On Sunday, October 15, 2017 at 6:47:40 AM UTC-4, Richard Závodný wrote:
>
> Hi, I have problem.
>
> I want/need encrypt file and put the encrypted data into the same one (not
> create new file).
>
> For example...
> I have unencrypted file (myfile.txt) and I want encrypt it, but don't
> create new file (let's say myfile.encrypted), but I want the encrypted data
> put into the same file (myfile.txt). If I use this: *CryptoPP::FileSource
> encryptFile("myfile.txt", true, new
> CryptoPP::StreamTransformationFilter(cbcEncryptor, new
> CryptoPP::FileSink("myfile.txt")));*, the FileSink will open the output
> file (myfile.txt) in trunc mode (delete all previous data), but that is the
> problem. The FileSource read files by 4096bytes chunk, so I need load chunk
> -> encrypt -> replace unencrypted chunk with encrypted one -> repeat.
>
The Source/Sink model is not really equipped for the requirement. Pipelines
can move forward when processing messages; they cannot move backwards or
backtrack. Or put another way, its not possible to Seek(...) backwards when
processing messages.
You may be able to do it by mapping a view of the file. When you map a view
of the file you get a pointer back that looks like a regular pointer, but
its really a view of the file mapped by the OS. With pointers in hand, you
can then use Sources/Sinks around those pointers.
In the bigger security engineering picture, you probably don't want the
plain text file to be stored on the disk. Some drives are notoriously
difficult to [keep] clean, and its next to impossible to clean the plain
text once its written to storage. Most SSDs with write leveling fall into
this category. Also see Reliably Erasing Data From Flash-Based Solid State
Drives, https://www.usenix.org/legacy/event/fast11/tech/full_papers/Wei.pdf
Jeff
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