draft-ietf-tls-psk-09.txt   rfc4279.txt 
TLS Working Group P. Eronen, Ed. Network Working Group P. Eronen, Ed.
Internet-Draft Nokia Request for Comments: 4279 Nokia
Expires: December 20, 2005 H. Tschofenig, Ed. Category: Standards Track H. Tschofenig, Ed.
Siemens Siemens
June 21, 2005 December 2005
Pre-Shared Key Ciphersuites for Transport Layer Security (TLS) Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)
draft-ietf-tls-psk-09.txt
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Abstract Abstract
This document specifies three sets of new ciphersuites for the This document specifies three sets of new ciphersuites for the
Transport Layer Security (TLS) protocol to support authentication Transport Layer Security (TLS) protocol to support authentication
based on pre-shared keys. These pre-shared keys are symmetric keys, based on pre-shared keys (PSKs). These pre-shared keys are symmetric
shared in advance among the communicating parties. The first set of keys, shared in advance among the communicating parties. The first
ciphersuites uses only symmetric key operations for authentication. set of ciphersuites uses only symmetric key operations for
The second set uses a Diffie-Hellman exchange authenticated with a authentication. The second set uses a Diffie-Hellman exchange
pre-shared key; and the third set combines public key authentication authenticated with a pre-shared key, and the third set combines
of the server with pre-shared key authentication of the client. public key authentication of the server with pre-shared key
authentication of the client.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction ....................................................2
1.1 Applicability statement . . . . . . . . . . . . . . . . . 4 1.1. Applicability Statement ....................................3
1.2 Conventions used in this document . . . . . . . . . . . . 4 1.2. Conventions Used in This Document ..........................4
2. PSK key exchange algorithm . . . . . . . . . . . . . . . . . . 5 2. PSK Key Exchange Algorithm ......................................4
3. DHE_PSK key exchange algorithm . . . . . . . . . . . . . . . . 7 3. DHE_PSK Key Exchange Algorithm ..................................6
4. RSA_PSK key exchange algorithm . . . . . . . . . . . . . . . . 8 4. RSA_PSK Key Exchange Algorithm ..................................7
5. Conformance requirements . . . . . . . . . . . . . . . . . . . 9 5. Conformance Requirements ........................................8
5.1 PSK identity encoding . . . . . . . . . . . . . . . . . . 9 5.1. PSK Identity Encoding ......................................8
5.2 Identity hint . . . . . . . . . . . . . . . . . . . . . . 10 5.2. Identity Hint ..............................................9
5.3 Requirements for TLS implementations . . . . . . . . . . 10 5.3. Requirements for TLS Implementations .......................9
5.4 Requirements for management interfaces . . . . . . . . . 10 5.4. Requirements for Management Interfaces .....................9
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations ............................................10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. Security Considerations ........................................10
7.1 Perfect forward secrecy (PFS) . . . . . . . . . . . . . . 11 7.1. Perfect Forward Secrecy (PFS) .............................10
7.2 Brute-force and dictionary attacks . . . . . . . . . . . 11 7.2. Brute-Force and Dictionary Attacks ........................10
7.3 Identity privacy . . . . . . . . . . . . . . . . . . . . 12 7.3. Identity Privacy ..........................................11
7.4 Implementation notes . . . . . . . . . . . . . . . . . . 12 7.4. Implementation Notes ......................................11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements ...............................................11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9. References .....................................................12
9.1 Normative References . . . . . . . . . . . . . . . . . . 13 9.1. Normative References ......................................12
9.2 Informative References . . . . . . . . . . . . . . . . . 13 9.2. Informative References ....................................12
Authors' and Contributors' Addresses . . . . . . . . . . . . . . . 15
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction 1. Introduction
Usually TLS uses public key certificates [TLS] or Kerberos [KERB] for Usually, TLS uses public key certificates [TLS] or Kerberos [KERB]
authentication. This document describes how to use symmetric keys for authentication. This document describes how to use symmetric
(later called pre-shared keys or PSKs), shared in advance among the keys (later called pre-shared keys or PSKs), shared in advance among
communicating parties, to establish a TLS connection. the communicating parties, to establish a TLS connection.
There are basically two reasons why one might want to do this: There are basically two reasons why one might want to do this:
o First, using pre-shared keys can, depending on the ciphersuite, o First, using pre-shared keys can, depending on the ciphersuite,
avoid the need for public key operations. This is useful if TLS avoid the need for public key operations. This is useful if TLS
is used in performance-constrained environments with limited CPU is used in performance-constrained environments with limited CPU
power. power.
o Second, pre-shared keys may be more convenient from a key o Second, pre-shared keys may be more convenient from a key
management point of view. For instance, in closed environments management point of view. For instance, in closed environments
where the connections are mostly configured manually in advance, where the connections are mostly configured manually in advance,
it may be easier to configure a PSK than to use certificates. it may be easier to configure a PSK than to use certificates.
