Network Working Group IPsec Working Group
INTERNET DRAFT C. Madson
Expires in Six Months Cisco Systems, Inc.
N. Doraswamy
Bay Networks, Inc.
November 1997
The ESP DES-CBC Cipher Algorithm
With Explicit IV
<draft-ietf-ipsec-ciph-des-expiv-01.txt>
Status of this Memo
This document is a submission to the IETF Internet Protocol Security
(IPSEC) Working Group. Comments are solicited and should be addressed
to the working group mailing list (ipsec@tis.com) or to the authors.
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Distribution of this memo is unlimited.
Abstract
This document describes the use of the DES Cipher algorithm in Cipher
Block Chaining Mode, with an explicit IV, as a confidentiality
mechanism within the context of the IPSec Encapsulating Security
Payload (ESP).
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1. Introduction
This document describes the use of the DES Cipher algorithm in Cipher
Block Chaining Mode as a confidentiality mechanism within the context
of the Encapsulating Security Payload.
DES is a symmetric block cipher algorithm. The algorithm is described
in [FIPS-46][FIPS-46-1][FIPS-74][FIPS-81]. [Schneier96] provides a
general description of Cipher Block Chaining Mode, a mode which is
applicable to several encryption algorithms.
As specified in this draft, DES-CBC is not an authentication
mechanism. [Although DES-MAC, described in [Schneier96] amongst other
places, does provide authentication, DES-MAC is not discussed here.]
For further information on how the various pieces of ESP fit together
to provide security services, refer to [ESP] and [Thayer97].
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 [RFC-2119].
2. Algorithm and Mode
DES-CBC is a symmetric secret-key block algorithm. It has a block
size of 64 bits.
[FIPS-46][FIPS-46-1][FIPS-74] and [FIPS-81] describe the DES
algorithm, while [Schneier96] provides a good description of CBC
mode.
2.1 Performance
Phil Karn has tuned DES-CBC software to achieve 10.45 Mbps with a 90
MHz Pentium, scaling to 15.9 Mbps with a 133 MHz Pentium. Other DES
speed estimates may be found in [Schneier96].
3. ESP Payload
DES-CBC requires an explicit Initialization Vector (IV) of 8 octets
(64 bits). This IV immediately precedes the protected (encrypted)
payload. The IV SHOULD be chosen at random.
Including the IV in each datagram ensures that decryption of each
received datagram can be performed, even when some datagrams are
dropped, or datagrams are re-ordered in transit.
Implementation note:
Common practice is to use random data for the first IV and the
last 8 octets of encrypted data from an encryption process as the
IV for the next encryption process; this logically extends the CBC
across the packets. It also has the advantage of limiting the
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leakage of information from the random number genrator. No matter
which mechnism is used, the receiver MUST NOT assume any meaning
for this value, other than that it is an IV.
The payload field, as defined in [ESP], is broken down according to
the following diagram:
+---------------+---------------+---------------+---------------+
| |
+ Initialization Vector (IV) +
| |
+---------------+---------------+---------------+---------------+
| |
~ Encrypted Payload (variable length) ~
| |
+---------------------------------------------------------------+
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
3.1 Block Size and Padding
The DES-CBC algorithm described in this document MUST use a block
size of 8 octets (64 bits).
When padding is required, it MUST be done according to the
conventions specified in [ESP].
4. Key Material
DES-CBC is a symmetric secret key algorithm. The key size is 64-bits.
[It is commonly known as a 56-bit key as the key has 56 significant
bits; these 56 bits are stored in an 8-byte (64- bit) value, where
each byte has seven significant bits from the 56-bit value and the
least significant bit is used as a parity bit.]
[ESP] describes the general mechanism to derive keying material for
the ESP transform. The derivation of the key from some amount of
keying material does not differ between the manually- and
automatically-keyed security associations.
The mechanism MUST derive a 64-bit key value for use by this cipher.
This derived value MUST be adjusted for parity as necessary. Weak key
checks will be performed; if a weak key is dicovered, the key will be
rejected and IPSEC will request a new SA.
