I'm using AES-GCM to send multiple messages (CryptoPP::GCM<CryptoPP::AES>)
> via AuthenticatedEncryptionFilter.
> It seems I need to resynchronize the underlying GCM cipher after each
> message with a call to Resynchronize which
> needs a new iv as argument.
>
> I see no reason why this new iv is neccessary. GCM uses a counter, so the
> "iv" is a nonce, not necessitating
> a fully random iv. Internally GCM increments the nonce for every AES
> block, so at the point one has to resynchronize it,
> it is already at a usefull last_iv+1.
>
> Does anything break by extending CryptoPP::GCM by a resynchronize method
> which does not change the iv, like:
>
> class CtrNonceGCMEncryption : public CryptoPP::GCM<CryptoPP::AES >::
> Encryption {
> public:
> void Resynchronize() { m_state = State_IVSet; }
> };
>
> and using this method instead (as well as in Decryption)? This would save
> on random nonce generation and transmission.
>
The reasoning makes sense to me. I don't believe you're violating security
requirements because the security context is unique per message.
The one thing I would verify is GCM's IncrementCounter() function gets
called when MessageEnd() is propagated to ensure you're not reusing your
accidentally reusing the last IV. That's the sort of optimization (defer on
the increment unless its needed) Wei would provide.
Also see GCM's source at
http://www.cryptopp.com/docs/ref/gcm_8cpp_source.html.
Jeff
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