Another case is when the parties already have a mechanism for Another case is when the parties already have a mechanism for
setting up a shared secret key, and that mechanism could be used setting up a shared secret key, and that mechanism could be used
to "bootstrap" a key for authenticating a TLS connection. to "bootstrap" a key for authenticating a TLS connection.
This document specifies three sets of new ciphersuites for TLS. This document specifies three sets of new ciphersuites for TLS.
These ciphersuites use new key exchange algorithms, and re-use These ciphersuites use new key exchange algorithms, and reuse
existing cipher and MAC algorithms from [TLS] and [AES]. A summary existing cipher and MAC algorithms from [TLS] and [AES]. A summary
of these ciphersuites is shown below. of these ciphersuites is shown below.
CipherSuite Key Exchange Cipher Hash CipherSuite Key Exchange Cipher Hash
TLS_PSK_WITH_RC4_128_SHA PSK RC4_128 SHA 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_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_128_CBC_SHA PSK AES_128_CBC SHA
TLS_PSK_WITH_AES_256_CBC_SHA PSK AES_256_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_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_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_128_CBC_SHA DHE_PSK AES_128_CBC SHA
TLS_DHE_PSK_WITH_AES_256_CBC_SHA DHE_PSK AES_256_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_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_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_128_CBC_SHA RSA_PSK AES_128_CBC SHA
TLS_RSA_PSK_WITH_AES_256_CBC_SHA RSA_PSK AES_256_CBC SHA TLS_RSA_PSK_WITH_AES_256_CBC_SHA RSA_PSK AES_256_CBC SHA
The first set of ciphersuites (with PSK key exchange algorithm), The ciphersuites in Section 2 (with PSK key exchange algorithm) use
defined in Section 2 use only symmetric key algorithms, and are thus only symmetric key algorithms and are thus especially suitable for
especially suitable for performance-constrained environments. performance-constrained environments.
The ciphersuites in Section 3 (with DHE_PSK key exchange algorithm) The ciphersuites in Section 3 (with DHE_PSK key exchange algorithm)
use a PSK to authenticate a Diffie-Hellman exchange. These use a PSK to authenticate a Diffie-Hellman exchange. These
ciphersuites protect against dictionary attacks by passive ciphersuites protect against dictionary attacks by passive
eavesdroppers (but not active attackers), and also provide Perfect eavesdroppers (but not active attackers) and also provide Perfect
Forward Secrecy (PFS). Forward Secrecy (PFS).
The third set of ciphersuites (with RSA_PSK key exchange algorithm), The ciphersuites in Section 4 (with RSA_PSK key exchange algorithm)
defined in Section 4, combine public key based authentication of the combine public-key-based authentication of the server (using RSA and
server (using RSA and certificates) with mutual authentication using certificates) with mutual authentication using a PSK.
a PSK.
1.1 Applicability statement 1.1. Applicability Statement
The ciphersuites defined in this document are intended for a rather The ciphersuites defined in this document are intended for a rather
limited set of applications, usually involving only a very small limited set of applications, usually involving only a very small
number of clients and servers. Even in such environments, other number of clients and servers. Even in such environments, other
alternatives may be more appropriate. alternatives may be more appropriate.
If the main goal is to avoid PKIs, another possibility worth If the main goal is to avoid Public-Key Infrastructures (PKIs),
considering is to use self-signed certificates with public key another possibility worth considering is using self-signed
fingerprints. Instead of manually configuring a shared secret in, certificates with public key fingerprints. Instead of manually
for instance, some configuration file, a fingerprint (hash) of the configuring a shared secret in, for instance, some configuration
other party's public key (or certificate) could be placed there file, a fingerprint (hash) of the other party's public key (or
instead. certificate) could be placed there instead.
It is also possible to use the SRP (Secure Remote Password) It is also possible to use the SRP (Secure Remote Password)
ciphersuites for shared secret authentication [SRP]. SRP was ciphersuites for shared secret authentication [SRP]. SRP was
designed to be used with passwords, and incorporates protection designed to be used with passwords, and it incorporates protection
against dictionary attacks. However, it is computationally more against dictionary attacks. However, it is computationally more
expensive than the PSK ciphersuites in Section 2. expensive than the PSK ciphersuites in Section 2.
1.2 Conventions used in this document 1.2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS]. document are to be interpreted as described in [KEYWORDS].
2. PSK key exchange algorithm 2. PSK Key Exchange Algorithm
This section defines the PSK key exchange algorithm and associated This section defines the PSK key exchange algorithm and associated
ciphersuites. These ciphersuites use only symmetric key algorithms. ciphersuites. These ciphersuites use only symmetric key algorithms.
It is assumed that the reader is familiar with ordinary TLS It is assumed that the reader is familiar with the ordinary TLS
handshake, shown below. The elements in parenthesis are not included handshake, shown below. The elements in parenthesis are not included
when PSK key exchange algorithm is used, and "*" indicates a when the PSK key exchange algorithm is used, and "*" indicates a
situation-dependent message that is not always sent. situation-dependent message that is not always sent.
