TLS Working Group P. Gutmann
Internet-Draft University of Auckland
Intended status: Standards Track October 04, 2013
Expires: April 07, 2014
Encrypt-then-MAC for TLS and DTLS
draft-gutmann-tls-encrypt-then-mac-04.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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 2
2. Negotiating Encrypt-then-MAC . . . . . . . . . . . . . . . . 2
2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Applying Encrypt-then-MAC . . . . . . . . . . . . . . . . . . 3
3.1. Rehandshake Issues . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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] 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
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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:
MAC(MAC_write_key, seq_num +
TLSCipherText.type +
TLSCipherText.version +
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TLSCipherText.length +
IV +
ENC(content + padding + padding_length));
for TLS 1.1 and greater with explicit IV. 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).
In [2] notation the overall packet is then:
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
GenericStream/BlockCipher fragment;
opaque MAC;
} TLSCiphertext;
This is identical to the existing TLS layout with the only difference
being that the MAC value is moved outside the encrypted data. The
change for DTLS follows similarly, the only difference being that in
place of the 64-bit implicit sequence number DTLS contains the two
32-bit fields 'epoch' and 'sequence_number' between the version and
length.
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
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. This 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].
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[Implementation note: There is a test server available for interop
testing at https://eid.vx4.net:443/. This uses the "extension_type"
value 0x10, which was the first unassigned TLS extension value at the
time the original specification was written, since this draft has
been stalled by the TLS WG chairs for a considerable amount of time
this value has now been assigned to another RFC but is currently
still used for interop-testing purposes. 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). If implementations wish to be more
flexible then the following rules apply:
+---------------------+----------------------+----------------------+
| Current Session | Renegotiated Session | Action to take |
+---------------------+----------------------+----------------------+
| 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
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
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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.
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
[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.
Author's Address
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Peter Gutmann
University of Auckland
Department of Computer Science
University of Auckland
New Zealand
Email: pgut001@cs.auckland.ac.nz
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