Internet Engineering Task Force R. Pereira
IP Security Working Group TimeStep Corporation
Internet Draft R. Adams
Expires in six months cisco Systems Inc.
September 30, 1997
The ESP CBC-Mode Cipher Algorithms
<draft-ietf-ipsec-ciph-cbc-00.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
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Distribution of this memo is unlimited.
Abstract
This document describes how to use CBC-mode cipher algorithms with
the IPSec ESP (Encapsulating Security Payload) Protocol. It not
only clearly states how to use certain cipher algorithms, but also
how to use all CBC-mode cipher algorithms.
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Table of Contents
1. Introduction...................................................2
1.1 Specification of Requirements...............................2
2. Cipher Algorithms..............................................3
2.1 Mode........................................................3
2.2 Key Size....................................................3
2.3 Weak Keys...................................................4
2.4 Block Size and Padding......................................5
2.5 Rounds......................................................6
2.6 Backgrounds.................................................6
2.7 Performance.................................................9
3. ESP Payload...................................................10
3.1 ESP Environmental Considerations...........................10
3.2 Keying Material............................................10
4. Security Considerations.......................................11
5. References....................................................11
6. Acknowledgments...............................................12
7. Editors' Addresses............................................13
8. Internet Draft Notes..........................................13
1. Introduction
The Encapsulating Security Payload (ESP) [Kent97] provides
confidentiality for IP datagrams by encrypting the payload data to
be protected. This specification describes the ESP use of CBC-mode
cipher algorithms.
While this document does no describe the use of the default cipher
algorithm DES, the reader should be familiar with that document.
[Madson97]
It is assumed that the reader is familiar with the terms and
concepts described in the "Security Architecture for the Internet
Protocol" [Atkinson95], "IP Security Document Roadmap" [Thayer97],
and "IP Encapsulating Security Payload (ESP)" [Kent97] documents.
Furthermore, this document is a companion to [Kent97] and MUST be
read in its context.
1.1 Specification of Requirements
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
NOT", and "MAY" that appear in this document are to be interpreted
as described in [Bradner97].
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2. Cipher Algorithms
All symmetric block cipher algorithms share common characteristics
and variables. These include mode, key size, weak keys, block
size, and rounds. All of which will be explained below.
While this document illustrates certain cipher algorithms such as
Blowfish [Schneier93], CAST-128 [Adams97], 3DES, IDEA [Lai], and
RC5 [Baldwin96], any other block cipher algorithm may be used with
ESP if all of the variables described within this document are
clearly defined.
2.1 Mode
All symmetric block cipher algorithms described or insinuated
within this document use Cipher Block Chaining (CBC) mode. This
mode requires an Initialization Vectors (IV) that is the same size
as the block size. The IV is used to scramble the initial block
that is the same as other initial block in other datagrams.
Without an IV, these initial blocks would all have the same
ciphertext, since they all use the same key.
The IV is XORd with the first plaintext block, before it is
encrypted. Then for successive blocks, the previous ciphertext
block is XORd with the current plaintext, before it is encrypted.
More information on CBC mode can be obtained in [Schneier95].
2.2 Key Size
Some cipher algorithms allow for variable sized keys, while other
only allow a specific key size. The length of the key correlates
with the strength of that algorithm, thus larger keys are always
harder to break than shorter ones.
This document stipulates that all key sizes MUST be a multiple of 8
bits.
+==============+==================+=================+==========+
| Algorithm | Key Sizes (bits) | Popular Sizes | Default |
+==============+==================+=================+==========+
| CAST-128 [1] | 40 to 128 | 40, 64, 80, 128 | 128 |
+--------------+------------------+-----------------+----------+
| RC5 | 40 to 2040 | 40, 128, 160 | 128 |
+--------------+------------------+-----------------+----------+
| IDEA | 128 | 128 | 128 |
+--------------+------------------+-----------------+----------+
| Blowfish | 40 to 448 | 128 | 128 |
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+--------------+------------------+-----------------+----------+
| 3DES [2] | 192 | 192 | 192 |
+--------------+------------------+-----------------+----------+
Notes:
[1] With CAST-128, keys less than 128 bits MUST be padded with
zeros in the rightmost, or least significant, positions out to 128
bits since the CAST-128 key schedule assumes an input key of 128
bits. Thus if you had a key with a size of 80 bits 3B5D831CFE,
it would be padded to produce a key with a size of 128 bits
3B5D831CFE000000.
[2] The first 3DES key is taken from the first 64 bits, the second
from the next 64 bits, and the third from the last 64 bits.
Implementations SHOULD take into consideration the parity bits when
initially accepting a new set of keys.