Client Server Client Server
------ ------ ------ ------
ClientHello --------> ClientHello -------->
ServerHello ServerHello
(Certificate) (Certificate)
ServerKeyExchange* ServerKeyExchange*
(CertificateRequest) (CertificateRequest)
skipping to change at page 5, line 48 skipping to change at page 5, line 12
ServerKeyExchange message (see below). The Certificate and ServerKeyExchange message (see below). The Certificate and
CertificateRequest payloads are omitted from the response. CertificateRequest payloads are omitted from the response.
Both clients and servers may have pre-shared keys with several Both clients and servers may have pre-shared keys with several
different parties. The client indicates which key to use by different parties. The client indicates which key to use by
including a "PSK identity" in the ClientKeyExchange message (note including a "PSK identity" in the ClientKeyExchange message (note
that unlike in [SHAREDKEYS], the session_id field in ClientHello that unlike in [SHAREDKEYS], the session_id field in ClientHello
message keeps its usual meaning). To help the client in selecting message keeps its usual meaning). To help the client in selecting
which identity to use, the server can provide a "PSK identity hint" which identity to use, the server can provide a "PSK identity hint"
in the ServerKeyExchange message. If no hint is provided, the in the ServerKeyExchange message. If no hint is provided, the
ServerKeyExchange message is omitted. See Section 5 for more ServerKeyExchange message is omitted. See Section 5 for a more
detailed description of these fields. detailed description of these fields.
The format of the ServerKeyExchange and ClientKeyExchange messages is The format of the ServerKeyExchange and ClientKeyExchange messages is
shown below. shown below.
struct { struct {
select (KeyExchangeAlgorithm) { select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */ /* other cases for rsa, diffie_hellman, etc. */
case psk: /* NEW */ case psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>; opaque psk_identity_hint<0..2^16-1>;
skipping to change at page 6, line 26 skipping to change at page 5, line 39
case psk: /* NEW */ case psk: /* NEW */
opaque psk_identity<0..2^16-1>; opaque psk_identity<0..2^16-1>;
} exchange_keys; } exchange_keys;
} ClientKeyExchange; } ClientKeyExchange;
The premaster secret is formed as follows: if the PSK is N octets 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 long, concatenate a uint16 with the value N, N zero octets, a second
uint16 with the value N, and the PSK itself. uint16 with the value N, and the PSK itself.
Note 1: All the ciphersuites in this document share the same Note 1: All the ciphersuites in this document share the same
general structure for the premaster secret, namely general structure for the premaster secret, namely,
struct { struct {
opaque other_secret<0..2^16-1>; opaque other_secret<0..2^16-1>;
opaque psk<0..2^16-1>; opaque psk<0..2^16-1>;
}; };
Here "other_secret" is either zeroes (plain PSK case), or comes Here "other_secret" either is zeroes (plain PSK case) or comes
from the Diffie-Hellman or RSA exchange (DHE_PSK and RSA_PSK, from the Diffie-Hellman or RSA exchange (DHE_PSK and RSA_PSK,
respectively). See Sections 3 and 4 for a more detailed respectively). See Sections 3 and 4 for a more detailed
description. description.
Note 2: Using zeroes for "other_secret" effectively means that 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 only the HMAC-SHA1 part (but not the HMAC-MD5 part) of the TLS PRF
is used when constructing the master secret. This was considered is used when constructing the master secret. This was considered
more elegant from an analytical viewpoint than, for instance, more elegant from an analytical viewpoint than, for instance,
using the same key for both the HMAC-MD5 and HMAC-SHA1 parts. See using the same key for both the HMAC-MD5 and HMAC-SHA1 parts. See
[KRAWCZYK] for a more detailed rationale. [KRAWCZYK] for a more detailed rationale.
The TLS handshake is authenticated using the Finished messages as The TLS handshake is authenticated using the Finished messages as
usual. usual.
If the server does not recognize the PSK identity, it MAY respond If the server does not recognize the PSK identity, it MAY respond
with an "unknown_psk_identity" alert message. Alternatively, if the with an "unknown_psk_identity" alert message. Alternatively, if the
server wishes to hide the fact that the PSK identity was not known, 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 it MAY continue the protocol as if the PSK identity existed but the
key was incorrect: that is, respond with a "decrypt_error" alert. key was incorrect: that is, respond with a "decrypt_error" alert.
3. DHE_PSK key exchange algorithm 3. DHE_PSK Key Exchange Algorithm
This section defines additional ciphersuites that use a PSK to This section defines additional ciphersuites that use a PSK to
authenticate a Diffie-Hellman exchange. These ciphersuites give some authenticate a Diffie-Hellman exchange. These ciphersuites give some
additional protection against dictionary attacks, and also provide additional protection against dictionary attacks and also provide
Perfect Forward Secrecy (PFS). See Section 7 for discussion of Perfect Forward Secrecy (PFS). See Section 7 for discussion of
related security considerations. related security considerations.
