Thread: C encryption/decryption auto cipher key

  1. #1
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    Jun 2017
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    C encryption/decryption auto cipher key

    Hi all,

    I am working on a encryption/decryption program that uses the auto key ciper method of encryption. Here is my encrypt func:

    Code:
    unsigned int encrypt(char x){
      char a;
        if ( i == 0 ){
        a = 'K';}
        else if( i == 1 ){
        a = 'E';}
        else if( i == 2 ){
        a = 'Y';}
        else if( i > 2 ){
            a = x;}
        i++;
      return x + a;
    }
    Here is my decrypt:

    Code:
    unsigned int decrypt(char x){
      char a;
        if( i == 0 ){
        a = 'K';}
        else if( i == 1 ){
        a = 'E';}
        else if( i == 2 ){
        a = 'Y';}
        else if( i > 2 ){
            a = (x/2);}
      i++;
      if (c != EOF && c != ('\n')){
      return x - a;}
      else {return 0;}
    }
    When I pipe a message through encrypt then through decrypt I should get the original message but I only get the first 3 characters (because of the first three K E Y keywords) and any numbers in the message. Any other characters don't come out correctly. What am I missing? Thanks in advance.

  2. #2
    Registered User
    Join Date
    May 2009
    Posts
    4,077
    Quote Originally Posted by arriver5 View Post
    Hi all,

    I am working on a encryption/decryption program that uses the auto key ciper method of encryption. Here is my encrypt func:


    When I pipe a message through encrypt then through decrypt I should get the original message but I only get the first 3 characters (because of the first three K E Y keywords) and any numbers in the message. Any other characters don't come out correctly. What am I missing? Thanks in advance.
    The code you posted is missing where you declared "i".

    Tim S.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  3. #3
    Registered User
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    Jun 2015
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    1,643
    You are doubling a value then halving it again and not getting the original value back. This is a sign of arithmetic overflow. char is a signed on your system. The (common) value for 'A' is 65. Doubling that gives 130, which is beyond 7-bits, i.e., the "sign" bit is now set, so it's interpreted as a negative number, which in the (common) 2's complement system for representing negative values is 130 - 256 = -126. When this is halved, you get -63 (or 256 - 63 = 193 unsigned).

    BTW, this is not an autokey cipher. You're supposed to add in the letter that appears n chars back in the message, where n is the length of the primary key ("KEY"). And if you want the ciphertext to be printable you'll have to limit it to printable characters (maybe ' ' to '~');

    And you should try to write it without the global variables.

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