Network Working Group W A Simpson [DayDreamer]
Internet Draft P Metzger [Piermont]
P Karn [Qualcomm]
Naganand Doraswamy [Bay]
expires in six months July 1998
The ESP Triple DES Transform
draft-simpson-esp-des3v2-03.txt
Status of this Memo
This document is an Internet-Draft. Internet Drafts are working doc-
uments of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups. Note that other groups may also distribute work-
ing documents as Internet Drafts.
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 not appropriate to use Internet Drafts as refer-
ence material, or to cite them other than as a ``working draft'' or
``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the internet-drafts Shadow
Directories on:
ftp.is.co.za (Africa)
nic.nordu.net (Northern Europe)
ftp.nis.garr.it (Southern Europe)
ftp.ietf.org (Eastern USA)
ftp.isi.edu (Western USA)
munnari.oz.au (Pacific Rim)
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) William Allen Simpson, Perry Metzger and Philip Karn
(1994-1995). Copyright (C) William Allen Simpson, Perry Metzger,
Philip Karn and Naganand Doraswamy (1997-1998). All Rights Reserved.
Simpson, Metzger expires in six months [Page i]
DRAFT ESP DES-EDE3-CBC July 1998
Abstract
This document describes the 'Triple' DES-EDE3-CBC block cipher trans-
form interface used with the IP Encapsulating Security Payload (ESP).
Simpson, Metzger expires in six months [Page ii]
DRAFT ESP DES-EDE3-CBC July 1998
1. Introduction
The Encapsulating Security Payload (ESP) [RFC-1827x] provides confi-
dentiality for IP datagrams by encrypting the payload data to be pro-
tected. This specification describes the ESP use of a variant of the
Cipher Block Chaining (CBC) mode of the US Data Encryption Standard
(DES) algorithm [FIPS-46, FIPS-46-1, FIPS-74, FIPS-81].
This variant, colloquially known as "Triple DES", processes each
block three times, each time with a different key [Tuchman79].
For an explanation of the use of CBC mode with this cipher, see [RFC-
wwww].
For more explanation and implementation information for Triple DES,
see [Schneier95].
This document assumes that the reader is familiar with the related
document "Security Architecture for the Internet Protocol"
[RFC-1825x], that defines the overall security plan for IP, and pro-
vides important background for this specification.
In this document, the key words "MAY", "MUST", "recommended",
"required", and "SHOULD", are to be interpreted as described in
[RFC-2119].
1.1. Availability
There were a number of US patents (see [Schneier95] for listing).
All patents have expired. Several freely available implementations
have been published world-wide.
1.2. Performance
As this specification requires "outer" chaining, it is not possible
to provide parallel computation for the same data stream.
The DES "initial" and "final" permutations are inverses of each
other, so the permutations effectively cancel between the first and
second and the second and third steps. A significant performance
improvement can be obtained by removing these "inner" permutations
entirely. Thus, "triple DES" is approximately 2.5 times slower than
"single DES".
Phil Karn has tuned DES-EDE3-CBC software to achieve 6.22 Mbps with a
133 MHz Pentium, compared with 15.9 Mbps for "single" DES-CBC on the
Simpson, Metzger expires in six months [Page 1]
DRAFT ESP DES-EDE3-CBC July 1998
same machine. This software is freely available on the
www.cryptography.org site. Other DES speed estimates may be found at
[Schneier95, page 279]. Your mileage may vary.
2. Description
2.1. Block Size
The US Data Encryption Standard (DES) algorithm operates on blocks of
64-bits (8 bytes). This often requires padding before encrypting,
and subsequent removal of padding after decrypting.
The output is the same number of bytes that are input. This facili-
tates in-place encryption and decryption.
2.2. Mode
The DES-EDE3-CBC algorithm is a simple variant of the DES-CBC algo-
rithm [RFC-wwww, RFC-1829].
In DES-EDE3-CBC, the algorithms are an encryption (Ek1), followed by
a decryption (Dk2), followed by an encryption (Ek3). Each step uses
an independant key: k1, k2 and k3.
