Crypto++ verify large file signature

Question

I use this code to verify a message with crypto++:

CryptoPP::ECDSA<CryptoPP::ECP, CryptoPP::SHA256>::Verifier verifier(key);
bool result = verifier.VerifyMessage( (const CryptoPP::byte*)message.data(), message.length(), (const CryptoPP::byte*)signature.data(), signature.length() );

Now I would like to verify a signature I previously created for a file. The file is quite big (GB) and I don't want to load it to memory in one piece (to message variable).

Is there a way crypto++ can verify large file's signature?

Why don't you use of `sha256` or `md5` hash codes to verify a file?? — BattleTested_закалённый в бою, Oct 24 '18 at 10:49
So, with a hash code of file you can do it. if you have file `hash` code and somebody modified just a `bit` of file, the file's `hash` code will be change... Actually a `hash` code is a sign for a file. — BattleTested_закалённый в бою, Oct 24 '18 at 10:58
Believe me, asymetric cryptography signature is the way I need to use. The file and the signature come to my verifier from unsecure environment. If I use just the hash, the attacker can simply supply modified file with recalculated hash. — Martin Dusek, Oct 24 '18 at 11:04
Have you tried [this](https://stackoverflow.com/questions/21024399/get-ecdsa-signature-with-crypto) modified for `FileSource` ? — Blacktempel, Oct 25 '18 at 04:47
Yes, I tried that. I need to modify that so the file is not loaded into RAM in one piece, but instead it is pumped into the verifier in smaller blocks (i.e. iit is read from file system in smaller blocks), as RAM of my device is limited. — Martin Dusek, Oct 25 '18 at 06:40
And also I need to pump signature to the verifier, which is not part of the file, but it is stored in local variable. — Martin Dusek, Oct 25 '18 at 06:50

score 1 · Accepted Answer · answered Oct 27 '18 at 17:28

This code does what I need:

 CryptoPP::ECDSA<CryptoPP::ECP, CryptoPP::SHA256>::PublicKey key; key.Load(queue);
 CryptoPP::ECDSA<CryptoPP::ECP, CryptoPP::SHA256>::Verifier verifier(key);

 CryptoPP::SignatureVerificationFilter verificationFilter(verifier, NULL, CryptoPP::SignatureVerificationFilter::SIGNATURE_AT_BEGIN);
 CryptoPP::FileSource fileSource( file, false, new CryptoPP::Redirector(verificationFilter));
 CryptoPP::StringSource signatureSource( (const CryptoPP::byte*)signature.data(), signature.length(), false, new CryptoPP::Redirector(verificationFilter));
 signatureSource.Pump(signature.length());
 fileSource.PumpAll();

 return verificationFilter.GetLastResult();

Crypto++ verify large file signature

1 Answers1