One document matched: draft-ietf-ipsec-ciph-desx-00.txt
Network Working Group W A Simpson
Internet Draft [DayDreamer]
R Baldwin
[RSA Data Security]
expires in six months July 1997
The ESP DES-XEX3-CBC Transform
draft-ietf-ipsec-ciph-desx-00.txt
Status of this Memo
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Abstract
This document describes the "DESX" DES-XEX3-CBC block cipher trans-
form interface used with the IP Encapsulating Security Payload (ESP).
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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, also known as "DESX", processes each block three times,
each time with a different key [Kaliski96]. The first and last pass
are a simple and fast XOR. This was originally proposed by Ron
Rivest in May of 1984 as a computationally cheap mechanism to protect
DES against exhaustive key-search attacks.
Although XOR of a constant value over multiple blocks would not nor-
mally be considered cryptographically secure, the use of DES-CBC in
the middle provides a background of highly random internal chaining.
The XOR values are combined with these random blocks to provide a
modest improvement in strength.
For an explanation of the use of CBC mode with this cipher, see [RFC-
wwww].
For more explanation and implementation information for DESX, 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
The DESX algorithm has been previously described in [Kaliski96,
Schneier95]. This algorithm is not protected by either patent or
trade secret laws, though the DESX name is a trademark of RSA Data
Security, a wholly owned subsidary of Security Dynamics Inc. Trade-
mark fair-use laws allow vendors to label a product as being compati-
ble with DESX. An implementation of DESX is available in RSA's BSAFE
cryptography toolkit and interoperable implementations have been cre-
ated outside of the United States.
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1.2. Performance
The additional computational cost beyond DES is negligible.
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
P1 P2 Pi
| | |
IV->->(X) +>->->->(X) +>->->->(X)
| ^ | ^ |
v ^ v ^ v
k1->->(X) ^ k1->->(X) ^ k1->->(X)
| ^ | ^ |
v ^ v ^ v
+-----+ ^ +-----+ ^ +-----+
k2->| E | ^ k2->| E | ^ k2->| E |
+-----+ ^ +-----+ ^ +-----+
| ^ | ^ |
v ^ v ^ v
k3->->(X) ^ k3->->(X) ^ k3->->(X)
| ^ | ^ |
+>->->+ +>->->+ +>->->
| | |
C1 C2 Ci
The DES-XEX3-CBC algorithm is a simple variant of the DES-CBC algo-
rithm [RFC-wwww, RFC-1829x].
In DES-XEX3-CBC, an Initialization Vector (IV) is XOR'd with the
first 64-bit (8 byte) plaintext block (P1), and with a block-sized
key (Xk1). A keyed DES encryption (Ek2) is followed by another XOR
(Xk3), and generates the ciphertext (C1) for the block. Each itera-
tion uses an independant key: k1, k2 and k3.
For successive blocks, the previous ciphertext block is XOR'd with
the current plaintext (Pi). The keyed DES-XEX3 encryption function
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generates the ciphertext (Ci) for that block.
To decrypt, the order of the functions is reversed: XOR with k3,
decrypt with k2, XOR with k1, and XOR the previous ciphertext block.
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-XEX3-CBC requires an Initialization Vector (IV) that is 64-bits
(8 bytes) in length [RFC-wwww].
By default, the 64-bit IV is generated from the 32-bit SPI field fol-
lowed by (concatenated with) the 32-bit Sequence Number field. Then,
the bit-wise complement of the 32-bit Sequence Number value is XOR'd
with the first 32-bits (SPI).
(SPI ^ -SN) || SN
Alternative IV generation techniques MAY be specified when dynami-
cally configured via a key management protocol.
Security Notes:
In a dynamic environment, the same data stream might be sent with
more than one key. Including the changed SPI in the IV generation
prevents analysis based on common leading blocks.
Using the Sequence Number provides an easy method for preventing
IV repetition, and is sufficiently robust for practical use with
the DES algorithm. Inclusion of the bit-wise complement ensures
that Sequence Number bit changes are reflected twice in the IV.
4. Keys
The secret DES-XEX3 key shared between the communicating parties is
effectively 184-bits long. This key consists of three independent
quantities: a 64-bit sub-key used by an XOR, a 56-bit sub-key used by
the DES algorithm, and another 64-bit sub-key used by an XOR. The
middle 56-bit sub-key is stored as a 64-bit (8 byte) quantity, with
the least significant bit of each byte used as a parity bit.
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4.1. Weak Keys
DES has 64 known weak keys, including so-called semi-weak keys and
possibly-weak keys [Schneier95, pp 280-282]. Implementations SHOULD
take care not to select weak keys [CN94], although the likelihood of
picking one at random is negligible.
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 [RFC-1829x] SHOULD be rejected.
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
the DES 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 [RFC-1829x] (for the DES key) 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 key. Depending on the average size of the datagrams, the
key SHOULD be changed at least as frequently as 2**30 datagrams.
