One document matched: draft-ietf-ipsec-ciph-3des-expiv-00.txt
The ESP 3DES-CBC Algorithm Using an Explicit IV
<draft-ietf-ipsec-ciph-3des-expiv-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
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Abstract
This document describes the "Triple" DES-EDE3-CBC block cipher
algorithm used with the IP Encapsulating Security Payload (ESP).
Use of an explicit IV is described.
R. Pereira, R. Thayer [Page 1]
Internet Draft draft-ietf-ipsec-ciph-3des-expiv-00.doc July 17, 1997
Table of Contents
1. Introduction...................................................2
1.1 Specification of Requirements...............................2
2. Cipher Algorithm...............................................2
2.1 Mode........................................................3
2.2 Performance.................................................4
3. Key Sizes......................................................4
3.1 Weak Keys...................................................4
4. ESP Payload....................................................4
4.1 Block Size and Padding......................................5
4.2 Interaction with Authentication Algorithms..................5
5. Keying Material................................................5
6. Security Considerations........................................5
7. References.....................................................6
8. Acknowledgments................................................6
9. Editors' Addresses.............................................7
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 3DES.
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].
2. Cipher Algorithm
This is a variant of the Cipher Block Chaining (CBC) mode of the US
Data Encryption Standard (DES) algorithm [FIPS-46].
This variant, colloquially known as "Triple DES", 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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Internet Draft draft-ietf-ipsec-ciph-3des-expiv-00.doc July 17, 1997
For more explanation and implementation information for Triple DES,
see [Schneier95].
2.1 Mode
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 [RFC-1829x]. 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 independant
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-
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EDE3 hardware implementations to operate in DES mode without
modification.
2.2 Performance
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
[Schneier95, page 279].
3. Key Sizes
The secret DES-EDE3 key shared between the communicating parties is
effectively 168-bits long. This key consists of three independent
56-bit quantities used by the DES algorithm. Each of the three 56-
bit sub-keys is stored as a 64-bit (8 byte) quantity, with the
least significant bit of each byte used as a parity bit.
Implementations of this transform SHOULD take into consideration
the parity bits when initially accepting a new set of keys.
3.1 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
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 independent 64-bit keys are equal (k1 ==
k2), then the 3DES operation is simply the same as DES.
Implementers MUST reject keys that exhibit this property.
4. ESP Payload
DES-EDE3-CBC requires an explicit Initialization Vector (IV) of 8
octets (64 bits). Thus the payload is made up of the 8 octet IV
followed by raw cipher-text. The IV SHOULD be chosen at random.
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.
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Internet Draft draft-ietf-ipsec-ciph-3des-expiv-00.doc July 17, 1997
The payload field, as defined in [Kent97], 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
4.1 Block Size and Padding
The ESP 3DES-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 [Kent97].
4.2 Interaction with Authentication Algorithms
This ESP 3DES-CBC document has no limitations on what
authentication algorithm is used in ESP.
5. Keying Material
The number of bits sent from the key exchange protocol to this ESP
algorithm must be equal to the key size.
The key is taken from the first 192 bits of the keying material,
where the first 64 bits represent the first key, the next 64 bits
represent the second key and the last 64 bits represent the third
key.
6. Security Considerations
As with other ESP Transforms there are common security
considerations, which are not discussed here. The ESP document and
the IPsec architecture document should be consulted. Also, as with
any other encryption technology, one should examine the current
literature for any new attack strategies discovered after this
document was published.
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Internet Draft draft-ietf-ipsec-ciph-3des-expiv-00.doc July 17, 1997
A discussion of security considerations specific to DES is also
relevant to this cipher, see [RFC-1829x] for this discussion.
Since Triple DES uses three times as much keying material as DES,
it places a larger burden on automatic keying systems that use such
devices as random number generators and entropy pools. Use of
automatic keying should be carefully configured to be aware of this
impact.
7. References
[Atkinson95] Atkinson, R., "Security Architecture for the Internet
Protocol", draft-ietf-ipsec-arch-sec-01
[Bradner97] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", RFC2119, March 1997
[Kent97] Kent, S., Atkinson, R., "IP Encapsulating Security Payload
(ESP)", draft-ietf-ipsec-new-esp-01
[Thayer97] R. Thayer, N. Doraswamy, R. Glenn, "IP Security Document
Roadmap", draft-ietf-ipsec-doc-roadmap-00.txt
[FIPS-46] US National Bureau of Standards, "Data Encryption
Standard", Federal Information Processing Standard (FIPS)
Publication 46, January 1977.
[RFC-1829x] Karn, P., Metzger, P., Simpson, W.A., "The ESP DES-CBC
Transform", work in progress.
[Schneier95] Schneier, B., "Applied Cryptography Second Edition",
John Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7.
[Tuchman79] Tuchman, W, "Hellman Presents No Shortcut Solutions to
DES", IEEE Spectrum, v. 16 n. 7, July 1979, pp. 40-41.
8. Acknowledgments
This document is based on the IETF work described in <draft-ietf-
ipsec-ciph-3des-00.txt> with he only major differences being an
explicit IV instead of a derived one and that padding is done as
the default method states in ESP.
Our thanks to all of the editors of the previous ESP 3DES
documents; W. Simpson, N. Doraswamy, P. Metzger, and P. Karn.
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9. Editors' Addresses
Roy Pereira
rpereira@timestep.com
TimeStep Corporation
(613) 599-3610 x 4808
Rodney Thayer
rodney@sabletech.com
Sable Technology Corporation
(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. Ts'o
tytso@MIT.EDU
Massachusetts Institute of Technology
R. Pereira, R. Thayer [Page 7]
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