Symmetric encryption using hash function and key derivation

hashcrypt.js

function hashCrypt(key, plain, hasher, workFactor=1) { // hasher is any sync string-output hash function
	var len=plain.length, keyLen = key.length, keyOrig = key,

	// calculate derivation count based on deterministic factors from the inputs:
        mx = hasher(key).split("").map((a, b)=> a.charCodeAt(0) + b).reduce((a, b)=> a + b, 0) * workFactor;

	// derive firstly for some hardness using cpu and ram hungry calculations:
	for(let i = 0; i < mx; i++) key = hasher(key.repeat( ((i+len) % ((workFactor || 1) * 4))+1  )); 

	// expand key to needed length by appending re-hashing (with counters):
	for(let i = 0, mx = (len / 32) - 1; i < mx; i++) key += hasher( key + i + keyLen + keyOrig );

	// turn key into array of numbers to perform the XOR in the next step:
	key = key.match(/\w{2}/g).map(a=>parseInt(a, 16));

	// encrypt the message using derived key of the same length:
	return plain.split("").map((c, i)=> String.fromCharCode(c.charCodeAt(0) ^ key[i])).join("");

} // end encrypt()


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// simple demo using sha3 implementation from https://github.com/emn178/js-sha3
var txt = "Hello World!"; // the plain text message to encrypt
var key = "123456789"; // not really a good key, but works for the demo...
var hash = sha3_512; // customize as needed with any good hash function
var enc = hashCrypt(key, txt, hash, 2); // encrypt txt with key using sha3-512 and a work factor of 2
var dec = hashCrypt(key, enc, hash, 2); // decrypt enc with key using sha3-512 and a work factor of 2
//  alert(hashCrypt(key, enc, hash, 2) === txt ); // quick proof of success
prompt(dec, enc); // show decoded and encoded versions