(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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#include <C_CkPrng.h>
#include <C_CkEcc.h>
#include <C_CkPrivateKey.h>
#include <C_CkPublicKey.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkPrng prngClient;
    HCkEcc eccClient;
    HCkPrivateKey privKeyClient;
    HCkPublicKey pubKeyClient;
    HCkPrng prngServer;
    HCkEcc eccServer;
    HCkPrivateKey privKeyServer;
    HCkPublicKey pubKeyServer;
    HCkPublicKey pubKeyFromServer;
    const char *sharedSecret1;
    HCkPublicKey pubKeyFromClient;
    const char *sharedSecret2;

    // 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...

    // -----------------------------------------------------------------
    // (Client-Side) Generate an ECC key, save the public part to a file.
    // -----------------------------------------------------------------
    prngClient = CkPrng_Create();
    eccClient = CkEcc_Create();
    privKeyClient = CkEcc_GenEccKey(eccClient,"secp256r1",prngClient);
    if (CkEcc_getLastMethodSuccess(eccClient) != TRUE) {
        printf("%s\n",CkEcc_lastErrorText(eccClient));
        CkPrng_Dispose(prngClient);
        CkEcc_Dispose(eccClient);
        return;
    }

    pubKeyClient = CkPrivateKey_GetPublicKey(privKeyClient);
    CkPublicKey_SavePemFile(pubKeyClient,FALSE,"qa_output/eccClientPub.pem");
    CkPublicKey_Dispose(pubKeyClient);

    // -----------------------------------------------------------------
    // (Server-Side) Generate an ECC key, save the public part to a file.
    // -----------------------------------------------------------------
    prngServer = CkPrng_Create();
    eccServer = CkEcc_Create();
    privKeyServer = CkEcc_GenEccKey(eccServer,"secp256r1",prngServer);
    if (CkEcc_getLastMethodSuccess(eccServer) != TRUE) {
        printf("%s\n",CkEcc_lastErrorText(eccServer));
        CkPrng_Dispose(prngClient);
        CkEcc_Dispose(eccClient);
        CkPrng_Dispose(prngServer);
        CkEcc_Dispose(eccServer);
        return;
    }

    pubKeyServer = CkPrivateKey_GetPublicKey(privKeyServer);
    CkPublicKey_SavePemFile(pubKeyServer,FALSE,"qa_output/eccServerPub.pem");
    CkPublicKey_Dispose(pubKeyServer);

    // -----------------------------------------------------------------
    // (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.
    pubKeyFromServer = CkPublicKey_Create();
    CkPublicKey_LoadFromFile(pubKeyFromServer,"qa_output/eccServerPub.pem");
    sharedSecret1 = CkEcc_sharedSecretENC(eccClient,privKeyClient,pubKeyFromServer,"base64");
    CkPrivateKey_Dispose(privKeyClient);

    // -----------------------------------------------------------------
    // (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.
    pubKeyFromClient = CkPublicKey_Create();
    CkPublicKey_LoadFromFile(pubKeyFromClient,"qa_output/eccClientPub.pem");
    sharedSecret2 = CkEcc_sharedSecretENC(eccServer,privKeyServer,pubKeyFromClient,"base64");
    CkPrivateKey_Dispose(privKeyServer);

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


    CkPrng_Dispose(prngClient);
    CkEcc_Dispose(eccClient);
    CkPrng_Dispose(prngServer);
    CkEcc_Dispose(eccServer);
    CkPublicKey_Dispose(pubKeyFromServer);
    CkPublicKey_Dispose(pubKeyFromClient);

    }