The cipher works as the following;

• Assume we have the plaintext : "helloworld"
• And the key : "key"
• The key will repeat alongside the plaintext like;

helloworld

keykeykeyk

• Then we simply need to XOR every byte in our plaintext and the key.

I have been struggling to find the desired index of the key for the encryption.

As an example, if I am at w I need to XOR it with y, at d my program need to know to XOR it with k. How can tell my program to XOR it with the needed element of the key?

CodePudding user response：

I believe you are asking about the formula you need to use. The formula for the vernam cipher is;

• For encrypting Ci = Pi ^ Ki
• For decrypting Pi = Ci ^ Ki

I believe there two approaches you can use. You can either;

• Have a second string where the key repeats until you reach the length of your plaintext.
• Use the length of the key as your modulo for the index of the plaintext you are at.

For the latter, it looks like the following;

while(str[i] != '\0'){
    cipher[i] = str[i] ^ key[i%strlen(key)];
    i  ;
}