OCB nonce length

After playing a bit with tthe OCB code, it seems that the nonce length 
is fixed at 12 bytes exactly. I wonder if this is what we want for 
SQRL, and whether it can be safely adjusted to some other value. For 
now, it is fixed in the ae_init function.


2/24/2014 4:55:55 AM
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[for the unabridged version, see John Milburn's post above]

> After playing a bit with the OCB code, it seems that the nonce
> length is fixed at 12 bytes exactly. I wonder if this is what
> we want for SQRL, and whether it can be safely adjusted to some
> other value. For now, it is fixed in the ae_init function.

Ah. You're a bit ahead of my documentation here, John.

In our application, we won't be using OCB's IV nonce at all. 
It'll simply be null.

An IV is only required to protect block ciphers from information 
leakage when an observer might be able to observe the cipher 
mode operate with the SAME KEY over DIFFERENT data. As long as 
that can never happen, an initialization vector adds nothing.

As we can see in our application...


.... in both places where we're using OCB, it's being keyed by 
our EnScrypt PBKDF... and THAT function (EnScrypt) is salted
to ensure that a different derived key is generated for OCB
any time the DATA being encrypted is changed.  (The EnScrypt 
salt and the encrypted key are always stored together.)

So, I think we're okay!  <g>

Steve.               Working on moving the SQRL project forward.
2/24/2014 4:57:49 PM

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