(Objective-C) How to Generate an Elliptic Curve Shared Secret

Demonstrates how to generate an ECC (Elliptic Curve Cryptography) shared secret. Imagine a cilent has one ECC private key, the server has another. A shared secret is computed by each side providing it's public key to the other. The private keys are kept private.

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#import <CkoPrng.h>
#import <CkoEcc.h>
#import <CkoPrivateKey.h>
#import <CkoPublicKey.h>
#import <NSString.h>

// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// This example includes both client-side and server-side code.
// Each code segment is marked as client-side or server-side.
// Imagine these segments are running on separate computers...

BOOL success;

// -----------------------------------------------------------------
// (Client-Side) Generate an ECC key, save the public part to a file.
// -----------------------------------------------------------------
CkoPrng *prngClient = [[CkoPrng alloc] init];
CkoEcc *eccClient = [[CkoEcc alloc] init];
CkoPrivateKey *privKeyClient = [eccClient GenEccKey: @"secp256r1" prng: prngClient];
if (eccClient.LastMethodSuccess != YES) {
    NSLog(@"%@",eccClient.LastErrorText);
    return;
}

CkoPublicKey *pubKeyClient = [privKeyClient GetPublicKey];
[pubKeyClient SavePemFile: NO path: @"qa_output/eccClientPub.pem"];

// -----------------------------------------------------------------
// (Server-Side) Generate an ECC key, save the public part to a file.
// -----------------------------------------------------------------
CkoPrng *prngServer = [[CkoPrng alloc] init];
CkoEcc *eccServer = [[CkoEcc alloc] init];
CkoPrivateKey *privKeyServer = [eccServer GenEccKey: @"secp256r1" prng: prngServer];
if (eccServer.LastMethodSuccess != YES) {
    NSLog(@"%@",eccServer.LastErrorText);
    return;
}

CkoPublicKey *pubKeyServer = [privKeyServer GetPublicKey];
[pubKeyServer SavePemFile: NO path: @"qa_output/eccServerPub.pem"];

// -----------------------------------------------------------------
// (Client-Side) Generate the shared secret using our private key, and the other's public key.
// -----------------------------------------------------------------

// Imagine that the server sent the public key PEM to the client.
// (This is simulated by loading the server's public key from the file.
CkoPublicKey *pubKeyFromServer = [[CkoPublicKey alloc] init];
[pubKeyFromServer LoadFromFile: @"qa_output/eccServerPub.pem"];
NSString *sharedSecret1 = [eccClient SharedSecretENC: privKeyClient pubKey: pubKeyFromServer encoding: @"base64"];

// -----------------------------------------------------------------
// (Server-Side) Generate the shared secret using our private key, and the other's public key.
// -----------------------------------------------------------------

// Imagine that the client sent the public key PEM to the server.
// (This is simulated by loading the client's public key from the file.
CkoPublicKey *pubKeyFromClient = [[CkoPublicKey alloc] init];
[pubKeyFromClient LoadFromFile: @"qa_output/eccClientPub.pem"];
NSString *sharedSecret2 = [eccServer SharedSecretENC: privKeyServer pubKey: pubKeyFromClient encoding: @"base64"];

// ---------------------------------------------------------
// Examine the shared secrets.  They should be the same.
// Both sides now have a secret that only they know.
// ---------------------------------------------------------
NSLog(@"%@",sharedSecret1);
NSLog(@"%@",sharedSecret2);