Re: Back Doors

From: Foo Bar (foobar965_at_hotmail.com)
Date: 12/20/03

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    Date: Sat, 20 Dec 2003 17:21:15 GMT

    mike4ty4@yahoo.com (mike3) writes:

    > Hi.
    >
    > How can one implement "back doors" in an encryption algorithm that
    > would allow someone to decode all messages encrypted with it provided
    > they know some secret "magic" formula? Is this a very diffcult problem
    > or an easy one?

    Depends on what the requirements are on the trapdoor. For example, how
    hard should it be to detect? Rijmen and Preneel suggested one way of
    designing hard-to-detect trapdoors into a cipher in "A family of
    trapdoor ciphers" (FSE'97) [1], but that proposal was broken by Wu, Bao,
    Deng and Ye in "Cryptanalysis on Rijmen-Preneel Trapdoor Ciphers"
    (Asiacrypt '98) [2].

    Note that a block cipher with a trapdoor that is hard to detect but easy
    to use can be used for public key encryption.

    Perhaps the Weis and Lucks paper "'All your key bit are belong to us'
    the truth about black box cryptography" [3] contains something
    interesting.

    [1] http://www.esat.kuleuven.ac.be/~cosicart/ps/VR-9703.ps.gz
    [2] http://icsd.i2r.a-star.edu.sg/publications/WuHongjun_1998_trapdoor.pdf
    [3] http://www.nluug.nl/events/sane2002/papers/WeisLucksAllYourKeybit.ps

    /FB 

    -- 
Foo Bar (foobar965@hotmail.com)
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