4.1 Weak Keys
DES has 64 known weak keys, including so-called semi-weak keys and
possibly-weak keys (from [Schneier96] -- corrected version provided
by William Allan Simpson -- shown here in hex with parity bits):
0101 0101 0101 0101
1f1f 1f1f 0e0e 0e0e
e0e0 e0e0 f1f1 f1f1
fefe fefe fefe fefe
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semi-weak key pairs:
01fe 01fe 01fe 01fe fe01 fe01 fe01 fe01
1fe0 1fe0 0ef1 0ef1 e0f1 e0f1 f10e f10e
01e0 01e0 01f1 01f1 e001 e001 f101 f101
1ffe 1ffe 0efe 0efe fe1f fe1f fe0e fe0e
011f 011f 010e 010e 1f01 1f01 0e01 0e01
e0fe e0fe f1fe f1fe fee0 fee0 fef1 fef1
possibly-weak keys:
1f1f 0101 0e0e 0101 e001 01e0 f101 01f1
011f 1f01 010e 0e01 fe1f 01e0 fe0e 01f1
1f01 011f 0e01 010e fe01 1fe0 fe01 0ef1
0101 1f1f 0101 0e0e e01f 1fe0 f10e 0ef1
--------------------
e0e0 0101 f1f1 0101 fe01 01fe fe01 01fe
fefe 0101 fefe 0101 e01f 01fe f10e 01fe
fee0 1f01 fef1 0e01 e001 1ffe f101 0efe
e0fe 1f01 f1fe 0e01 fe1f 1ffe fe0e 0efe
--------------------
fee0 011f fef1 010e 1ffe 01e0 0efe 01f1
e0fe 011f f1fe 010e 01fe 1fe0 01fe 0ef1
e0e0 1f1f f1f1 0e0e 1fe0 01fe 0ef1 01fe
fefe 1f1f fefe 0e0e 01e0 1ffe 01f1 0efe
fe1f e001 fe0e f101 0101 e0e0 0101 f1f1
e01f fe01 f10e fe01 1f1f e0e0 0e0e f1f1
fe01 e01f fe01 f1e0 1f01 fee0 0e01 fef1
e001 fe1f f101 fe0e 011f fee0 010e fef1
--------------------
01e0 e001 01f1 f101 1f01 e0fe 0e01 f1fe
1ffe e001 0efe f101 011f e0fe 010e f1fe
1fe0 fe01 0ef1 fe01 0101 fefe 0101 fefe
01fe fe01 01fe fe01 1f1f fefe 0e0e fefe
--------------------
1fe0 e01f 0ef1 f10e fefe e0e0 fefe f1f1
01fe e01f 01fe f10e e0fe fee0 f1fe fef1
01e0 fe1f 01f1 fe0e fee0 e0fe fef1 f1fe
1ffe fe1f 0efe fe0e e0e0 fefe f1f1 fefe
Implementations SHOULD take care not to select weak keys [CN94],
although the likelihood of picking one at random is negligible.
4.2 Key Lifetime
There are no current recommendations for key lifetime.
5. Interaction with Authentication Algorithms
As of this writing, there are no known issues which preclude the use
of the DES-CBC algorithm with any specific authentication algorithm.
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6. Security Considerations
[Much of this section was originally written by William Allen Simpson
and Perry Metzger.]
Users need to understand that the quality of the security provided by
this specification depends completely on the strength of the 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.
[Bell95] and [Bell96] describe a cut and paste splicing attack which
applies to all Cipher Block Chaining algorithms. This attack can be
addressed with the use of an authentication mechanism.
The use of the cipher mechanism without any corresponding
authentication mechanism is strongly discouraged. This cipher can be
used in an ESP transform that also includes authentication; it can
also be used in an ESP transform that doesn't include authentication
provided there is an companion AH header. Refer to [ESP], [AH],
[arch], and [Thayer97] for more details.
When the default ESP padding is used, the padding bytes have a
predictable value. They provide a small measure of tamper detection
on their own block and the previous block in CBC mode. This makes it
somewhat harder to perform splicing attacks, and avoids a possible
covert channel. This small amount of known plaintext does not create
any problems for modern ciphers.
At the time of writing of this document, [BS93] demonstrated a dif-
ferential cryptanalysis based chosen-plaintext attack requiring 2^47
plaintext-ciphertext pairs, where the size of a pair is the size of a
DES block (64 bits). [Matsui94] demonstrated a linear cryptanalysis
based known-plaintext attack requiring only 2^43 plain- text-
ciphertext pairs. Although these attacks are not considered
practical, they must be taken into account.