To decrypt, the order of the functions is reversed: decrypt with k3,
encrypt with k2, decrypt with k1.
Note that when a pair of keys (k1 and k2 or k2 and k3) are the same,
DES-EDE3-CBC is equivalent to DES-CBC. This property allows the DES-
EDE3 hardware implementations to operate in DES mode without modifi-
cation.
2.3. Interaction with Authentication
There is no known interaction of DES with any currently specified
Authenticator algorithm. Never-the-less, any Authenticator MUST use
a separate and independently generated key.
3. Initialization Vector
DES-EDE3-CBC requires an Initialization Vector (IV) that is 64-bits
(8 bytes) in length. By default, the IV is carried immediately fol-
lowing the ESP Sequence Number.
Simpson, Metzger expires in six months [Page 2]
DRAFT ESP DES-EDE3-CBC July 1998
4. Keys
The secret DES-EDE3 keys shared between the communicating parties are
effectively 168-bits long, but are represented as a 192-bit (24 byte)
quantity.
The keys consist of three independent 56-bit quantities used by the
DES algorithm. Each of the three 56-bit keys is stored as a 64-bit
(8 byte) quantity, with the least significant bit of each byte used
as a parity bit.
4.1. Weak Keys
DES has 64 known weak keys, including so-called semi-weak keys and
possibly-weak keys [Schneier95, pp 280-282]. The likelihood of pick-
ing one at random is negligible.
For DES-EDE3, there is no known need to reject weak or complementa-
tion keys. Any weakness is likely to be obviated by the other keys.
However, since checking for weak keys is quite easy, conformant
implementations MUST test for weak DES keys.
Moreover, when operating as DES-EDE3, each key MUST NOT be the same
as the preceding key.
4.2. Manual Key Management
When configured manually, three independently generated keys are
required, in the order used for encryption, and 64-bits (8 bytes) are
configured for each individual key.
Keys with incorrect parity SHOULD be rejected by the configuration
utility, ensuring that the keys have been correctly configured.
Each key is examined sequentially, in the order used for encryption.
A key that is identical to a previous key MAY be rejected. The 64
known weak DES keys SHOULD be rejected. In either case, a warning
SHOULD be displayed.
Simpson, Metzger expires in six months [Page 3]
DRAFT ESP DES-EDE3-CBC July 1998
4.3. Automated Key Management
When configured via a Security Association management protocol, three
independently generated keys are required, in the order used for
encryption, and 64-bits (8 bytes) are returned for each individual
key.
The key manager MAY be required to generate the correct parity for
each key. Alternatively, the least significant bit of each key byte
is ignored, or locally set to parity by the DES implementation.
Each key is examined sequentially, in the order used for encryption.
A key that is identical to a previous key MUST be rejected. The 64
known weak DES keys MUST be rejected.
4.4. Refresh Rate
To prevent differential and linear cryptanalysis of collisions [RFC-
wwww], no more than 2**32 plaintext blocks SHOULD be encrypted with
the same keys. Depending on the average size of the datagrams, the
keys SHOULD be changed at least as frequently as 2**30 datagrams.
Operational Considerations
The specification provides only a few manually configurable parame-
ters:
SPI
Manually configured SPIs are limited in range to aid operations.
Automated SPIs are pseudo-randomly distributed throughout the
remaining 2**32 values.
Default: 0 (none). Range: 256 to 65,535.
SPI LifeTime (SPILT)
Although automated LifeTimes are internally maintained in seconds,
manually configured LifeTimes are generally measured in days to
promote ease of configuration.
Default: 32 days (2,764,800 seconds). Maximum: 182 days
(15,724,800 seconds).
Key
Three 56-bit keys, with parity included as appropriate, are con-
figured in order as a 192-bit quantity.
Simpson, Metzger expires in six months [Page 4]
DRAFT ESP DES-EDE3-CBC July 1998
Each party configures a list of known SPIs and symmetric secret-keys.
In addition, each party configures local policy that determines what
access (if any) is granted to the holder of a particular SPI. For
example, a party might allow FTP, but prohibit Telnet. Such consid-
erations are outside the scope of this document.
