Re: [saag] Revision of "Attacks on Cryptographic Hashes in Internet Protocols"

Paul Hoffman <paul.hoffman@vpnc.org> Wed, 28 November 2012 15:40 UTC

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From: Paul Hoffman <paul.hoffman@vpnc.org>
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Date: Wed, 28 Nov 2012 07:40:46 -0800
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Subject: Re: [saag] Revision of "Attacks on Cryptographic Hashes in Internet Protocols"
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On Nov 28, 2012, at 7:23 AM, David McGrew (mcgrew) <mcgrew@cisco.com> wrote:

> Hi Paul,
> 
> A quick comment.   The draft says that uses for hash algorithms include
> 
>   o  Integrity protection.  It is common to compare a hash value that
>      is received out-of-band for a file with the hash value of the file
>      after it is received over an unsecured protocol such as FTP.
> 
> ... and it then says that collision resistance is not needed for the
> integrity protection use case.   However, it seems to me that there are
> possible threats against hash-based integrity protection that would be
> possible if the person/system generating the hash can generate collisions,
> and is untrustworthy.  Consider the case of an md5sum hash on a debian ISO
> image.  If the person responsible for generating the hash can create a
> malware-laden ISO imagine that has a hash collision with the actual ISO
> image, then they could substitute the bad one for the good one regardless
> of the hash checking step.  An attacker might leave the good ISO image in
> place to avoid detection, while foisting the bad one on some victims.  I
> am assuming that there are different ways of distributing ISO images, or
> that the substitution of bad for good can take place after the good one
> has been investigated and found to be good.
> 
> It is debatable how important this scenario is in practice, but it seems
> that collision resistance is at least desirable for integrity protection.

Collision resistance is not needed if you trust the party who created the hash; it is needed if you do not trust that party.

Proposed addition to the paragraph following the list:

Integrity protection is affected by collision attacks if the party checking the hash does not completely trust the party who published the hash.

--Paul Hoffman