Twitter oauth/access_token 401 Error - ios

Question

I've been working on an all that uses the full OAuth app flow, and I have been running into an issue where I only get back a 401 = "Invalid or expired token" error. I've checked my request with the documentation, and everything looks correct, and I'm stumped. Below is the details of my request.
URL
https://api.twitter.com/1.1/oauth/access_token
HTTP Method
POST
Headers
Content-Type: application/x-www-form-urlencoded
Authorization: OAuth oauth_consumer_key="CONSUMER_API_KEY", oauth_nonce="B4D43B0C-A348-4EB6-9C0B-8B0F4FE8", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1397724299", oauth_version="1.0", oauth_token="TOKEN_FROM_REQUEST_TOKEN_API", oauth_signature="ulYIzTacwC%2FeGUdoCPYsrFEqg4A%3D"
HTTP Body
oauth_verifier=OAUTH_VERIFIER_FROM_REQUEST_TOKEN_API
I have no issues getting the oauth/request_token API to work, and I have appropriately set the permissions in Twitter's app settings. Does anyone have any idea what's going on with this?
Thanks for any help you all may be able to offer.
-- UPDATE --
Another thing to note is that I'm using the STTwitter library to make the requests. It did not have a built-in method to handle the oauth/authorize or oath/authenticate API methods so I'm using the code below.
// get request token, and present login screen
STTwitterAPI *twitter = [STTwitterAPI twitterAPIWithOAuthConsumerKey:twitterApiKey consumerSecret:twitterApiSecret];
[twitter postTokenRequest:^(NSURL *url, NSString *oauthToken)
{
authWebViewController = [[TWAuthWebViewController alloc] init];
authWebViewController.url = [NSURL URLWithString:[NSString stringWithFormat:#"https://api.twitter.com/oauth/authorize?oauth_token=%#", oauthToken]];
authWebViewController.completion = ^(TWAuthWebViewController *authViewController, NSURL *oauthCallbackURL)
{
// get the request token and verifier from the URL
NSString *oauthToken = [TWLoginBusiness getOAuthTokenFromURL:oauthCallbackURL];
NSString *oauthVerifier = [TWLoginBusiness getOAuthVerifierFromURL:oauthCallbackURL];
// get user data with the oauth token
[twitter postResource:#"oauth/access_token"
baseURLString:#"https://api.twitter.com/1.1"
parameters:#{#"oauth_verifier" : oauthVerifier}
uploadProgressBlock:nil
downloadProgressBlock:nil
successBlock:^(NSDictionary *rateLimits, id response)
{
NSLog(#"Reponse: %#", response);
completion(nil, nil);
} errorBlock:^(NSError *error)
{
NSLog(#"Error: %#", error);
completion(nil, error);
}];
};
presentAuthController(authWebViewController);
} oauthCallback:twitterApiCallback errorBlock:^(NSError *error)
{
completion(nil, error);
}];
One last note. The part that actually displays the web view controller is not listed here. I wanted to keep the code snippet here focused on the actual API methods, and not the UI logic. Just be assured that the line right after authWebViewController.url.... displays the web view, and then the completion block is called after the user completes the authentication on the Twitter web page. Also the two methods getOauthTokenFromURL and getOauthVerifierFromUrl do in fact return the correct token and verifier. The STTwitter library actually saves out the token it's self, so that is why it's not manually passed into the logic below. The logic generates the request above.
Thanks

Answer 1

The full OAuth flow is already implemented in STTwitter library.
Check out iOS demo for a working example of web-based authentication through Safari.
What you are trying to do here is a web-based authentication inside the application.
Although it is totally feasible, I did not include this workflow in the demo project because it is considered a bad practice.
Indeed, the point of OAuth is that the user does not want to enter her Twitter credentials in your own application, but asks Twitter to send your app dedicated OAuth tokens instead.
Let me know if you need more help with STTwitter.