On Fri, Jan 8, 2021 at 8:05 PM Jeffrey Walton <noloa...@gmail.com> wrote:
>
> On Fri, Jan 8, 2021 at 10:44 AM Xamix <gmax...@gmail.com> wrote:
> >
> > I'm using the library with the templated AES CTR decryptor:
> >
> > CryptoPP::CTR_Mode<CryptoPP::AES>::Decryption decryptor_ctr_;
> >
> > I decrypt input data which is previouslly encoded data blocks by using the
> > following function:
> >
> > decryptor_ecb_.ProcessData(output, input, input_len);
> >
> > Now I want to get get the IV value after encoding, in order to save it.
> > I haven't found any function to retrieve the current IV value which is
> > normally incremented by 1 after each block encoding.
> > I can compute it myself but the decryptor should have the value currently
> > after encoding.
> >
> > Is there a solution to get back the current IV value after encoding?
>
> No, there is no function to retrieve the IV.
>
> There is a function to retrieve the next IV, which only generates a
> random block with size of IVsize().
By the way, for CTR mode, it is pretty easy to calculate the next IV.
Something like:
// AES block size
byte iv[AES::BLOCKSIZE] = ...;
// i-th block
size_t i = ...;
for (size_t b=0; b < i; ++i)
IncrementCounterByOne(iv, 1);
// The mask is AES_enc(iv)
byte mask[16];
AES::Encryption enc(key, key.size());
enc.ProcessBlock(mask, iv);
Then XOR mask with the plaintext or ciphertext. AES::Encryption is
used for both the forward and reverse directions.
Jeff
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