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I'm having a strange problem with converting a string to a byte array to hash it. Right now, my code is something like this:

    String textToHash = "test"; 
    MessageDigest messageDigest = MessageDigest.getInstance("MD5");
    messageDigest.update(textToHash.getBytes("UTF-8"));
    hash = messageDigest.digest();

Even though the string is the same, on different runs, the byte array (generated from textToHash.getBytes("UTF-8")) changes. Sometimes it will have one value, and other times it will change even though the string is static. Why is this happening and how can I make it reliably hash a string?

Thanks!

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I'd think this has to be a case of "what you think is happening is not what is actually happening". String.getBytes should return the same thing for a given input string. What exactly leads you to believe that the input String is the same but that getBytes returns a different result? – davmac Jan 24 at 15:18
My input string is always "test". When I print the value of textToHash.getBytes, it will sometimes return a different value. – Andrew Hassan Jan 24 at 16:26

1 Answer

up vote 1 down vote accepted

call MessageDigest.reset before update

http://docs.oracle.com/javase/1.4.2/docs/api/java/security/MessageDigest.html#reset()

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That still doesn't work. It breaks before hashing. Essentially it's the getBytes call that's not working. The byte array that that produces is what varies. Not the output of the hash function. – Andrew Hassan Jan 24 at 15:05
@AndrewHassan sorry. Are you sure byte[] form for String.getBytes are different each time ? plz paste the testcase – farmer1992 Jan 24 at 15:20
I wasn't using any test cases, just debugging. If I print or view the value of the bytes, it will output different values from time to time. Here is one of my outputs: Test #0 String: test Bytes: [B@68111f9b Test #1 String: test Bytes: [B@21c8dfe6 Test #2 String: test Bytes: [B@ae3865e Test #3 String: test Bytes: [B@5f8a8ae7 Test #4 String: test Bytes: [B@a574b2 Test #5 String: test Bytes: [B@6e905272 Test #6 String: test Bytes: [B@4514f313 Test #7 String: test Bytes: [B@626287d3 Test #8 String: test Bytes: [B@32c3601b Test #9 String: test Bytes: [B@38daa6a6 – Andrew Hassan Jan 24 at 16:24
@AndrewHassan [B@xx means byte[] at mem addr xx . you should do like this byte[] b = new byte[]{1,2,3}; System.out.println(b); // wrong System.out.println(Arrays.toString(b)); – farmer1992 Jan 24 at 16:27
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Oh, I can't believe I missed that. I feel really dumb right now haha. Thanks for your help! – Andrew Hassan Jan 24 at 16:53

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