
TLS Working Group                                              P.Gutmann
Internet-Draft                                    University of Auckland
Expires: April 2004                                         October 2003

                  Use of Shared Keys in the TLS Protocol
                       draft-ietf-tls-sharedkeys-02

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Copyright (C) The Internet Society (2003).  All Rights Reserved.

1. Abstract

The TLS handshake requires the use of CPU-intensive public-key algorithms with
a considerable overhead in resource-constrained environments or ones such as
mainframes where users are charged for CPU time.  This document describes a
means of employing TLS using symmetric keys or passwords shared in advance
among communicating parties.  As an additional benefit, this mechanism
provides cryptographic authentication of both client and server without
requiring the transmission of passwords or the use of certificates.  No
modifications or alterations to the TLS protocol are required for this
process.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document (in
uppercase, as shown) are to be interpreted as described in [RFC 2119].

2. Problem analysis

TLS is frequently used with devices with little CPU power available, for
example mobile and embedded devices.  In these situations the initial TLS
handshake can take as long as half a minute with a 1Kbit RSA key.  In many
cases a fully general public-key-based handshake is unnecessary, since the
device is only syncing to a host PC or contacting a fixed base station, which
would allow a pre-shared symmetric key to be used instead.  An example of this
kind of use is using 3GPP cellular mechanisms to establish keys used to secure
a TLS tunnel to a mobile device.

In a slight variation of this case, CPU power is available but is too
expensive to devote to public-key operations.  This situation is common in
mainframe environments, where users are charged for CPU time.  As with mobile
devices, mainframe-to-mainframe or client-to-mainframe communications are
generally fixed in advance, allowing shared symmetric keys to be employed.

In order to solve these problems, we require a means of eliminating the
expensive public-key operations in the TLS handshake, while providing an
equivalent level of security using shared symmetric keys.  The solution is
fairly straightforward.  Observe that after the initial handshake phase, TLS
is operating with a quantity of symmetric keying material derived from the
information exchanged during the initial handshake.  Using shared symmetric
keys involves explicitly deriving the TLS master secret from the shared key,
rather than sharing it implicitly via the public-key-based key agreement
process.  TLS already contains such a mechanism built into the protocol in the
form of the session cacheing mechanism, which allows a TLS session to be
resumed without requiring a full public-key-based re-handshake.

The solution to the problem then is obvious: We need to seed the TLS session
cache with the shared symmetric key.  When the client connects, the session
cacheing mechanism takes over and the client and server "resume" the phantom
session created by seeding the cache.  This mechanism requires an absolute
minimum of code changes to existing TLS implementations (it can be bolted onto
any existing TLS engine without needing to change the engine itself), and no
changes to the TLS protocol itself.

2.1 Design considerations

In order to work within the existing TLS protocol design, we require a means
of identifying a particular session (the session ID in TLS terminology), and
the keying material required to protect the session.  The { ID, key }
combination is analogous to the { user name, password } combination
traditionally used to secure access to computer systems.

In TLS, the session ID is a variable-length value of up to 32 bytes, but is
typically 32 or less frequently 16 bytes long.  For our use we don't really
care about its form.  A (somewhat unsound) practice would be to use the user
name as the session ID.  A more secure alternative would be to employ a value
derived from the user name in such a way that it can't be directly connected
to it, for example a MAC of the user name.

Normally the exact format of the session ID is determined explicitly by the
server and remembered by the client for use during session resumption.
However, when "resuming" a phantom session in the manner described here, both
the client and the server must be able to implicitly generate identical
session ID values in order to identify the phantom session to be resumed.  To
create a canonical session ID value, we pad the variable-length value out to a
fixed length by appending zero bytes.

The TLS master secret is a 48-byte value, which is unlikely to correspond to
the value of the shared symmetric key or password, which would typically be a
128-bit key or a text password/passphrase.  In order to transform this into
the type of keying material required by TLS, we need to apply the TLS
pseudorandom function (PRF) to produce the master secret with which we seed
the session cache.  The shared secret thus takes the place of the 48-byte
premaster secret usually used to derive the master secret.  As with the
variable-length session ID, we need to canonicalise the variable-length
secret.

