Re: Get current IV value in decrypt AES256 CTR mode

On Fri, Jan 8, 2021 at 8:05 PM Jeffrey Walton <[email protected]> wrote:
>
> On Fri, Jan 8, 2021 at 10:44 AM Xamix <[email protected]> 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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