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Re: [Cfrg] Attacker changing tag length in OCB

"Blumenthal, Uri - 0558 - MITLL" <uri@ll.mit.edu> Wed, 29 May 2013 15:03 UTC

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From: "Blumenthal, Uri - 0558 - MITLL" <uri@ll.mit.edu>
To: Ted Krovetz <ted@krovetz.net>
Date: Wed, 29 May 2013 11:03:32 -0400
Thread-Topic: [Cfrg] Attacker changing tag length in OCB
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Subject: Re: [Cfrg] Attacker changing tag length in OCB
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Ted,

Your explanation is perfectly satisfactory.

But I'd personally feel better if James M.'s suggestion was accepted -
i.e., if the tag length selection was explicitly encoded, perhaps the way
James proposed.

Tnx!
--
Regards,
Uri Blumenthal




On 5/29/13 10:28 , "Ted Krovetz" <ted@krovetz.net> wrote:

>James,
>
>Thanks for looking over the document. I didn't mean for this announcement
>to get posted here, but a few small edits caused an auto-announcement.
>Sorry.
>
>As for your concerns... AEAD_AES_128_OCB_TAGLEN128 and
>AEAD_AES_128_OCB_TAGLEN64 are different algorithms and a protocol will
>specify or negotiate one or the other, but not allow both simultaneously
>under the same key.
>
>If for some reason a protocol did want to allow tags valid under either
>algorithm, then the two algorithms would need to operate under different
>keys. Just like if you wanted to accept messages with tags valid under
>either HMAC or CBC-MAC, you'd use different keys for each.
>
>In other words, you are right. Shorter tags are simple truncations of
>longer tags. But, only a very poorly engineered system would be affected
>by this fact.
>
>-Ted
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