either untested or non-
existent, and experience with possible implementation or operation
pitfalls simply doesn't exist.

These reasons are particularly relevant when individual human users of a
service are being authenticated over the Internet, and as a result,
virtually all authentication of (human) clients of such services is
currently performed using shared passwords.  Typically, servers
implementing one of the aforementioned transport-layer security
protocols, and needing client authentication, simply accept secure
(i.e., encrypted and server-authenticated) connections from each client,
who then provides a password (or engages in a challenge-response
authentication protocol based on a password) over the secure connection
to authenticate to the server.

Unfortunately, such "secure" connections are often not secure enough to
protect passwords, because of the various international legal
restrictions that have been placed on the use of encryption.  Obviously,
secret keys such as passwords should not be sent over weakly encrypted
connections.  In fact, even a challenge-response protocol which never
reveals the password is vulnerable, if a poorly chosen, guessable
password is used; an attacker can obtain the (weakly protected)
transcript of the challenge-response protocol, then attempt to guess the
password, verifying each guess against the transcript.

However, it is possible to protect even badly-chosen passwords against
such attacks by incorporating shared-key authentication into the
transport-layer security protocol itself.  These protocols already
involve the exchange of long keys for message authentication, and those
same keys can be used (without the legal restraints associated with
encryption) to provide very strong protection for shared-key-based
challenge-response authentications, provided that the mechanism used
cannot be diverted for use as a strong encryption method.  This latter
requirement makes it essential that the shared-key-based authentication
occur at the protocol level, rather than above it (as is normally the
case today), so that the implementation can carefully control use of the
long authentication key.


3.  Protocol Additions

Starting from SSL version 3.0 notation and formats, the following three
new HandshakeTypes are added, and included in the Handshake message
definition:

shared_keys(30), shared_key_request(31), shared_key_verify(32)

A new CipherSuite is also included, to allow the client to signal
support for shared-key authentication to the server:

TLS_AUTH_SHARED_KEY = {x01, x01};

The client's inclusion of this CipherSuite is independent of other
listed CipherSuites, and simply indicates to the server the
client's support for shared-key authentication.


3.1  SharedKeys message

The SharedKeys message has the following structure:

struct {
     DistinguishedName auth_services_client<1..65535>;
} SharedKeys;

This optional message may be sent by the client immediately following
the ClientHello message; in fact, if sent, it is actually enclosed
within the ClientHello message, immediately following the last defined
field of the ClientHello message.  (For forward compatibility reasons,
the SSL 3.0 ClientHello message is allowed to contain data beyond its
defined fields, and because there is no ClientHelloDone message, the
server cannot know that an extra message follows the ClientHello unless
it is actually included in the ClientHello message itself.  A server
that does not support shared-key authentication will simply ignore the
extra data in the ClientHello message.)  Although enclosed within the
ClientHello, the SharedKeys message retains the normal structure and
headers of a Handshake message.

The SharedKeys message contains a list of distinguished names of
authentication services to which the client is willing to authenticate.
This list need not be exhaustive; if the server cannot find an
acceptable authentication service from the list in the SharedKeys
message, then the server is free to reply with a list of acceptable
services in a subsequent SharedKeyRequest message.

In cases where pass-through authentication is used, this message allows
clients to be able to notify servers in advance of one or more
authentication services sharing a key with the client, so that the
server need only fetch (or use up) a challenge from a single service for
that client.  This message may also be useful in non-pass-through
situations; for example, the client may share several keys with the
server, associated with identities on different systems (corresponding
to different "authentication services" residing on the same server).
If a server receives a SharedKeys message, then any subsequent
SharedKeyRequest message can contain a single authentication service
selected from the client's list.

Note that sending a SharedKeys message does not in itself normally
reveal significant information about the client's as-yet-unspecified
identity or identities.  However, if information about the set of
authentication services supported by a particular client is at all
sensitive, then the client should not send this message.


3.2  SharedKeyRequest message

The SharedKeyRequest message has the following structure:

struct {
         DistinguishedName auth_service_name;
         opaque display_string<0..65535>;
         opaque challenge<0..255>;
} AuthService;

struct {
     AuthService auth_services_server<1..65535>;
} SharedKeyRequest;

This optional message may be sent immediately following the server's
first set of consecutive messsages, which includes the ServerHello and
(possibly) the Certificate, CertificateRequest and ServerKeyExchange
messages, but before the ServerHelloDone message.  The
auth_services_server field contains a list of distinguished names of
shared-key authentication services by which the client can authenticate.
The challenge field accompanying each authentication service name
contains an optional extra authentication challenge, in case the server
needs to obtain one from an authentication service for pass-through
authentication.  If none is required, then it would simply be an empty
(zero-length) field.  Similarly, the display_string field may contain
information to be used (displayed to the user, for example) during
authentication, if needed; its interpretation is left to the
implementation.


