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

Status of This Memo

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Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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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 [TLS] and DTLS [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 [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 [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 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.

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 +
    TLSCipherText.type +
    TLSCipherText.version +
    TLSCipherText.length +
    ENC(content + padding + padding_length));
        

MAC(MAC_write_key, seq_num +
    TLSCipherText.type +
    TLSCipherText.version +
    TLSCipherText.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 [TLS] notation the MAC calculation is: DTLS [DTLS]). 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).

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

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

The overall TLS packet [2] is then: [4] is then:

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.

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 [2]. 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 [TLS-Ext].

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). While implementations SHOULD NOT change the state during a rehandshake, if they wish to be more flexible then the following rules apply: