b64.js

/**
 * Created by Jacob Strieb
 * May 2020
 */

var b64 = (function() {

  // Generate a dictionary with {key: val} as {character: index in input string}
  function generateIndexDict(a) {
    let result = {};
    for (let i = 0; i < a.length; i++) {
      result[a[i]] = i;
    }
    return result;
  }

  // Decode URL safe even though it is not the primary encoding mechanism
  const _a = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  const _aRev = generateIndexDict(_a);
  _aRev["-"] = _aRev["+"];
  _aRev["_"] = _aRev["/"];

  const _enc = new TextEncoder("utf-8");
  const _dec = new TextDecoder("utf-8");

  return {

    // Encode a string as base64
    decode: function(s) {
      return this.binaryToAscii(this.base64ToBinary(s));
    },

    // Decode base64 to a string
    encode: function(s) {
      return this.binaryToBase64(this.asciiToBinary(s));
    },

    // Convert a string to a Uint8Array
    // FIXME: TextEncoding and TextDecoding are not actually inverses
    asciiToBinary: function(text) {
      return _enc.encode(text);
    },


    // Convert a Uint8Array to a string
    // FIXME: TextEncoding and TextDecoding are not actually inverses
    binaryToAscii: function(binary) {
      return _dec.decode(binary);
    },


    // Return a base64-encoded string from a Uint8Array input
    binaryToBase64: function(originalBytes) {
      // Pad the output array to a multiple of 3 bytes
      let length = originalBytes.length;
      let added = (length % 3 == 0) ? 0 : (3 - length % 3);
      let bytes = new Uint8Array(length + added);
      bytes.set(originalBytes);

      let output = "";
      for (let i = 0; i < bytes.length; i += 3) {
        // Convert 3 8-bit bytes into 4 6-bit indices and get a character from
        // the master list based on each 6-bit index
        //    3 x 8-bit:  |------ --|---- ----|-- ------|
        // => 4 x 6-bit:  |------|-- ----|---- --|------|

        // Get the first 6 bits of the first byte
        output += _a[ bytes[i] >>> 2 ];
        // Merge the end 2 bits of the first byte with the first 4 of the second
        output += _a[ ((bytes[i] & 0x3) << 4) | (bytes[i + 1] >>> 4) ];
        // Merge the end 4 bits of the second byte with the first 2 of the third
        output += _a[ ((bytes[i + 1] & 0xF) << 2) | (bytes[i + 2] >>> 6) ];
        // Get the last 6 bits of the third byte
        output += _a[ bytes[i + 2] & 0x3F ];
      }

      // Turn the final "A" characters into "=" depending on necessary padding
      if (added > 0) {
        output = output.slice(0, -added) + ("=".repeat(added));
      }

      return output;
    },


    // Takes a Base64 encoded string and returns a decoded Uint8Array. Throws
    // an error if the input string does not appear to be a valid base64
    // encoding. Attempts to add padding to un-padded base64 strings.
    base64ToBinary: function(s) {
      let bytes = [];

      // Base64 strings have at most 2 padding characters to make their length
      // a multiple of 4, so they could be missing up to 2 characters and still
      // be valid. But if 3 padding characters would be needed, the input
      // cannot be valid. Try and add padding characters if necessary/possible.
      if (s.length % 4 == 1) {
        throw "Invalid base64 input";
      } else if (s.length % 4 != 0) {
        s += "=".repeat(4 - (s.length % 4));
      }

      for (let i = 0; i <= (s.length - 4); i += 4) {
        // Check that each character in this group of 4 is valid
        for (let j = 0; j < 4; j++) {
          if (s[i + j] != "=" && !(s[i + j] in _aRev)) {
            throw "Invalid base64 input";
          } else if (s[i + j] == "=" && Math.abs(s.length - (i + j)) > 2) {
            throw "Invalid base64 input";
          }
        }

        // Convert 4 6-bit indices into 3 8-bit bytes by finding the index of
        // each 6-bit character in the master list and combining
        //    4 x 6-bit:  |------|-- ----|---- --|------|
        // => 3 x 8-bit:  |------ --|---- ----|-- ------|

        // Get all 6 bits of the first byte and first 2 bits of the second byte
        bytes.push((_aRev[s[i]] << 2) | (_aRev[s[i + 1]] >>> 4));
        if (s[i + 2] != "=") {
          // If not padding, merge end 4 bits of the second byte and first 4 of
          // the third
          bytes.push(((_aRev[s[i + 1]] & 0xF) << 4) | (_aRev[s[i + 2]] >>> 2));
        }
        if (s[i + 3] != "=") {
          // If not padding, take the last 2 bits of the third byte and all 6 of
          // the fourth. Note that if the fourth byte is padding, then certainly
          // the third byte is, so we only have to check the fourth
          bytes.push(((_aRev[s[i + 2]] & 0x3) << 6) | _aRev[s[i + 3]]);
        }
      }

      return new Uint8Array(bytes);
    }

  }
})();