Getting Request Token From Dropbox OAuth "Invalid OAuth Request" - oauth

Question

I'm trying to get a response data from dropbox oauth as described here
Now i dont know why it keeps saying {"error": "Invalid OAuth request."}
Here is a my complete code for requesting token:
-(void)requestTokenSecret
{
NSString* oauthVersion=#"1";
NSString* oauth_signature_method=#"PLAINTEXT";
NSString* postHeader = [NSString stringWithFormat: #"Authorization: OAuth oauth_version=\"%#\", oauth_signature_method=\"%#\", oauth_consumer_key=\"%#\", oauth_signature=\"%#&\"",oauthVersion,oauth_signature_method,APP_KEY,APP_SECRET];
NSLog(#"URL HEADER: %#",postHeader);
NSData* postHeaderData = [postHeader dataUsingEncoding:NSUTF8StringEncoding];
NSMutableURLRequest* request= [[NSMutableURLRequest alloc]init];
[request setURL:[NSURL URLWithString:#"https://api.dropbox.com/1/oauth/request_token"]];
[request setHTTPMethod:#"POST"];
[request setValue:#"application/json" forHTTPHeaderField:#"Content-Type"];
[request setValue:[NSString stringWithFormat:#"%d",[postHeaderData length]] forHTTPHeaderField:#"Content-Length"];
[request setHTTPBody:postHeaderData];
NSURLConnection* conn = [[NSURLConnection alloc]initWithRequest:request delegate:self];
[conn start];
if(conn)
{
NSLog(#"Connected");
}
else
NSLog(#"Connected");
}

Answer 1

It looks like you're putting "Authorization: OAuth ..." in the body of the POST request instead of putting "OAuth ..." in the Authorization header.
Give this code a try instead (untested, sorry):
NSString* oauthVersion=#"1";
NSString* oauth_signature_method=#"PLAINTEXT";
NSString* authorizationHeader = [NSString stringWithFormat: #"OAuth oauth_version=\"%#\", oauth_signature_method=\"%#\", oauth_consumer_key=\"%#\", oauth_signature=\"%#&\"",oauthVersion,oauth_signature_method,APP_KEY,APP_SECRET];
NSMutableURLRequest* request= [[NSMutableURLRequest alloc]init];
[request setURL:[NSURL URLWithString:#"https://api.dropbox.com/1/oauth/request_token"]];
[request setHTTPMethod:#"POST"];
[request setValue:authorizationHeader forHTTPHeaderField:#"Authorization"];
NSURLConnection* conn = [[NSURLConnection alloc]initWithRequest:request delegate:self];
[conn start];
Also, I would strongly suggest using OAuth 2 instead of OAuth 1. It's considerably simpler.