Re: HMACs of files

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 <[email protected]> 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;
}