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Versions: (draft-gutmann-tls-encrypt-then-mac) 00 01 02 03 RFC 7366

TLS Working Group                                             P. Gutmann
Internet-Draft                                    University of Auckland
Intended status: Standards Track                           July 22, 2014
Expires: January 23, 2015


                   Encrypt-then-MAC for TLS and DTLS
                 draft-ietf-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

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

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   This Internet-Draft will expire on January 23, 2015.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Applying Encrypt-then-MAC . . . . . . . . . . . . . . . . . .   3
     3.1.  Rehandshake Issues  . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   TLS [2] and DTLS [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 TLS [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 SHALL be 22 (0x16) and the "extension_data" field of
   this extension SHALL be empty.  The client and server MUST NOT use
   encrypt-then-MAC unless both sides have successfully exchanged
   encrypt_then_mac extensions.





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2.1.  Rationale

   The use of TLS/DTLS 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.

   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 TLS [2] notation the MAC calculation for TLS 1.0 without
   the explicit IV is:

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

   and for TLS 1.1 and greater with explicit IV is:





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

   (for DTLS the sequence number is replaced by the combined epoch and
   sequence number as per DTLS [4]).  The final MAC value is then
   appended to the encrypted data and padding.  This calculation is
   identical to the existing one with the exception that the MAC
   calculation is run over the payload ciphertext (the TLSCipherText
   PDU) rather than the plaintext (the TLSCompressed PDU).

   The overall TLS packet [2] is then:

   struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          GenericBlockCipher fragment;
          opaque MAC;
          } TLSCiphertext;

   The equivalent DTLS packet [4] is then:

   struct {
          ContentType type;
          ProtocolVersion version;
          uint16 epoch;
          uint48 sequence_number;
          uint16 length;
          GenericBlockCipher fragment;
          opaque MAC;
          } TLSCiphertext;

   This is identical to the existing TLS/DTLS layout with the only
   difference being that the MAC value is moved outside the encrypted
   data.

   Note from the GenericBlockCipher annotation that this only applies to
   standard block ciphers that have distinct encrypt and MAC operations.
   It does not apply to GenericStreamCiphers, or 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 a stream or AEAD cipher suite, it MUST NOT send an
   encrypt-then-MAC response extension back to the client.




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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.  For TLS, a fatal bad_record_mac MUST be generated [2].
   For DTLS, the record MUST be discarded and a fatal bad_record_mac MAY
   be generated [4].  This immediate response to a bad MAC eliminates
   any timing channels that may be available through the use of
   manipulated packet data.

   Some implementations may prefer to use a truncated MAC rather than a
   full-length one.  In this case they MAY negotiate the use of a
   truncated MAC through the TLS truncated_hmac extension as defined in
   TLS-Ext [3].

3.1.  Rehandshake Issues

   The status of encrypt-then-MAC vs. MAC-then-encrypt can potentially
   change during one or more rehandshakes.  Implementations SHOULD
   retain the current session state across all rehandshakes for that
   session (in other words if the mechanism for the current session is X
   then the renegotiated session should also use X).  While
   implementations SHOULD NOT change the state during a rehandshake, if
   they wish to be more flexible then the following rules apply:

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

               Table 1: Encrypt-then-MAC with Renegotiation

   As the above table points out, implementations MUST NOT renegotiate a
   downgrade from Encrypt-then-MAC to MAC-then-Encrypt.  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.

   Note that these rules apply across potentially many rehandshakes.
   For example if a session were in the Encrypt-then-MAC state and a



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   rehandshake selected a GenericAEADCiphers ciphersuite and a
   subsequent rehandshake then selected a MAC-then-Encrypt ciphersuite,
   this is an error since the renegotiation process has resulted in a
   downgrade from Encrypt-then-MAC to MAC-then-Encrypt (via the AEAD
   ciphersuite).

   (As the text above has already pointed out, implementations SHOULD
   avoid having to deal with these cipher-suite calisthenics by
   retaining the initially-negotiated mechanism across all
   rehandshakes).

   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).
   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.

   An active attacker who can emulate a client or server with extension
   intolerance may cause some implementations to fall back to older
   protocol versions that don't support extensions, which will in turn
   force a fallback to non-Encrypt-then-MAC behaviour.  A
   straightforward solution to this problem is to avoid fallback to
   older, less secure protocol versions.  If fallback behaviour is
   unavoidable then mechanisms to address this issue, which affects all
   capabilities that are negotiated via TLS extensions, are being
   developed by the TLS working group [7].  Anyone concerned about this
   type of attack should consult the TLS working group documents for
   guidance on appropriate defence mechanisms.

5.  IANA Considerations

   IANA has added the extension code point 22 (0x16) for the
   encrypt_then_mac extension to the TLS ExtensionType values registry
   as specified in TLS [2].







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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]        Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions", RFC 6066, January 2011.

   [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.

   [7]        Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC XXXX, November 2013.

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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