On Sat, Jan 9, 2021 at 5:07 AM Xamix <gmax...@gmail.com> wrote:
>
> Thank you for your answer,
>
> For the moment I will do something like the following:
>
> // Create vector to store result
> std::vector<CryptoPP::byte> decoded(data_len);
>
> // Set counter before decoding
> decryptor_.Resynchronize(counter, 16);
> // Decrypt data
> decryptor_.ProcessData(decoded.data(), data, data_len);
>
> // For the moment we compute ourself the value of IV after decrypting
> // But maybe we could optimize by retrieving it from decryptor object?
> for(std::size_t i = 0; i < data_len / 16; ++i)
> {
> for(int j = 15; j >= 0; --j)
> {
> if (++counter[j] != 0)
> break;
> }
> }
>
> Note that data_len is a multiple of block size (i.e 16).
>
> Do you think it is possible to me to patch your library to get back the
> current IV in order to optimize the code a bit?
> I haven't take a look in the underling code, but is the current IV available
> in private member at one moment?
> Or maybe it is not available because you are using some Hardware function
> which do not return the IV?
The problem is not so much hardware. The problem is Filters and
Buffering. The current IV is held in an appropriate buffer provided by
a mode object, like CBC_Mode or CTR_Mode. The mode may only use a few
bytes of the iv, so the question becomes - what do you return?
Jeff
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