Network Working Group P. Eronen, Ed.

Network Working Group P. Eronen, Ed.
Request for Comments: 4279 Nokia
Category: Standards Track H. Tschofenig, Ed.
Siemens
December 2005

Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

This document specifies three sets of new ciphersuites for the
Transport Layer Security (TLS) protocol to support authentication
based on pre-shared keys (PSKs). These pre-shared keys are symmetric
keys, shared in advance among the communicating parties. The first
set of ciphersuites uses only symmetric key operations for
authentication. The second set uses a Diffie-Hellman exchange
authenticated with a pre-shared key, and the third set combines
public key authentication of the server with pre-shared key
authentication of the client.

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Table of Contents

1. Introduction ....................................................2
1.1. Applicability Statement ....................................3
1.2. Conventions Used in This Document ..........................4
2. PSK Key Exchange Algorithm ......................................4
3. DHE_PSK Key Exchange Algorithm ..................................6
4. RSA_PSK Key Exchange Algorithm ..................................7
5. Conformance Requirements ........................................8
5.1. PSK Identity Encoding ......................................8
5.2. Identity Hint ..............................................9
5.3. Requirements for TLS Implementations .......................9
5.4. Requirements for Management Interfaces .....................9
6. IANA Considerations ............................................10
7. Security Considerations ........................................10
7.1. Perfect Forward Secrecy (PFS) .............................10
7.2. Brute-Force and Dictionary Attacks ........................10
7.3. Identity Privacy ..........................................11
7.4. Implementation Notes ......................................11
8. Acknowledgements ...............................................11
9. References .....................................................12
9.1. Normative References ......................................12
9.2. Informative References ....................................12

1. Introduction

Usually, TLS uses public key certificates [TLS] or Kerberos [KERB]
for authentication. This document describes how to use symmetric
keys (later called pre-shared keys or PSKs), shared in advance among
the communicating parties, to establish a TLS connection.

There are basically two reasons why one might want to do this:

o First, using pre-shared keys can, depending on the ciphersuite,
avoid the need for public key operations. This is useful if TLS
is used in performance-constrained environments with limited CPU
power.

o Second, pre-shared keys may be more convenient from a key
management point of view. For instance, in closed environments
where the connections are mostly configured manually in advance,
it may be easier to configure a PSK than to use certificates.
Another case is when the parties already have a mechanism for
setting up a shared secret key, and that mechanism could be used
to "bootstrap" a key for authenticating a TLS connection.

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This document specifies three sets of new ciphersuites for TLS.
These ciphersuites use new key exchange algorithms, and reuse
existing cipher and MAC algorithms from [TLS] and [AES]. A summary
of these ciphersuites is shown below.

CipherSuite Key Exchange Cipher Hash

TLS_PSK_WITH_RC4_128_SHA PSK RC4_128 SHA
TLS_PSK_WITH_3DES_EDE_CBC_SHA PSK 3DES_EDE_CBC SHA
TLS_PSK_WITH_AES_128_CBC_SHA PSK AES_128_CBC SHA
TLS_PSK_WITH_AES_256_CBC_SHA PSK AES_256_CBC SHA
TLS_DHE_PSK_WITH_RC4_128_SHA DHE_PSK RC4_128 SHA
TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA DHE_PSK 3DES_EDE_CBC SHA
TLS_DHE_PSK_WITH_AES_128_CBC_SHA DHE_PSK AES_128_CBC SHA
TLS_DHE_PSK_WITH_AES_256_CBC_SHA DHE_PSK AES_256_CBC SHA
TLS_RSA_PSK_WITH_RC4_128_SHA RSA_PSK RC4_128 SHA
TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA RSA_PSK 3DES_EDE_CBC SHA
TLS_RSA_PSK_WITH_AES_128_CBC_SHA RSA_PSK AES_128_CBC SHA
TLS_RSA_PSK_WITH_AES_256_CBC_SHA RSA_PSK AES_256_CBC SHA

The ciphersuites in Section 2 (with PSK key exchange algorithm) use
only symmetric key algorithms and are thus especially suitable for
performance-constrained environments.

