rsa-csr.js/lib/asn1.js

'use strict';

//
// A dumbed-down, minimal ASN.1 parser / packer combo
//
// Note: generally I like to write congruent code
// (i.e. output can be used as input and vice-versa)
// However, this seemed to be more readable and easier
// to use written as-is, asymmetrically.
// (I also generally prefer to export objects rather
// functions but, yet again, asthetics one in this case)

var Enc = require('./encoding.js');

//
// Packer
//

// Almost every ASN.1 type that's important for CSR
// can be represented generically with only a few rules.
var ASN1 = module.exports = function ASN1(/*type, hexstrings...*/) {
  var args = Array.prototype.slice.call(arguments);
  var typ = args.shift();
  var str = args.join('').replace(/\s+/g, '').toLowerCase();
  var len = (str.length/2);
  var lenlen = 0;
  var hex = typ;

  // We can't have an odd number of hex chars
  if (len !== Math.round(len)) {
    console.error(arguments);
    throw new Error("invalid hex");
  }

  // The first byte of any ASN.1 sequence is the type (Sequence, Integer, etc)
  // The second byte is either the size of the value, or the size of its size

  // 1. If the second byte is < 0x80 (128) it is considered the size
  // 2. If it is > 0x80 then it describes the number of bytes of the size
  //    ex: 0x82 means the next 2 bytes describe the size of the value
  // 3. The special case of exactly 0x80 is "indefinite" length (to end-of-file)

  if (len > 127) {
    lenlen += 1;
    while (len > 255) {
      lenlen += 1;
      len = len >> 8;
    }
  }

  if (lenlen) { hex += Enc.numToHex(0x80 + lenlen); }
  return hex + Enc.numToHex(str.length/2) + str;
};

// The Integer type has some special rules
ASN1.UInt = function UINT() {
  var str = Array.prototype.slice.call(arguments).join('');
  var first = parseInt(str.slice(0, 2), 16);

  // If the first byte is 0x80 or greater, the number is considered negative
  // Therefore we add a '00' prefix if the 0x80 bit is set
  if (0x80 & first) { str = '00' + str; }

  return ASN1('02', str);
};

// The Bit String type also has a special rule
ASN1.BitStr = function BITSTR() {
  var str = Array.prototype.slice.call(arguments).join('');
  // '00' is a mask of how many bits of the next byte to ignore
  return ASN1('03', '00' + str);
};