Network Working Group P Metzger
Internet Draft [Piermont]
W A Simpson
[DayDreamer]
expires in six months July 1997
The ESP DES-CBC Transform
draft-ietf-ipsec-ciph-des-derived-00.txt
Status of this Memo
Follows draft-simpson-esp-des1-v2-00.txt.
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Abstract
This document describes the DES-CBC block cipher transform interface
used with the IP Encapsulating Security Payload (ESP). It provides
compatible migration from RFC-1829.
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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 the Cipher Block
Chaining (CBC) mode of the US Data Encryption Standard (DES) algo-
rithm [FIPS-46, FIPS-46-1, FIPS-74, FIPS-81].
The level of privacy provided by use of ESP DES-CBC in the Internet
environment is far greater than sending the datagram as cleartext.
However, in view of the current analysis of DES, it is suggested that
DES is not a good encryption algorithm for the protection of even
moderate value information for any length of time.
For an explanation of the use of CBC mode with this cipher, see [RFC-
wwww].
For more explanation and implementation information for 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
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 at [Schneier95, page 279]. Your milage
may vary.
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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. 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-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 ESP Sequence
Number field followed by (concatenated with) the bit-wise complement
of the same 32-bit value:
SN || -SN
Alternative IV generation techniques MAY be specified when dynami-
cally configured via a key management protocol.
Security Notes:
Using the Sequence Number provides an easy method for preventing
IV repetition, and is sufficiently robust for practical use with
the DES algorithm. But, when used alone, cryptanalysis might be
aided by the rare serendipitous occurrence when the Sequence Num-
ber increments in exactly the same fashion as a corresponding bit
position in the first block.
No commonly used IP (Next Header) Protocols exhibit this property.
Never-the-less, inclusion of the bit-wise complement ensures that
Sequence Number bit changes are reflected twice in the IV.
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4. Keys
DES-CBC is a symmetric secret key algorithm. The secret DES key
shared between the communicating parties is 56-bits in length. The
56-bit key 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] (shown in hex with parity
bits):
0101 0101 0101 0101
1f1f 1f1f 0e0e 0e0e
e0e0 e0e0 f1f1 f1f1
fefe fefe fefe fefe
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
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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. Manual Key Management
When configured manually, 64-bits (8 bytes) are configured.
Keys with incorrect parity SHOULD be rejected by the configuration
utility, ensuring that the keys have been correctly configured.
The 64 known weak keys SHOULD be rejected.
4.3. Automated Key Management
When configured via a Security Association management protocol,
64-bits (8 bytes) are returned for the key.
The key manager MAY be required to generate the correct parity.
Alternatively, the least significant bit of each key byte is ignored,
or locally set to parity by the DES implementation.
The 64 known weak 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 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.
Also, some earlier implementations used pseudo-random values.
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
New implementations use verifiable values. However, some earlier
implementations used pseudo-random values. This check must only
be used with those peers that have implemented this feature.
Also, some operations desire additional padding to inhibit traffic
analysis.
Default: 0 (checking off). Range: 7 to 255.
Key
The 56-bit key is configured as a 64-bit quantity, with parity
included as appropriate.
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 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 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 block pairs, and [Matsui94] demonstrated a lin-
ear cryptanalysis based known-plaintext attack requiring only 2**43
plaintext-ciphertext block 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.
Changes from RFC-1829:
This specification results in the same default bits-on-the-wire as
the 32-bit IV calculation method of RFC-1829. The 32-bit field is
semantically identical to a Sequence Number when implemented as a
counter (the recommended method).
The 64-bit explicit IV option is deprecated, as no hardware manufac-
turers were found that required it. It does not meet 64-bit field
alignment expectations of IPv6, it is a cryptographically weaker con-
struct than a calculated IV [Bellovin96], and it conflicts with the
use of a Sequence Number immediately following the SPI.
Padding is a known series of integers, that may be checked upon
receipt.
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Many implementation details by Karn were found to be common to all
ESP Ciphers, and are awaiting consolidation in the ESP specification.
Added an operational section.
Updated acknowledgements, references, and contacts.
Reorganized according to the new "road map" document.
Acknowledgements
The basic field naming and layout is based on "swIPe" [IBK93, IB93].
Participants in the IP Security Working Group modified this to a
variable number of variable length fields. After a digression span-
ning 4 years, actual implementors mandated a return to these fewer
well-known fields.
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, the IV calculation technique(s), editing and formatting.
The use of known padding values was suggested in various forms by
Robert Baldwin, Phil Karn, and David Wagner. This specification uses
Self-Describing-Padding [RFC-1570].
Robert Baldwin, Steve Bellovin, Steve Deering, Karl Fox, Charles
Lynn, Cheryl Madson, Craig Metz, Dave Mihelcic, Jeffrey Schiller,
Norman Shulman and David Wagner provided useful critiques of earlier
versions of this document.
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References
[Bellovin96]
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 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.
[IB93] Ioannidis, J., and Blaze, M., "The Architecture and Imple-
mentation of Network-Layer Security Under Unix", Proceedings
of the Fourth Usenix Security Symposium, Santa Clara Cali-
fornia, 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:
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Springer-Verlag, 1994.
[RFC-1570]
Simpson, W., "PPP LCP Extensions", DayDreamer, January 1994.
[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", 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.
[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.
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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:
Perry Metzger
Piermont Information Systems Inc.
160 Cabrini Blvd., Suite #2
New York, NY 10033
perry@piermont.com
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
bsimpson@MorningStar.com
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