The ciphersuites in Section 3 (with DHE_PSK key exchange algorithm)
use a PSK to authenticate a Diffie-Hellman exchange. These
ciphersuites protect against dictionary attacks by passive
eavesdroppers (but not active attackers) and also provide Perfect
Forward Secrecy (PFS).

The ciphersuites in Section 4 (with RSA_PSK key exchange algorithm)
combine public-key-based authentication of the server (using RSA and
certificates) with mutual authentication using a PSK.

1.1. Applicability Statement

The ciphersuites defined in this document are intended for a rather
limited set of applications, usually involving only a very small
number of clients and servers. Even in such environments, other
alternatives may be more appropriate.

If the main goal is to avoid Public-Key Infrastructures (PKIs),
another possibility worth considering is using self-signed
certificates with public key fingerprints. Instead of manually
configuring a shared secret in, for instance, some configuration
file, a fingerprint (hash) of the other party's public key (or
certificate) could be placed there instead.

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It is also possible to use the SRP (Secure Remote Password)
ciphersuites for shared secret authentication [SRP]. SRP was
designed to be used with passwords, and it incorporates protection
against dictionary attacks. However, it is computationally more
expensive than the PSK ciphersuites in Section 2.

1.2. Conventions Used in This Document

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 [KEYWORDS].

2. PSK Key Exchange Algorithm

This section defines the PSK key exchange algorithm and associated
ciphersuites. These ciphersuites use only symmetric key algorithms.

It is assumed that the reader is familiar with the ordinary TLS
handshake, shown below. The elements in parenthesis are not included
when the PSK key exchange algorithm is used, and "*" indicates a
situation-dependent message that is not always sent.

Client Server
------ ------

ClientHello -------->
ServerHello
(Certificate)
ServerKeyExchange*
(CertificateRequest)
<-------- ServerHelloDone
(Certificate)
ClientKeyExchange
(CertificateVerify)
ChangeCipherSpec
Finished -------->
ChangeCipherSpec
<-------- Finished
Application Data <-------> Application Data

The client indicates its willingness to use pre-shared key
authentication by including one or more PSK ciphersuites in the
ClientHello message. If the TLS server also wants to use pre-shared
keys, it selects one of the PSK ciphersuites, places the selected
ciphersuite in the ServerHello message, and includes an appropriate
ServerKeyExchange message (see below). The Certificate and
CertificateRequest payloads are omitted from the response.

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Both clients and servers may have pre-shared keys with several
different parties. The client indicates which key to use by
including a "PSK identity" in the ClientKeyExchange message (note
that unlike in [SHAREDKEYS], the session_id field in ClientHello
message keeps its usual meaning). To help the client in selecting
which identity to use, the server can provide a "PSK identity hint"
in the ServerKeyExchange message. If no hint is provided, the
ServerKeyExchange message is omitted. See Section 5 for a more
detailed description of these fields.

The format of the ServerKeyExchange and ClientKeyExchange messages is
shown below.

struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>;
};
} ServerKeyExchange;

struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case psk: /* NEW */
opaque psk_identity<0..2^16-1>;
} exchange_keys;
} ClientKeyExchange;

The premaster secret is formed as follows: if the PSK is N octets
long, concatenate a uint16 with the value N, N zero octets, a second
uint16 with the value N, and the PSK itself.

Note 1: All the ciphersuites in this document share the same
general structure for the premaster secret, namely,

struct {
opaque other_secret<0..2^16-1>;
opaque psk<0..2^16-1>;
};

Here "other_secret" either is zeroes (plain PSK case) or comes
from the Diffie-Hellman or RSA exchange (DHE_PSK and RSA_PSK,
respectively). See Sections 3 and 4 for a more detailed
description.

Note 2: Using zeroes for "other_secret" effectively means that
only the HMAC-SHA1 part (but not the HMAC-MD5 part) of the TLS PRF

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is used when constructing the master secret. This was considered
more elegant from an analytical viewpoint than, for instance,
using the same key for both the HMAC-MD5 and HMAC-SHA1 parts. See
[KRAWCZYK] for a more detailed rationale.

The TLS handshake is authenticated using the Finished messages as
usual.

