TLS Working Group P. Gutmann
Internet-Draft University of Auckland
Intended status: Standards Track May 2, 2014
Expires: November 3, 2014
Encrypt-then-MAC for TLS and DTLS
draft-ietf-tls-encrypt-then-mac-01.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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 3, 2014.
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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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.
Gutmann Expires November 3, 2014 [Page 1]
Internet-Draft Encrypt-then-MAC-for-TLS May 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 . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[2] and [4] 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 [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 is [TBD - expected value is 22, 0x16] and the
"extension_data" field of this extension SHALL be empty.
Gutmann Expires November 3, 2014 [Page 2]
Internet-Draft Encrypt-then-MAC-for-TLS May 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 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
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 [2] notation the MAC calculation is:
MAC(MAC_write_key, seq_num +
TLSCipherText.type +
TLSCipherText.version +
TLSCipherText.length +
ENC(content + padding + padding_length));
for TLS 1.0 without the explicit IV and:
Gutmann Expires November 3, 2014 [Page 3]
Internet-Draft Encrypt-then-MAC-for-TLS May 2014
MAC(MAC_write_key, seq_num +
TLSCipherText.type +
TLSCipherText.version +
TLSCipherText.length +
IV +
ENC(content + padding + padding_length));
for TLS 1.1 and greater with explicit IV (for DTLS the sequence
number is replaced by the combined epoch and sequence number as per
[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 AEAD cipher suite, it MUST NOT send an
encrypt-then-MAC response extension back to the client.
Gutmann Expires November 3, 2014 [Page 4]
Internet-Draft Encrypt-then-MAC-for-TLS May 2014
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
[3].
[(To be removed by RFC Editor)
Implementation note: There is a test server available for
interop testing at https://eid.vx4.net:443/. This uses the
"extension_type" value 0x15 for the encrypt_then_MAC extension,
which was the first unassigned TLS extension value at the time
the original specification was written. It is expected that
the final "extension_type" value will be 22, 0x16. The 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). While implementations SHOULD NOT change
the state during a rehandshake, if they wish to be more flexible then
the following rules apply:
Gutmann Expires November 3, 2014 [Page 5]
Internet-Draft Encrypt-then-MAC-for-TLS May 2014
+------------------+---------------------+--------------------------+
| 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.
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).
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 [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
Gutmann Expires November 3, 2014 [Page 6]
Internet-Draft Encrypt-then-MAC-for-TLS May 2014
type of attack should consult the TLS working group documents for
guidance on appropriate defence mechanisms.
5. IANA Considerations
This document defines a new extension for TLS/DTLS. IANA[should
allocate extension number 22 (0x16) for this extension.
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
[5] Bellare, M. and C. Namprempre, "Authenticated Encryption:
Relations among notions and analysis of the generic
composition paradigm", Springer-Verlag LNCS 1976, December
2000.
[6] Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (or: How Secure Is SSL?)",
Springer-Verlag LNCS 2139, August 2001.
[7] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC XXXX, November 2013.
Gutmann Expires November 3, 2014 [Page 7]
Internet-Draft Encrypt-then-MAC-for-TLS May 2014
Author's Address
Peter Gutmann
University of Auckland
Department of Computer Science
University of Auckland
New Zealand
Email: pgut001@cs.auckland.ac.nz
Gutmann Expires November 3, 2014 [Page 8]