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It seems to me there is a bug in the base64 decode.
How could (enc3 == 64) on line 88, since you removed all paddings on line 59?
enc3 can never be 64 so that check seems unnecessary.

Similarly, the line
enc4 = this._keyStr.indexOf(input.charAt(j++));
seems wrong. If j>input.length, then input.charAt() = "" and indexOf("") =0, which is wrong since indexOf("A") = 0 as well. But your code works because in this case, i is already equal to bytes, so writing to uarray[i+1] writes outside of the actual array so it adds no new characters there.

1. line 44 and 56: var bytes = Math.ceil( (3*input.length) / 4.0);
   change to var bytes = (input.length/4) * 3;
   because input.length must be input.length % 4 = 0
2. line 54 var lkey2 = this._keyStr.indexOf(input.charAt(input.length-1));
   change to var lkey2 = this._keyStr.indexOf(input.charAt(input.length-2));
   because you need to analyze two last characters
3. Don`t understand why you need decodeArrayBuffer function, remove it and use decode

I'm trying to use your script to decode canvas data in android and get the subject error. I'm reading that this is not supported in Android 2.3.

Is there any known workaround?

I read issue #2 which was asking about removing decodeArrayBuffer. After reading your answer what's its purpose is still not clear to me. It returns an array with size always divisible by 3, which I don't understand if it is by design or a bug: please clarify.

at lines 53, 54

var lkey1 = this._keyStr.indexOf(input.charAt(input.length-1));
var lkey2 = this._keyStr.indexOf(input.charAt(input.length-1));

surely line 54 should read

var lkey2 = this._keyStr.indexOf(input.charAt(input.length-2));

as there can be two pad characters at positions length -1 and length -2

http://blog.danguer.com/2011/10/24/base64-binary-decoding-in-javascript/

So,I decide to stat it