RFC1829 - The ESP DES-CBC Transform

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　　Network Working Group P. Karn
Request for Comments: 1829 Qualcomm
Category: Standards Track P. Metzger
Piermont
W. Simpson
Daydreamer
August 1995

The ESP DES-CBC Transform

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document describes the DES-CBC security transform for the IP
Encapsulating Security Payload (ESP).

Table of Contents

1. Introduction .......................................... 1
1.1 Keys ............................................ 1
1.2 Initialization Vector ........................... 1
1.3 Data Size ....................................... 2
1.4 Performance ..................................... 2

2. Payload Format ........................................ 3

3. Algorithm ............................................. 5
3.1 Encryption ...................................... 5
3.2 Decryption ...................................... 5

SECURITY CONSIDERATIONS ...................................... 6
ACKNOWLEDGEMENTS ............................................. 7
REFERENCES ................................................... 8
AUTHOR'S ADDRESS ............................................. 10

1. Introduction

The Encapsulating Security Payload (ESP) [RFC-1827] provides
confidentiality for IP datagrams by encrypting the payload data to be
protected. This specification describes the ESP use of the Cipher
Block Chaining (CBC) mode of the US Data Encryption Standard (DES)
algorithm [FIPS-46, FIPS-46-1, FIPS-74, FIPS-81].

All implementations that claim conformance or compliance with the
Encapsulating Security Payload specification MUST implement this
DES-CBC transform.

This document assumes that the reader is familiar with the related
document "Security Architecture for the Internet Protocol"
[RFC-1825], which defines the overall security plan for IP, and
provides important background for this specification.

1.1. Keys

The secret DES key shared between the communicating parties is eight
octets in length. This key consists of a 56-bit quantity used by the
DES algorithm. The 56-bit key is stored as a 64-bit (eight octet)
quantity, with the least significant bit of each octet used as a
parity bit.

1.2. Initialization Vector

This mode of DES requires an Initialization Vector (IV) that is eight
octets in length.

Each datagram contains its own IV. Including the IV in each datagram
ensures that decryption of each received datagram can be performed,
even when other datagrams are dropped, or datagrams are re-ordered in
transit.

The method for selection of IV values is implementation dependent.

Notes:
A common acceptable technique is simply a counter, beginning with
a randomly chosen value. While this provides an easy method for
preventing repetition, and is sufficiently robust for practical
use, cryptanalysis may use the rare serendipitous occurrence when
a corresponding bit position in the first DES block increments in
exactly the same fashion.

Other implementations exhibit unpredictability, usually through a
pseudo-random number generator. Care should be taken that the
periodicity of the number generator is long enough to prevent
repetition during the lifetime of the session key.

1.3. Data Size

The DES algorithm operates on blocks of eight octets. This often
requires padding after the end of the unencrypted payload data.

Both input and output result in the same number of octets, which
facilitates in-place encryption and decryption.

On receipt, if the length of the data to be decrypted is not an
integral multiple of eight octets, then an error is indicated, as
described in [RFC-1825].

1.4. Performance

At the time of writing, at least one hardware implementation can
encrypt or decrypt at about 1 Gbps [Schneier94, p. 231].

2. Payload Format

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initialization Vector (IV) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Payload Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | Pad Length | Payload Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Security Parameters Index (SPI)

A 32-bit value identifying the Security Parameters for this
datagram. The value MUST NOT be zero.

Initialization Vector (IV)

The size of this field is variable, although it is constant for
all DES-CBC datagrams of the same SPI and IP Destination. Octets
are sent in network order (most significant octet first)
[RFC-1700].

The size MUST be a multiple of 32-bits. Sizes of 32 and 64 bits
are required to be supported. The use of other sizes is beyond
the scope of this specification. The size is expected to be
indicated by the key management mechanism.

When the size is 32-bits, a 64-bit IV is formed from the 32-bit
value followed by (concatenated with) the bit-wise complement of
the 32-bit value. This field size is most common, as it aligns
the Payload Data for both 32-bit and 64-bit processing.

All conformant implementations MUST also correctly process a
64-bit field size. This provides strict compatibility with
existing hardware implementations.

It is the intent that the value not repeat during the lifetime
of the encryption session key. Even when a full 64-bit IV is
used, the session key SHOULD be changed at least as frequently
as 2**32 datagrams.

Payload Data

The size of this field is variable.

Prior to encryption and after decryption, this field begins with
the IP Protocol/Payload header specified in the Payload Type
field. Note that in the case of IP-in-IP encapsulation (Payload
Type 4), this will be another IP header.

Padding

The size of this field is variable.

Prior to encryption, it is filled with unspecified implementation
dependent (preferably random) values, to align the Pad Length and
Payload Type fields at an eight octet boundary.

After decryption, it MUST be ignored.

Pad Length

This field indicates the size of the Padding field. It does not
include the Pad Length and Payload Type fields. The value
typically ranges from 0 to 7, but may be up to 255 to permit
hiding of the actual data length.

This field is opaque. That is, the value is set prior to
encryption, and is examined only after decryption.

Payload Type

This field indicates the contents of the Payload Data field, using
the IP Protocol/Payload value. Up-to-date values of the IP
Protocol/Payload are specified in the most recent "Assigned
Numbers" [RFC-1700].