When these ciphersuites are used, the ServerKeyExchange and When these ciphersuites are used, the ServerKeyExchange and
ClientKeyExchange messages also include the Diffie-Hellman ClientKeyExchange messages also include the Diffie-Hellman
parameters. The PSK identity and identity hint fields have the same parameters. The PSK identity and identity hint fields have the same
meaning as in the previous section (note that the ServerKeyExchange meaning as in the previous section (note that the ServerKeyExchange
message is always sent even if no PSK identity hint is provided). message is always sent, even if no PSK identity hint is provided).
The format of the ServerKeyExchange and ClientKeyExchange messages is The format of the ServerKeyExchange and ClientKeyExchange messages is
shown below. shown below.
struct { struct {
select (KeyExchangeAlgorithm) { select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */ /* other cases for rsa, diffie_hellman, etc. */
case diffie_hellman_psk: /* NEW */ case diffie_hellman_psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>; opaque psk_identity_hint<0..2^16-1>;
ServerDHParams params; ServerDHParams params;
skipping to change at page 7, line 39 skipping to change at page 7, line 4
} ServerKeyExchange; } ServerKeyExchange;
struct { struct {
select (KeyExchangeAlgorithm) { select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */ /* other cases for rsa, diffie_hellman, etc. */
case diffie_hellman_psk: /* NEW */ case diffie_hellman_psk: /* NEW */
opaque psk_identity<0..2^16-1>; opaque psk_identity<0..2^16-1>;
ClientDiffieHellmanPublic public; ClientDiffieHellmanPublic public;
} exchange_keys; } exchange_keys;
} ClientKeyExchange; } ClientKeyExchange;
The premaster secret is formed as follows. First, perform the The premaster secret is formed as follows. First, perform the
Diffie-Hellman computation in the same way as for other Diffie-Hellman computation in the same way as for other
Diffie-Hellman based ciphersuites in [TLS]. Let Z be the value Diffie-Hellman-based ciphersuites in [TLS]. Let Z be the value
produced by this computation (with leading zero bytes stripped as in produced by this computation (with leading zero bytes stripped as in
other Diffie-Hellman based ciphersuites). Concatenate a uint16 other Diffie-Hellman-based ciphersuites). Concatenate a uint16
containing the length of Z (in octets), Z itself, a uint16 containing containing the length of Z (in octets), Z itself, a uint16 containing
the length of the PSK (in octets), and the PSK itself. the length of the PSK (in octets), and the PSK itself.
This corresponds to the general structure for the premaster secrets This corresponds to the general structure for the premaster secrets
(see Note 1 in Section 2) in this document, with "other_secret" (see Note 1 in Section 2) in this document, with "other_secret"
containing Z. containing Z.
4. RSA_PSK key exchange algorithm 4. RSA_PSK Key Exchange Algorithm
The ciphersuites in this section use RSA and certificates to The ciphersuites in this section use RSA and certificates to
authenticate the server, in addition to using a PSK. authenticate the server, in addition to using a PSK.
As in normal RSA ciphersuites, the server must send a Certificate As in normal RSA ciphersuites, the server must send a Certificate
message. The format of the ServerKeyExchange and ClientKeyExchange message. The format of the ServerKeyExchange and ClientKeyExchange
messages is shown below. If no PSK identity hint is provided, the messages is shown below. If no PSK identity hint is provided, the
ServerKeyExchange message is omitted. ServerKeyExchange message is omitted.
struct { struct {
skipping to change at page 8, line 51 skipping to change at page 8, line 14
This corresponds to the general structure for the premaster secrets This corresponds to the general structure for the premaster secrets
(see Note 1 in Section 2) in this document, with "other_secret" (see Note 1 in Section 2) in this document, with "other_secret"
containing both the 2-byte version number and the 46-byte random containing both the 2-byte version number and the 46-byte random
value. value.
Neither the normal RSA ciphersuites nor these RSA_PSK ciphersuites Neither the normal RSA ciphersuites nor these RSA_PSK ciphersuites
themselves specify what the certificates contain (in addition to the themselves specify what the certificates contain (in addition to the
RSA public key), or how the certificates are to be validated. In RSA public key), or how the certificates are to be validated. In
particular, it is possible to use the RSA_PSK ciphersuites with particular, it is possible to use the RSA_PSK ciphersuites with
unvalidated self-signed certificates to provide somewhat similar unvalidated self-signed certificates to provide somewhat similar
protection against dictionary attacks as the DHE_PSK ciphersuites protection against dictionary attacks, as the DHE_PSK ciphersuites
defined in Section 3. define in Section 3.
5. Conformance requirements 5. Conformance Requirements
It is expected that different types of identities are useful for It is expected that different types of identities are useful for
different applications running over TLS. This document does not different applications running over TLS. This document does not
therefore mandate the use of any particular type of identity (such as therefore mandate the use of any particular type of identity (such as
IPv4 address or FQDN). IPv4 address or Fully Qualified Domain Name (FQDN)).
