<script type="text/javascript">
function Encrypt(){
    plaintext = document.getElementById("p").value.toLowerCase().replace(/[^a-z]/g, "");
    k = document.getElementById("k").value.toLowerCase().replace(/[^0-9 ]/g, "");
    keys = k.split(" ");
    // do some error checking
    if(plaintext.length < 1){ alert("please enter some plaintext (letters and numbers only)"); return; }
    if(plaintext.length % 2 == 1){ plaintext = plaintext + "x"; }
    if(keys.length != 4){ alert("key should consist of 4 integers"); return; }
    for(i=0;i<4;i++) keys[i] = keys[i]%26;
    ciphertext="";
    for(i=0; i<plaintext.length; i+=2){
      ciphertext += String.fromCharCode((keys[0]*(plaintext.charCodeAt(i)-97) + keys[1]*(plaintext.charCodeAt(i+1)-97))%26 + 97);
      ciphertext += String.fromCharCode((keys[2]*(plaintext.charCodeAt(i)-97) + keys[3]*(plaintext.charCodeAt(i+1)-97))%26 + 97);
    }
    document.getElementById("c").value = ciphertext;
}
function Decrypt(){
    ciphertext = document.getElementById("c").value.toLowerCase().replace(/[^a-z]/g, "");
    k = document.getElementById("k").value.toLowerCase().replace(/[^0-9 ]/g, "");
    keys = k.split(" ");
    // do some error checking
    if(ciphertext.length < 1){ alert("please enter some ciphertext (letters only, numbers should be spelled)"); return; }
    if(ciphertext.length % 2 == 1){ alert("ciphertext is not divisible by 2 (wrong algorithm?)"); return; }
    if(keys.length != 4){ alert("key should consist of 4 integers"); return; }
    for(i=0;i<4;i++) keys[i] = keys[i]%26;
    // calc inv matrix
    det = keys[0]*keys[3] - keys[1]*keys[2];
    det = ((det%26)+26)%26;
    di=0;
    for(i=0;i<26;i++){ if((det*i)%26 == 1) di = i; }
    if(di == 0){alert("could not invert, try different key"); return; }
    ikeys = new Array(4);
    ikeys[0] = (di*keys[3])%26; ikeys[1] = (-1*di*keys[1])%26;
    ikeys[2] = (-1*di*keys[2])%26; ikeys[3] = di*keys[0];
    for(i=0;i<4;i++){ if(ikeys[i] < 0) ikeys[i] += 26; }
    plaintext="";
    for(i=0; i<ciphertext.length; i+=2){
      plaintext += String.fromCharCode((ikeys[0]*(ciphertext.charCodeAt(i)-97) + ikeys[1]*(ciphertext.charCodeAt(i+1)-97))%26 + 97);
      plaintext += String.fromCharCode((ikeys[2]*(ciphertext.charCodeAt(i)-97) + ikeys[3]*(ciphertext.charCodeAt(i+1)-97))%26 + 97);
    }
    document.getElementById("p").value = plaintext;
}
</script>