This field is opaque. That is, the value is set prior to
encryption, and is examined only after decryption.

For example, when encrypting an entire IP datagram (Tunnel-
Mode), this field will contain the value 4, which indicates
IP-in-IP encapsulation.

3. Algorithm

In DES-CBC, the base DES encryption function is applied to the XOR of
each plaintext block with the previous ciphertext block to yield the
ciphertext for the current block. This provides for
re-synchronization when datagrams are lost.

For more explanation and implementation information for DES, see
[Schneier94].

3.1. Encryption

Append zero or more octets of (preferably random) padding to the
plaintext, to make its modulo 8 length equal to 6. For example, if
the plaintext length is 41, 5 octets of padding are added.

Append a Pad Length octet containing the number of padding octets
just added.

Append a Payload Type octet containing the IP Protocol/Payload value
which identifies the protocol header that begins the payload.

Provide an Initialization Vector (IV) of the size indicated by the
SPI.

Encrypt the payload with DES in CBC mode, producing a ciphertext of
the same length.

Octets are mapped to DES blocks in network order (most significant
octet first) [RFC-1700]. Octet 0 (modulo 8) of the payload
corresponds to bits 1-8 of the 64-bit DES input block, while octet 7
(modulo 8) corresponds to bits 57-64 of the DES input block.

Construct an appropriate IP datagram for the target Destination, with
the indicated SPI, IV, and payload.

The Total/Payload Length in the encapsulating IP Header reflects the
length of the encrypted data, plus the SPI, IV, padding, Pad Length,
and Payload Type octets.

3.2. Decryption

First, the SPI field is removed and examined. This is used as an
index into the local Security Parameter table to find the negotiated

parameters and decryption key.

The negotiated form of the IV determines the size of the IV field.
These octets are removed, and an appropriate 64-bit IV value is
constructed.

The encrypted part of the payload is decrypted using DES in the CBC
mode.

The Payload Type is removed and examined. If it is unrecognized, the
payload is discarded with an appropriate ICMP message.

The Pad Length is removed and examined. The specified number of pad
octets are removed from the end of the decrypted payload, and the IP
Total/Payload Length is adjusted accordingly.

The IP Header(s) and the remaining portion of the decrypted payload
are passed to the protocol receive routine specified by the Payload
Type field.

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 key management mechanism and its implementation, the
strength of the key [CN94], and upon the correctness of the
implementations in all of the participating nodes.

Among other considerations, applications may wish to take care not to
select weak keys, although the odds of picking one at random are low
[Schneier94, p 233].

The cut and paste attack described by [Bell95] exploits the nature of
all Cipher Block Chaining algorithms. When a block is damaged in
transmission, on decryption both it and the following block will be
garbled by the decryption process, but all subsequent blocks will be
decrypted correctly. If an attacker has legitimate access to the
same key, this feature can be used to insert or replay previously
encrypted data of other users of the same engine, revealing the
plaintext. The usual (ICMP, TCP, UDP) transport checksum can detect
this attack, but on its own is not considered cryptographically
strong. In this situation, user or connection oriented integrity
checking is needed [RFC-1826].

At the time of writing of this document, [BS93] demonstrated a

differential cryptanalysis based chosen-plaintext attack requiring
2^47 plaintext-ciphertext pairs, and [Matsui94] demonstrated a linear
cryptanalysis based known-plaintext attack requiring only 2^43
plaintext-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.

Acknowledgements

This document was reviewed by the IP Security Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the ipsec@ans.net mailing list.

Some of the text of this specification was derived from work by
Randall Atkinson for the SIP, SIPP, and IPv6 Working Groups.

The use of DES for confidentiality is closely modeled on the work
done for SNMPv2 [RFC-1446].

Steve Bellovin, Steve Deering, Karl Fox, Charles Lynn, Craig Metz,
Dave Mihelcic and Jeffrey Schiller provided useful critiques of
earlier versions of this draft.

References

[Bell95] Bellovin, S., "An Issue With DES-CBC When Used Without
Strong Integrity", Proceedings of the 32nd IETF, Danvers,
MA, April 1995.

[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 dor DES Cipher,"
Advances in Cryptology -- Eurocrypt '93 Proceedings, Berlin:
Springer-Verlag, 1994.

[RFC-1446]
Galvin, J., and McCloghrie, K., "Security Protocols for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC-1446, DDN Network Information Center, April
1993.

[RFC-1700]
Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,

RFC-1700, USC/Information Sciences Institute, October 1994.

[RFC-1800]
Postel, J., "Internet Official Protocol Standards", STD 1,
RFC-1800, USC/Information Sciences Institute, July 1995.

[RFC-1825]
Atkinson, R., "Security Architecture for the Internet
Protocol", RFC-1825, Naval Research Laboratory, July 1995.

[RFC-1826]
Atkinson, R., "IP Authentication Header", RFC-1826, Naval
Research Laboratory, July 1995.

[RFC-1827]
Atkinson, R., "IP Encapsulating Security Protocol (ESP)",
RFC-1827, Naval Research Laboratory, July 1995.

[Schneier94]
Schneier, B., "Applied Cryptography", John Wiley & Sons, New
York, NY, 1994. ISBN 0-471-59756-2

[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.

Author's Address

Questions about this memo can also be directed to:

Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779

karn@unix.ka9q.ampr.org

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

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