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MessageDigest Length

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  MessageDigest Length "sean" <sean@THRWHITE.remove-dii-this> - 2011-04-27 16:07 +0000
    Re: MessageDigest Length "Robert Kochem" <robert.kochem@THRWHITE.remove-dii-this> - 2011-04-27 16:07 +0000
      Re: MessageDigest Length "Roedy Green" <roedy.green@THRWHITE.remove-dii-this> - 2011-04-27 16:07 +0000
    Re: MessageDigest Length "Roedy Green" <roedy.green@THRWHITE.remove-dii-this> - 2011-04-27 16:07 +0000
    Re: MessageDigest Length "Maarten Bodewes" <maarten.bodewes@THRWHITE.remove-dii-this> - 2011-04-27 16:08 +0000
      Re: MessageDigest Length "Roedy Green" <roedy.green@THRWHITE.remove-dii-this> - 2011-04-27 16:08 +0000
        Re: MessageDigest Length "Maarten Bodewes" <maarten.bodewes@THRWHITE.remove-dii-this> - 2011-04-27 16:08 +0000
          Re: MessageDigest Length "Roedy Green" <roedy.green@THRWHITE.remove-dii-this> - 2011-04-27 16:08 +0000
          Re: MessageDigest Length "Roedy Green" <roedy.green@THRWHITE.remove-dii-this> - 2011-04-27 16:08 +0000
          Re: MessageDigest Length "rmoldskr+usenet" <rmoldskr+usenet@THRWHITE.remove-dii-this> - 2011-04-27 16:08 +0000

#54 — MessageDigest Length

  To: comp.lang.java.security
Does anyone know if there is a way of setting the length of the hashed
value returned from a message digest?

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#56

  To: comp.lang.java.security
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

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#67

  To: comp.lang.java.security
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

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#66

  To: comp.lang.java.security
On Tue, 11 Dec 2007 17:18:45 -0800 (PST), sean@onset.com.au 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?

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

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#78

  To: comp.lang.java.security
sean@onset.com.au wrote:
> 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

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#81

  To: comp.lang.java.security
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".

-- 

Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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#87

  To: comp.lang.java.security
Roedy Green wrote:
> 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".
> 

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

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#96

  To: comp.lang.java.security
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.
-- 

Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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#97

  To: comp.lang.java.security
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.
-- 

Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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#99

  To: comp.lang.java.security
Maarten Bodewes <maarten.bodewes@xs4all.nl> wrote:

> 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.

-- 
Leif Roar Moldskred
Got Sfik?

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