Question

I am having trouble with my affine caesar cipher decoding. I'm taking ascii values, subtracting 97 to get 0-25, applying the shift, then adding 97 back to get the cipher text, and that encodes just fine. But my decoding is getting a wrong output every 5th and 6th number, and I'm not sure why.

Answer 1

Here is the code

public class KeyGen {

public void encrypt(ArrayList plainText, ArrayList cipherText) {
for(int i = 0; i < plainText.size(); i++){
int ct = (int) plainText.get(i);
ct = ((((ct-97) *5) +8)%26)+97;
cipherText.add(ct);
System.out.print((char)(int)cipherText.get(i));

}
}

public void decrypt(ArrayList cipherText, ArrayList decodedText){
for(int i = 0; i < cipherText.size(); i++){
int ct = (int) cipherText.get(i);
ct = ct-97;
ct = (abs((ct-8))*21 %26)+97;
decodedText.add(ct);

}
}
}