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Versions: 00 01 02 03 04 05 draft-ietf-tls-encrypt-then-mac

TLS Working Group                                             P. Gutmann
Internet-Draft                                    University of Auckland
Intended status: Standards Track                        February 8, 2013
Expires: August 12, 2013


                        Encrypt-then-MAC for TLS
               draft-gutmann-tls-encrypt-then-mac-00.txt

Abstract

   This document describes a means of negotiating the use of the
   encrypt-then-MAC security mechanism in place of TLS' existing MAC-
   then-encrypt one, which has been the subject of a number of security
   vulnerabilities over a period of many years.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 12, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Negotiating Encrypt-then-MAC . . . . . . . . . . . . . . . . .  4
     2.1.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Applying Encrypt-then-MAC  . . . . . . . . . . . . . . . . . .  5
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10






































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1.  Introduction

   [TLS] uses a MAC-then-encrypt construction that was regarded as
   secure at the time the original SSL protocol was specified in the
   mid-1990s, but that is no longer regarded as secure
   [EncryptThenAuth].  This construction, as used in TLS, has been the
   subject of numerous security vulnerabilities and attacks stretching
   over a period of many years.  This document specifies a means of
   switching to the more secure encrypt-then-MAC construction as part of
   the TLS handshake, replacing the current MAC-then-encrypt
   construction.

1.1.  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 [RFC2119].


































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2.  Negotiating Encrypt-then-MAC

   The use of encrypt-then-MAC is negotiated via TLS extensions as
   defined in [TLS].  On connecting, the client includes the
   encrypt_then_MAC extension in its client_hello if it wishes to use
   encrypt-then-MAC rather than the default MAC-then-encrypt.  If the
   server is capable of meeting this requirement, it responds with an
   encrypt_then_MAC in its server_hello.  The "extension_type" value for
   this extension is [TBD] and the "extension_data" field of this
   extension SHALL be empty.

2.1.  Rationale

   The use of TLS extensions to negotiate an overall switch is
   preferable to defining new ciphersuites because the latter would
   result in a Cartesian explosion of suites, potentially requiring
   duplicating every single existing suite with a new one that uses
   encrypt-then-MAC.  In contrast the approach presented here requires
   just a single new extension type with a corresponding minimal-length
   extension sent by client and server.

   The use of extensions precludes use with SSL 3.0, but then it's
   likely that anything still using this nearly two decades-old protocol
   will be vulnerable to any number of other attacks anyway, so there
   seems little point in bending over backwards to accomodate SSL 3.0.


























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3.  Applying Encrypt-then-MAC

   Once the use of encrypt-then-MAC has been negotiated, processing of
   TLS packets switches from the standard:

   encrypt( data || MAC || pad )

   to the new:

   encrypt( data || pad ) || MAC

   with the MAC covering the entire packet up to the start of the MAC
   value.  In [TLS] notation the MAC calculation is:

   MAC(MAC_write_key, seq_num +
       TLSCompressed.type +
       TLSCompressed.version +
       TLSCompressed.length +
       ENC(content + padding + padding_length));

   for TLS 1.0 without the explicit IV and:

   MAC(MAC_write_key, seq_num +
       TLSCompressed.type +
       TLSCompressed.version +
       TLSCompressed.length +
       IV +
       ENC(content + padding + padding_length));

   for TLS 1.1 and greater with explicit IV.  The final MAC value is
   then appended to the encrypted data and padding.  Note that this
   calculation is identical to the existing one with the exception that
   the MAC calculation is run over the payload ciphertext rather than
   the plaintext.

   In [TLS] notation the overall packet is then:

   struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          GenericStream/BlockCipher fragment;
          opaque MAC;
          } TLSCiphertext;

   Note that this is identical to the existing TLS layout with the
   single exception being that the MAC value is moved outside the
   encrypted data.



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   Decryption reverses this processing.  The MAC SHALL be evaluated
   before any further processing such as decryption is performed, and if
   the MAC verification fails then processing SHALL terminate
   immediately.  This eliminates any timing channels that may be
   available through the use of manipulated packet data.














































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4.  Security Considerations

   This document defines an improved security mechanism encrypt-then-MAC
   to replace the current MAC-then-encrypt one.  This is regarded as
   more secure than the current mechanism [EncryptThenAuth], and should
   mitigate or eliminate a number of attacks on the current mechanism,
   provided that the instructions on MAC processing given in Section 3
   are applied.











































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5.  IANA Considerations

   This document defines a new extension for TLS.
















































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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [TLS-Ext]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

6.2.  Informative References

   [EncryptThenAuth]
              Krawczyk, H., "The Order of Encryption and Authentication
              for Protecting Communications (or: How Secure Is SSL?)",
              Springer-Verlag LNCS 2139, August 2001.































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Author's Address

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   New Zealand

   Email: pgut001@cs.auckland.ac.nz











































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