key derivation using sha3 and variable loop count

derive.js

/* password derivation that forces a re-creation of this routine by making slight mods to a normal hash-based kdf:
requires https://github.com/emn178/js-sha3

Example timings on a i7 in chrome:
cost ms
1 - 108
2 - 408
3 - 1067
4 - 2351
5 - 3927
6 - 7113
7 - 9462
8 - 12055
9 - 16270
10 - 20796

*/

function derive(strKey, cost){
	cost = +cost || 1;
	strKey=sha3_512(String(strKey));
	var rounds = (cost*1000) + // 1000 per cost, plus a pseudo-random number between ~ -50 to 1200 :
                     Math.floor((sha3_256( strKey+cost+strKey ).match(/\w{2}/g).map(a=>parseInt(a,16)).reduce(function(a,b){return a+Math.abs(b)})-2500)/5),
        pad = Array(33).join(sha3_512( cost + strKey )); // create a 4kb pad to make derivation require more work
	for(var i=0; i < rounds; i++) strKey=  sha3_256(  (cost+(i*rounds)) + pad.slice(-i).repeat(cost) + strKey + i); // make derivation do math and string work
	return strKey;
}