Generate an RSA key pair, sign a message and verify it using crypto++ 5.6.1 or later. By Tim Sheerman-Chase, 2013. This code is in the public domain and CC0.

gistfile1.cpp

//Generate an RSA key pair, sign a message and verify it using crypto++ 5.6.1 or later.
//By Tim Sheerman-Chase, 2013
//This code is in the public domain and CC0
//To compile: g++ gen.cpp -lcrypto++ -o gen

#include <string>
using namespace std;
#include <crypto++/rsa.h>
#include <crypto++/osrng.h>
#include <crypto++/base64.h>
#include <crypto++/files.h>
using namespace CryptoPP;

void GenKeyPair()
{
	// InvertibleRSAFunction is used directly only because the private key
	// won't actually be used to perform any cryptographic operation;
	// otherwise, an appropriate typedef'ed type from rsa.h would have been used.
	AutoSeededRandomPool rng;
	InvertibleRSAFunction privkey;
	privkey.Initialize(rng, 1024);

	// With the current version of Crypto++, MessageEnd() needs to be called
	// explicitly because Base64Encoder doesn't flush its buffer on destruction.
	Base64Encoder privkeysink(new FileSink("privkey.txt"));
	privkey.DEREncode(privkeysink);
	privkeysink.MessageEnd();
	 
	// Suppose we want to store the public key separately,
	// possibly because we will be sending the public key to a third party.
	RSAFunction pubkey(privkey);
	
	Base64Encoder pubkeysink(new FileSink("pubkey.txt"));
	pubkey.DEREncode(pubkeysink);
	pubkeysink.MessageEnd();

}

void Sign()
{
	string strContents = "A message to be signed";
	//FileSource("tobesigned.dat", true, new StringSink(strContents));
	
	AutoSeededRandomPool rng;
	
	//Read private key
	CryptoPP::ByteQueue bytes;
	FileSource file("privkey.txt", true, new Base64Decoder);
	file.TransferTo(bytes);
	bytes.MessageEnd();
	RSA::PrivateKey privateKey;
	privateKey.Load(bytes);

	//Sign message
	RSASSA_PKCS1v15_SHA_Signer privkey(privateKey);
	SecByteBlock sbbSignature(privkey.SignatureLength());
	privkey.SignMessage(
		rng,
		(byte const*) strContents.data(),
		strContents.size(),
		sbbSignature);

	//Save result
	FileSink sink("signed.dat");
	sink.Put((byte const*) strContents.data(), strContents.size());
	FileSink sinksig("sig.dat");
	sinksig.Put(sbbSignature, sbbSignature.size());
}

void Verify()
{
	//Read public key
	CryptoPP::ByteQueue bytes;
	FileSource file("pubkey.txt", true, new Base64Decoder);
	file.TransferTo(bytes);
	bytes.MessageEnd();
	RSA::PublicKey pubKey;
	pubKey.Load(bytes);

	RSASSA_PKCS1v15_SHA_Verifier verifier(pubKey);

	//Read signed message
	string signedTxt;
	FileSource("signed.dat", true, new StringSink(signedTxt));
	string sig;
	FileSource("sig.dat", true, new StringSink(sig));

	string combined(signedTxt);
	combined.append(sig);

	//Verify signature
	try
	{
		StringSource(combined, true,
			new SignatureVerificationFilter(
				verifier, NULL,
				SignatureVerificationFilter::THROW_EXCEPTION
		   )
		);
		cout << "Signature OK" << endl;
	}
	catch(SignatureVerificationFilter::SignatureVerificationFailed &err)
	{
		cout << err.what() << endl;
	}

}

int main()
{
	GenKeyPair();
	Sign();
	Verify();
}

Hi,
I tried to load the private key generated by Putty, using the above code. But it throw an exception while loading. What am I missing here, ? Logically Is it the right thing am trying to do here ?

"Unhandled exception at 0x00007FF95EC23C58 in RSAEncryption.exe: Microsoft C++ exception: CryptoPP::BERDecodeErr at memory location 0x0000005CA573EF10."

Thank you

Putty has its own format just like openssh has its own key format that is probably why it is throwing the error