I think I understand a little better now what GCM does. Probably what I'm 
looking for is XTS; but this doesn't appear supported in cryptopp (?). 

On Tuesday, August 26, 2014 11:18:21 AM UTC+1, [email protected] wrote:
>
> For example, when using CTR mode, I can use something like the following 
> code fragment to randomly seek to a position in a data buffer:
>
>         CryptoPP::CTR_Mode<CryptoPP::AES>::Encryption encryptor;
>         encryptor.SetKeyWithIV(key, sizeof(key), iv);
>         encryptor.Seek(startPosition);
>         encryptor.ProcessData(out, in, length);
>
> This works very nicely but I would like to use GCM instead.
>
> Can I therefore do something like the following?
>
>         CryptoPP::GCM<CryptoPP::AES>::Encryption encryptor;
>         encryptor.SetKeyWithIV(key, sizeof(key), iv);
>         encryptor.Seek(startPosition);
>         encryptor.ProcessData(out, in, length);
>
> As you can probably tell, I'm not a cryptographer.
>
> Many thanks,
>
> Ben.
>
>
>

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