Base 64 decode to String not as expected-CodePudding

I have a String that I want to decode to a binary object (BigInteger or its String) Say for instance I got the String

String str = "LTEwMTAwMTEwMTExMA==";

An now I want to decode it using the Base64.getDecoder() method, such as:

String result = new BigInteger(Base64.getDecoder().decode(str.getBytes("UTF-8"))).toString(2);

According to that, the value of result is

101101001100010011000000110001001100000011000000110001001100010011000000110001001100010011000100110000

which does not match with the result that I obtain using any online decoder:

-101001101110

Could you please help me? what am I doing wrong? Thank you very much!

CodePudding user response：

You have an issue with your String conversions - try this instead:

String result = new BigInteger(new String(Base64.getDecoder().decode(encoded.getBytes("UTF-8")))).toString();

By converting the byte array to String using a new String constructor you're getting a simple String representation which BigInteger constructor knows how to parse. The result will be the expected -101001101110

CodePudding user response：

Base64.Decoder.decode() produces a byte array, not a string, so you need to convert it. Then you can use the BigInteger constructor that accepts a radix to specify base 2:

import java.io.IOException;
import java.math.BigInteger;
import java.util.Base64;

public class Test
{
    public static void main(String args[]) throws IOException
    {
        String str = "LTEwMTAwMTEwMTExMA==";
        String str2 = new String(Base64.getDecoder().decode(str), "UTF-8");
        BigInteger big = new BigInteger(str2, 2);
        System.out.println(big.toString(2));
    }
}

Output:

-101001101110