If the server does not recognize the PSK identity, it MAY respond
with an "unknown_psk_identity" alert message. Alternatively, if the
server wishes to hide the fact that the PSK identity was not known,
it MAY continue the protocol as if the PSK identity existed but the
key was incorrect: that is, respond with a "decrypt_error" alert.

3. DHE_PSK Key Exchange Algorithm

This section defines additional ciphersuites that use a PSK to
authenticate a Diffie-Hellman exchange. These ciphersuites give some
additional protection against dictionary attacks and also provide
Perfect Forward Secrecy (PFS). See Section 7 for discussion of
related security considerations.

When these ciphersuites are used, the ServerKeyExchange and
ClientKeyExchange messages also include the Diffie-Hellman
parameters. The PSK identity and identity hint fields have the same
meaning as in the previous section (note that the ServerKeyExchange
message is always sent, even if no PSK identity hint is provided).

The format of the ServerKeyExchange and ClientKeyExchange messages is
shown below.

struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case diffie_hellman_psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>;
ServerDHParams params;
};
} ServerKeyExchange;

struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case diffie_hellman_psk: /* NEW */
opaque psk_identity<0..2^16-1>;
ClientDiffieHellmanPublic public;
} exchange_keys;
} ClientKeyExchange;

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The premaster secret is formed as follows. First, perform the
Diffie-Hellman computation in the same way as for other
Diffie-Hellman-based ciphersuites in [TLS]. Let Z be the value
produced by this computation (with leading zero bytes stripped as in
other Diffie-Hellman-based ciphersuites). Concatenate a uint16
containing the length of Z (in octets), Z itself, a uint16 containing
the length of the PSK (in octets), and the PSK itself.

This corresponds to the general structure for the premaster secrets
(see Note 1 in Section 2) in this document, with "other_secret"
containing Z.

4. RSA_PSK Key Exchange Algorithm

The ciphersuites in this section use RSA and certificates to
authenticate the server, in addition to using a PSK.

As in normal RSA ciphersuites, the server must send a Certificate
message. The format of the ServerKeyExchange and ClientKeyExchange
messages is shown below. If no PSK identity hint is provided, the
ServerKeyExchange message is omitted.

struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case rsa_psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>;
};
} ServerKeyExchange;

struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case rsa_psk: /* NEW */
opaque psk_identity<0..2^16-1>;
EncryptedPreMasterSecret;
} exchange_keys;
} ClientKeyExchange;

The EncryptedPreMasterSecret field sent from the client to the server
contains a 2-byte version number and a 46-byte random value,
encrypted using the server's RSA public key as described in Section
7.4.7.1 of [TLS]. The actual premaster secret is formed by both
parties as follows: concatenate a uint16 with the value 48, the
2-byte version number and the 46-byte random value, a uint16
containing the length of the PSK (in octets), and the PSK itself.
(The premaster secret is thus 52 octets longer than the PSK.)

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This corresponds to the general structure for the premaster secrets
(see Note 1 in Section 2) in this document, with "other_secret"
containing both the 2-byte version number and the 46-byte random
value.

Neither the normal RSA ciphersuites nor these RSA_PSK ciphersuites
themselves specify what the certificates contain (in addition to the
RSA public key), or how the certificates are to be validated. In
particular, it is possible to use the RSA_PSK ciphersuites with
unvalidated self-signed certificates to provide somewhat similar
protection against dictionary attacks, as the DHE_PSK ciphersuites
define in Section 3.

5. Conformance Requirements

It is expected that different types of identities are useful for
different applications running over TLS. This document does not
therefore mandate the use of any particular type of identity (such as
IPv4 address or Fully Qualified Domain Name (FQDN)).

However, the TLS client and server clearly have to agree on the
identities and keys to be used. To improve interoperability, this
document places requirements on how the identity is encoded in the
protocol, and what kinds of identities and keys implementations have
to support.

The requirements for implementations are divided into two categories,
requirements for TLS implementations and management interfaces. In
this context, "TLS implementation" refers to a TLS library or module
that is intended to be used for several different purposes, while
"management interface" would typically be implemented by a particular
application that uses TLS.

