(C) Get Google API Access Token using JSON Private Key

Demonstrates how to get a Google API access token using a JSON service account private key.

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#include <C_CkFileAccess.h>
#include <C_CkAuthGoogle.h>
#include <C_CkSocket.h>
#include <C_CkMailMan.h>
#include <C_CkEmail.h>

void ChilkatSample(void)
    {
    HCkFileAccess fac;
    const char *jsonKey;
    HCkAuthGoogle gAuth;
    HCkSocket tlsSock;
    BOOL success;
    const char *accessToken;
    HCkMailMan mailman;
    HCkEmail email;

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

    // --------------------------------------------------------------------------------
    // For a step-by-step guide for setting up your Google Workspace service account,
    // see Setup Google Workspace Account for Sending SMTP GMail from a Service Account
    // --------------------------------------------------------------------------------

    // First load the JSON key into a string.
    fac = CkFileAccess_Create();
    jsonKey = CkFileAccess_readEntireTextFile(fac,"qa_data/googleApi/chilkat25-b4214220e565.json","utf-8");
    if (CkFileAccess_getLastMethodSuccess(fac) != TRUE) {
        printf("%s\n",CkFileAccess_lastErrorText(fac));
        CkFileAccess_Dispose(fac);
        return;
    }

    // A Google service account JSON private key looks like this:

    // {
    //   "type": "service_account",
    //   "project_id": "chilkat25",
    //   "private_key_id": "b4214220f565881e19eeb97c2699bf5a0d1e3e0b",
    //   "private_key": "-----BEGIN PRIVATE KEY-----\nMIIEvQ...NXcM=\n-----END PRIVATE KEY-----\n",
    //   "client_email": "chilkatsvc@chilkat25.iam.gserviceaccount.com",
    //   "client_id": "109122032928932715958",
    //   "auth_uri": "https://accounts.google.com/o/oauth2/auth",
    //   "token_uri": "https://oauth2.googleapis.com/token",
    //   "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
    //   "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/chilkatsvc%40chilkat25.iam.gserviceaccount.com",
    //   "universe_domain": "googleapis.com"
    // }

    gAuth = CkAuthGoogle_Create();
    CkAuthGoogle_putJsonKey(gAuth,jsonKey);

    // Specify a scope.
    CkAuthGoogle_putScope(gAuth,"https://mail.google.com/");

    // Request an access token that is valid for this many seconds.
    CkAuthGoogle_putExpireNumSeconds(gAuth,3600);

    // When using a Google Workspace account with Gmail APIs, a service account can impersonate a user 
    // via a process called domain-wide delegation — and the "sub" claim in the JWT is what enables this.
    // Domain-wide delegation allows a Google Workspace administrator to authorize a service account to 
    // act on behalf of any user in the domain, without user interaction.

    // This is required for server-to-server access to user data — such as reading/sending Gmail from a background service.
    // This is your company email address.
    CkAuthGoogle_putSubEmailAddress(gAuth,"info@chilkat.xyz");

    // Connect to www.googleapis.com using TLS
    tlsSock = CkSocket_Create();
    success = CkSocket_Connect(tlsSock,"www.googleapis.com",443,TRUE,5000);
    if (success != TRUE) {
        printf("%s\n",CkSocket_lastErrorText(tlsSock));
        CkFileAccess_Dispose(fac);
        CkAuthGoogle_Dispose(gAuth);
        CkSocket_Dispose(tlsSock);
        return;
    }

    // Send the request to obtain the access token.
    success = CkAuthGoogle_ObtainAccessToken(gAuth,tlsSock);
    if (success != TRUE) {
        printf("%s\n",CkAuthGoogle_lastErrorText(gAuth));
        CkFileAccess_Dispose(fac);
        CkAuthGoogle_Dispose(gAuth);
        CkSocket_Dispose(tlsSock);
        return;
    }

    // Examine the access token:
    accessToken = CkAuthGoogle_accessToken(gAuth);
    printf("Access Token: %s\n",accessToken);

    // Sample output:
    // ya29.a0AW4XtxjGTD67Z8 .... IRw0218

    // The access token allows us to send unlimited emails while it's valid. Once it expires, we must obtain and use a new one.

    // -----------------------------------------------------------------------
    mailman = CkMailMan_Create();

    // Set the properties for the GMail SMTP server:
    CkMailMan_putSmtpHost(mailman,"smtp.gmail.com");
    CkMailMan_putSmtpPort(mailman,587);
    CkMailMan_putStartTLS(mailman,TRUE);

    CkMailMan_putSmtpUsername(mailman,"info@chilkat.xyz");
    CkMailMan_putOAuth2AccessToken(mailman,accessToken);

    // Create a new email object
    email = CkEmail_Create();

    CkEmail_putSubject(email,"This is a test");
    CkEmail_putBody(email,"This is a test");
    CkEmail_putFrom(email,"Chilkat Test <info@chilkat.xyz>");
    success = CkEmail_AddTo(email,"Chilkat Software","info@chilkatsoft.com");
    // To add more recipients, call AddTo, AddCC, or AddBcc once per recipient.

    success = CkMailMan_SendEmail(mailman,email);
    if (success != TRUE) {
        printf("%s\n",CkMailMan_lastErrorText(mailman));
        CkFileAccess_Dispose(fac);
        CkAuthGoogle_Dispose(gAuth);
        CkSocket_Dispose(tlsSock);
        CkMailMan_Dispose(mailman);
        CkEmail_Dispose(email);
        return;
    }

    success = CkMailMan_CloseSmtpConnection(mailman);
    if (success != TRUE) {
        printf("Connection to SMTP server not closed cleanly.\n");
    }

    printf("Successfully sent email using Gmail with a service account key.\n");


    CkFileAccess_Dispose(fac);
    CkAuthGoogle_Dispose(gAuth);
    CkSocket_Dispose(tlsSock);
    CkMailMan_Dispose(mailman);
    CkEmail_Dispose(email);

    }