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RE: aes128gcm: why verify padding?

"Manger, James" <James.H.Manger@team.telstra.com> Fri, 27 January 2017 01:54 UTC

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From: "Manger, James" <James.H.Manger@team.telstra.com>
To: Martin Thomson <martin.thomson@gmail.com>
CC: "ilariliusvaara@welho.com" <ilariliusvaara@welho.com>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>
Thread-Topic: aes128gcm: why verify padding?
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Subject: RE: aes128gcm: why verify padding?
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>> If we go with this approach, we should also distinguish the 1st record. For instance, for the end-of-padding byte:


> I think that might be going too far.  Truncating the first record
requires that you are able to find an input to HKDF that provides the
same key, and a nonce that has just the lower bit flipped (you can
truncate more with higher probability).  That requires several
preimage attacks on SHA-256 in series.  Mostly though, marking the
start also increases complexity.  I think that it's enough to say 0x01
= keep going, 0x02 = all done.


I agree that Start-or-middle vs End is enough in this case due to the KDF.
I was hoping for an anti-truncation mechanism that didn't depend in a not-completely-obvious-to-me manner on a seemingly quite separate aspect: the KDF.
For instance, even with no KDF (for key or nonce) having a byte distinguishing start/middle/end would be sufficient to authenticate you have received an authentic prefix or suffix or complete message.

--
James Manger

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