Feistel Help

This is a discussion on Feistel Help within the C++ Programming forums, part of the General Programming Boards category; I never have seen a plain feistel structure, but implementations like skipjack and DES etc... So based on theories and ...

  1. #1
    *this
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    Feistel Help

    I never have seen a plain feistel structure, but implementations like skipjack and DES etc... So based on theories and drawings this is my VERY simple implementation. Am I on the right track? I know I will be adding a better rounding function (F) but using an easy one for now...

    Code:
    typedef unsigned int  WORD;
    typedef unsigned char BYTE;
    
    inline WORD F (BYTE k) {
       return (((~k) << 8) + (k));
    }
    
    void PEA (const bool encrypt, string &d, const string k, const int begin) {
       const int ROUNDS = 32;       /* Number of Rounds */
       const int SIZE = k.size();   /* Size of key */
       WORD temp, left, right;      /* 32 Bit Words */
       int index, step;             /* For Feistel Structure */
       
       /* Change direction depending on encrypt or decrypt */
       if (encrypt) index = 0, step = 1;
       else index = ROUNDS-1, step = -1;
       
       /* Push data bytes into words */
       left =  ((d[begin] << 8) + d[begin+1]);
       right = ((d[begin+2] << 8) + d[begin+3]);
       
       /* Feistel Structure */
       for (int loop = 0; loop < ROUNDS; loop++) {
          temp = right;
          right = left ^ F(k[index%(SIZE-1)]);
          left = temp;
          index += step;
       }
       
       /* Unpack words into bytes */
       d[begin+3] = (left & 255), d[begin+2] = (left >> 8) & 255,
       d[begin+1] = (right & 255), d[begin] = (right >> 8) & 255;
    }
    Last edited by JoshR; 07-09-2005 at 01:13 PM.

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    Code:
    for (int loop = 0; loop < ROUNDS; loop++) {
    Was that supposed to use index and step?

    I'm sorry, I don't know what "feistel structure" is, so I can't help too much.
    dwk

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  3. #3
    Frequently Quite Prolix dwks's Avatar
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    It seems to me that you don't need the parameter begin. You would just call the function with string+begin rather than func(string, begin).
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
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  4. #4
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    I know but it only uses 4 bytes of data at a time so i had a string that i would pass the current location through PEA.

    In response to your first post, no. Because step changes depending on encrypt or decrypt.

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    Code:
    void func1(char *s, int begin) {
        cout << (s+begin);
    }
    
    func1(string, 3);
    is the same as
    Code:
    void func2(char *s) {
        cout << s;
    }
    
    func2((string+3));
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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  6. #6
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    Ok well I wanted to know if the structure was right, it doesnt matter about the begin etc...

    Begin does what I want it to so ill worry about it later.

  7. #7
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    Quote Originally Posted by dwks
    Code:
    void func1(char *s, int begin) {
        cout << (s+begin);
    }
    
    func1(string, 3);
    is the same as
    Code:
    void func2(char *s) {
        cout << s;
    }
    
    func2((string+3));
    Well thats not the concept either. its not s + begin, its s[begin] so you cant do the second thing...

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    No, you just go *s.

    [edit]
    Or *(s+0) or s[0] etc.
    [/edit]
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  9. #9
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    Well I'm not using char indexing. I'm using c++ strings. So you cant add 3 to string...

  10. #10
    Frequently Quite Prolix dwks's Avatar
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    Oops, sorry, I missed that.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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