On Fri, Apr 12, 2019 at 10:28 AM <zhupf...@gmail.com> wrote: > > Hi there! I'm looking into using Crypto++ for an open source project I > contribute to. We need to implement a function which encrypts/decrypts some > data using the AES-CTR or AES-CBC method (the CTR/IV is provided), and then > return the encrypted data and **the new CTR/IV to be used if the crypto > operation is to continue, as if done as a whole block**. For example if the > input CTR is 0 and I encrypted a 32-byte data, I would need to return the new > CTR 2. > > Actually, calculating the CTR myself is easy, but I don't think there would > be an easy way to calculate the IV for AES-CBC. So I wonder if Crypto++ > provides a way to fetch the new CTR/IV after crypto operations? I can see > some "GetNextIV()" function in the document but I don't think that would > satisfy my need as it seems to use a RNG. >
For CBC mode the next IV is the previous ciphertext block. I think the tricky part may be operating a stream. That is, you don't use blocks of 16 for AES. In this case, you have a partial block that gets padded, and that mucks up the continuous blocks of text, and complicates continuing a cipher. If you post some code we may be able to offer more suggestions. Jeff -- You received this message because you are subscribed to "Crypto++ Users". More information about Crypto++ and this group is available at http://www.cryptopp.com and http://groups.google.com/forum/#!forum/cryptopp-users. --- You received this message because you are subscribed to the Google Groups "Crypto++ Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to cryptopp-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