However, the TLS client and server clearly have to agree on the However, the TLS client and server clearly have to agree on the
identities and keys to be used. To improve interoperability, this identities and keys to be used. To improve interoperability, this
document places requirements on how the identity is encoded in the document places requirements on how the identity is encoded in the
protocol, and what kinds of identities and keys implementations have protocol, and what kinds of identities and keys implementations have
to support. to support.
The requirements for implementations are divided to two categories, The requirements for implementations are divided into two categories,
requirements for TLS implementations and management interfaces. In requirements for TLS implementations and management interfaces. In
this context, "TLS implementation" refers to a TLS library or module this context, "TLS implementation" refers to a TLS library or module
that is intended to be used for several different purposes, while that is intended to be used for several different purposes, while
"management interface" would typically be implemented by a particular "management interface" would typically be implemented by a particular
application that uses TLS. application that uses TLS.
This document does not specify how the server stores the keys and This document does not specify how the server stores the keys and
identities, or how exactly it finds the key corresponding to the identities, or how exactly it finds the key corresponding to the
identity it receives. For instance, if the identity is a domain identity it receives. For instance, if the identity is a domain
name, it might be appropriate to do a case-insensitive lookup. It is name, it might be appropriate to do a case-insensitive lookup. It is
RECOMMENDED that before looking up the key, the server processes the RECOMMENDED that before looking up the key, the server processes the
PSK identity with a stringprep profile [STRINGPREP] appropriate for PSK identity with a stringprep profile [STRINGPREP] appropriate for
the identity in question (such as Nameprep [NAMEPREP] for components the identity in question (such as Nameprep [NAMEPREP] for components
of domain names or SASLprep for usernames [SASLPREP]). of domain names or SASLprep for usernames [SASLPREP]).
5.1 PSK identity encoding 5.1. PSK Identity Encoding
The PSK identity MUST be first converted to a character string, and The PSK identity MUST be first converted to a character string, and
then encoded to octets using UTF-8 [UTF8]. For instance, then encoded to octets using UTF-8 [UTF8]. For instance,
o IPv4 addresses are sent as dotted-decimal strings (e.g., o IPv4 addresses are sent as dotted-decimal strings (e.g.,
"192.0.2.1"), not as 32-bit integers in network byte order. "192.0.2.1"), not as 32-bit integers in network byte order.
o Domain names are sent in their usual text form (e.g., o Domain names are sent in their usual text form [DNS] (e.g.,
"www.example.com" or "embedded.dot.example.net"), not in DNS "www.example.com" or "embedded\.dot.example.net"), not in DNS
protocol format. protocol format.
o X.500 Distinguished Names are sent in their string representation o X.500 Distinguished Names are sent in their string representation
[LDAPDN], not as BER-encoded ASN.1. [LDAPDN], not as BER-encoded ASN.1.
This encoding is clearly not optimal for many types of identities. This encoding is clearly not optimal for many types of identities.
It was chosen to avoid identity type specific parsing and encoding It was chosen to avoid identity-type-specific parsing and encoding
code in implementations where the identity is configured by a person code in implementations where the identity is configured by a person
using some kind of management interface. Requiring such identity using some kind of management interface. Requiring such identity-
type specific code would also increase the chances for type-specific code would also increase the chances for
interoperability problems resulting from different implementations interoperability problems resulting from different implementations
supporting different identity types. supporting different identity types.
5.2 Identity hint 5.2. Identity Hint
In the absence of an application profile specification specifying In the absence of an application profile specification specifying
otherwise, servers SHOULD NOT provide an identity hint and clients otherwise, servers SHOULD NOT provide an identity hint and clients
MUST ignore the identity hint field. Applications that do use this MUST ignore the identity hint field. Applications that do use this
field MUST specify its contents, how the value is chosen by the TLS 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. server, and what the TLS client is expected to do with the value.
5.3 Requirements for TLS implementations 5.3. Requirements for TLS Implementations
TLS implementations supporting these ciphersuites MUST support TLS implementations supporting these ciphersuites MUST support
arbitrary PSK identities up to 128 octets in length, and arbitrary arbitrary PSK identities up to 128 octets in length, and arbitrary
PSKs up to 64 octets in length. Supporting longer identities and PSKs up to 64 octets in length. Supporting longer identities and
keys is RECOMMENDED. keys is RECOMMENDED.
5.4 Requirements for management interfaces 5.4. Requirements for Management Interfaces
In the absence of an application profile specification specifying In the absence of an application profile specification specifying
otherwise, a management interface for entering the PSK and/or PSK otherwise, a management interface for entering the PSK and/or PSK
identity MUST support the following: identity MUST support the following:
o Entering PSK identities consisting of up to 128 printable Unicode o Entering PSK identities consisting of up to 128 printable Unicode
characters. Supporting as wide character repertoire and as long characters. Supporting as wide a character repertoire and as long
identities as feasible is RECOMMENDED. identities as feasible is RECOMMENDED.
o Entering PSKs up to 64 octets in length as ASCII strings and in o Entering PSKs up to 64 octets in length as ASCII strings and in
hexadecimal encoding. hexadecimal encoding.
