javascript:
function base64_decode( data ) {
// Decodes data encoded with MIME base64
//
// + original by: Tyler Akins (http://rumkin.com)
var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
var o1, o2, o3, h1, h2, h3, h4, bits, i=0, enc='';
do { // unpack four hexets into three octets using index points in b64
h1 = b64.indexOf(data.charAt(i++));
h2 = b64.indexOf(data.charAt(i++));
h3 = b64.indexOf(data.charAt(i++));
h4 = b64.indexOf(data.charAt(i++));
bits = h1<<18 | h2<<12 | h3<<6 | h4;
o1 = bits>>16 & 0xff;
o2 = bits>>8 & 0xff;
o3 = bits & 0xff;
if (h3 == 64) enc += String.fromCharCode(o1);
else if (h4 == 64) enc += String.fromCharCode(o1, o2);
else enc += String.fromCharCode(o1, o2, o3);
} while (i < data.length);
return enc;
}
//Примеры:
alert ( base64_decode('S2V2aW4gdmFuIFpvbm5ldmVsZA==') );