On Wednesday, September 8, 2021 at 6:47:45 AM UTC-4 Jeffrey Walton wrote:
> On Tue, Sep 7, 2021 at 7:45 AM Tom <thoma...@gmail.com> wrote:
> >
> > I can create HMACs of files using pipelines via filesources but... I
> can't seem to figure out to verify the HMAC without throwing the file into
> a string in memory.
> >
> > like this:
> >
> > StringSource(plain + mac, true, new HashVerificationFilter(hmac, NULL,
> flags) ); // StringSource
> >
> > Is there a way to use a FileSource without loading the file fully into
> memory?
> >
> > I think its possible but do I append the hmac if I use a file?
>
> Yeah, that's a problem. We should have some documentation covering it.
>
> I think you need a custom source that takes two sources - the existing
> HMAC wrapped in a StringSource and the FileSource. The custom source
> then pumps the data to the attached filter.
>
> Another option is a HashVerificationFilter that takes two sources. It
> could be tricky since the source is expected to pump its data. I did
> not test this option.
>
> Attached is an example. It uses a hash rather than HMAC to simplify the
> code.
>
> The example has a bug, though. HashVerificationFilter is failing...
>
Attached is a corrected example that works as expected. Unfortunately, I
was not able to get the CombinedSource class to work as expected. Instead,
I had to manually fiddle with both Sources. It is not as elegant, but it
should get you through your task.
Jeff
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#include <iostream>
#include <string>
#include "cryptlib.h"
#include "filters.h"
#include "files.h"
#include "sha.h"
#include "hex.h"
int main(int argc, char* argv[])
{
using namespace CryptoPP;
// Create a file of all 0's with:
// dd if=/dev/zero of=./zero.dat bs=4096 count=1
std::string digest;
SHA256 sha256;
// Create the digest on the file
FileSource("zero.dat", true, new HashFilter(sha256, new
StringSink(digest)));
// Print the digest
std::cout << "Digest: ";
StringSource(digest, true, new HexEncoder(new FileSink(std::cout)));
std::cout << std::endl;
// Create a verifier
byte result = 0;
HashVerificationFilter verifier(sha256, new ArraySink(&result,
sizeof(result)));
// Wrap the data in sources
StringSource ss(digest, true);
FileSource fs("zero.dat", true);
// Add the data to the filter
ss.TransferTo(verifier);
fs.TransferTo(verifier);
// Signal end of data. The verifier will finish calculating
// the digest, and compare the expected and calculated digests.
verifier.MessageEnd();
if (result)
std::cout << "Verified hash on file" << std::endl;
else
std::cout << "Failed to verify hash on file" << std::endl;
return 0;
}
