(C++) About RSA Public/Private Keys

This example provides some additional information for understanding public/private key pairs. In demonstrates how a private key is a superset of the public key. A public key contains the modulus and exponent. The matching private key also contains the modulus and exponent, but also contains the additional private key parts.

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#include <CkCert.h>
#include <CkPublicKey.h>
#include <CkPrivateKey.h>

void ChilkatSample(void)
    {
    CkCert cert;

    bool success;

    // Load a digital certificate.
    success = cert.LoadFromFile("digitalCert.cer");
    if (success != true) {
        std::cout << cert.lastErrorText() << "\r\n";
        return;
    }

    CkPublicKey *pubKey = 0;

    // A .cer file does not contain the private key.  It should contain
    // the public key...
    pubKey = cert.ExportPublicKey();

    // Let's have a look at it (in XML format).
    std::cout << "Public Key from Certificate:" << "\r\n";
    std::cout << pubKey->getXml() << "\r\n";

    // An RSA public key consists of a modulus and exponent.

    // An RSA private key includes both the modulus and exponent,
    // as well as other "big" numbers: P, Q, D, etc.

    // Let's load an RSA private key from a DER-encoded file:
    CkPrivateKey privKey;

    success = privKey.LoadAnyFormatFile("PrivateKey.key","");
    if (success != true) {
        std::cout << privKey.lastErrorText() << "\r\n";
        delete pubKey;
        return;
    }

    // If this private key is the matching half to the public key from
    // the certificate, then the modulus and exponent should
    // be identical.  (Thus, a "private key" really contains both the public part as well as the private parts...).
    std::cout << "Private Key from DER:" << "\r\n";
    std::cout << privKey.getXml() << "\r\n";

    delete pubKey;
    }