(MFC) 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.

Chilkat C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

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

void ChilkatSample(void)
    {
    CkString strOut;

    CkCert cert;

    bool success;

    // Load a digital certificate.
    success = cert.LoadFromFile("digitalCert.cer");
    if (success != true) {
        strOut.append(cert.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        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).
    strOut.append("Public Key from Certificate:");
    strOut.append("\r\n");
    strOut.append(pubKey->getXml());
    strOut.append("\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.LoadRsaDerFile("PrivateKey.key");
    if (success != true) {
        strOut.append(privKey.lastErrorText());
        strOut.append("\r\n");
        delete pubKey;
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        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...).
    strOut.append("Private Key from DER:");
    strOut.append("\r\n");
    strOut.append(privKey.getXml());
    strOut.append("\r\n");

    delete pubKey;


    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }