(C) Get OAuth 2.0 Access Token using G_SvcOauthAccessToken2

Obtains an OAUTH2 access token from the Google OAuth 2.0 Authorization Server using the G_SvcOauthAccessToken2 method. This is for Server to server applications using Google API's that need an access token. See https://developers.google.com/accounts/docs/OAuth2ServiceAccount

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#include <C_CkHttp.h>
#include <C_CkCert.h>
#include <C_CkHashtable.h>

void ChilkatSample(void)
    {
    HCkHttp http;
    HCkCert cert;
    BOOL success;
    HCkHashtable claimParams;
    int numSec;
    const char *accessToken;

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

    http = CkHttp_Create();

    // When a service account (Client ID) is created at https://code.google.com/apis/console/
    // Google will generate a P12 key.  This is a PKCS12 (PFX) file that you will download
    // and save.  The password to access the contents of this file is "notasecret".
    // NOTE: The Chilkat Pfx API provides the ability to load a PFX/P12 and re-save
    // with a different password.

    // Begin by loading the downloaded .p12 into a Chilkat certificate object:
    cert = CkCert_Create();
    success = CkCert_LoadPfxFile(cert,"/myDir/API Project-1c43a291e2a1-notasecret.p12","notasecret");
    if (success != TRUE) {
        printf("%s\n",CkCert_lastErrorText(cert));
        CkHttp_Dispose(http);
        CkCert_Dispose(cert);
        return;
    }

    // The next (and final) step is to request the access token.  Chilkat internally
    // does all the work of forming the JWT header and JWT claim set, encoding and
    // signing the JWT, and sending the access token request.
    // The application need only provide the inputs: The iss, scope(s), sub, and the
    // desired duration with a max of 3600 seconds (1 hour).
    // 
    // Each of these inputs is defined as follows 
    // (see https://developers.google.com/accounts/docs/OAuth2ServiceAccount
    // iss: The email address of the service account.
    // scope: A space-delimited list of the permissions that the application requests.
    // sub: The email address of the user for which the application is requesting delegated access.
    // numSec: The number of seconds for which the access token will be valid (max 3600).

    claimParams = CkHashtable_Create();

    // The required claim parameters that must be provided by the application are
    // the "iss" and "scope" params:
    success = CkHashtable_AddStr(claimParams,"iss","761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com");
    success = CkHashtable_AddStr(claimParams,"scope","https://mail.google.com/");

    // The "sub" param is required if there is an email address, such as for a
    // Google Apps domain—if you use Google Apps for Work, where the administrator of the Google Apps domain 
    // can authorize an application to access user data on behalf of users in the Google Apps domain.
    // 
    // Comment out this line if there is no such email address.
    // The typical case is that there is no such email address.
    success = CkHashtable_AddStr(claimParams,"sub","user@your-domain.com");

    // The "aud" parameter is optional.  If not set, it defaults to the value shown here:
    success = CkHashtable_AddStr(claimParams,"aud","https://accounts.google.com/o/oauth2/token");

    numSec = 3600;

    accessToken = CkHttp_g_SvcOauthAccessToken2(http,claimParams,numSec,cert);
    if (CkHttp_getLastMethodSuccess(http) != TRUE) {
        printf("%s\n",CkHttp_lastErrorText(http));
    }
    else {
        printf("access token: %s\n",accessToken);
    }



    CkHttp_Dispose(http);
    CkCert_Dispose(cert);
    CkHashtable_Dispose(claimParams);

    }