Encrypt-then-MAC for TLS and DTLS
draft-gutmann-tls-encrypt-then-mac-02.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.

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Table of Contents

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

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 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 
         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.