[op setCredentialPersistence:NSURLCredentialPersistenceNone];
[op setUsername:username password:password];

- (void) clearNSURLCredentialCache {

// clear any URL caches
[[NSURLCache sharedURLCache] removeAllCachedResponses];
NSLog(@"Deleting NSURLCache.");

// reset the credentials cache...
NSDictionary *credentialsDict = [[NSURLCredentialStorage sharedCredentialStorage] allCredentials];

if ([credentialsDict count] > 0) {
    // the credentialsDict has NSURLProtectionSpace objs as keys and dicts of userName => NSURLCredential
    NSEnumerator *protectionSpaceEnumerator = [credentialsDict keyEnumerator];
    id urlProtectionSpace;

    // iterate over all NSURLProtectionSpaces
    while (urlProtectionSpace = [protectionSpaceEnumerator nextObject]) {
        NSEnumerator *userNameEnumerator = [[credentialsDict objectForKey:urlProtectionSpace] keyEnumerator];
        id userName;

        // iterate over all usernames for this protectionspace, which are the keys for the actual NSURLCredentials
        while (userName = [userNameEnumerator nextObject]) {
            NSURLCredential *cred = [[credentialsDict objectForKey:urlProtectionSpace] objectForKey:userName];
            NSLog(@"cred to be removed: %@", cred);
            [[NSURLCredentialStorage sharedCredentialStorage] removeCredential:cred forProtectionSpace:urlProtectionSpace];
        }
    }
}

// clear all cookies

NSHTTPCookieStorage *cookieStorage = [NSHTTPCookieStorage sharedHTTPCookieStorage];
for (NSHTTPCookie *sessionCookie in cookieStorage.cookies) {
    NSLog(@"Deleting cookie: %@", sessionCookie);
    [cookieStorage deleteCookie:sessionCookie];
}
}

[op setCredentialPersistence:NSURLCredentialPersistenceNone];
[op addHeader:@"Cookie" withValue:@""];