(Swift 3,4,5...) 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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func chilkatTest() {
    let cert = CkoCert()!

    var success: Bool

    // Load a digital certificate.
    success = cert.load(fromFile: "digitalCert.cer")
    if success != true {
        print("\(cert.lastErrorText!)")
        return
    }

    var pubKey: CkoPublicKey?

    // 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).
    print("Public Key from Certificate:")
    print("\(pubKey!.getXml()!)")

    // 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:
    let privKey = CkoPrivateKey()!

    success = privKey.loadAnyFormatFile("PrivateKey.key", password: "")
    if success != true {
        print("\(privKey.lastErrorText!)")
        pubKey = nil
        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...).
    print("Private Key from DER:")
    print("\(privKey.getXml()!)")

    pubKey = nil

}