< draft-hoffman-des40-00.txt		draft-hoffman-des40-01.txt >
	Internet Draft Paul Hoffman		Internet Draft Paul Hoffman
	draft-hoffman-des40-00.txt Internet Mail Consortium		draft-hoffman-des40-01.txt Internet Mail Consortium
	Russ Housley		Russ Housley
	SPYRUS		SPYRUS
	May 14, 1996 Expires six months later		April 20, 1998 Expires six months later
	Creating 40-Bit Keys for DES		Creating 40-Bit Keys for DES
	Status of this memo		Status of this memo
	This document is an Internet-Draft. Internet-Drafts are working		This document is an Internet-Draft. Internet-Drafts are working
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	1. Introduction		1. Introduction
	This document describes an method for shortening DES keys from 56 bits		This document describes an method for shortening DES keys from 56 bits
	to 40 bits. The shortened keys are generally known as "DES-40". The		to 40 bits. The shortened keys are generally known as "DES-40". The
	motivation for this weakening is that some localities (such as the		motivation for this weakening is that some localities (such as the
	United States) give special preference to applications that use 40-bit		United States) give special preference to applications that use 40-bit
	keys. The weakened keys are then used with the DES encryption		keys. The weakened keys are then used with the DES encryption
	algorithm in the same manner as full-strength keys.		algorithm in the same manner as full-strength keys.
	There are many possible methods for reducing a 56-bit key to a 40-bit		There are many possible methods for reducing a 56-bit key to a 40-bit
	key. The method in this draft was chosen because one method is needed		key. The method in this draft was chosen because one method is needed
	for interoperability. Further, this method has been known to		for interoperability. Further, this method has been known to
	occaisionally have been approved for export from the United States.		occasionally have been approved for export from the United States.
	1.1 Discussion of this Draft
	This draft is being discussed on the "ietf-smime" mailing list. To
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	2. Creating 40-Bit Keys for DES		2. Creating 40-Bit Keys for DES
	DES [DES] uses a 56-bit key. The key consists of eight 8-bit bytes;		DES [DES] uses a 56-bit key. The key consists of eight 8-bit bytes;
	however the last (eighth) bit of each byte is used for parity, leaving		however the last (eighth) bit of each byte is used for parity, leaving
	56 bits of key.		56 bits of key.
	To weaken the 8-byte, 56-bit key into a 40-bit key, you set to zero		To weaken the 8-byte, 56-bit key into a 40-bit key, you set to zero
	the first four bits of every other byte in the key, starting with the		the first four bits of every other byte in the key, starting with the
	first byte. Stated a different way, you take the bitwise logical AND		first byte. Stated a different way, you take the bitwise logical AND
	skipping to change at line 114 ¶		skipping to change at line 105 ¶
	due to the short key length.		due to the short key length.
	There are other methods for converting longer keys to shorter ones.		There are other methods for converting longer keys to shorter ones.
	For example, IBM has created a patented (and significantly more		For example, IBM has created a patented (and significantly more
	complex) method called "Commercial Data Masking Facility", or CDMF		complex) method called "Commercial Data Masking Facility", or CDMF
	[CDMF]; other methods probably exist. These methods might result in		[CDMF]; other methods probably exist. These methods might result in
	keys that produce cyphertext that is harder (or easier) to determine		keys that produce cyphertext that is harder (or easier) to determine
	through brute-force. A quick comparison of CDMF and DES-40 shows that		through brute-force. A quick comparison of CDMF and DES-40 shows that
	the brute-force attack against CDMF require one additional DES		the brute-force attack against CDMF require one additional DES
	operation. Saving one DES operation does not seem to warrant the		operation. Saving one DES operation does not seem to warrant the
	additonal complexity.		additional complexity.
	A. References		A. References
	[CDMF] "Design of the Commercial Data Masking Facility Data Privacy		[CDMF] "Design of the Commercial Data Masking Facility Data Privacy
	Algorithm", 1st ACM Conference on Computer and Communications		Algorithm", 1st ACM Conference on Computer and Communications
	Security, ACM Press, 1993.		Security, ACM Press, 1993.
	[DES] ANSI X3.106, "American National Standard for Information		[DES] ANSI X3.106, "American National Standard for Information
	Systems-Data Link Encryption," American National Standards		Systems-Data Link Encryption," American National Standards
	Institute, 1983.		Institute, 1983.
	skipping to change at line 137 ¶		skipping to change at line 128 ¶
	Paul Hoffman		Paul Hoffman
	Internet Mail Consortium		Internet Mail Consortium
	127 Segre Place		127 Segre Place
	Santa Cruz, CA 95060		Santa Cruz, CA 95060
	(408) 426-9827		(408) 426-9827
	phoffman@imc.org		phoffman@imc.org
	Russ Housley		Russ Housley
	SPYRUS		SPYRUS
	PO Box 1198		381 Elden Street, Suite 1120
	Herndon, VA 20172		Herndon, VA 20170
	(703) 435-7344
	housley@spyrus.com		housley@spyrus.com
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