
/**
 * Javascript implementation of mask generation function MGF1.
 *
 * @author Stefan Siegl
 * @author Dave Longley
 *
 * Copyright (c) 2012 Stefan Siegl <stesie@brokenpipe.de>
 * Copyright (c) 2014 Digital Bazaar, Inc.
 */
var forge = require('./forge');
require('./util');

forge.mgf = forge.mgf || {};
var mgf1 = module.exports = forge.mgf.mgf1 = forge.mgf1 = forge.mgf1 || {};

/**
 * Creates a MGF1 mask generation function object.
 *
 * @param md the message digest API to use (eg: forge.md.sha1.create()).
 *
 * @return a mask generation function object.
 */
mgf1.create = function(md) {
  var mgf = {
    /**
     * Generate mask of specified length.
     *
     * @param {String} seed The seed for mask generation.
     * @param maskLen Number of bytes to generate.
     * @return {String} The generated mask.
     */
    generate: function(seed, maskLen) {
      /* 2. Let T be the empty octet string. */
      var t = new forge.util.ByteBuffer();

      /* 3. For counter from 0 to ceil(maskLen / hLen), do the following: */
      var len = Math.ceil(maskLen / md.digestLength);
      for(var i = 0; i < len; i++) {
        /* a. Convert counter to an octet string C of length 4 octets */
        var c = new forge.util.ByteBuffer();
        c.putInt32(i);

        /* b. Concatenate the hash of the seed mgfSeed and C to the octet
         * string T: */
        md.start();
        md.update(seed + c.getBytes());
        t.putBuffer(md.digest());
      }

      /* Output the leading maskLen octets of T as the octet string mask. */
      t.truncate(t.length() - maskLen);
      return t.getBytes();
    }
  };

  return mgf;
};