Internet DRAFT - draft-ietf-tls-encrypt-then-mac
draft-ietf-tls-encrypt-then-mac
TLS Working Group P. Gutmann
Internet-Draft University of Auckland
Intended status: Standards Track July 22, 2014
Expires: January 23, 2015
Encrypt-then-MAC for TLS and DTLS
draft-ietf-tls-encrypt-then-mac-03.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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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
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
TLS [2] and DTLS [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 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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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 TLS [2] notation the MAC calculation for TLS 1.0 without
the explicit IV is:
MAC(MAC_write_key, seq_num +
TLSCipherText.type +
TLSCipherText.version +
TLSCipherText.length +
ENC(content + padding + padding_length));
and for TLS 1.1 and greater with explicit IV is:
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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 AEAD cipher suite, it MUST NOT send an
encrypt-then-MAC response extension back to the client.
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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). While
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.
Note that these rules apply across potentially many rehandshakes.
For example if a session were in the Encrypt-then-MAC state and a
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rehandshake selected a GenericAEADCiphers ciphersuite and a
subsequent rehandshake then selected a MAC-then-Encrypt ciphersuite,
this is 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 cipher-suite 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).
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
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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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 3rd, D., "Transport Layer Security (TLS)
Extensions", 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", 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.
Author's Address
Peter Gutmann
University of Auckland
Department of Computer Science
University of Auckland
New Zealand
Email: pgut001@cs.auckland.ac.nz
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