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Java Forum / Security / April 2008MessageDigest Length
Does anyone know if there is a way of setting the length of the hashed value returned from a message digest? sean@onset.com.au schrieb: > Does anyone know if there is a way of setting the length of the hashed > value returned from a message digest? Sorry, but this does not make sense to me. Each digest algorithm has a specified output length, e.g. MD5 128-Bit, SHA-1 160bit ... Truncating this output may result in collisions and extending the output is also not destined. If you want a deterministic variable length output generated by a given input you should look for a PRNG (Pseudo Random Number Generator) using it's seed as input value. Robert On Sat, 15 Dec 2007 13:07:47 +0100, Robert Kochem <robert@mailueberfall.de> wrote, quoted or indirectly quoted someone who said : >If you want a deterministic variable length output generated by a given >input you should look for a PRNG (Pseudo Random Number Generator) using >it's seed as input value. see http://mindprod.com/jgloss/digest.html why you would not want to do that. -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com >Does anyone know if there is a way of setting the length of the hashed >value returned from a message digest? each algorithm has a standard fixed length. If you want a different length, use a different algorithm. See http://mindprod.com/jgloss/digest.html -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com > Does anyone know if there is a way of setting the length of the hashed > value returned from a message digest? There isn't, but in general a good message digest spreads all the information over all the bits of the result. So you can just cut stuff off of the message digest. In protocols, you generally take the first bytes from the returned byte array (e.g. within Key Generation or 4 byte 3DES MAC). Maarten On Sat, 22 Mar 2008 16:25:42 +0100, Maarten Bodewes <maarten.bodewes@xs4all.nl> wrote, quoted or indirectly quoted someone who said : >> Does anyone know if there is a way of setting the length of the hashed >> value returned from a message digest? There is not much point in generating a 256 bit digest then using only the first 16 bits. You can same yourself considerable CPU cycles by generating a 16 bit digest directly . Just pick an algorithm with the size you want. For a list of algorithms see http://mindprod.com/jgloss/digest.html Short ones are usually called "checksums".  SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > On Sat, 22 Mar 2008 16:25:42 +0100, Maarten Bodewes > <maarten.bodewes@xs4all.nl> wrote, quoted or indirectly quoted someone [quoted text clipped - 11 lines] > > Short ones are usually called "checksums". That's the worst advice I've seen for years. The strength is in the algorithm mostly, not in the length of the value that you use. A CRC does not provide cryptographic security *at all*. Of course, you cannot delete bits without destroying the chances of creating hash collisions, but at least you would not be able to just calculate them. Actually, you should not be posting here if you know this little about cryptography. Regards, Maarten On Mon, 31 Mar 2008 00:06:26 +0200, Maarten Bodewes <maarten.bodewes@xs4all.nl> wrote, quoted or indirectly quoted someone who said : >Actually, you should not be posting here if you know this little about >cryptography. I did not see any mention of cryptography. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com On Mon, 31 Mar 2008 00:06:26 +0200, Maarten Bodewes <maarten.bodewes@xs4all.nl> wrote, quoted or indirectly quoted someone who said : >Actually, you should not be posting here if you know this little about >cryptography. If he wants only 16 bits, the purpose is not going to be cryptography. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > That's the worst advice I've seen for years. The strength is in the > algorithm mostly, not in the length of the value that you use. No matter what algorithm you use, a single bit digest won't be very secure. SignatureLeif Roar Moldskred Got Sfik? Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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