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Thread: S-boxes in Blowfish

  1. #1
    Join Date
    Sep 2003

    S-boxes in Blowfish

    Hey there, Im just working on a project for a crypto class and I decided to implement blowfish on VHDL(Hardware Description Language) to run on a hardware piece. Anyways, the real question is that the blowfish algorithm initializes its famous 4 S-boxes and a P array with the hex values of the decimal part of pi. I have found some programs which calculate pi with enough accuracy to get enough values for initialization of these S-boxes (they are 4 each with 256 32 bit entries). Problem is I dont know how to change this number later to hex or even binary. Any help here would be greatly appreciated.
    I know I haven't given much information on the algorithm but it is not really necessary to understand what I need.
    I Speak in frequencies even dogs have trouble hearing

  2. #2
    Senior Member
    Join Date
    Jul 2003
    hey sounds like a cool project, are you gonna have two cpld or fpga's? one encrypting and on decrypting?

    I found this page,

    http://personal.sip.fi/~lm/c2txt2c/blowfish.txt <---hehehe

    dunno whether there any help...


  3. #3
    Senior Member
    Join Date
    Mar 2004

    An excellent list of the initialisation numbers for the 18x32bit P and the 4 256x32bit S-boxes
    can be obtained here[1], in a useful format.

    How is this computed?

    The starting point chosen by Bruce is the number: Pi - 3 = 0.14159265...

    How can this number be assembled in hex-base? As follows:

    First digit: 1/16 = 0.0625

    Q: How often does that fit into this number 0.14159...?
    A: 2 times.

    0.14159265 - 2* 1/16 = 0.01659265...
    Note: This number 0.01659265... is smaller than 1/16, and thus has to
    be "approximated" using the next smaller possibility: 1/(16^2)

    Second digit: 1/(16^2) = 0.00390625

    Q: How often does that fit into the rest-term 0.01659265...
    A: 4 times

    0.14159265 - 2* 1/16 - 4*1/(16^2) = 0.0009676535...

    Third digit: 1/(16^3) = 0.00024414...

    Q: How often does that fit into the rest-term 0.000967653...
    A: 3 times

    and the fourth digit: 15 times. 15 in hexadezimal notation is F.

    Thus: 24 3F ...

    The initialisation starts with P, then S0, S1, S2 and finally S3.


    [1] http://www.sstorie.com/school/cs4821...owfishSPInit.h
    If the only tool you have is a hammer, you tend to see every problem as a nail.
    (Abraham Maslow, Psychologist, 1908-70)

  4. #4
    Join Date
    Sep 2003
    Thanks a lot guys, definitely going to help. Oh and i2c, ill be using fpga's .
    I Speak in frequencies even dogs have trouble hearing

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