Subject: RE: [Cfrg] Derivation of an AES key from hash function

From: "Blumenthal, Uri" <[email-masked]>
Date: Fri, 21 Jul 2006 09:56:51 -0400

No. But it is possible to prove the opposite. As Helger did for you.

________________________________

	From: ngaga Gisse [mailto:[email-masked]] 
	Sent: Friday, July 21, 2006 9:05 AM
	To: Helger Lipmaa
	Cc: [email-masked]
	Subject: Re: [Cfrg] Derivation of an AES key from hash function

	Thank you Helger for the comments,
	I reformulate my question to make it clear,
	 
	Generally, is it possible to show/prove  that a collision
restant hash function gives a random output? (Assume that there are  no
bits padded or truncated from the hash result of  the  CRHF).
	 
	If Yes,  what are the possible sources (papers or books)?
	 
	Regards

	Helger Lipmaa <[email-masked]> wrote:

		No!
		
		For example if h is a collision-resistant hf then also
the next function 
		is crhf: take h(x), concatenate it with 50 zeros. The
result is clearly 
		not random.
		
		CRHF-s are not supposed to be random, randomness is
guaranteed by 
		different primitives.
		
		Helger
		PS I did not read the previous discussion...
		
		On Fri, 21 Jul 2006, ngaga Gisse wrote:
		
		> Dear all,
		> Is there a way to prove that, a collision resistant
hash function gives 
		> a random output?. That is, given that you are using a
collision 
		> resistant hash function, can it be proved that the
output hash result is 
		> random?
		> Regards

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