var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
var input = "PGlmcmFtZSBzcmM9Imh0dHA6Ly93d3cueHh4c2V4ZmlsbXMubmwvdm9pY2UvZTFjZmU4LzAwMDAwMC8wMDAwMDAvZmZmZmZmLzY5NF9rb3J0ZXNleGZpbG1iZS5odG1sP3Q9MSIgd2lkdGg9IjI3NCIgaGVpZ2h0PSIzNzgiIGZyYW1lYm9yZGVyPSIwIiBzY3JvbGxpbmc9Im5vIj48L2lmcmFtZT4=";
var output = "";
var chr1, chr2, chr3;
var enc1, enc2, enc3, enc4;
var i = 0;

// remove all characters that are not A-Z, a-z, 0-9, +, /, or =
input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");

do {
  enc1 = keyStr.indexOf(input.charAt(i++));
  enc2 = keyStr.indexOf(input.charAt(i++));
  enc3 = keyStr.indexOf(input.charAt(i++));
  enc4 = keyStr.indexOf(input.charAt(i++));

  chr1 = (enc1 << 2) | (enc2 >> 4);
  chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
  chr3 = ((enc3 & 3) << 6) | enc4;

  output = output + String.fromCharCode(chr1);

  if (enc3 != 64) {
     output = output + String.fromCharCode(chr2);
  }
  if (enc4 != 64) {
     output = output + String.fromCharCode(chr3);
  }
} while (i < input.length);
document.write(output);