Encrypt-then-MAC for TLS and DTLS
draft-gutmann-tls-encrypt-then-mac-03

TLS Working Group                                             P. Gutmann
Internet-Draft                                    University of Auckland
Intended status: Standards Track                           June 29, 2013
Expires: December 31, 2013

                   Encrypt-then-MAC for TLS and DTLS
               draft-gutmann-tls-encrypt-then-mac-03.txt

Abstract

   This document describes a means of negotiating the use of the
   encrypt-then-MAC security mechanism in place of TLS'/DTLS' 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

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   This Internet-Draft will expire on December 31, 2013.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   2
   2.  Negotiating Encrypt-then-MAC  . . . . . . . . . . . . . . . .   2
     2.1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Applying Encrypt-then-MAC . . . . . . . . . . . . . . . . . .   3
     3.1.  Rehandshake Issues  . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   [2] and [4] use 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 [5] [6].  This
   construction, as used in TLS and later DTLS, 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/
   DTLS 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 [1].

2.  Negotiating Encrypt-then-MAC

   The use of encrypt-then-MAC is negotiated via TLS/DTLS extensions as
   defined in [2].  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/DTLS extensions to negotiate an overall switch is
   preferable to defining new ciphersuites because the latter would

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   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.

   Another possibility for introducing encrypt-then-MAC would be to make
   it part of TLS 1.3, however this would require the implementation and
   deployment of all of TLS 1.2 just to support a trivial code change in
   the order of encryption and MAC'ing.  In contrast deploying encrypt-
   then-MAC via the TLS/DTLS extension mechanism required changing less
   than a dozen lines of code in one implementation (not including the
   handling for the new extension type, which was a further 50 or so
   lines of code).

   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.

3.  Applying Encrypt-then-MAC

   Once the use of encrypt-then-MAC has been negotiated, processing of
   TLS/DTLS 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 [2] 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 +

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       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 [2] notation the overall packet is then:

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

   This is identical to the existing TLS layout with the single
   exception being that the MAC value is moved outside the encrypted
   data.  The change for DTLS follows similarly, the only difference
   being that in place of the 64-bit implicit sequence number DTLS
   contains the two 32-bit fields 'epoch' and 'sequence_number' between
   the version and length.

   Note from the GenericStreamCipher/GenericBlockCipher annotation that
   this only applies to standard stream and block ciphers that have
   distinct encrypt and MAC operations.  It does not apply to
   GenericAEADCiphers that already include integrity protection with the
   cipher.  If a server receives an encrypt-then- MAC request extension
   from a client and then selects an AEAD cipher suite, it MUST NOT send
   a encrypt-then-MAC response extension back to the client.

   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.

        [Implementation note: There is currently a test server
         available for interop testing at https://eid.vx4.net:443/.
         This uses the "extension_type" value 0x10, which is the
         first currently unassigned TLS extension value.  This
         server has been tested successfully with several different

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         implementations].

3.1.  Rehandshake Issues

   The status of encrypt-then-MAC vs. MAC-then-encrypt can potentially
   change during a rehandshake.  Implementations SHOULD retain the
   current session state for the renegotiated session (in other words if
   the mechanism for the current session is X then the renegotiated
   session should also use X).  If implementations wish to be more
   flexible then the following rules apply:

   +---------------------+----------------------+----------------------+
   |   Current Session   | Renegotiated Session |    Action to take    |
   +---------------------+----------------------+----------------------+
   |   MAC-then-encrypt  |   MAC-then-encrypt   |      No change       |
   |                     |                      |                      |
   |   MAC-then-encrypt  |   Encrypt-then-MAC   | Upgrade to MAC-then- |
   |                     |                      |       encrypt        |
   |                     |                      |                      |
   |   Encrypt-then-MAC  |   MAC-then-encrypt   |        Error         |
   |                     |                      |                      |
   |   Encrypt-then-MAC  |   Encrypt-then-MAC   |      No change       |
   +---------------------+----------------------+----------------------+

               Table 1: Encrypt-then-MAC with Renegotiation

   Note that a client or server that doesn't wish to implement the
   mechanism-change-during-rehandshake ability can (as a client) not
   request a mechanism change and (as a server) deny the mechanism
   change.

   If an upgrade from MAC-then-encrypt to Encrypt-then-MAC is negotiated
   as per the second line in the table above then the change will take
   place in the first message that follows the Change Cipher Spec (CCS)
   as per established TLS practice.  In other words all messages up to
   and including the CCS will use MAC-then-encrypt, and then the message
   that follows will continue with Encrypt-then-MAC.

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 [5] [6], 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/DTLS.

6.  Acknowledgements

   The author would like to thank Martin Rex, Dan Shumow, and the
   members of the TLS mailing list for their feedback on this document.

7.  References

7.1.  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 Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

   [4]        Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

7.2.  Informative References

   [5]        Bellare, M. and C. Namprempre, "Authenticated Encryption:
              Relations among notions and analysis of the generic
              composition paradigm", Springer-Verlag LNCS 1976, December
              2000.

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

Author's Address

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   University of Auckland
   New Zealand

   Email: pgut001@cs.auckland.ac.nz

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