2 July 1998
To: firstname.lastname@example.org Subject: Eli Biham notches the pressure up a bit higher. Date: Thu, 02 Jul 1998 19:39:15 -0400 From: "Perry E. Metzger" <email@example.com> This just in before the holiday weekend. Quite a stunning result. One can only wonder what he finds in coming days. Perry Date: Fri, 3 Jul 1998 01:33:41 +0300 From: biham@CS.Technion.AC.IL (Eli Biham) Message-Id: <199807022233.BAA16757@CS.Technion.AC.IL> To: firstname.lastname@example.org Dear colleages, We have just released new results on the cryptanalysis of SkipJack variants. They can be found in http://www.cs.technion.ac.il/~biham/Reports/SkipJack/. Its summary is enclosed below. Sincerely, Eli Biham, Alex Biryukov, Orr Dunkelman, Eran Richardson, Adi Shamir Cryptanalysis of SkipJack-3XOR in 2^20 time using 2^9 chosen plaintexts July 2nd, 1998 (DRAFT) This note can be found in http://www.cs.technion.ac.il/~biham/Reports/SkipJack/ Feel free to distribute Summary SkipJack is the secret key encryption algorithm developed in 1993 by the NSA for the Clipper chip and Fortezza PC card. Its description was made public on June 24th 1998 at NIST's web site. It uses an 80 bit key, 32*4=128 table lookup operations, and 32*10=320 XOR operations to map a 64 bit plaintext into a 64 bit ciphertext in 32 rounds. This note summarizes our first week of analysis. The main result is an attack on a variant, which we call SkipJack-3XOR (SkipJack minus 3 XORs). The only difference between SkipJack and SkipJack-3XOR is the removal of 3 out of the 320 XOR operations. The attack uses the ciphertexts derived from about 500 plaintexts which are identical except for the second 16 bit word. Its total running time is equivalent to about one million SkipJack encryptions, which can be carried out in seconds on a personal computer. This is still a preliminary result, but it reiterates our earlier comment that SkipJack does not have a conservative design with a large margin of safety.
See two previous notes: http://jya.com/skipjack-bs.htm http://jya.com/skipjack-bs2.htm