Add MGF1.js file - mask generation utility class

lib/MGF1.js

@@ -0,0 +1,67 @@
+'use strict'
+
+const crypto = require("crypto");
+const BN     = require('bn.js');
+
+module.exports = class MGF1 {
+
+	constructor() {
+		this._len = 32;
+	}
+
+	/**
+	 *
+	 * @param {Buffer} seed
+	 * @param {Number} masklen
+	 *
+	 * @returns Buffer
+	 */
+	static generate(seed, masklen) {
+		const mgf1 = new MGF1();
+
+		if ( masklen > 4294967296 * this._len) {
+			throw new Error('Mask too long');
+		}
+
+		const b = [];
+
+		for (let i = 0; i < mgf1._divceil(masklen, mgf1._len); i++) {
+			b[i] = crypto.createHash('sha256').update(Buffer.concat([seed, mgf1._i2osp(i, 4)])).digest();
+		}
+
+		return (Buffer.concat(b)).slice(0, masklen);
+	}
+
+	static xor(a, b) {
+		if ( a.length != b.length ) {
+			throw new Error('Different length for a and b');
+		}
+
+		for ( let i = 0; i < a.length; i++ ) {
+			a[i] ^= b[i];
+		}
+
+		return a;
+	}
+
+	_divceil(a, b) {
+		return ~~((a + b - 1) / b);
+	}
+
+	_i2osp(x, len) {
+		if ( x >= 256 ** len ) {
+			throw new Error('Integer too large');
+		}
+
+		return Buffer.from(this._rjust( (Buffer.from((new BN(x)).toArray('be', 4)).toString()).replace(/\x00/gi, ''), len, "\x00" ));
+	}
+
+	_rjust( string, width, padding ) {
+		padding = padding || " ";
+		padding = padding.substr( 0, 1 );
+		if ( string.length < width )
+			return padding.repeat( width - string.length ) + string;
+		else
+			return string;
+	}
+}