string cipher = getCipher();

string toBeCiphered = "blahcipherme";

int[] keysToUse = new int[strlen(cipher)];
for(int i = 0;i<keysToUse.Length;i++) {
    keysToUse[i] = getCharAt(cipher,i);
}

string output = "";
int cipherPos = 0;
for(int i = 0;i<toBeCiphered.Length;i++) {
    cipherPos++;
    output += doTheCipherThing(keysToUse[cipherPos],toBeCiphered.charAt(i));
    if(cipherPos>=keysToUse.Length) {
        cipherPos = 0;
    }
}