This document does not specify how the server stores the keys and
identities, or how exactly it finds the key corresponding to the
identity it receives. For instance, if the identity is a domain
name, it might be appropriate to do a case-insensitive lookup. It is
RECOMMENDED that before looking up the key, the server processes the
PSK identity with a stringprep profile [STRINGPREP] appropriate for
the identity in question (such as Nameprep [NAMEPREP] for components
of domain names or SASLprep for usernames [SASLPREP]).

5.1. PSK Identity Encoding

The PSK identity MUST be first converted to a character string, and
then encoded to octets using UTF-8 [UTF8]. For instance,

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o IPv4 addresses are sent as dotted-decimal strings (e.g.,
"192.0.2.1"), not as 32-bit integers in network byte order.

o Domain names are sent in their usual text form [DNS] (e.g.,
"www.example.com" or "embedded\.dot.example.net"), not in DNS
protocol format.

o X.500 Distinguished Names are sent in their string representation
[LDAPDN], not as BER-encoded ASN.1.

This encoding is clearly not optimal for many types of identities.
It was chosen to avoid identity-type-specific parsing and encoding
code in implementations where the identity is configured by a person
using some kind of management interface. Requiring such identity-
type-specific code would also increase the chances for
interoperability problems resulting from different implementations
supporting different identity types.

5.2. Identity Hint

In the absence of an application profile specification specifying
otherwise, servers SHOULD NOT provide an identity hint and clients
MUST ignore the identity hint field. Applications that do use this
field MUST specify its contents, how the value is chosen by the TLS
server, and what the TLS client is expected to do with the value.

5.3. Requirements for TLS Implementations

TLS implementations supporting these ciphersuites MUST support
arbitrary PSK identities up to 128 octets in length, and arbitrary
PSKs up to 64 octets in length. Supporting longer identities and
keys is RECOMMENDED.

5.4. Requirements for Management Interfaces

In the absence of an application profile specification specifying
otherwise, a management interface for entering the PSK and/or PSK
identity MUST support the following:

o Entering PSK identities consisting of up to 128 printable Unicode
characters. Supporting as wide a character repertoire and as long
identities as feasible is RECOMMENDED.

o Entering PSKs up to 64 octets in length as ASCII strings and in
hexadecimal encoding.

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6. IANA Considerations

IANA does not currently have a registry for TLS ciphersuite or alert
numbers, so there are no IANA actions associated with this document.

For easier reference in the future, the ciphersuite numbers defined
in this document are summarized below.

CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0x8A };
CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8B };
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x8C };
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x8D };
CipherSuite TLS_DHE_PSK_WITH_RC4_128_SHA = { 0x00, 0x8E };
CipherSuite TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8F };
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x90 };
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x91 };
CipherSuite TLS_RSA_PSK_WITH_RC4_128_SHA = { 0x00, 0x92 };
CipherSuite TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x93 };
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x94 };
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x95 };

This document also defines a new TLS alert message,
unknown_psk_identity(115).

7. Security Considerations

As with all schemes involving shared keys, special care should be
taken to protect the shared values and to limit their exposure over
time.

7.1. Perfect Forward Secrecy (PFS)

The PSK and RSA_PSK ciphersuites defined in this document do not
provide Perfect Forward Secrecy (PFS). That is, if the shared secret
key (in PSK ciphersuites), or both the shared secret key and the RSA
private key (in RSA_PSK ciphersuites), is somehow compromised, an
attacker can decrypt old conversations.

The DHE_PSK ciphersuites provide Perfect Forward Secrecy if a fresh
Diffie-Hellman private key is generated for each handshake.

7.2. Brute-Force and Dictionary Attacks

Use of a fixed shared secret of limited entropy (for example, a PSK
that is relatively short, or was chosen by a human and thus may
contain less entropy than its length would imply) may allow an
attacker to perform a brute-force or dictionary attack to recover the
secret. This may be either an off-line attack (against a captured

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TLS handshake messages) or an on-line attack where the attacker
attempts to connect to the server and tries different keys.

For the PSK ciphersuites, an attacker can get the information
required for an off-line attack by eavesdropping on a TLS handshake,
or by getting a valid client to attempt connection with the attacker
(by tricking the client to connect to the wrong address, or by
intercepting a connection attempt to the correct address, for
instance).

