Source: packet/public_key_encrypted_session_key.js

// GPG4Browsers - An OpenPGP implementation in javascript
// Copyright (C) 2011 Recurity Labs GmbH
// 
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
// 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
// 
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA

/**
 * Public-Key Encrypted Session Key Packets (Tag 1)<br/>
 * <br/>
 * RFC4880 5.1: A Public-Key Encrypted Session Key packet holds the session key
 * used to encrypt a message. Zero or more Public-Key Encrypted Session Key
 * packets and/or Symmetric-Key Encrypted Session Key packets may precede a
 * Symmetrically Encrypted Data Packet, which holds an encrypted message. The
 * message is encrypted with the session key, and the session key is itself
 * encrypted and stored in the Encrypted Session Key packet(s). The
 * Symmetrically Encrypted Data Packet is preceded by one Public-Key Encrypted
 * Session Key packet for each OpenPGP key to which the message is encrypted.
 * The recipient of the message finds a session key that is encrypted to their
 * public key, decrypts the session key, and then uses the session key to
 * decrypt the message.
 * @requires crypto
 * @requires enums
 * @requires type/keyid
 * @requires type/mpi
 * @requires util
 * @module packet/public_key_encrypted_session_key
 */

module.exports = PublicKeyEncryptedSessionKey;

var type_keyid = require('../type/keyid.js'),
  util = require('../util'),
  type_mpi = require('../type/mpi.js'),
  enums = require('../enums.js'),
  crypto = require('../crypto');

/**
 * @constructor
 */
function PublicKeyEncryptedSessionKey() {
  this.version = 3;

  this.publicKeyId = new type_keyid();
  this.publicKeyAlgorithm = 'rsa_encrypt';

  this.sessionKey = null;
  this.sessionKeyAlgorithm = 'aes256';

  /** @type {Array<module:type/mpi>} */
  this.encrypted = [];
}

/**
 * Parsing function for a publickey encrypted session key packet (tag 1).
 *
 * @param {String} input Payload of a tag 1 packet
 * @param {Integer} position Position to start reading from the input string
 * @param {Integer} len Length of the packet or the remaining length of
 *            input at position
 * @return {module:packet/public_key_encrypted_session_key} Object representation
 */
PublicKeyEncryptedSessionKey.prototype.read = function (bytes) {

  this.version = bytes.charCodeAt(0);
  this.publicKeyId.read(bytes.substr(1));
  this.publicKeyAlgorithm = enums.read(enums.publicKey, bytes.charCodeAt(9));

  var i = 10;

  var integerCount = (function(algo) {
    switch (algo) {
      case 'rsa_encrypt':
      case 'rsa_encrypt_sign':
        return 1;

      case 'elgamal':
        return 2;

      default:
        throw new Error("Invalid algorithm.");
    }
  })(this.publicKeyAlgorithm);

  this.encrypted = [];

  for (var j = 0; j < integerCount; j++) {
    var mpi = new type_mpi();
    i += mpi.read(bytes.substr(i));
    this.encrypted.push(mpi);
  }
};

/**
 * Create a string representation of a tag 1 packet
 *
 * @param {String} publicKeyId
 *             The public key id corresponding to publicMPIs key as string
 * @param {Array<module:type/mpi>} publicMPIs
 *            Multiprecision integer objects describing the public key
 * @param {Integer} pubalgo
 *            The corresponding public key algorithm // See RFC4880 9.1
 * @param {Integer} symmalgo
 *            The symmetric cipher algorithm used to encrypt the data
 *            within an encrypteddatapacket or encryptedintegrity-
 *            protecteddatapacket
 *            following this packet //See RFC4880 9.2
 * @param {String} sessionkey
 *            A string of randombytes representing the session key
 * @return {String} The string representation
 */
PublicKeyEncryptedSessionKey.prototype.write = function () {

  var result = String.fromCharCode(this.version);
  result += this.publicKeyId.write();
  result += String.fromCharCode(
    enums.write(enums.publicKey, this.publicKeyAlgorithm));

  for (var i = 0; i < this.encrypted.length; i++) {
    result += this.encrypted[i].write();
  }

  return result;
};

PublicKeyEncryptedSessionKey.prototype.encrypt = function (key) {
  var data = String.fromCharCode(
    enums.write(enums.symmetric, this.sessionKeyAlgorithm));

  data += this.sessionKey;
  var checksum = util.calc_checksum(this.sessionKey);
  data += util.writeNumber(checksum, 2);

  var mpi = new type_mpi();
  mpi.fromBytes(crypto.pkcs1.eme.encode(
    data,
    key.mpi[0].byteLength()));

  this.encrypted = crypto.publicKeyEncrypt(
    this.publicKeyAlgorithm,
    key.mpi,
    mpi);
};

/**
 * Decrypts the session key (only for public key encrypted session key
 * packets (tag 1)
 *
 * @param {module:packet/secret_key} key
 *            Private key with secMPIs unlocked
 * @return {String} The unencrypted session key
 */
PublicKeyEncryptedSessionKey.prototype.decrypt = function (key) {
  var result = crypto.publicKeyDecrypt(
    this.publicKeyAlgorithm,
    key.mpi,
    this.encrypted).toBytes();

  var checksum = util.readNumber(result.substr(result.length - 2));

  var decoded = crypto.pkcs1.eme.decode(
    result,
    key.mpi[0].byteLength());

  key = decoded.substring(1, decoded.length - 2);

  if (checksum != util.calc_checksum(key)) {
    throw new Error('Checksum mismatch');
  } else {
    this.sessionKey = key;
    this.sessionKeyAlgorithm =
      enums.read(enums.symmetric, decoded.charCodeAt(0));
  }
};