(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 <C_CkCert.h>
#include <C_CkPublicKey.h>
#include <C_CkPrivateKey.h>

void ChilkatSample(void)
    {
    HCkCert cert;
    BOOL success;
    HCkPublicKey pubKey;
    HCkPrivateKey privKey;

    cert = CkCert_Create();

    // Load a digital certificate.
    success = CkCert_LoadFromFile(cert,"digitalCert.cer");
    if (success != TRUE) {
        printf("%s\n",CkCert_lastErrorText(cert));
        CkCert_Dispose(cert);
        return;
    }

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

    // Let's have a look at it (in XML format).
    printf("Public Key from Certificate:\n");
    printf("%s\n",CkPublicKey_getXml(pubKey));

    // 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:
    privKey = CkPrivateKey_Create();

    success = CkPrivateKey_LoadAnyFormatFile(privKey,"PrivateKey.key","");
    if (success != TRUE) {
        printf("%s\n",CkPrivateKey_lastErrorText(privKey));
        CkPublicKey_Dispose(pubKey);
        CkCert_Dispose(cert);
        CkPrivateKey_Dispose(privKey);
        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...).
    printf("Private Key from DER:\n");
    printf("%s\n",CkPrivateKey_getXml(privKey));

    CkPublicKey_Dispose(pubKey);


    CkCert_Dispose(cert);
    CkPrivateKey_Dispose(privKey);

    }