6. IANA considerations 6. IANA Considerations
IANA does not currently have a registry for TLS ciphersuite or alert IANA does not currently have a registry for TLS ciphersuite or alert
numbers, so there are no IANA actions associated with this document. numbers, so there are no IANA actions associated with this document.
For easier reference in the future, the ciphersuite numbers defined For easier reference in the future, the ciphersuite numbers defined
in this document are summarized below. in this document are summarized below.
CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0x8A }; CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0x8A };
CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8B }; 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_128_CBC_SHA = { 0x00, 0x8C };
skipping to change at page 11, line 35 skipping to change at page 10, line 35
This document also defines a new TLS alert message, This document also defines a new TLS alert message,
unknown_psk_identity(115). unknown_psk_identity(115).
7. Security Considerations 7. Security Considerations
As with all schemes involving shared keys, special care should be As with all schemes involving shared keys, special care should be
taken to protect the shared values and to limit their exposure over taken to protect the shared values and to limit their exposure over
time. time.
7.1 Perfect forward secrecy (PFS) 7.1. Perfect Forward Secrecy (PFS)
The PSK and RSA_PSK ciphersuites defined in this document do not The PSK and RSA_PSK ciphersuites defined in this document do not
provide Perfect Forward Secrecy (PFS). That is, if the shared secret provide Perfect Forward Secrecy (PFS). That is, if the shared secret
key (in PSK ciphersuites), or both the shared secret key and the RSA key (in PSK ciphersuites), or both the shared secret key and the RSA
private key (in RSA_PSK ciphersuites), is somehow compromised, an private key (in RSA_PSK ciphersuites), is somehow compromised, an
attacker can decrypt old conversations. attacker can decrypt old conversations.
The DHE_PSK ciphersuites provide Perfect Forward Secrecy if a fresh The DHE_PSK ciphersuites provide Perfect Forward Secrecy if a fresh
DH private key is generated for each handshake. Diffie-Hellman private key is generated for each handshake.
7.2 Brute-force and dictionary attacks 7.2. Brute-Force and Dictionary Attacks
Use of a fixed shared secret of limited entropy (for example, a PSK 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 that is relatively short, or was chosen by a human and thus may
contain less entropy than its length would imply) may allow an contain less entropy than its length would imply) may allow an
attacker to perform a brute-force or dictionary attack to recover the attacker to perform a brute-force or dictionary attack to recover the
secret. This may be either an off-line attack (against a captured secret. This may be either an off-line attack (against a captured
TLS handshake messages), or an on-line attack where the attacker TLS handshake messages) or an on-line attack where the attacker
attempts to connect to the server and tries different keys. attempts to connect to the server and tries different keys.
For the PSK ciphersuites, an attacker can get the information For the PSK ciphersuites, an attacker can get the information
required for an off-line attack by eavesdropping a TLS handshake, or required for an off-line attack by eavesdropping on a TLS handshake,
by getting a valid client to attempt connection with the attacker (by or by getting a valid client to attempt connection with the attacker
tricking the client to connect to wrong address, or intercepting a (by tricking the client to connect to the wrong address, or by
connection attempt to the correct address, for instance). intercepting a connection attempt to the correct address, for
instance).
For the DHE_PSK ciphersuites, an attacker can obtain the information For the DHE_PSK ciphersuites, an attacker can obtain the information
by getting a valid client to attempt connection with the attacker. by getting a valid client to attempt connection with the attacker.
Passive eavesdropping alone is not sufficient. Passive eavesdropping alone is not sufficient.
For the RSA_PSK ciphersuites, only the server (authenticated using For the RSA_PSK ciphersuites, only the server (authenticated using
RSA and certificates) can obtain sufficient information for an RSA and certificates) can obtain sufficient information for an
off-line attack. off-line attack.
It is RECOMMENDED that implementations that allow the administrator It is RECOMMENDED that implementations that allow the administrator
to manually configure the PSK also provide a functionality for to manually configure the PSK also provide a functionality for
generating a new random PSK, taking [RANDOMNESS] into account. generating a new random PSK, taking [RANDOMNESS] into account.
7.3 Identity privacy 7.3. Identity Privacy
The PSK identity is sent in cleartext. While using a user name or The PSK identity is sent in cleartext. Although using a user name or
other similar string as the PSK identity is the most straightforward other similar string as the PSK identity is the most straightforward
option, it may lead to problems in some environments since an option, it may lead to problems in some environments since an
eavesdropper is able to identify the communicating parties. Even eavesdropper is able to identify the communicating parties. Even
when the identity does not reveal any information itself, reusing the when the identity does not reveal any information itself, reusing the
same identity over time may eventually allow an attacker to perform same identity over time may eventually allow an attacker to perform
traffic analysis to identify the parties. It should be noted that traffic analysis to identify the parties. It should be noted that
this is no worse than client certificates, since they are also sent this is no worse than client certificates, since they are also sent
in cleartext. in cleartext.
