# vigenere_cipher

This is based on the classic Vigenère cipher. Using a repeating key, the letters in the target text are shifted in the alphabet by varying amounts. While effective, this is not a strong encryption. A determined person would likely be able to crack it. The cipher becomes more effective as the length of the key increases. The biggest weakness with Vigenère ciphers is in cases where many spaces or zero-value characters exist in the source data. When that happens the enciphering key can be exposed. Using long keys which appear random can help hide the exposure.

G  A  M  E  M  A  K  E  R        7  1 13  5 13  1 11  5 18
K  E  Y  K  E  Y  K  E  Y       11  5 25 11  5 25 11  5 25
-  -  -  -  -  -  -  -  -       -- -- -- -- -- -- -- -- --
R  F  L  P  R  Z  V  J  Q       18  6 12 16 18 26 22 10 17

WARNING: This script is NOT SUITABLE FOR GM:STUDIO. Because GM:Studio strings are null terminated, there is a very high chance that the encrypted strings generated by this script will be truncated resulting in data loss. The script must be modified to perform its actions using a different data type. Buffers are the recommended type for arbitrary binary data such as that produced by this script. Alternatively, you may wish to try vigenere_ascii, although it may have other problems caused by GM:Studio's string handling.

vigenere_cipher(in,key,mode)
Returns the given string enciphered or deciphered using a Vigenere style cipher.
COPY/// vigenere_cipher(in,key,mode)
//
//  Returns the given string enciphered or deciphered
//  using a Vigenere style cipher.
//
//      in          input, string
//      key         enciphering key, string
//      mode        0 = decipher, 1 = encipher
//
{
var in, key, mode, out;
in = argument0;
key = argument1;
mode = argument2;
out = "";
var inLen, keyLen, pos, inChar, keyChar, outChar;
var inVal, keyVal, outVal;
inLen = string_length(in);
keyLen = string_length(key);
for (pos=0; pos<inLen; pos+=1) {
inChar = string_char_at(in, pos+1);
keyChar = string_char_at(key, (pos mod keyLen)+1);
inVal = ord(inChar);
keyVal = ord(keyChar);
if (mode) {
outVal = (inVal + keyVal) mod 256;
}else{
outVal = (256 + inVal - keyVal) mod 256;
}
outChar = chr(outVal);
out = out + outChar;
}
return out;
}


Contributors: xot

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