Generating a License Key based on a particular string, and then given the License key how to get back the string

[rameshdk12]

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HI,
I am trying to generate license keys for a software that I have written.
how can I Generate a License Key based on a particular string, and then given the License key how to get back the string
I know very little cryptography .I just need small piece of code to accomplish this.
Using internet managed to get the below code for the first part:
#include <cryptopp/rsa.h>
#include <cryptopp/osrng.h>
#include <cryptopp/base64.h>
#include <cryptopp/filters.h>
#include <cryptopp/hex.h>
#include
#include

using namespace CryptoPP;

std::string generateLicenseKey(const std::string& email) {
AutoSeededRandomPool rng;

// Generate RSA keys
InvertibleRSAFunction params;
params.GenerateRandomWithKeySize(rng, 2048);

RSA::PrivateKey privateKey(params);
RSA::PublicKey publicKey(params);

// Sign the email
RSASSA_PKCS1v15_SHA_Signer signer(privateKey);
std::string signature;
StringSource(email, true,
	new SignerFilter(rng, signer,
		new StringSink(signature)
	)
);

// Encode the signature in base64
std::string encoded;
StringSource(signature, true,
	new /*Base64Encoder*/HexEncoder(
		new StringSink(encoded)
	)
);

return encoded;

}
int main() {
std::string email = "user@example.com";
std::string licenseKey = generateLicenseKey(email);
std::cout << "License Key: " << licenseKey << std::endl;
return 0;
}

secondpart(to be done):
How do I decode and get back the string(email) given the license key?
Regards,
Ramesh D