SecretKeySpec mac and base64

Posted on 2012-03-15
Last Modified: 2012-03-15
I have a secret key which is already in base64 format.

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import javax.crypto.SecretKey;

              String key = "<base 64 encoded string>";
              String key_1 =  Base64.decode(key);
              try {
                  Mac mac = Mac.getInstance("HmacSHA1");
                  SecretKey secretKey  = new SecretKeySpec(key_1.getBytes("UTF-8"),"HmacSHA1");
                  String enc = new String(mac.doFinal());

                  // Base 64 Encode the results
                  String retVal = Base64Codec.encode(enc);
                  System.out.println ("result: " + retVal);
              } catch (Exception e) {

The output I am getting  "result: "  is not matching with the output when I execute an equivalent code in python

I am sure something to do with byte array conversion in java and string but cant figure it

NOTE: If I don't do base64 decode and use "new SecretKeySpec(key.getBytes)"
then the output of "result: " is same in java and output from python code.
Question by:learningunix
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LVL 35

Expert Comment

ID: 37727439
I am not sure what the question is then since you have solved it with your statement...

If I don't do base64 decode and use "new SecretKeySpec(key.getBytes)"
then the output of "result: " is same in java and output from python code.

Your python code is maybe automatically doing the Base64 encode when it create the SecretKeySpec equivalent? I don't know because a) you have posted the python code, and b) I don't know a lot of python anyway ;)

Author Comment

ID: 37727488
The python code does base64decode first. I am trying to mimic the code in java.
python uses "base64" and "hmac" library

Author Comment

ID: 37727499
I am sure something to do with base64decode as it coverts to binary on windows and something is getting messed up.  The python I am runing is on linux machine
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LVL 35

Expert Comment

ID: 37727503
Ok, maybe your statement is a bit unclear... when you said
If I don't do base64 decode and use "new SecretKeySpec(key.getBytes)"
did you mean that when you don't do base64 decode in both java and python the results come out the same?

(I thought that you were saying that when the python code DOES do the bas64 decode and the Java DOESN'T do the decode, that the results line up)
LVL 35

Expert Comment

ID: 37727516
What are you using to do your Base64 conversion?

I would say that you are on the mark with byte array conversions being the issue. It is strange that your Base64.decode(key) returns a String, it should really return a byte[] so that no further conversion needs to happen. Yeah, the problem that you are having is because the bytes will be converted to String and then back again at .getBytes("UTF-8") but various encoding schemes will conspire to change the actual bytes on the way through.

Author Comment

ID: 37727519
may be i confused u. let me explain it again.

the exact above Java code is running on windows. An exact identical code in python runs from unix.

the value of result does not match.

not sure if something to do with charset on windows

Author Comment

ID: 37727544
The issue is definitely with java. I just installed python on y windows machine and I get same value for "result" when I run python on unix

Not sure why Java won't give me same reuslt. something to with getBytes or new String() which is messing up
LVL 35

Expert Comment

ID: 37727563
Yeah, I understood that part ok. The part that wasn't clear was the NOTE: right at the end of the original question. (That's what I was referring to in the above comment)

Anyway, regardless, I think the issue is still what I wrote about above... So again, what are you using to do the Base64 conversion? A library? Your own code?

Hang on a sec, you just mentioned "new String()" and this made me check your code again, and yes there is an issue there to. The byte[] returned from mac.doFinal() is being converted to a String aswell. This byte[] should also just be passed directly from mac.doFinal() to base64 encode(), not via a String!

Author Comment

ID: 37727583
I found the solution, I knew it was charset issue.
the correct solution is

SecretKey secretKey  = new SecretKeySpec(key_1.getBytes("ISO-8859-1"),"HmacSHA1");
String enc = new String(mac.doFinal(), "ISO-8859-1");

PS: I have my own base 64 which accepts string only
LVL 35

Accepted Solution

mccarl earned 500 total points
ID: 37727639
NO, the solution is to NOT convert to string at all. You will only be setting yourself for more issues down the track, ie. remembering to use the correct encoding everywhere, or someone changing the encoding unknowingly because it isn't clear what is going. On top of that is the lack of efficiency as you needlessly convert to string and back again.

Change you Base64 routines to encode from a byte[] and to decode back to a byte[] and save yourself all this unnecessary heart ache!!

Author Closing Comment

ID: 37727666

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