Internet Engineering Task Force (IETF) P. Gutmann

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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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RFC 7366 Encrypt-then-MAC for TLS and DTLS September 2014

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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RFC 7366 Encrypt-then-MAC for TLS and DTLS September 2014

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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RFC 7366 Encrypt-then-MAC for TLS and DTLS September 2014

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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RFC 7366 Encrypt-then-MAC for TLS and DTLS September 2014

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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RFC 7366 Encrypt-then-MAC for TLS and DTLS September 2014

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: [email protected]

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