3.3  SharedKeyVerify message

The SharedKeyVerify message is sent in response to a SharedKeyRequest
message from the server, at the same point at which a CertificateVerify
message would be sent in response to a CertificateRequest message.  (If
both a CertificateRequest and a SharedKeyRequest are sent by the server,
then the client may respond with either a CertificateVerify message or a
SharedKeyVerify message.  Only one of the two messages is ever sent in
the same handshake, however.)  The SharedKeyVerify message has the
following structure:

struct {
     AuthService auth_service;
     opaque identity<1..65535>;
     opaque shared_key_response<1..255>;
} SharedKeyVerify;

The value of auth_service must be identical to one of the AuthService
values on the list in SharedKeyRequest.auth_services_server.  If the
client does not share a key with any of the authentication services
listed in the SharedKeyRequest message (and cannot supply a certificate
matching the requirements specified in the accompanying
CertificateRequest message, if one was sent), then the client returns a
"no certificate" alert message (in its normal place in the protocol).

The format of the identity field is left to the implementation, and must
be inferable from the accompanying value of auth_service.  The value of
shared_key_response is defined as

SharedKeyVerify.shared_key_response
     hash (auth_write_secret + pad_2 +
            hash (auth_write_secret + pad_1 + hash (handshake_messages)
                  + SharedKeyVerify.auth_service.auth_service_name
                  + SharedKeyVerify.auth_service.display_string
                  + SharedKeyVerify.auth_service.challenge
                  + SharedKeyVerify.identity + shared_key) )

Here "+" denotes concatenation.  The hash function used (hash) is
taken from the pending cipher spec.  The client_auth_write_secret and
server_auth_write_secret values are obtained by extending the
key_block by CipherSpec.hash_size bytes beyond the server_write_key (or
the server_write_IV, if it is derived from key_block as well), and using
this extended portion as the client_auth_write_secret value.  (Only the
client_auth_write_secret is used, since only the client ever sends a
SharedKeyVerify message.)  The value of handshake_messages is the
concatenation of all handshake messages from the first one sent up to
(but not including) the shared_key_verify message.  The pad_1 and pad_2
values correspond to the ones used for MAC computation in the
application_data message.  The fields from the SharedKeyVerify message
are input with their length prefixes included.


4.  Normal Authentication

A shared-key-based client authentication may proceed as follows:  the
client includes the TLS_AUTH_SHARED_KEY CipherSuite in its list of
CipherSuites in its ClientHello message.  It also may or may not send a
SharedKeys message along with the ClientHello message, listing the
authentication services with which the client shared a key for
authentication purposes.  In any event, the server sends a
SharedKeyRequest handshake message following the ServerHello and
accompanying messages containing a list of names of one or more
authentication services; if a SharedKeys message was sent, then this
list will contain a single choice from the client's SharedKeys message.
The client, on receiving the SharedKeyRequest message, selects an
authentication service from the server's list (if more than one is
offered) and constructs the appropriate authentication response as
described above, sending it back, along with its identity and choice of
authentication service, in a SharedKeyVerify handshake message.  The
server itself also constructs the correct authentication response using
the known shared key, and checks it against the one provided by the
client.  The authentication is successful if the two match exactly.
Note that if the shared key is password-based, then it would typically
be derived from the password using a one-way cryptographic hash
function, rather than being the password itself, so that the original
password need not be remembered by anyone but the client.


5.  Pass-through Authentication

In some circumstances, it is preferable for shared keys to be stored in
one place (a central, well-protected site, for instance) while servers
that actually communicate with clients are elsewhere (possibly widely
distributed, but maintaining secure connections to the central shared-
key server).  One of the advantages of the shared-key authentication
method proposed here is that it allows "pass-through" authentication by
a third party, if the server accepting the public-key key exchange and
the server sharing the key with the client happen to be different.  (The
use of a separately derived authentication key in the response
computation makes this possible.)

Pass-through authentication might work as follows:  The server would
either collect random challenges in advance from its authentication
services, or request them as needed.  (If the client sends a SharedKeys
message, then the server can select an authentication service from the
client's list, and obtain a challenge from that service alone.)
Assuming that the client indicates support for shared-key authentication
by including the TLS_AUTH_SHARED_KEY CipherSuite in its list, the server
would then send a list of one or more authentication services and
associated challenges in a SharedKeyRequest message.  The client would
then select an authentication service (if more than one is offered),
compute the correct authentication response using the above proposed
formula, and send it to the server in a SharedKeyVerify message.

The server, on receiving a response from a client, would pass it through
to the authentication service, along with the values necessary to
recalculate it:  the client_auth_write_key, the hash of all the
handshake messages and the identity field from the certificate verify
message.  The authentication service would then use the values provided,
along with the secret key it shares with the client and the challenge it
supplied, to reconstruct the correct value of the response.  If this
value exactly matches the one provided by the server, then the
authentication would succeed; otherwise it would fail.


References

[1]  K. Hickman and T. Elgamal, "The SSL Protocol", Internet Draft
<draft-hickman-netscape-ssl-01.txt> (deleted), February 1995.

[2]  A. Freier, P. Karlton and P. Kocher, "The SSL Protocol Version
3.0", Internet Draft <draft-freier-ssl-version3-01.txt>, March 1996.

[3]  J. Benaloh, B. Lampson, D. Simon, T. Spies and B. Yee, The PCT
Protocol", Internet Draft <draft-benaloh-pct-00.txt>, November 1995.


Author's Address

Daniel Simon <dansimon@microsoft.com>


Microsoft Corporation
One Microsoft Way
Redmond, WA  98052
Phone: (206) 936-6711
Fax:   (206) 936-7329


Draft-ietf-tls-passauth-00.txt
November 20, 1996
Expires:  May 25, 1997


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