For the DHE_PSK ciphersuites, an attacker can obtain the information
by getting a valid client to attempt connection with the attacker.
Passive eavesdropping alone is not sufficient.

For the RSA_PSK ciphersuites, only the server (authenticated using
RSA and certificates) can obtain sufficient information for an
off-line attack.

It is RECOMMENDED that implementations that allow the administrator
to manually configure the PSK also provide a functionality for
generating a new random PSK, taking [RANDOMNESS] into account.

7.3. Identity Privacy

The PSK identity is sent in cleartext. Although using a user name or
other similar string as the PSK identity is the most straightforward
option, it may lead to problems in some environments since an
eavesdropper is able to identify the communicating parties. Even
when the identity does not reveal any information itself, reusing the
same identity over time may eventually allow an attacker to perform
traffic analysis to identify the parties. It should be noted that
this is no worse than client certificates, since they are also sent
in cleartext.

7.4. Implementation Notes

The implementation notes in [TLS11] about correct implementation and
use of RSA (including Section 7.4.7.1) and Diffie-Hellman (including
Appendix F.1.1.3) apply to the DHE_PSK and RSA_PSK ciphersuites as
well.

8. Acknowledgements

The protocol defined in this document is heavily based on work by Tim
Dierks and Peter Gutmann, and borrows some text from [SHAREDKEYS] and
[AES]. The DHE_PSK and RSA_PSK ciphersuites are based on earlier
work in [KEYEX].

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Valuable feedback was also provided by Bernard Aboba, Lakshminath
Dondeti, Philip Ginzboorg, Peter Gutmann, Sam Hartman, Russ Housley,
David Jablon, Nikos Mavroyanopoulos, Bodo Moeller, Eric Rescorla, and
Mika Tervonen.

When the first version of this document was almost ready, the authors
learned that something similar had been proposed already in 1996
[PASSAUTH]. However, this document is not intended for web password
authentication, but rather for other uses of TLS.

9. References

9.1. Normative References

[AES] Chown, P., "Advanced Encryption Standard (AES)
Ciphersuites for Transport Layer Security (TLS)", RFC
3268, June 2002.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RANDOMNESS] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.

[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.

[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

9.2. Informative References

[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[KERB] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999.

[KEYEX] Badra, M., Cherkaoui, O., Hajjeh, I. and A. Serhrouchni,
"Pre-Shared-Key key Exchange methods for TLS", Work in
Progress, August 2004.

[KRAWCZYK] Krawczyk, H., "Re: TLS shared keys PRF", message on
[email protected] mailing list 2004-01-13,
http://www.imc.org/ietf-tls/mail-archive/msg04098.html.

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[LDAPDN] Zeilenga, K., "LDAP: String Representation of
Distinguished Names", Work in Progress, February 2005.

[NAMEPREP] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.

[PASSAUTH] Simon, D., "Addition of Shared Key Authentication to
Transport Layer Security (TLS)", Work in Progress,
November 1996.

[SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.

[SHAREDKEYS] Gutmann, P., "Use of Shared Keys in the TLS Protocol",
Work in Progress, October 2003.

[SRP] Taylor, D., Wu, T., Mavroyanopoulos, N. and T. Perrin,
"Using SRP for TLS Authentication", Work in Progress,
March 2005.

[STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.

[TLS11] Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.1", Work in Progress, June 2005.

Authors' and Contributors' Addresses

Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland

EMail: [email protected]

Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany

EMail: [email protected]

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RFC 4279 PSK Ciphersuites for TLS December 2005

Mohamad Badra
ENST Paris
46 rue Barrault
75634 Paris
France

EMail: [email protected]

Omar Cherkaoui
UQAM University
Montreal (Quebec)
Canada

EMail: [email protected]

Ibrahim Hajjeh
ESRGroups
17 passage Barrault
75013 Paris
France

EMail: [email protected]

Ahmed Serhrouchni
ENST Paris
46 rue Barrault
75634 Paris
France

EMail: [email protected]

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RFC 4279 PSK Ciphersuites for TLS December 2005

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