(Objective-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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#import <CkoCert.h>
#import <CkoPublicKey.h>
#import <CkoPrivateKey.h>

CkoCert *cert = [[CkoCert alloc] init];

BOOL success;

// Load a digital certificate.
success = [cert LoadFromFile: @"digitalCert.cer"];
if (success != YES) {
    NSLog(@"%@",cert.LastErrorText);
    return;
}

CkoPublicKey *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).
NSLog(@"%@",@"Public Key from Certificate:");
NSLog(@"%@",[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:
CkoPrivateKey *privKey = [[CkoPrivateKey alloc] init];

success = [privKey LoadAnyFormatFile: @"PrivateKey.key" password: @""];
if (success != YES) {
    NSLog(@"%@",privKey.LastErrorText);

    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...).
NSLog(@"%@",@"Private Key from DER:");
NSLog(@"%@",[privKey GetXml]);