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


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.

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Table of Contents

1. Introduction

[TLS] and [DTLS] 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 [EncryptThenMAC] [EncryptThenAuth]. 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 [RFC2119].

2. Negotiating Encrypt-then-MAC

The use of encrypt-then-MAC is negotiated via TLS/DTLS 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/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

encrypt( data || MAC || pad )

encrypt( data || pad ) || MAC

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

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

Once the use of encrypt-then-MAC has been negotiated, processing of TLS/DTLS packets switches from the standard: [TLS] notation the MAC calculation is:

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

In [TLS] notation the overall packet is then:

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  
         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 

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:

Encrypt-then-MAC with Renegotiation
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

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 [EncryptThenMAC] [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.

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

[1] Bellare, M. and C. Namprempre, "Authenticated Encryption: Relations among notions and analysis of the generic composition paradigm", Springer-Verlag LNCS 1976, December 2000.
[2] 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: