ber.js

// http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
var UNIVERSAL_TAG_CLASS = 0;

var EOC_TAG = 0;
var BOOLEAN_TAG = 1;
var INTEGER_TAG = 2;
var NULL_TAG = 5;
var OBJECT_ID_TAG = 6;
var RELATIVE_OBJECT_ID_TAG = 13;
var PRINTABLE_STRING_TAG = 19;
var IA5_STRING_TAG = 22;
var UTC_TIME_TAG = 23;

function readBerPrimitive(tagClass, tagNumber, data, start, end) {
  if (tagClass === UNIVERSAL_TAG_CLASS) {
    switch (tagNumber) {
      case EOC_TAG:
        if (start !== end) {
          throw new Error('Invalid EOC tag');
        }
        return undefined;
      case BOOLEAN_TAG:
        if (start + 1 !== end) {
          throw new Error('Invalid boolean tag');
        }
        return !!data[start];
      case NULL_TAG:
        if (start !== end) {
          throw new Error('Invalid null tag');
        }
        return null;
      case INTEGER_TAG:
        if (start === end) {
          throw new Error('Invalid integer tag');
        } else if (start + 1 === end) {
          return (data[start] << 24) >> 24;
        } else if (start + 2 === end) {
          return ((data[start] << 24) | (data[start + 1] << 16)) >> 16;
        } else if (start + 3 === end) {
          return ((data[start] << 24) | (data[start + 1] << 16) |
                  (data[start + 2] << 8)) >> 8;
        } else if (start + 4 === end) {
          return (data[start] << 24) | (data[start + 1] << 16) |
                 (data[start + 2] << 8) | data[start + 3];
        }
        // to long for us
        break;
      case OBJECT_ID_TAG:
      case RELATIVE_OBJECT_ID_TAG:
        if (start === end) {
          throw new Error('Invalid identifier tag');
        }
        var id = [];
        for (var i = start; i < end; ) {
          var tmp = decodeCompressedNumber(data, i, end);
          id.push(tmp.value);
          i += tmp.octetLength;
        }
        if (tagNumber === OBJECT_ID_TAG) {
          var firstNumber = +id.shift();
          id.unshift(((firstNumber / 40) | 0).toString(),
                     (firstNumber % 40).toString());
        }
        return id.join('.');
      case PRINTABLE_STRING_TAG:
      case IA5_STRING_TAG:
      case UTC_TIME_TAG:
        return String.fromCharCode.apply(null, data.subarray(start, end));
    }
  }
  return data.subarray(start, end);
}

function decodeCompressedNumber(data, start, end) {
  var j = start, k;
  while (j < end && (data[j] & 0x80)) {
    j++;
  }
  if (j >= end) {
    throw new Error('Invalid identifier bytes encoding');
  }
  var str = '', mult = 128, buffer = data[j];
  k = j;
  var partSize = 10000;
  while (start < k) {
    buffer += (data[--k] & 0x7F) * mult;
    mult <<= 7;
    if (mult >= partSize) {
      str = ((buffer % partSize) + partSize).toString().substr(1) + str;
      mult = (mult / partSize) | 0;
      buffer = (buffer / partSize) | 0;
    }
  }
  if (buffer !== 0) {
    str = buffer.toString() + str;
  } else {
    while (str.length > 1 && str[0] === '0') {
      str = str.substr(1);
    }
  }
  return {
    value: str,
    octetLength: j - start + 1
  };
}

function readBer(data, start, end) {
  var i = start, k;
  // 8.1.2 Identifier octets
  var tagOctet = data[i++];
  var tagNumber = tagOctet & 0x1F;
  if (tagNumber === 0x1F) {
    // 8.1.2.4
    var tmpNumber = decodeCompressedNumber(data, i, end);
    tagNumber = tmpNumber.value;
    i += tmpNumber.octetLength;
  }
  var tagClass = tagOctet >> 6;
  var constructed = !!(tagOctet & 0x20); // 8.1.2.3
  var contents;
  var b = data[i];
  if (b === 0x80) {
    // 8.1.3.6 indefinite form
    if (!constructed) {
      throw new Error('primitive cannot have indefinite form length');
    }
    i++;
    contents = [];
    var eocFound = false;
    while (i < end) {
      child = readBer(data, i, end);
      i += child.length;
      if (child.class === UNIVERSAL_TAG_CLASS && child.number === EOC_TAG) {
        eocFound = true; //  end-of-contents octets
        break;
      }
      contents.push(child);
    }
    if (!eocFound) {
      throw new Error('EOC is not found');
    }
  } else {
    // 8.1.3.3
    var shortForm = !(b & 0x80);
    var length;
    if (shortForm) {
      length = b;
      i++;
    } else {
      // 8.1.3.5
      var lengthOfLength = b & 0x7F;
      if (lengthOfLength >= 4) {
        throw new Error('length is too long');
      }
      i++;
      length = 0;
      for (k = 0; k < lengthOfLength; k++) {
        length = (length << 8) | data[i++];
      }
      length = length >>> 0;
    }
    if (i + length > end) {
      throw new Error('data tag overflow');
    }
    if (!constructed) {
      contents = readBerPrimitive(tagClass, tagNumber, data, i, i + length);
      i += length;
    } else {
      contents = [];
      while (i < end) {
        child = readBer(data, i, end);
        i += child.length;
        contents.push(child);
      }
    }
  }
  return {
    class: tagClass,
    number: tagNumber,
    constructed: constructed,
    contents: contents,
    offset: start,
    length: i - start
  };
}