The obvious way to do this would be to by zero-pad it to the standard 48-byte
length usually used for the premaster secret, as for the session ID.
Unfortunately this straightforward approach doesn't work.  Unlike the SSL PRF,
which uses the full secret for both the MD5 and SHA-1 halves, the TLS PRF
isn't a pure black-box design because it splits the secret into two halves
before using it.  This would result in the second (SHA-1) half in most cases
end up with only the zero padding bytes as its "secret".  The reasoning behind
this splitting of the secret was that there might be some interaction between
the two algorithms that could cause security problems.

As a result, it's necessary to be aware of the PRF's internal structure and
pre-process the input in a way that negates what the PRF does.  Some of the
possible options to fix the problem are:

1. Synthesise a new PRF from HMAC to pre-PRF the input to the TLS PRF.  Apart
   from just being an awful approach, this violates the minimal code-change
   requirement for TLS implementations that the shared-keys mechanism is
   supposed to provide.  Instead of simply feeding data in via a standard
   mechanism, implementors would now need to extend their TLS implementation
   to introduce new crypto mechanisms.

2. Repeat the input (or some variant thereof) to fill the 48-byte secret
   value.  This is problematic in that it creates key equivalence classes,
   for example "ABCD" == "ABCDABCD".

3. Unsplit the input, so that instead of arranaging it as 1 x 48 bytes it's
   done as 2 x 24 bytes.  This limits the overall key size, and is specific to
   the PRF being used - a future PRF design may not split the input in this
   manner, negating the un-splitting step.

The least ugly solution is a variation of 2, prepending a single length byte
to the secret, then repeating it to fill 48 bytes, to fix the problem of key
equivalence classes.  This is the approach used here.

Currently the shared-key mechanism always uses the TLS PRF (even if it's used
with SSL, since this is purely a TLS mechanism).  If in the future a new PRF
is introduced, it will be necessary to provide some means of switching over to
the new PRF if both it and the current one are in active use.  Presumably the
only reason to introduce a new, incompatible PRF would be a successful attack
on the current one, in which case the point is moot.  However, if for some
reason it's necessary to keep both PRFs in active use at the same time, then
some mechanism such as adding the session ID and shared key in the standard
manner using the TLS PRF and some transformation of the session ID and the
shared key using the new PRF can be adopted.  Since the details of a possible
PRF switch are impossible to predict (it may entail a complete protocol
overhaul for example), this document does not attempt to guess at the details
beyond providing this implementation hint.

Finally, we need a means of injecting the resulting session ID and master
secret into the session cache.  This is the only modification required to
existing TLS implementations.  Once the cache is seeded, all further details
are handled automatically by the TLS protocol.

It should be noted that this mechanism is best suited for situations where a
small number of clients/servers are communicating.  While seeding a session
cache with IDs and keys for 10,000 different users is certainly possible, this
is rather wasteful of server resources, not to mention the accompanying key
management nightmare involved in handling such a large number of shared
symmetric keys.

3. TLS using shared keys

[Note: The following is phrased fairly informally, since it's really an
 application note rather than a standards-track RFC]

Before any exchange takes place, the client and server session caches are
seeded with a session ID identifying the user/session, and a master secret
derived from the shared secret key or password/passphrase.  The exact form of
the data used to create the session ID is application specific (but see the
comment in the security considerations).  The data used to create the session
ID is zero-padded to 16 bytes (128 bits) if necessary to meet the requirements
given in section 2.1.  In C this may be expressed as:

  memset( session_id, 0, 16 );
  memcpy( session_id, input_data, min( input_data_length, 16 ) );

The master secret used to seed the cache is computed in the standard manner
using the TLS PRF:

  master_secret = PRF(shared_secret, "shared secret", "")[0..47];

The shared secret or password/passphrase takes the place of the premaster
secret that is normally used at this point, arranged as follows: First, the
shared secret/password has a single length byte prepended to it.  The length +
secret value is then repeated as required to fill the standard 48 bytes.  In C
this may be expressed as:

  for( premaster_index = 0; premaster_index < 48; )
    {
    int i;

    premaster_secret[ premaster_index++ ] = shared_secret_length;
    for( i = 0; i < shared_secret_length && premaster_index < 48; i++ )
      premaster_secret[ premaster_index++ ] = shared_secret[ i ];
    }

This formats the shared secret in a manner that allows it to be used directly
in place of the standard premaster secret derived from the public-key-based
key agreement process.

