AFNetworking and Cookies

You do not need to bother with NSUserDefaults nor any keychain wrapper if you use NSURLCredential.
Indeed NSURLCredential is much simpler to use, as it allows you to store both username and password in the keychain in two lines of code.

Your code would be something like that once the user is logged in:

NSURLCredential *credential;

credential = [NSURLCredential credentialWithUser:username password:password persistence:NSURLCredentialPersistencePermanent];
[[NSURLCredentialStorage sharedCredentialStorage] setCredential:credential forProtectionSpace:self.loginProtectionSpace];

Then, each time the app is launched, you can check whether your user was already logged in by searching for any credential in order to automatically log back your user (if need be):

NSURLCredential *credential;
NSDictionary *credentials;

credentials = [[NSURLCredentialStorage sharedCredentialStorage] credentialsForProtectionSpace:self.loginProtectionSpace];
credential = [credentials.objectEnumerator nextObject];
NSLog(@"User %@ already connected with password %@", credential.user, credential.password);

You also need to clean the credential when the user wants to log out:

NSURLCredential *credential;
NSDictionary *credentials;

credentials = [[NSURLCredentialStorage sharedCredentialStorage] credentialsForProtectionSpace:self.loginProtectionSpace];
credential = [credentials.objectEnumerator nextObject];
[[NSURLCredentialStorage sharedCredentialStorage] removeCredential:credential forProtectionSpace:self.loginProtectionSpace];

loginProtectionSpace is created once for all. Please, note this sample code assumes there is only one credential in this space, which is usually the case unless you manage several accounts.

Here is an example of how you would create a NSURLProtectionSpace:

NSURL *url = [NSURL URLWithString:@"http://www.example.com"];
self.loginProtectionSpace = [[NSURLProtectionSpace alloc] initWithHost:url.host
                                                                  port:[url.port integerValue]
                                                              protocol:url.scheme
                                                                 realm:nil
                                                  authenticationMethod:NSURLAuthenticationMethodHTTPDigest];