The reader should note that the minimum key size for all of the
above cipher algorithms is 40 bits, and that the authors strongly
advise that implementations do NOT use key sizes smaller than 40
bits.
2.3 Weak Keys
Some cipher algorithms have weak keys or keys that MUST not be used
due to their weak nature. [Kent97] describes what to do when such
a key size is generated.
CAST-128:
No known weak keys.
RC5:
No known weak keys when used with 16 rounds.
IDEA:
IDEA has weak keys of the following form [Crypto93]:
0000,0000,0x00,0000,0000,000x,xxxx,x000
where "x" can be any hexadecimal number.
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Keys of this form guarantee the value of bit-wise XOR of resultant
ciphertext pairs from the bit-wise XOR of certain plaintext pairs.
Blowfish:
Weak keys for Blowfish have been discovered. Weak keys are keys
that produce the identical entries in a given S-box.
Unfortunately, there is no way to test for weak keys before the S-
box values are generated. However, the chances of randomly
generating such a key are small.
3DES:
DES has 64 known weak keys, including so-called semi-weak keys and
possibly-weak keys [Schneier95, pp 280-282]. The likelihood of
picking one at random is negligible.
For DES-EDE3, there is no known need to reject weak or
complementation keys. Any weakness is obviated by the other keys.
However, if the first two or last two independent 64-bit keys are
equal (k1 == k2 or k2 == k3), then the 3DES operation is simply the
same as DES. Implementers MUST reject keys that exhibit this
property.
2.4 Block Size and Padding
All of the algorithms described in this document use a block size
of eight octets (64 bits).
Padding is used to align the payload type and pad length octets as
specified in [Kent97]. Padding must be sufficient to align the
data to be encrypted to an eight octet (64 bit) boundary.
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2.5 Rounds
This variable determines how many times a block is encrypted.
While this variable MAY be negotiated, a default value MUST always
exist when it is not negotiated.
+====================+============+======================+
| Algorithm | Negotiable | Default Rounds |
+====================+============+======================+
| CAST-128 | No | key<=80 bits, 12 |
| | | key>80 bits, 16 |
+--------------------+------------+----------------------+
| RC5 | No | 16 |
+--------------------+------------+----------------------+
| IDEA [1] | 4, 8 | 8 |
+--------------------+------------+----------------------+
| Blowfish | No | 16 |
+--------------------+------------+----------------------+
| 3DES | No | 48 (16x3) |
+--------------------+------------+----------------------+
Notes:
[1] Although there are no known attacks against four round IDEA,
those choosing to use four round IDEA for performance reasons, may
wish shorten key lifetimes via site specific policy.
2.6 Backgrounds
CAST-128:
The CAST design procedure was originally developed by Carlisle
Adams and Stafford Travares at Queens University, Kingston,
Ontario, Canada. Subsequent enhancements have been made over the
years by Carlisle Adams and Michael Wiener of Entrust Technologies.
CAST-128 is the result of applying the CAST Design Procedure as
outlined in [Adams97].
RC5:
The RC5 encryption algorithm was developed by Ron Rivest for RSA
Data Security Inc. in order to address the need for a high-
performance software and hardware ciphering alternative to DES.
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IDEA:
Xuejia Lai and James Massey developed the IDEA (International Data
Encryption Algorithm) algorithm. The algorithm is described in
detail in [Lai] and [Schneier].
The IDEA algorithm is patented in Europe and in the United States
with patent application pending in Japan. Licenses are required
for commercial uses of IDEA.
For patent and licensing information, contact:
Ascom Systec AG, Dept. CMVV
Gewerbepark, CH-5506
Magenwil, Switzerland
Phone: +41 64 56 59 83
Fax: +41 64 56 59 90
idea@ascom.ch
http://www.ascom.ch/Web/systec/policy/normal/exhibit1.html
Blowfish:
Bruce Schneier of Counterpane Systems developed the Blowfish block
cipher algorithm. The algorithm is described in detail in
[Schneier93], [Schneier95] and [Schneier].
3DES:
This DES variant, colloquially known as "Triple DES" or as DES-
EDE3, processes each block three times, each time with a different
key. This technique of using more than one DES operation was
proposed in [Tuchman79].
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P1 P2 Pi
| | |
IV->->(X) +>->->->(X) +>->->->(X)
v ^ v ^ v
+-----+ ^ +-----+ ^ +-----+
k1->| E | ^ k1->| E | ^ k1->| E |
+-----+ ^ +-----+ ^ +-----+
| ^ | ^ |
v ^ v ^ v
+-----+ ^ +-----+ ^ +-----+
k2->| D | ^ k2->| D | ^ k2->| D |
+-----+ ^ +-----+ ^ +-----+
| ^ | ^ |
v ^ v ^ v
+-----+ ^ +-----+ ^ +-----+
k3->| E | ^ k3->| E | ^ k3->| E |
+-----+ ^ +-----+ ^ +-----+
| ^ | ^ |
+>->->+ +>->->+ +>->->
| | |
C1 C2 Ci
The DES-EDE3-CBC algorithm is a simple variant of the DES-CBC
algorithm [FIPS-46]. The "outer" chaining technique is used.