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5. ESP Alterations
5.1. ESP Sequence Number
The Sequence Number is a 32-bit (4 byte) unsigned counter. This
field protects against replay attacks, and may also be used for syn-
chronization by stream or block-chaining ciphers.
When configured manually, the first value sent SHOULD be a random
number. The limited anti-replay security of the sequence of data-
grams depends upon the unpredictability of the values.
When configured via an automated Security Association management pro-
tocol, the first value sent is 1, unless otherwise negotiated.
Thereafter, the value is monotonically increased for each datagram
sent. A replacement SPI SHOULD be established before the value
repeats. That is, no more than 2**32 datagrams SHOULD be sent with
any single key.
5.2. ESP Padding
The Padding field may be zero or more bytes in length.
Prior to encryption, this field is filled with a series of integer
values to align the Pad Length and Payload Type fields at the end of
a 64-bit (8 byte) block boundary (measured from the beginning of the
Transform Data).
By default, each byte contains the index of the byte. For example,
three pad bytes would contain the values 1, 2, 3.
After decryption, this field MAY be examined for a valid series of
integer values. Verification of the sequence of values is at the
discretion of the receiver.
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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)
Manually configured LifeTimes are generally measured in days.
Automated LifeTimes are specified in seconds.
Default: 32 days (2,764,800 seconds). Maximum: 182 days
(15,724,800 seconds).
Replay Window
Long term replay prevention requires automated configuration.
This check must only be used with those peers that have imple-
mented this feature.
Default: 0 (checking off). Range: 32 to 256.
Pad Values
All implementations use verifiable values.
Also, some operations desire additional padding to inhibit traffic
analysis.
Default: 7 (checking on). Range: 7 to 255.
Key
A 64-bit key, a 56-bit key with parity included as appropriate,
and another 64-bit key, are configured in order as a 192-bit quan-
tity.
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.
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Security Considerations
Users need to understand that the quality of the security provided by
this specification depends completely on the strength of the DESX
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 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.
It has been shown that DES-XEX3 is substantially stronger than DES
alone, as it is less amenable to brute force attack with an exhaus-
tive key search. When the number of plaintext blocks are limited to
2**32 as recommended, the time complexity of the idealized random
permutation block cipher model is increased from an order 2**86 (for
DES) to 2**134 (for DES-EXE3) [Kilian96, Rogaway96].
It should be noted that real cryptanalysis of DES-XEX3 might not use
brute force methods at all. Instead, it might be performed using
variants on differential [BS93] or linear [Matsui94] cryptanalysis.
It has been estimated that differential cryptanalysis is increased
from 2**47 (for DES) to 2**61 chosen-plaintext blocks, and linear
cryptanalysis is increased from 2**43 (for DES) to 2**60 known-
plaintext blocks [Kaliski96]. Although these attacks are not consid-
ered practical, this offers only a small improvement over DES alone.
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-XEX3 prove weak.
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Acknowledgements
Most of the text of this specification was derived from earlier work
by Perry Metzger and William Allen Simpson in multiple Request for
Comments.
Use of DES-XEX3 was proposed by William Allen Simpson and various
other participants in the IETF IP Security Working Group in 1995 and
1996, but was prevented from publication through disregard of the
IETF Standards Process.
References
[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 Implement-
ing and Using the Data Encryption Standard", Federal Infor-
mation 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.
[Kaliski96]
Kaliski, B., and Robshaw, M., "Multiple Encryption: Weighing
Security and Performance", Dr. Dobbs Journal, January 1996.
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[Kilian96]
Kilian J., and Rogaway, P., "How to protect DES against
exhaustive key search", Advances in Cryptology -- Crypto '96
Proceedings, Berlin: Springer-Verlag, 1996,
http://wwwcsif.cs.ucdavis.edu/~rogaway.
[Matsui94]
Matsui, M., "Linear Cryptanalysis method for DES Cipher,"
Advances in Cryptology -- Eurocrypt '93 Proceedings, Berlin:
Springer-Verlag, 1994.
[Rogaway96]
Rogaway, P., "The Security of DESX", CryptoBytes, v 2 n 2,
RSA Laboratories, Redwood City, CA, USA, Summer 1996.
[RFC-1825x]
Atkinson, R., "Security Architecture for the Internet Proto-
col", Naval Research Laboratory, July 1995.
[RFC-1827x]
Simpson, W., "IP Encapsulating Security Protocol (ESP) for
implementors",
[RFC-1829x]
Karn, P., Metzger, P., Simpson, W.A., "The ESP DES-CBC
Transform", work in progress.
[RFC-2119]
Bradner, S., "Key words for use in RFCs to Indicate Require-
ment 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.
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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)
bsimpson@MorningStar.com
Robert Baldwin
RSA Data Security Inc.
100 Marine Parkway
Redwood City, California 94065
baldwin@rsa.com
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