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my code gives the wrong hash value

Tim McGuire
Ranch Hand

Joined: Apr 30, 2003
Posts: 820

Hello,
I'm trying to follow directions to generate a hash of a string to get the "digestValue" portion of a digitally signed XML file.

the code below works and gives me a string. but the string doesn't match the digestValue in the xml example file, part of which is given below.
I guess my main question: Is MessageDigest the correct class to be using in this situation?




xml file I'm trying to match:


finally, here are the instructions I am trying to follow:
1) Apply a hash algorithm over the specified content to be digitally signed. In this case, the content to be signed is <wsu:Timestamp>. The hash algorithm that is used is SHA-1. The result of the hashing operation is stored in the DigestValue. The SHA-1 hash value is 160 bits in length and when converted into Base64 it is precisely 28 characters, which is exactly what you see in DigestValue.
Ulf Dittmer
Marshal

Joined: Mar 22, 2005
Posts: 41613
    
  55
Could it be a matter of whitespace? In the Java app, you're concatenating the elements w/o line breaks, while the XML you quote has them, as well as leading spaces in the two lines in the middle.


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Robin Wilson
Greenhorn

Joined: May 10, 2006
Posts: 22
It is almost certainly related to whitespace... The information you are supposed to be hashing is explicitly the 4 lines specified. However, you are manually selecting 4 separate strings, without getting all the whitespace in the 4 specified lines. You are missing "\n" at the end of the first 3 lines, then " " at the beginning of lines 2 and 3... That does make a difference.

(Keep in mind that simply because you can't see a character doesn't mean it isn't there - and if it is there, it has a value that will significantly alter the resulting hash value. We see a file as a series of independent "lines" of data. The computer sees a file as a continuous stream of bit (bytes), including the bytes that represent line breaks and leading/trailing spaces.)


--<br />Robin D. Wilson
Tim McGuire
Ranch Hand

Joined: Apr 30, 2003
Posts: 820

Thank you, guys.
That was it.
In fact, there is a step called "canonicalization" or "c14n" that I was missing. It must be done when signing things. This is handled by the WSS4J libraries and it strips whitespace before signing an element.
 
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