Re: How is .net generating extra bits in MD5?
From: Shawn Farkas [MS] (shawnfa_at_online.microsoft.com)
Date: 01/26/05
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Date: Wed, 26 Jan 2005 18:38:23 GMT
Right ... PasswordDeriveBytes does extend beyond the MD5 hash using an
algorithm similar to:
1. Reset the hash
2. Increment a prefix (starting with byte[] { (byte)'0', (byte)'0',
(byte)'0'} )
3. If the prefix wraps around (gets bigger than 999), fail
4. Hash prefix + original hash
You can try that out and see if it works for you.
-Shawn
http://blogs.msdn.com/shawnfa
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> From: "Pete" <jpeteb@hotmail.com>
> Newsgroups: microsoft.public.dotnet.security
> Subject: Re: How is .net generating extra bits in MD5?
> Date: 25 Jan 2005 17:59:26 -0800
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>
> Hi Shawn,
>
> OK, I hope I am not being incredibly stupid here, but this is the
> disconnect I'm having in my head. I've looked at PDKDF1, which says
> that the length of the derived key is bounded by the length of the hash
> function output, which is 16 octets or 128 bits in MD5. That I can
> generate fine, but the code fragment is requesting 32 octets or 256
> bits.
>
> >From your blog, it says "PBKDF1 will only produce the number of bytes
> that the hash algorithm generates, but GetBytes will extend the result
> further, allowing you to get a large number of bytes out of your
> password."
>
> There's my question - how is GetBytes() actually producing 16 octets
> more than PDKDF1 says it should? I don't understand how GetBytes() is
> extending the result past what PBKDF1 says. It seems that it must be
> doing another hash to create the data, but how is that derived?
>
> I need to make this work exactly like the .net version as we are using
> this code to pass information between different OS'es, and if we can't
> decode and encode on the foreign systems, that is a big problem.
> Thanks for any more insight you can provide.
>
> --pete
>
>
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