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2 July 1998

    To: cryptography@c2.net
    Subject: Eli Biham notches the pressure up a bit higher.
    Date: Thu, 02 Jul 1998 19:39:15 -0400
    From: "Perry E. Metzger" <perry@piermont.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: perry@piermont.com

    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