On Tue, Sep 7, 2021 at 7:45 AM Tom <thomasb9...@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.
Instead of PUT_RESULT (or THROW), I used PUT_HASH to see the hash that
was calculated during verification. When using PUT_HASH, the
calculated digest has 32 0's appending to it. So the calculated digest
in the example program is
AD7FACB2586FC6E966C004D7D1D16B024F5805FF7CB47C7A85DABD8B48892CA7
0000000000000000000000000000000000000000000000000000000000000000.
I'll add the example to the wiki once I get the problem sorted out.
Jeff
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#include <iostream>
#include <string>
#include "cryptlib.h"
#include "filters.h"
#include "files.h"
#include "sha.h"
#include "hex.h"
using namespace CryptoPP;
class CombinedSource : public Source
{
public:
// Pumps s1 first, and then s2
CombinedSource(Source& s1, Source& s2, bool pumpAll,
BufferedTransformation* attachment)
: Source(attachment), m_s1(s1), m_s2(s2) {
if (pumpAll)
PumpAll();
}
virtual size_t Pump2(lword &byteCount, bool blocking=true)
{
std::cout << "Pump2: " << byteCount << std::endl;
if (byteCount)
m_s1.TransferTo2(*AttachedTransformation(), byteCount,
DEFAULT_CHANNEL, blocking);
if (byteCount)
m_s2.TransferTo2(*AttachedTransformation(), byteCount,
DEFAULT_CHANNEL, blocking);
return static_cast<size_t>(byteCount);
}
virtual size_t PumpMessages2(unsigned int &messageCount, bool blocking=true)
{
std::cout << "PumpMessages2: " << messageCount << std::endl;
if (messageCount)
m_s1.TransferMessagesTo2(*AttachedTransformation(), messageCount,
DEFAULT_CHANNEL, blocking);
if (messageCount)
m_s2.TransferMessagesTo2(*AttachedTransformation(), messageCount,
DEFAULT_CHANNEL, blocking);
return messageCount;
}
virtual bool SourceExhausted() const {
return m_s1.SourceExhausted() && m_s2.SourceExhausted();
}
private:
Source &m_s1, &m_s2;
};
int main(int argc, char* argv[])
{
// 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 hash on the file
FileSource("zero.dat", true, new HashFilter(sha256, new
StringSink(digest)));
std::cout << "Digest: ";
StringSource(digest, true, new HexEncoder(new FileSink(std::cout)));
std::cout << std::endl;
StringSource ss(digest, true);
FileSource fs("zero.dat", true);
byte result = 0;
std::string digest2;
CombinedSource cs(ss, fs, true,
new HashVerificationFilter(sha256,
// new ArraySink(&result, sizeof(result))));
new StringSink(digest2), 4));
std::cout << "Digest: ";
StringSource(digest2, true, new HexEncoder(new FileSink(std::cout)));
std::cout << std::endl;
if (result)
std::cout << "Verified hash on file" << std::endl;
else
std::cout << "Failed to verify hash on file" << std::endl;
return 0;
}