7.4 Implementation notes 7.4. Implementation Notes
The implementation notes in [TLS11] about correct implementation and The implementation notes in [TLS11] about correct implementation and
use of RSA (including Section 7.4.7.1) and Diffie-Hellman (including 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 Appendix F.1.1.3) apply to the DHE_PSK and RSA_PSK ciphersuites as
well. well.
8. Acknowledgments 8. Acknowledgements
The protocol defined in this document is heavily based on work by Tim The protocol defined in this document is heavily based on work by Tim
Dierks and Peter Gutmann, and borrows some text from [SHAREDKEYS] and Dierks and Peter Gutmann, and borrows some text from [SHAREDKEYS] and
[AES]. The DHE_PSK and RSA_PSK ciphersuites are based on earlier [AES]. The DHE_PSK and RSA_PSK ciphersuites are based on earlier
work in [KEYEX]. work in [KEYEX].
Valuable feedback was also provided by Bernard Aboba, Lakshminath Valuable feedback was also provided by Bernard Aboba, Lakshminath
Dondeti, Philip Ginzboorg, Peter Gutmann, Sam Hartman, Russ Housley, Dondeti, Philip Ginzboorg, Peter Gutmann, Sam Hartman, Russ Housley,
David Jablon, Nikos Mavroyanopoulos, Bodo Moeller, Eric Rescorla, and David Jablon, Nikos Mavroyanopoulos, Bodo Moeller, Eric Rescorla, and
Mika Tervonen. Mika Tervonen.
When the first version of this draft was almost ready, the authors When the first version of this document was almost ready, the authors
learned that something similar had been proposed already in 1996 learned that something similar had been proposed already in 1996
[PASSAUTH]. However, this draft is not intended for web password [PASSAUTH]. However, this document is not intended for web password
authentication, but rather for other uses of TLS. authentication, but rather for other uses of TLS.
9. References 9. References
9.1 Normative References 9.1. Normative References
[AES] Chown, P., "Advanced Encryption Standard (AES) [AES] Chown, P., "Advanced Encryption Standard (AES)
Ciphersuites for Transport Layer Security (TLS)", RFC Ciphersuites for Transport Layer Security (TLS)", RFC
3268, June 2002. 3268, June 2002.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RANDOMNESS] [RANDOMNESS] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC
"Randomness Requirements for Security", BCP 106, RFC 4086, 4086, June 2005.
June 2005.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", [TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999. RFC 2246, January 1999.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003. 10646", STD 63, RFC 3629, November 2003.
9.2 Informative References 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 [KERB] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712, Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999. October 1999.
[KEYEX] Badra, M., Cherkaoui, O., Hajjeh, I. and A. Serhrouchni, [KEYEX] Badra, M., Cherkaoui, O., Hajjeh, I. and A. Serhrouchni,
"Pre-Shared-Key key Exchange methods for TLS", "Pre-Shared-Key key Exchange methods for TLS", Work in
draft-badra-tls-key-exchange-00 (expired), August 2004. Progress, August 2004.
[KRAWCZYK] Krawczyk, H., "Re: TLS shared keys PRF", message on [KRAWCZYK] Krawczyk, H., "Re: TLS shared keys PRF", message on
ietf-tls@lists.certicom.com mailing list 2004-01-13, ietf-tls@lists.certicom.com mailing list 2004-01-13,
http://www.imc.org/ietf-tls/mail-archive/msg04098.html. http://www.imc.org/ietf-tls/mail-archive/msg04098.html.
[LDAPDN] Zeilenga, K., "LDAP: String Representation of [LDAPDN] Zeilenga, K., "LDAP: String Representation of
Distinguished Names", draft-ietf-ldapbis-dn-16 (work in Distinguished Names", Work in Progress, February 2005.
progress), February 2005.
[NAMEPREP] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep [NAMEPREP] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003. 3491, March 2003.
[PASSAUTH] Simon, D., "Addition of Shared Key Authentication to [PASSAUTH] Simon, D., "Addition of Shared Key Authentication to
Transport Layer Security (TLS)", Transport Layer Security (TLS)", Work in Progress,
draft-ietf-tls-passauth-00 (expired), November 1996. November 1996.
[SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for User Names [SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for User
and Passwords", RFC 4013, February 2005. Names and Passwords", RFC 4013, February 2005.
[SHAREDKEYS] [SHAREDKEYS] Gutmann, P., "Use of Shared Keys in the TLS Protocol",
Gutmann, P., "Use of Shared Keys in the TLS Protocol", Work in Progress, October 2003.
draft-ietf-tls-sharedkeys-02 (expired), October 2003.
[SRP] Taylor, D., Wu, T., Mavroyanopoulos, N. and T. Perrin, [SRP] Taylor, D., Wu, T., Mavroyanopoulos, N. and T. Perrin,
"Using SRP for TLS Authentication", draft-ietf-tls-srp-09 "Using SRP for TLS Authentication", Work in Progress,
(work in progress), March 2005. March 2005.
