Re: Skipping last few bytes of file when FileSource
Thanks much! The last example works well.. :) -- -- You received this message because you are subscribed to the "Crypto++ Users" Google Group. To unsubscribe, send an email to cryptopp-users-unsubscr...@googlegroups.com. More information about Crypto++ and this group is available at http://www.cryptopp.com. --- 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.
Re: Skipping last few bytes of file when FileSource
On Monday, November 6, 2017 at 2:27:30 PM UTC-5, Richard Závodný wrote:
>
> Okay. I think, there is another problem. The file is actually splitted
> into two parts. First is encrypted content using AES and the second is
> encrypted Aes Key and Iv using RSA. The encrypted Aes Key and Iv is 512
> bytes big on the end. First I needed to get 512 bytes from the end, decrypt
> using RSA then I get the AES key and iv. That isn't problem. I did what you
> proposed. Code following:
> CryptoPP::FileSource sourceFile("first.txt", false, new CryptoPP::
> StreamTransformationFilter(cbcDecryptor, new CryptoPP::FileSink(
> "second.txt")));
> sourceFile.Pump(fileSize - 512); //Fetch only encrypted content with AES
> (without the RSA encrypted key)
> sourceFile.Skip(512); //Skip last 512 bytes
>
> But when I do this, the problem occur. My decrypted file is 15 bytes
> shorter and I don't know why! Original file size is 351 bytes, encrypted is
> 864 bytes (351 bytes + 1byte AES padding + 512 bytes key), decrypted file
> size is 336 bytes.
>
It sounds like data is being buffered: 351 -336 = 15.
Try calling sourceFile.MessageEnd(). If you get an exception, then you will
need to use a different technique.
Since you are at the end of your data stream you may be able to induce a
MessageEnd() with a call to PumpAll().
You can wrap the code above in a C++ block to cause the destructors to run,
which will implicitly call MessageEnd():
{
FileSource sourceFile("first.txt", false, ...);
sourceFile.Pump(fileSize - 512);
sourceFile.Skip(512);
}
If you want to explicitly control MessageEnd(), then use the following. It
is how the library does it when needed:
StreamTransformationFilter* x = NULL;
FileSource sourceFile("first.txt", false, x=new
StreamTransformationFilter(...));
sourceFile.Pump(fileSize - 512);
sourceFile.Skip(512);
x->MessageEnd();
Jeff
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Re: Skipping last few bytes of file when FileSource
Okay. I think, there is another problem. The file is actually splitted into
two parts. First is encrypted content using AES and the second is encrypted
Aes Key and Iv using RSA. The encrypted Aes Key and Iv is 512 bytes big on
the end. First I needed to get 512 bytes from the end, decrypt using RSA
then I get the AES key and iv. That isn't problem. I did what you proposed.
Code following:
CryptoPP::FileSource sourceFile("first.txt", false, new CryptoPP::
StreamTransformationFilter(cbcDecryptor, new CryptoPP::FileSink("second.txt"
)));
sourceFile.Pump(fileSize - 512); //Fetch only encrypted content with AES
(without the RSA encrypted key)
sourceFile.Skip(512); //Skip last 512 bytes
But when I do this, the problem occur. My decrypted file is 15 bytes
shorter and I don't know why! Original file size is 351 bytes, encrypted is
864 bytes (351 bytes + 1byte AES padding + 512 bytes key), decrypted file
size is 336 bytes.
On Monday, 6 November 2017 14:37:30 UTC+1, Jeffrey Walton wrote:
>
>
>
> On Sunday, November 5, 2017 at 10:00:06 AM UTC-5, Richard Závodný wrote:
>>
>> Hi. I need skip last 512bytes from the file. This solution not working:
>> int fileSize = boost::filesystem::file_size("first.txt");
>>
>> CryptoPP::FileSource sourceFile("first.txt", false, new CryptoPP::
>> FileSink("second.txt"));
>> sourceFile.Pump(fileSize - 512);
>>
>> How could i achieve this?
>>
>
> Pump as many bytes as you wish. Then, call sourceFile.Skip(...) to skip
> the bytes.
>
> Also see BufferedTransformation in the docs at
> https://www.cryptopp.com/docs/ref/class_buffered_transformation.html.
>
> Jeff
>
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Re: Skipping last few bytes of file when FileSource
On Sunday, November 5, 2017 at 10:00:06 AM UTC-5, Richard Závodný wrote:
>
> Hi. I need skip last 512bytes from the file. This solution not working:
> int fileSize = boost::filesystem::file_size("first.txt");
>
> CryptoPP::FileSource sourceFile("first.txt", false, new CryptoPP::FileSink
> ("second.txt"));
> sourceFile.Pump(fileSize - 512);
>
> How could i achieve this?
>
Pump as many bytes as you wish. Then, call sourceFile.Skip(...) to skip the
bytes.
Also see BufferedTransformation in the docs at
https://www.cryptopp.com/docs/ref/class_buffered_transformation.html.
Jeff
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