Can two different BASE 64 encoded strings result into same string if decoded?

Question

On client side I am doing an MD5 encryption of a string and then a BASE 64 encoding on the hash generated.

This final encoded string is then used for comparison on server side.

I was able to do this successfully for many test cases but it failed for the below one.

String for MD5

  2679:07071960:09348448:3:08912206:3:EXPRESS:1:EU4NW31E7INEC1X

My MD5 hash string:

  291423A531148527A9524EA0924CDF68

My Base64 encoded string:

  KRQjpTEUhSepUk6gkkzfaA==

When I try to put the MD5 hash string for BASE64 encoding on http://www.opinionatedgeek.com/dotnet/tools/base64encode/ it generates following string:

  MjkxNDIzQTUzMTE0ODUyN0E5NTI0RUEwOTI0Q0RGNjg=

But, when I try to decode my Base64 string, that is "KRQjpTEUhSepUk6gkkzfaA==", here http://www.opinionatedgeek.com/dotnet/tools/Base64Decode/Default.aspx I am getting my Hash Code(opened the .bin file being generated in hex editor).

So, is it possible that a single string may have multiple Base64 encoded value?

I am using the below code for generating the encoded string:

  public static String getHashCode(String text) 
 { 
    MessageDigest md;
    byte[] md5hash = new byte[32];
    try{
    md = MessageDigest.getInstance("MD5");

    md.update(text.getBytes("iso-8859-1"), 0, text.length());
    md5hash = md.digest();
    }
    catch(Exception e)
    {
        return "-1";
        }
    String encoded = Base64.encode(md5hash);
    String retValue = new String(encoded);


    return retValue;

   } 

Kindly, suggest what is going wrong and how to make sure that we use same digest both on client and server side.

Answer 1

The accepted answer solves the problem but does not answer the question.

For example, these base64 values QzNWwq== and QzNWwr== encode the same binary value (hex) 433356c2

You can check it at http://kjur.github.io/jsjws/tool_b64udec.html or using the command

echo <<BASE64>> | base64 -d | xxd

In summary

• Two different base 64 encode same value --> true
• Two different values generate the same base64 value --> false (see this)

Answer 2

You have the MD5 Hash 291423A531148527A9524EA0924CDF68 that generates the Base64 encoded string MjkxNDIzQTUzMTE0ODUyN0E5NTI0RUEwOTI0Q0RGNjg= , which is fine. You have converted the MD5 hash to an ascii hex representation, and base64 encoded that.

However your own getHashCode() works differently, it creates a base64 encoding of the binary representation of your hash code, you have not converted your hash to an ascii hex representation, and that's why you see different base64 encoded strings.