(Objective-C) OAuth2 for GMail using a Service Account Key

Demonstrates how to use GMail with OAuth2 for a Google Service Account. The 1st step is to obtain a temporary access token from the Google OAuth 2.0 Authorization Server. This can be accomplished using Chilkat HTTP.

Once the access token is obtained, it may be used in IMAP XOAUTH2 authentication. The access token may be used while it remains valid to send as many emails as desired. Once it expires, a new access token needs to be obtained (using the same procedure).

Note: This is for OAuth2 using Google Service account keys.

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#import <CkoCert.h>
#import <NSString.h>
#import <CkoHttp.h>
#import <CkoImap.h>

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

// --------------------------------------------------------------------
// Important:  In most cases, this example is not what you are looking for.
// This example is for OAuth2 using a Google Service Account Key.
// 
// It is more likely that the example you actually need is here:
// Get GMail IMAP OAuth2 Access Token 
// --------------------------------------------------------------------

// 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:
CkoCert *cert = [[CkoCert alloc] init];
BOOL success = [cert LoadPfxFile: @"/myDir/API Project-1c43a291e2a1-notasecret.p12" password: @"notasecret"];
if (success != YES) {
    NSLog(@"%@",cert.LastErrorText);
    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.
//      The sub may be empty if there is no delegation. (This is typical.)
// numSec: The number of seconds for which the access token will be valid (max 3600).

NSString *iss = @"761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com";
NSString *scope = @"https://mail.google.com/";
NSString *sub = @"";
int numSec = 3600;

CkoHttp *http = [[CkoHttp alloc] init];
NSString *accessToken = [http G_SvcOauthAccessToken: iss scope: scope subEmail: sub numSec: [NSNumber numberWithInt: numSec] cert: cert];
if (http.LastMethodSuccess != YES) {
    NSLog(@"%@",http.LastErrorText);
    return;
}
else {
    NSLog(@"%@%@",@"access token: ",accessToken);
}

// Now that we have the access token, it may be used to authenticate via XOAUTH2 with GMail:

CkoImap *imap = [[CkoImap alloc] init];

// GMail's IMAP service uses SSL and port 993.
imap.Ssl = YES;
imap.Port = [NSNumber numberWithInt:993];

// Connect to the GMail IMAP server.
success = [imap Connect: @"imap.gmail.com"];
if (success != YES) {
    NSLog(@"%@",imap.LastErrorText);
    return;
}

// Indicate that XOAUTH2 authentication is to be used:
imap.AuthMethod = @"XOAUTH2";

// Login
// The username must be the email address used for the "sub" argument when getting
// the access token.  Instead of using a password, pass the access token in the 2nd argument:
success = [imap Login: @"user@your-domain.com" password: accessToken];
if (success != YES) {
    NSLog(@"%@",imap.LastErrorText);
}
else {
    NSLog(@"%@",@"Successfully authenticate with GMail IMAP using XOAUTH2!");
}