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Versions: 00                                                            
Network Working Group                                        E. Rescorla
Internet-Draft                                         Network Resonance
Expires: August 29, 2006                               February 25, 2006


         Transport Layer Security (TLS) Partial Encryption Mode
                   draft-rescorla-tls-partial-00.txt

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   This Internet-Draft will expire on August 29, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes an extension to TLS to allow partial
   encryption of record bodies.  This allows the beginning of the record
   body to be in the clear, thus facilitating debugging and header
   compression.








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used In This Document . . . . . . . . . . . . . . . 3
   3.  Negotiating the Partial Encryption Extension  . . . . . . . . . 3
   4.  Record Processing . . . . . . . . . . . . . . . . . . . . . . . 3
     4.1.  Record Transmission . . . . . . . . . . . . . . . . . . . . 4
     4.2.  Record Reception  . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7






































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1.  Introduction

   Encryption in Transport Layer Security (TLS) [2] is currently an all-
   or-nothing proposition.  The choices are a cipher suite that has
   encryption or one of the NULL cipher suites which offer no
   encryption.  This has disadvantages in settings where the application
   layer itself has some data (such as a header) that it wishes to have
   in the clear (e.g., for debugging purposes) and some data (such as a
   payload) that it wishes to have encrypted.  This document describes
   an extension to TLS that allows for the initial portion of the record
   to remain uncompressed and unencrypted.


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 [1].


3.  Negotiating the Partial Encryption Extension

   The client requests support for the partial encryption feature by
   sending the "partial_encryption" extension in its ClientHello.  The
   "extension_data" field contains a PartialEncryption field:

       struct {
         uint16 InitialClearBytes;
       } PartialEncryption

   The InitialClearBytes value contains the number of bytes which will
   be in the clear for each application_data record.  This value will
   obtain for the entire life of this association.

   The server indicates support for the partial encryption feature
   sending a "partial_encryption" extension with an empty
   "extension_data" field.  This indicates its acceptance of the
   extension and of the the number of bytes to be sent in the clear.  If
   the server does not support the extension or does not accept the
   InitialClearBytes value, it MUST ignore the extension.  The first
   application_data record in the new association (after the
   change_cipher_spec message) MUST use the new encryption mode as
   described below.


4.  Record Processing

   The partial encryption extension only matters for records of type



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   "application_data".  All other records should be processed via the
   usual TLS/DTLS rules.

4.1.  Record Transmission

   When the partial encryption extension is in effect, the
   TLSCiphertext.fragment struct becomes:

       select (CipherSpec.cipher_type) {
               opaque plaintext_bytes[InitialClearBytes];  // New field

               case stream: GenericStreamCipher;
               case block:  GenericBlockCipher;
       } fragment;

   The first InitialClearBytes bytes of the TLSPlaintext.fragment are
   inserted in the TLSCiphertext.plaintext_bytes value.  The rest are
   passed through compression and encryption to form the
   GenericStreamCipher or GenericBlockCipher values.  If the
   TLSPlaintext.fragment is less than InitialClearBytes then the entire
   plaintext is left un-encrypted.  The same processing applies to DTLS
   [4].

   The TLS MAC remains unchanged and is applied to both the
   plaintext_bytes and the TLSCompressed.fragment.  Where length is
   computed as InitialClearBytes + TLSCompressed.length.

4.2.  Record Reception

   Record reception is relatively simple.  The receiver knows whether
   the partial_plaintext extension is in effect and simply treats the
   first InitialClearBytes of what would otherwise be the ciphertext as
   plaintext.  After those bytes are removed, the rest of the record can
   be processed as usual.


5.  Security Considerations

   There are two security concerns introduced by these extensions.  The
   first involves the security of the negotiation and the second the
   security of the transport protocol.  Because the negotiation is
   protected by the TLS/DTLS handshake, attackers can neither force the
   use of these extensions nor block them while allowing the negotiation
   to succeed.

   The second concern is the security if the data.  Obviously, no
   confidentiality is provided for any data in the initial plaintext.
   However, because the length of the initial plaintext is fixed in the



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   negotiation and the MAC covers the total length, an active attacker
   cannot convince the receiver to accept values which are encrypted as
   if they were plaintext or vice versa.

   One concern that applies solely to DTLS is that an active attacker
   might manipulate MTU values to attempt to force the sender to split
   data across multiple records and thus have some application layer
   data which would otherwise be encrypted sent in the clear.  DTLS
   itself does not do any fragmentation and applications which use this
   extension MUST NOT fragment the data that they send to DTLS in such a
   way that sensitive data could be transmitted unencrypted.


6.  IANA Considerations

   This document defines an extension to TLS, in accordance with [3]:

     enum { partial_encryption (??) } ExtensionType;

   [[ NOTE: These values need to be assigned by IANA ]]

7.  Normative References

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

   [2]  Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
        draft-ietf-tls-rfc2246-bis-13 (work in progress), June 2005.

   [3]  Blake-Wilson, S., "Transport Layer Security (TLS) Extensions",
        draft-ietf-tls-rfc3546bis-02 (work in progress), October 2005.

   [4]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
        Security", draft-rescorla-dtls-05 (work in progress), June 2005.

















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Author's Address

   Eric Rescorla
   Network Resonance
   2483 E. Bayshore Rd., #212
   Palo Alto, CA  94303
   USA

   Email: ekr@networkresonance.com










































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