base64.js

// adapted from here: http://ostermiller.org/calc/encode.html
var base64 = {};

(function () {
  var END_OF_INPUT = -1,
      base64Chars = new Array(
        'A','B','C','D','E','F','G','H',
        'I','J','K','L','M','N','O','P',
        'Q','R','S','T','U','V','W','X',
        'Y','Z','a','b','c','d','e','f',
        'g','h','i','j','k','l','m','n',
        'o','p','q','r','s','t','u','v',
        'w','x','y','z','0','1','2','3',
        '4','5','6','7','8','9','+','/'),
      reverseBase64Chars = new Array(),
      base64Str,
      base64Count;
      
  for (var i=0; i < base64Chars.length; i++){
      reverseBase64Chars[base64Chars[i]] = i;
  }

  function setBase64Str(str){
      base64Str = str;
      base64Count = 0;
  }
  function readBase64(){    
      if (!base64Str) return END_OF_INPUT;
      if (base64Count >= base64Str.length) return END_OF_INPUT;
      var c = base64Str.charCodeAt(base64Count) & 0xff;
      base64Count++;
      return c;
  }
  function readReverseBase64(){   
      if (!base64Str) return END_OF_INPUT;
      while (true){      
          if (base64Count >= base64Str.length) return END_OF_INPUT;
          var nextCharacter = base64Str.charAt(base64Count);
          base64Count++;
          if (reverseBase64Chars[nextCharacter]){
              return reverseBase64Chars[nextCharacter];
          }
          if (nextCharacter == 'A') return 0;
      }
      return END_OF_INPUT;
  }

  function ntos(n){
      n=n.toString(16);
      if (n.length == 1) n="0"+n;
      n="%"+n;
      return unescape(n);
  }

  base64.encode = function(str){
      setBase64Str(str);
      var result = '';
      var inBuffer = new Array(3);
      var lineCount = 0;
      var done = false;
      while (!done && (inBuffer[0] = readBase64()) != END_OF_INPUT){
          inBuffer[1] = readBase64();
          inBuffer[2] = readBase64();
          result += (base64Chars[ inBuffer[0] >> 2 ]);
          if (inBuffer[1] != END_OF_INPUT){
              result += (base64Chars [(( inBuffer[0] << 4 ) & 0x30) | (inBuffer[1] >> 4) ]);
              if (inBuffer[2] != END_OF_INPUT){
                  result += (base64Chars [((inBuffer[1] << 2) & 0x3c) | (inBuffer[2] >> 6) ]);
                  result += (base64Chars [inBuffer[2] & 0x3F]);
              } else {
                  result += (base64Chars [((inBuffer[1] << 2) & 0x3c)]);
                  result += ('=');
                  done = true;
              }
          } else {
              result += (base64Chars [(( inBuffer[0] << 4 ) & 0x30)]);
              result += ('=');
              result += ('=');
              done = true;
          }
          lineCount += 4;
          if (lineCount >= 76){
              result += ('\n');
              lineCount = 0;
          }
      }
      return result;
  }
  
  base64.decode = function(str){
      setBase64Str(str);
      var result = "";
      var inBuffer = new Array(4);
      var done = false;
      while (!done && (inBuffer[0] = readReverseBase64()) != END_OF_INPUT
          && (inBuffer[1] = readReverseBase64()) != END_OF_INPUT){
          inBuffer[2] = readReverseBase64();
          inBuffer[3] = readReverseBase64();
          result += ntos((((inBuffer[0] << 2) & 0xff)| inBuffer[1] >> 4));
          if (inBuffer[2] != END_OF_INPUT){
              result +=  ntos((((inBuffer[1] << 4) & 0xff)| inBuffer[2] >> 2));
              if (inBuffer[3] != END_OF_INPUT){
                  result +=  ntos((((inBuffer[2] << 6)  & 0xff) | inBuffer[3]));
              } else {
                  done = true;
              }
          } else {
              done = true;
          }
      }
      return result;
  }
})()

usage.js

base64.decode(base64.encode("open sesame"));
// "open sesame"