Security Considerations
Users need to understand that the quality of the security provided by
this specification depends completely on the strength of the Triple
DES algorithm, the correctness of that algorithm's implementation,
the security of the Security Association management mechanism and its
implementation, the strength of the key [CN94], and upon the correct-
ness of the implementations in all of the participating nodes.
The biggest practical vulnerability of any software security system
is protecting the memory containing the keys. No system can remain
secure when its key storage is insecure. If this key storage is man-
aged by an operating system, then the combination is no more secure
than the operating system itself. Even when the OS is secure against
external attack, there is always the possibility of physical compro-
mise.
It was originally thought that DES might be a group, but it has been
demonstrated that it is not [CW92]. Since DES is not a group, compo-
sition of multiple rounds of DES is not equivalent to simply using
DES with a different key.
Triple DES with independent keys is not, as naively might be
expected, as difficult to break by brute force as a cryptosystem with
three times the keylength. A space/time tradeoff has been shown
which can brute-force break triple block encryptions in the time
naively expected for double encryption [MH81].
However, "double" DES (DES-EE2) can be broken with a meet-in-the-
middle attack, without significantly more complexity than breaking
DES requires [ibid]. DES-EDE3 with three independant keys is actu-
ally needed to provide significantly more security than "single" DES.
An attack has been shown on DES-EDE2 (using only two independent
keys) [Tuchman79] that is somewhat (sixteen times) faster than
exhaustive search [OW91]. Again, DES-EDE3 with three independant
keys is actually needed to provide the expected level of security.
Although it is widely believed that DES-EDE3 is substantially
stronger than single DES alone, as it is less amenable to brute force
Simpson, Metzger expires in six months [Page 5]
DRAFT ESP DES-EDE3-CBC July 1998
attack, it should be noted that real cryptanalysis of DES-EDE3 might
not use brute force methods at all. Instead, it might be performed
using variants on differential [BS93] or linear [Matsui94] cryptanal-
ysis.
It should also be noted that no encryption algorithm is permanently
safe from brute force attack, because of the increasing speed of mod-
ern computers.
As with all cryptosystems, those responsible for applications with
substantial risk when security is breeched should pay close attention
to developments in cryptology, and especially cryptanalysis, and
switch to other transforms should DES-EDE3 prove weak.
Changes from RFC-1851:
Updated performance estimates. Replaced erroneous text about paral-
lel computation.
Details of CBC are moved to an informational document, to be used by
multiple algorithms.
The explicit 64-bit version of the IV is specified as a default.
Compatibility and enhancement options are moved to an informational
document.
Implementation details by Karn were found to be common to many ESP
Ciphers, and are awaiting consolidation in the ESP specification.
Updated security considerations.
Added an operational section.
Updated acknowledgements, references, and contacts.
Reorganized according to the "road map" document.
Simpson, Metzger expires in six months [Page 6]
DRAFT ESP DES-EDE3-CBC July 1998
Acknowledgements
The basic field naming and layout is based on "swIPe" [IBK93, IB93].
Some of the text of this specification was derived from work by Ran-
dall Atkinson for the SIP, SIPP, and IPv6 Working Groups.
Perry Metzger provided the original Security Considerations text,
some of which is distributed throughout the document.
William Allen Simpson was responsible for the name and semantics of
the SPI, editing and formatting.
Steve Bellovin, Angelos Keromytis, Holger Kummert, and Rodney Thayer
provided useful critiques of earlier versions of this document.
References
[Bellovin96]
Bellovin, S., "Problem Areas for the IP Security Proto-
cols", Proceedings of the Sixth Usenix Security Sympo-
sium, July 1996.
[BS93] Biham, E., and Shamir, A., "Differential Cryptanalysis of
the Data Encryption Standard", Berlin: Springer-Verlag,
1993.
[CN94] Carroll, J.M., and Nudiati, S., "On Weak Keys and Weak
Data: Foiling the Two Nemeses", Cryptologia, Vol. 18 No.
23 pp. 253-280, July 1994.
[CW92] Campbell, K.W., and Wiener, M.J., "Proof that DES Is Not
a Group", Advances in Cryptology -- Crypto '92 Proceed-
ings, Berlin: Springer-Verlag, 1993, pp 518-526.
