Re: [babel] Babel-MAC: Blake2s is 128-bits by default

Juliusz Chroboczek <jch@irif.fr> Wed, 25 November 2020 18:28 UTC

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Date: Wed, 25 Nov 2020 19:28:03 +0100
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From: Juliusz Chroboczek <jch@irif.fr>
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Subject: Re: [babel] Babel-MAC: Blake2s is 128-bits by default
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> This does raise one question, though (as I just found out while
> implementing the option for a 128-bit version): How does this impact the
> recommendations for key size?

I suggest that they remain the same (SHOULD be 32 bytes).

The digest size impacts the overhead carried by every packet.  Not so the
key size: increasing the key size is essentially free.  As a matter of
fact, Blake2s will pad the key with zeroes before it is used.

Thus:

  - allowing shorter keys gains no generality (the user may achieve the
    same result by padding with zeroes before passing the key to Blake2s);

  - disallowing shorter keys causes confusion (by using a value different
    from both HMAC-SHA-256 and the Blake2s maximum), and has no measurable
    benefits.

I do not believe that it impacts the security in any way (2^128 ns is
roughly 10^12 times the age of the universe), but I could be wrong.

-- Juliusz