The 'seed' component of the calculation (normally occupied by the client and
server nonces) is empty in this case, however applications may choose to use
an application or system-specific value to ensure that the same shared secret
used with another application or system yields a different master secret.
When the 'seed' component is non-empty, it should not contain information
computed from the shared_secret value [SIGMA].  Note that the use of the
client and server nonces will always produce different keys for each session,
even if the same master secret is employed.

The final step involves injecting the session ID and master secret into the
session cache.  This is an implementation-specific issue beyond the scope of
this document.  All further steps are handled automatically by the TLS
protocol, which will "resume" the phantom session created by the above steps
without going through the full public-key-based handshake.

Session cache entries are normally expired after a given amount of time, or
overwritten on an LRU basis.  In order to prevent shared secret-based entries
from vanishing after a certain amount of time, these cache entries will need
to be distinguished from standard cache entries and made more persistent then
the latter, for example by giving them a longer expiry time when they are
added or periodically touching them to update their last-access time.  Again,
this is an implementation issue beyond the scope of this document.

3.1 Use of shared keys with SSLv3

If this key management mechanism is used with an implementation that supports
SSLv3 alongside TLS (as most do), the TLS PRF must be used for both SSLv3 and
TLS.  This is required in order to allow the mechanism to function for both
SSLv3 and TLS, since using different PRFs would require a different session ID
for each PRF used.

3.2 Test vectors

The following test vectors are derived from the transformation of the password
"test" into a master_secret value to be added to the session cache:

  Shared secret:

    74 65 73 74   ("test")

  Shared secret expanded to 48-byte premaster secret size:

    04 74 65 73 74 04 74 65
    73 74 04 74 65 73 74 04
    ...

  Master secret added to session cache:

    F5 CE 30 92 B8 09 70 D9
    22 D5 A1 2C EB 7C 43 FA
    9C 46 A8 83 EA 6E EF 98
    EB A5 15 12 FD B1 B6 5A
    5A 47 B8 C4 C5 63 5B 30
    86 96 F4 FC FB D5 45 78

4. Security considerations

The session ID used to identify sessions is visible to observers.  While using
a user name as the session ID is the most straightforward option, it may lead
to problems with traffic analysis, with an attacker being able to track the
identities of communicating parties.  In addition since the session ID is
reused over time, traffic analysis may eventually allow an attacker to
identify parties even if an opaque session ID is used.  [RFC 2246] contains a
similar warning about the contents of session IDs with TLS in general.  It
should be noted though that even a worse-case non-opaque session ID results in
no more exposure than the use of client certificates during a handshake.

As with all schemes involving shared keys, special care should be taken to
protect the shared values and to limit their exposure over time.  Documents
covering other shared-key protocols such as Kerberos [RFC 1510] contain
various security suggestions in this regard.

Use of a fixed shared secret of limited entropy (for example a password)
allows an attacker to perform an online password-guessing attack by trying to
resume a session with a master secret derived from each possible password.
This results in a fatal decrypt_error alert (or some equivalent such as
handshake_failure or bad_record_mac) which makes the session non-resumable
(that is, it clears the phantom session from the session cache).
Implementations should limit the enthusiasm with which they re-seed the
session cache after such an event; standard precautions against online
password-guessing attacks apply.

This mechanism is purely a shared-key session establishment mechanism and does
not provide perfect forward secrecy (PFS) by negotiating additional new keying
material for each session.  Users requiring PFS can either use a shared-key
mechanism that also provides PFS such as SRP [SRP], or perform a rehandshake
using a standard PFS-providing mechanism over the shared key-protected
channel.  Note though that both of these mechanisms negate the two main
advantages of the shared-key mechanism, requiring both considerable re-
engineering of an existing TLS implementation and considerable CPU time to
perform the PFS cryptographic operations.

