Encrypt-then-MAC for Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)
RFC 7366

Internet Engineering Task Force (IETF)                        P. Gutmann
Request for Comments: 7366                        University of Auckland
Category: Standards Track                                 September 2014
ISSN: 2070-1721

        Encrypt-then-MAC for Transport Layer Security (TLS) and
                Datagram Transport Layer Security (DTLS)

Abstract

   This document describes a means of negotiating the use of the
   encrypt-then-MAC security mechanism in place of the existing MAC-
   then-encrypt mechanism in Transport Layer Security (TLS) and Datagram
   Transport Layer Security (DTLS).  The MAC-then-encrypt mechanism has
   been the subject of a number of security vulnerabilities over a
   period of many years.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7366.

Copyright Notice

   Copyright (c) 2014 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  . . . . . . . . . . . . . . . . . . . . . . . .   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

1.  Introduction

   TLS [2] and DTLS [4] use a MAC-then-encrypt construction that was
   regarded as secure at the time the original Secure Socket Layer (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.  (In this document, "MAC" refers to
   "Message Authentication Code".)

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 that protocol, which is nearly two
   decades old, will be vulnerable to any number of other attacks
   anyway, so there seems little point in bending over backwards to
   accommodate 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 Initialization Vector (IV) is:

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

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   and for TLS 1.1 and greater with an explicit IV is:

   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 Authenticated Encryption with Associated

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   Data (AEAD) ciphersuite, it MUST NOT send an 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.  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.)  Although
   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.

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   Note that these rules apply across potentially many rehandshakes.
   For example, if a session were in the encrypt-then-MAC state and a
   rehandshake selected a GenericAEADCiphers ciphersuite and a
   subsequent rehandshake then selected a MAC-then-encrypt ciphersuite,
   this would be 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 ciphersuite 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)
   message.  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 encrypt-then-MAC, an improved security
   mechanism to replace the current MAC-then-encrypt one.  Encrypt-then-
   MAC 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, D., "Transport Layer Security (TLS) Extensions:
        Extension Definitions", 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", Proceedings of AsiaCrypt '00, Springer-Verlag LNCS
        No. 1976, p. 531, December 2000.

   [6]  Krawczyk, H., "The Order of Encryption and Authentication for
        Protecting Communications (or: How Secure Is SSL?)", Proceedings
        of Crypto '01, Springer-Verlag LNCS No. 2139, p. 310, August
        2001.

   [7]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Suite
        Value (SCSV) for Preventing Protocol Downgrade Attacks", Work in
        Progress, July 2014.

Author's Address

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   New Zealand

   EMail: pgut001@cs.auckland.ac.nz

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