On Wednesday, October 21, 2015 at 2:05:27 AM UTC-4, lasl...@gmail.com wrote:
>
>
> OK, I see.
> But what if I want to get the IV of 123th block? Do I have to do it 123
> times or there is an IncrementCounterByX(byte*,unsigned int, unsignet int )
> function?
>
You need to find the i-th block because your block cipher operates on
blocks. So you need a floor function; maybe something like:
size_t block = SaturatingSubtract(X, T::BLOCKSIZE - 1) / T::BLOCKSIZE;
Saturating arithmetic clamps at lower and upper values, so you don't have
to worry about underflow or overflow. Plus, good compilers will produce
fast code based on bit operations.
With the block in hand, you can add seek to it, and then add it to your
counter for a decryption operation.
But for GCM mode, I don't believe its seekable. See
https://groups.google.com/d/msg/cryptopp-users/UHlnZ8r-0Gc/yctuSeSn57gJ.
Jeff
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