One document matched: draft-urien-badra-eap-tls-identity-protection-00.txt
Internet Engineering Task Force P. Urien
M. Badra
ENST Paris
Expires: December 2006 June, 2006
Identity Protection within EAP-TLS
<draft-urien-badra-eap-tls-identity-protection-00.txt>
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Copyright (C) The Internet Society (2006). All Rights Reserved.
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INTERNET-DRAFT EAP-TLS Identity Protection June 2006
Abstract
This document defines a mechanism providing EAP-TLS identity
protection.
It defines new TLS extension, in order to negotiate the symmetric
encryption algorithm that is used to encrypt or decrypt the client's
certificate.
Table of Contents
Abstract...........................................................2
1 Introduction.....................................................3
1.2 Requirements language..........................................3
2 EAP-TLS Identity Protection overview.............................4
3 Certificate Encryption...........................................5
4 Identity protection extension....................................5
5 Security Considerations..........................................6
6 References.......................................................6
7 Author's Addresses...............................................6
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1 Introduction
According to [EAP], "EAP Methods deriving keys MUST provide for
mutual authentication between the EAP peer and the EAP Server".
Consequently, in [EAP-TLS], which is a quite transparent transport
of TLS [TLS] over EAP, "the mutual authentication will occur between
the peer and the EAP server". For that, when the EAP server sent a
certificate_request message, then the peer MUST reply with
certificate and certificate_verify handshake messages.
peer EAP server
ClientHello -------->
ServerHello
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate // peer identity is sent in clear text
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Figure 1. Mutual authentication exchanges within TLS
The peer always sends its certificate in clear text, and therefore
exposes its identity (e.g. an X509 certificate) to eavesdropping.
Thus, an intruder can easily get the certificate and then derive the
certificate owner's real identity.
This document describes how identity protection may be integrated as
an optional service in EAP-TLS. It is based on TLS and TLS
extensions standards and then it does not require significant
changes to the actual specifications.
1.2 Requirements language
The key words "MUST", "SHALL", "SHOULD", and "MAY", in this document
are to be interpreted as described in RFC-2119.
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2 EAP-TLS Identity Protection overview
In order to allow an EAP-TLS peer to request identity protection
exchange, a new extension type is added to the Extended Client and
Server Hello messages.
TLS clients and servers include an extension of type 'identity-
protection (TBD)' in the Extended Client and Server Hello messages.
The 'extension_data' field of this extension contains a list of
encryption algorithms supported by the client, ordered by
preference.
If the server is willing to accept using the extension, the client
and the server negotiate the symmetric algorithm that will be used
to encrypt/decrypt the client certificate.
At the end of the hello phase, the client generates the
pre_master_secret, encrypts it under the server's public key, and
sends the result to the server.
According to [TLS] a shared secret, called master_secret is computed
from, among others, the pre_master_secret.
Hence, the client and the server have all the security parameters to
generate the session keys.
Before sending its certificate message, the client encrypts its
certificate using the negotiated symmetric algorithm through the
anonymity extension and a key derived as follows.
enc_key = PRF(SecurityParameters.master_secret,
"client_certificate",
SecurityParameters.client_random +
SecurityParameters.server_random);
The server repeats the same operation by decrypting the certificate
message using the negotiated symmetric algorithm through the
anonymity extension and a key derived in the same manner.
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Client Server
ExtendedClientHello -------->
ExtendedServerHello
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate // peer identity is sent encrypted
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Figure 2. Mutual authentication exchanges with identity protection
3 Certificate Encryption
If a stream cipher is chosen, then the peer's certificate is
encrypted with the enc_key, without any padding byte.
If a block cipher is selected, then padding bytes are added to force
the length of the certificate message to be an integral multiple of
the bloc cipher's length.
4 Identity protection extension
The TLS extension is sent by the client to indicate to the server
that the client certificate will be sent encrypted using a symmetric
algorithm negotiated through that extension.
The symmetric algorithm uses a key derived from the random values
and from the master_secret.
struct {
SymmetricAlgorithm symmetric_alg_list<0..2^16-1>;
} IdentityProtection;
enum { rc4_128(0), (255) } SymmetricAlgorithm;
The "extension_data" field of this extension shall contain the
symmetric algorithms (and their key length) supported by the client.
Encryption algorithms are sent in order of the client's preference
(favorite choice first).
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5 Security Considerations
Security issues are discussed throughout this memo and in [EAP-TLS],
[EAP-TLS] and [TLS-EXT]
6 References
[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[EAP-TLS] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[TLS] Dierks, T., et. al, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[TLS-EXT] Blake-Wilson, S., et. al, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
7 Author's Addresses
Pascal Urien
ENST Paris
France Email: Pascal.Urien@enst.fr
Mohamad Badra
ENST Paris
France Email: Mohamad.Badra@enst.fr
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