In DES-EDE3-CBC, an Initialization Vector (IV) is XOR'd with the
first 64-bit (8 byte) plaintext block (P1). The keyed DES function
is iterated three times, an encryption (Ek1) followed by a
decryption (Dk2) followed by an encryption (Ek3), and generates the
ciphertext (C1) for the block. Each iteration uses an independent
key: k1, k2 and k3.
For successive blocks, the previous ciphertext block is XOR'd with
the current plaintext (Pi). The keyed DES-EDE3 encryption function
generates the ciphertext (Ci) for that block.
To decrypt, the order of the functions is reversed: decrypt with
k3, encrypt with k2, decrypt with k1, and XOR the previous
ciphertext block.
Note that when all three keys (k1, 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
modification.
For more explanation and implementation information for Triple DES,
see [Schneier95].
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2.7 Performance
For a comparison table of the speed of any of these and other
cipher algorithms, please see [Schneier97].
CAST-128:
CAST runs approximately 3 times faster than a highly optimized DES
implementation and runs 5-6 times faster than the DES
implementations found in typical applications. This is based on a
non optimized C++ implementation of CAST-128. It can therefore be
tuned to give even higher performance, if this is required.
The following performance tests were run on a Pentium 90 MHz
running the Windows NT operating system using 20 Kbyte buffers and
do not include file I/O. The DES-CBC implementation was not
optimized for a 32 bit environment.
CAST-128 64 bit key CBC encryption ............ 2,640,000 bytes/sec
DES CBC encryption .............................. 504,000 bytes/sec
RC5:
Benchmark numbers from RSA Data Security suggest that RC5-CBC runs
about twice as fast as Eric Young's DES-CBC implementation from
SSLeay on the popular 32-bit CPUs.
IDEA:
Normal eight round IDEA is approximately twice as fast as DES on
386 and 486 processors. However on a Pentium, both eight round
IDEA and 56 bit key, 16 round DES require about the same number of
clock cycles per byte encrypted. Four round IDEA is twice as fast
as eight round IDEA.
Blowfish:
Blowfish is designed to encrypt data very efficiently on 32 bit
processors. Although setting up the keys for Blowfish is complex
and time consuming, actual encryption is efficient. Sixteen round
Blowfish uses only 18 clock cycles per byte encrypted on a Pentium
versus 45 clock cycles for 16 round DES with a 56 bit key, and 108
for 48 round Triple-DES.
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3DES:
Triple DES is approximately 2.5 times slower than "single" DES
(rather than 3 times), because inner permutations may be removed.
Phil Karn has tuned DES-EDE3-CBC software to achieve 6.22 Mbps with
a 133 MHz Pentium. Other DES speed estimates may be found at
[Schneier, page 279].
3. ESP Payload
The ESP payload is made up of the IV followed by raw cipher-text.
Thus the payload field, as defined in [Kent97], is broken down
according to the following diagram:
+---------------+---------------+---------------+---------------+
| |
+ Initialization Vector (8 octets) +
| |
+---------------+---------------+---------------+---------------+
| |
~ 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
The IV field MUST be same size as the block size of the cipher
algorithm being used. The IV SHOULD be chosen at random. Common
practice is to use random data for the first IV and the last block
of encrypted data from an encryption process as the IV for the next
encryption process.
3.1 ESP Environmental Considerations
Currently, there are no known issues regarding interactions between
these algorithms and other aspects of ESP, such as use of certain
authentication schemes.
3.2 Keying Material
The minimum number of bits sent from the key exchange protocol to
this ESP algorithm must be greater or equal to the key size.
The ciphers encryption and decryption key is taken from the first
<x> bits of the keying material, where <x> represents the required
key size.
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4. Security Considerations
Care should be taken when using small key sizes. Smaller key sizes
of 56 bits and below make brute force type attacks practical
regardless of the cipher algorithm used. It is therefore
recommended for variable sized keyed algorithms, that the key size
be at least 80 bits. Use of key sizes less than 80 bits is
permitted, but careful considerations should be taken before its
use.
For further security considerations, the reader is encouraged to
read the documents that describe the actual cipher algorithms.
5. References
[Adams97] Adams, C., "The CAST-128 Encryption Algorithm
, RFC2144,
1997.