Since it does not contain an innate cryptographic mechanism to provide PFS,
the shared-key mechanism is vulnerable to an offline password-guessing attack
as follows: An attacker who records all of the handshake messages and knows
the plaintext for at least one encrypted message can perform the TLS key-
derivation using a selection of guessed passwords, perform the cryptographic
operations required to process the TLS handshake exchange, and then apply the
resulting cryptographic keys to the known-plaintext message.  Such an attack
consumes considerable time and CPU resources, but is nevertheless possible.

There are three possible defences against this type of attack, the first two
of which are standard defences against password-guessing attacks:

1. Don't use weak, easily-guess passwords or keys.

2. Perform iterated pre-processing of the password/key before adding it to the
   session cache.  This has the disadvantage that it negates the shared-key
   advantage of low CPU consumption during the handshake phase, however the
   preprocessing can be performed offline on a one-off basis and only the
   preprocessed key stored by the two communicating parties.  An attacker can,
   however, also generate a dictionary of pre-processed keys offline, given
   sufficient CPU and storage space.  The use of a per-server diversifier
   ('seed' in the PRF process) makes use of a precomputed dictionary
   impractical, and a secret diversifier makes a general offline attack
   considerably more difficult through to impossible depending on the
   circumstances.

3. Use a mechanism that allows for the use of shared keys but also provides
   PFS, with the advantages and disadvantages described earlier.

Note that the two password-guessing attacks possible against the shared-key
mechanism, while superficially similar, have quite different requirements on
the attacker's side.  An online attack merely requires that the attacker know
the URL of the server that they wish to attack.  An offline attack requires
that an attacker both know the URL of the server that they wish to attack and
be able to record complete sessions between the client and the server in order
to provide the material required for the offline attack.

The TLS specification requires that when a session is resumed, the resumed
session use the same cipher suite as the original one.  Since with a shared-
secret session there is no actual session being resumed, it's not possible to
meet this requirement.  Two approaches are possible to resolve this:

1. When the session cache is seeded on the server, a cipher suite acceptable
   to the server is specified for the resumed session.  This complies with the
   requirements, but requires that the server know that the client is capable
   of supporting this particular suite.  In closed environments (for example
   syncing to a host PC or a fixed base station, or in a mainframe
   environment) this is likely to be the case.

2. The requirements are relaxed to allow the client and server to negotiate a
   cipher suite in the usual manner.  In order to subvert this, an attacker
   would have to be able to perform a real-time simultaneous break of both
   HMAC-MD5 and HMAC-SHA1.  In particular the attacker would need to be able
   to subvert:

     HMAC( secret, PRF( secret, MD5+SHA1 hash ) )

   in the Finished message, which expands to:

     HMAC( secret, HMAC-MD5^HMAC-SHA1( secret, MD5+SHA1 hash ) )

   Because of the unlikeliness of this occurring (an attacker capable of doing
   this can subvert any TLS session, with or without shared secrets), it
   appears safe to relax the requirement for resuming with the same cipher
   suite.

References (Normative)

  [RFC 2119] "Key words for use in RFCs to Indicate Requirement Levels",
             Scott Bradner, RFC 2119, March 1997.

  [RFC 2246] "The TLS Protocol", RFC 2246, Tim Dierks and Christopher
             Allen, January 1999.

References (Informative)

  [RFC 1510] "The Kerberos Network Authentication Service (V5)",
             RFC 1510, John Kohl and B. Clifford Neuman, September
             1993.

  [SIGMA] "SIGMA: the `SIGn-and-MAc' Approach to Authenticated
          Diffie-Hellman and its Use in the IKE Protocols", Hugo
          Krawczyk, Proceedings of of Crypto'03, Springer-Verlag
          Lecture Notes in Computer Science No.2729, p.399.

  [SRP] "Using SRP for TLS Authentication", David Taylor, IETF draft,
        November 2002.

Author's Address

Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand

Email: pgut001@cs.auckland.ac.nz

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