[STRINGPREP] [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, Internationalized Strings ("stringprep")", RFC 3454,
December 2002. December 2002.
[TLS11] Dierks, T. and E. Rescorla, "The TLS Protocol Version [TLS11] Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.1", draft-ietf-tls-rfc2246-bis-12 (work in progress), 1.1", Work in Progress, June 2005.
June 2005.
Authors' and Contributors' Addresses Authors' and Contributors' Addresses
Pasi Eronen Pasi Eronen
Nokia Research Center Nokia Research Center
P.O. Box 407 P.O. Box 407
FIN-00045 Nokia Group FIN-00045 Nokia Group
Finland Finland
Email: pasi.eronen@nokia.com
EMail: pasi.eronen@nokia.com
Hannes Tschofenig Hannes Tschofenig
Siemens Siemens
Otto-Hahn-Ring 6 Otto-Hahn-Ring 6
Munich, Bayern 81739 Munich, Bayern 81739
Germany Germany
Email: Hannes.Tschofenig@siemens.com
EMail: Hannes.Tschofenig@siemens.com
Mohamad Badra Mohamad Badra
ENST Telecom ENST Paris
46 rue Barrault 46 rue Barrault
75634 Paris 75634 Paris
France France
Email: Mohamad.Badra@enst.fr
EMail: Mohamad.Badra@enst.fr
Omar Cherkaoui Omar Cherkaoui
UQAM University UQAM University
Montreal (Quebec) Montreal (Quebec)
Canada Canada
Email: cherkaoui.omar@uqam.ca
EMail: cherkaoui.omar@uqam.ca
Ibrahim Hajjeh Ibrahim Hajjeh
ENST Telecom ESRGroups
46 rue Barrault 17 passage Barrault
75634 Paris 75013 Paris
France France
Email: Ibrahim.Hajjeh@enst.fr
EMail: Ibrahim.Hajjeh@esrgroups.org
Ahmed Serhrouchni Ahmed Serhrouchni
ENST Telecom ENST Paris
46 rue Barrault 46 rue Barrault
75634 Paris 75634 Paris
France France
Email: Ahmed.Serhrouchni@enst.fr
Appendix A. Changelog
(This section should be removed by the RFC Editor before
publication.)
Changes from -08 to -09:
o Clarified internationalization of PSK identities in Section 5.
o Corrected the example IP address in Section 5.1.
o Small clarification to IANA considerations on Section 6.
o Editorial: changed numeric references to symbolic ones, updated
references to latest versions.
Changes from -07 to -08:
o Added table of contents and updated I-D boilerplate.
o Small clarification to motivation in Section 1.
o Small clarification to note 2 in Section 2.
o Corrected all instances of "an uint16" to "a uint16".
o Updated references to latest versions.
Changes from -06 to -07: EMail: Ahmed.Serhrouchni@enst.fr
o Small clarifications to management interface requirements.
Changes from -05 to -06:
o Small clarifications to how the premaster secret is formed.
o Added a section about conformance requirements, and moved existing
text about identity formats there.
Changes from -04 to -05:
o Omit ServerKeyExchange message (in PSK/RSA_PSK versions) if no
identity hint is provided.
Changes from -03 to -04:
o Added a note about premaster secret "general structure" in
Sections 3 and 4.
o Something in the I-D submission procedure had removed all
circumflexes from -03 version, turning e.g. "2^16" (two-to- the
sixteenth power) to "216" (two hundred and sixteen). Let's try
again.
Changes from -02 to -03:
o Aligned the way the premaster secret is derived.
o Specified that identities must be sent as human-readable UTF-8
strings, not in binary formats. Changed reference to RFC 3629
from informative to normative.
o Selected ciphersuite and alert numbers, and updated IANA
considerations section to match this.
o Reworded some text about dictionary attacks in Section 6.2.
Changes from -01 to -02:
o Clarified text about DHE_PSK ciphersuites in Section 1.
o Clarified explanation of HMAC-SHA1/MD5 use of PRF in Section 2.
o Added note about certificate validation and self-signed
certificates in Section 4.
o Added Mohamad Badra et al. as contributors.
Changes from draft-ietf-tls-psk-00 to -01:
o Added DHE_PSK and RSA_PSK key exchange algorithms, and updated
other text accordingly
o Removed SHA-1 hash from PSK key exchange premaster secret
construction (since premaster secret doesn't need to be 48 bytes).
o Added unknown_psk_identity alert message.
o Updated IANA considerations section.
Changes from draft-eronen-tls-psk-00 to draft-ietf-tls-psk-00:
o Updated dictionary attack considerations based on comments from
David Jablon.
o Added a recommendation about using UTF-8 in the identity field. Full Copyright Statement
o Removed Appendix A comparing this document with Copyright (C) The Internet Society (2005).
draft-ietf-tls-sharedkeys-02.
o Removed IPR comment about SPR. This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
o Minor editorial changes. This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgement
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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