[Atkinson95] Atkinson, R., "Security Architecture for the Internet
Protocol", draft-ietf-ipsec-arch-sec-01
[Baldwin96] Baldwin, R.W., Rivest, R., "The RC5, RC5-CBC, RC5-CBC-
Pad, and RC5-CTS Algorithms", RFC2040, October 1996
[Bradner97] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", RFC2119, March 1997
[Crypto93] Daeman, J., Govaerts, R., Vandewalle, J., "Weak Keys for
IDEA", Advances in Cryptology, CRYPTO 93 Proceedings, Springer-
Verlag, pp. 224-230.
[FIPS-46] US National Bureau of Standards, "Data Encryption
Standard", Federal Information Processing Standard (FIPS)
Publication 46, January 1977.
[Kent97] Kent, S., Atkinson, R., "IP Encapsulating Security Payload
(ESP)", draft-ietf-ipsec-esp-v2-00
[Lai] Lai, X. "On the Design and Security of Block Ciphers", ETH
Series in Information Processing, v. 1, Konstanz: Hartung-Gorre
Verlag, 1992.
[Madson97] Madson, C., Dorswamy, N., "The ESP DES-CBC Cipher
Algorithm With Explicit IV", draft-ietf-ipsec-ciph-des-expiv-00
[Schneier] Schneier, B., "Applied Cryptography Second Edition",
John Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7
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[Schneier93] Schneier, B., "Description of a New Variable-Length
Key, 64-Bit Block Cipher", from "Fast Software Encryption,
Cambridge Security Workshop Proceedings", Springer-Verlag, 1994,
pp. 191-204. http://www.counterpane.com/bfsverlag.html
[Schneier95] Schneier, B., "The Blowfish Encryption Algorithm - One
Year Later", Dr. Dobb's Journal, September 1995,
http://www.counterpane.com/bfdobsoyl.html
[Schneier97] Scheier, B. "Speed Comparisons of Block Ciphers on a
Pentium." February 1997, http://www.counterpane.com/speed.html
[Thayer97] R. Thayer, N. Doraswamy, R. Glenn, "IP Security Document
Roadmap", draft-ietf-ipsec-doc-roadmap-00
[Tuchman79] Tuchman, W, "Hellman Presents No Shortcut Solutions to
DES", IEEE Spectrum, v. 16 n. 7, July 1979, pp. 40-41.
6. Acknowledgments
This document is a merger of most of the ESP cipher algorithm
documents. This merger was done to facilitate greater
understanding of the commonality of all of the ESP algorithms and
to further the development of these algorithm within ESP.
The content of this document is based on suggestions originally
from Stephen Kent and subsequent discussions from the IPSec mailing
list as well as other IPSec drafts.
For CAST, special thanks to Carlisle Adams and Paul Van Oorschot
both of Entrust Technologies who provided input and review.
For 3DES, thanks to all of the editors of the previous ESP 3DES
documents; W. Simpson, N. Doraswamy, P. Metzger, and P. Karn.
For RC5, thanks to Brett Howard from TimeStep for his original
work.
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7. Editors' Addresses
Roy Pereira
<rpereira@timestep.com>
TimeStep Corporation
+1 (613) 599-3610 x 4808
Rob Adams
<adams@cisco.com>
cisco Systems Inc.
+1 (408) 457 5397
Contributors:
Robert W. Baldwin
<baldwin@rsa.com> or <baldwin@lcs.mit.edu>
RSA Data Security, Inc.
+1 (415) 595-8782
Greg Carter
<carterg@entrust.com>
Entrust Technologies
+1 (613) 763-1358
Rodney Thayer
rodney@sabletech.com
Sable Technology Corporation
+1 (617) 332-7292
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. Tso
tytso@MIT.EDU
Massachusetts Institute of Technology
8. Internet Draft Notes
This document obsoletes the following documents:
draft-ietf-ipsec-ciph-cast-128cbc-00.txt, R. Pereira, G. Carter
draft-ietf-ipsec-ciph-rc5-cbc-00.txt, R. Pereira, R. Baldwin
draft-ietf-ipsec-ciph-3des-expiv-00.txt, R. Pereira, R. Thayer
draft-ietf-ipsec-ciph-idea-cbc-00.txt, R. Adams
draft-ietf-ipsec-ciph-blowfish-cbc-00.txt, R. Adams
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The key size for IDEA was restricted for "ease of use" purposes.
Furthermore, the use of setting the sub-keys directly was removed,
again for ease of use.
IDEAs weak key derivation was removed as it is the responsibility
of the ESP document to describe actions when there is a weak key.
DES-CBC could be part of this document with very little effort.
Should it be?
R. Pereira, R. Adams [Page 14]