[FIPS-46] US National Bureau of Standards, "Data Encryption Stan-
dard", Federal Information Processing Standard (FIPS)
Publication 46, January 1977.
[FIPS-46-1] US National Bureau of Standards, "Data Encryption Stan-
dard", Federal Information Processing Standard (FIPS)
Publication 46-1, January 1988.
[FIPS-74] US National Bureau of Standards, "Guidelines for Imple-
menting and Using the Data Encryption Standard", Federal
Information Processing Standard (FIPS) Publication 74,
April 1981.
Simpson, Metzger expires in six months [Page 7]
DRAFT ESP DES-EDE3-CBC July 1998
[FIPS-81] US National Bureau of Standards, "DES Modes of Operation"
Federal Information Processing Standard (FIPS) Publica-
tion 81, December 1980.
[IB93] Ioannidis, J., and Blaze, M., "The Architecture and
Implementation of Network-Layer Security Under Unix",
Proceedings of the Fourth Usenix Security Symposium,
Santa Clara California, October 1993.
[IBK93] Ioannidis, J., Blaze, M., and Karn, P., "swIPe: Network-
Layer Security for IP", Presentation at the 26th Internet
Engineering Task Force, Columbus Ohio, March 1993.
[Matsui94] Matsui, M., "Linear Cryptanalysis method for DES Cipher,"
Advances in Cryptology -- Eurocrypt '93 Proceedings,
Berlin: Springer-Verlag, 1994.
[MH81] Merkle, R.C., and Hellman, M., "On the Security of Multi-
ple Encryption", Communications of the ACM, v. 24 n. 7,
1981, pp. 465-467.
[OW91] van Oorschot, P.C., and Weiner, M.J. "A Known-Plaintext
Attack on Two-Key Triple Encryption", Advances in Cryp-
tology -- Eurocrypt '90 Proceedings, Berlin: Springer-
Verlag, 1991, pp. 318-325.
[RFC-1570] Simpson, W., "PPP LCP Extensions", DayDreamer, January
1994.
[RFC-1825x] Atkinson, R., "Security Architecture for the Internet
Protocol", Naval Research Laboratory, July 1995.
[RFC-1827x] Simpson, W., "IP Encapsulating Security Protocol (ESP)
for implementors", work in progress.
[RFC-1829] Karn, P., Metzger, P., Simpson, W.A., "The ESP DES-CBC
Transform", August 1995.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, Harvard University, March
1997.
[RFC-wwww] Simpson, W.A, "ESP with Cipher Block Chaining (CBC)",
work in progress.
[Schneier95]
Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7.
Simpson, Metzger expires in six months [Page 8]
DRAFT ESP DES-EDE3-CBC July 1998
[Tuchman79] Tuchman, W, "Hellman Presents No Shortcut Solutions to
DES", IEEE Spectrum, v. 16 n. 7, July 1979, pp. 40-41.
Contacts
Comments about this document should be discussed on the ipsec@tis.com
mailing list.
Questions about this document can also be directed to:
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
Perry Metzger
Piermont Information Systems Inc.
160 Cabrini Blvd., Suite #2
New York, NY 10033
perry@piermont.com
Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779
karn@qualcomm.com
karn@unix.ka9q.ampr.org (preferred)
Naganand Doraswamy
Bay Networks
3 Federal Street #BL3-04
Billerica, Massachusetts 01821
naganand@BayNetworks.Com
Simpson, Metzger expires in six months [Page 9]
DRAFT ESP DES-EDE3-CBC July 1998
Full Copyright Statement
Copyright (C) William Allen Simpson, Perry Metzger and Philip Karn
(1994-1995). Copyright (C) William Allen Simpson, Perry Metzger,
Philip Karn and Naganand Doraswamy (1997-1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this doc-
ument itself may not be modified in any way, except as required to
translate it into languages other than English.
This document and the information contained herein is provided on an
"AS IS" basis and the author(s) DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING (BUT NOT LIMITED TO) ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Simpson, Metzger expires in six months [Page 10]