More disturbingly, [Weiner94] has shown the design of a DES cracking
machine costing $1 Million that can crack one key every 3.5 hours.
This is an extremely practical attack.
One or two blocks of known plaintext suffice to recover a DES key.
Because IP datagrams typically begin with a block of known and/or
guessable header text, frequent key changes will not protect against
this attack.
It is suggested that DES is not a good encryption algorithm for the
protection of even moderate value information in the face of such
equipment. Triple DES is probably a better choice for such purposes.
However, despite these potential risks, the level of privacy provided
by use of ESP DES-CBC in the Internet environment is far greater than
sending the datagram as cleartext.
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7. References
[Bell95] Bellovin, S., "An Issue With DES-CBC When Used Without
Strong Integrity", Presentation at the 32nd Internet Engineering
Task Force, Danvers Massachusetts, April 1995.
[Bell96] Bellovin, S., "Problem Areas for the IP Security Protocols",
Proceedings of the Sixth Usenix Security Symposium, 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.
[FIPS-46] US National Bureau of Standards, "Data Encryption Standard",
Federal Information Processing Standard (FIPS) Publication 46,
January 1977.
[FIPS-46-1] US National Bureau of Standards, "Data Encryption Standard",
Federal Information Processing Standard (FIPS) Publication 46-1,
January 1988.
[FIPS-74] US National Bureau of Standards, "Guidelines for
Implementing and Using the Data Encryption Standard", Federal
Information Processing Standard (FIPS) Publication 74, April 1981.
[FIPS-81] US National Bureau of Standards, "DES Modes of Operation"
Federal Information Processing Standard (FIPS) Publication 81,
December 1980.
[Matsui94] Matsui, M., "Linear Cryptanalysis method for DES Cipher,"
Advances in Cryptology -- Eurocrypt '93 Proceedings, Berlin:
Springer-Verlag, 1994.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119/BCP 14, March, 1997.
[Schneier96] Schneier, B., "Applied Cryptography Second Edition",
John Wiley & Sons, New York, NY, 1996. ISBN 0-471-12845-7.
[Weiner94] Wiener, M.J., "Efficient DES Key Search", School of
Computer Science, Carleton University, Ottawa, Canada, TR-244, May
1994. Presented at the Rump Session of Crypto '93.
[ESP] Kent, S., Atkinson, R., "IP Encapsulating Security Payload
(ESP)", draft-ietf-ipsec-esp-v2-02.txt, work in progress, November
1997.
[AH] Kent, S., Atkinson, R., "IP Authentication Header (AH)",
draft-ietf-ipsec-auth-header-03.txt, work in progress, November
1997.
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[arch] Kent, S., Atkinson, R., "Security Architecture for the
Internet Protocol", draft-ietf-ipsec-arch-sec-02.txt, work in
progress, November 1997.
[Thayer97] Thayer, R., Doraswamy, N., Glenn, R., "IP Security
Document Roadmap", draft-ietf-ipsec-doc-roadmap-02.txt, work in
progress, November, 1997.
8. Acknowledgments
Much of the information provided here originated with various ESP-DES
documents authored by Perry Metzger and William Allen Simpson,
including the data entry of the known weak key values, and especially
the Security Considerations section.
This document is also derived in part from previous works by Jim
Hughes, those people that worked with Jim on the combined DES-
CBC+HMAC-MD5 ESP transforms, the ANX bakeoff participants, and the
members of the IPsec working group.
Thanks also to Rob Glenn for assisting with the nroff formatting.
The IPSec working group can be contacted via the IPSec working
group's mailing list (ipsec@tis.com) or through its chairs:
Robert Moskowitz
<rgm@chrysler.com>
Chrysler Corporation
Theodore Y. Ts'o
<tytso@MIT.EDU>
Massachusetts Institute of Technology
9. Editors' Addresses
Cheryl Madson
<cmadson@cisco.com>
Cisco Systems, Inc.
Naganand Doraswamy
<naganand@baynetworks.com>
Bay Networks, Inc.
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