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Re: MD5: hashing, > 1->1

To: Karl Lui Barrus <[email protected]>
Subject: Re: MD5: hashing, > 1->1
From: "Perry E. Metzger" <[email protected]>
Date: Wed, 06 Jul 1994 08:21:20 -0400
Cc: [email protected]
In-Reply-To: Your message of "Tue, 05 Jul 1994 18:24:54 CDT." <[email protected]>
Reply-To: [email protected]
Sender: [email protected]


Karl Lui Barrus says:
> The birthday paradox situation corresponds to just finding two
> messages with the same hash.  In this case the expected work is 2^64,
> but then the two messages that you discover with the same hash may be
> random (and thus worthless).

You can engineer them, actually.

Imagine that you had a 64 bit hash function, and the birthday paradox
thus provided you with a 2^32 difficulty in finding a collision.

Prepare two versions of the document you want to fake the signature
on. Adjust the documents over and over again (trivia like spacing will
do -- find 32 locations and either add or don't add a space) until you
get a colliding pair of hashes. 

This illustrates that hash collisions are actually quite a problem if
you have an insufficiently large hash.

Perry

References:
- MD5: hashing, > 1->1
  - From: Karl Lui Barrus <[email protected]>

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