Q:

NSURLSession + server with self signed cert

I have an app that is production along with a development server that has a self signed certificate. 
I am attempting to test NSURLSession and background downloading but can't seem to get past - (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler
When I use NSURLConnection I am able to bypass it using: 
- (BOOL)connection:(NSURLConnection *)connection canAuthenticateAgainstProtectionSpace:(NSURLProtectionSpace *)protectionSpace {

    NSLog(@"canAuthenticateAgainstProtectionSpace %@", [protectionSpace authenticationMethod]);

    return [protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust];
}

- (void)connection:(NSURLConnection *)connection didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {

    NSLog(@"didReceiveAuthenticationChallenge %@ %zd", [[challenge protectionSpace] authenticationMethod], (ssize_t) [challenge previousFailureCount]);

    [challenge.sender useCredential:[NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust] forAuthenticationChallenge:challenge];
}

But I can't figure out how to get this to work with NSURLSession >:(
This is what I have currently (that doesn't work): 
- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
    NSLog(@"NSURLSession did receive challenge.");

    completionHandler(NSURLSessionAuthChallengeUseCredential, [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust]);
}

I also tried creating a category of NSURLSession that would allow any certificate for a host: 
#import "NSURLRequest+IgnoreSSL.h"

@implementation NSURLRequest (IgnoreSSL)

+ (BOOL)allowsAnyHTTPSCertificateForHost:(NSString*)host {
    return YES;
}

+ (void)setAllowsAnyHTTPSCertificate:(BOOL)allow forHost:(NSString*)host {}

@end

Which also doesn't seem to help. 

EDIT
I've updated this method to return: 
- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {

    //Creates credentials for logged in user (username/pass)
    NSURLCredential *cred = [[AuthController sharedController] userCredentials];

    completionHandler(NSURLSessionAuthChallengeUseCredential, cred);

}

Which still does nothing. 

A:

For me your first example is working fine. I have tested with the following code without problems (it is of course very insecure since it allows any server certificate).
@implementation SessionTest

- (void) startSession
{
    NSURL *url = [NSURL URLWithString:@"https://self-signed.server.url"];

    NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];

    NSURLSessionDataTask * dataTask = [defaultSession dataTaskWithURL:url
                                                completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                                    if(error == nil)
                                                    {
                                                        NSString * text = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
                                                        NSLog(@"Data: %@",text);
                                                    }
                                                    else
                                                    {
                                                        NSLog(@"Error: %@", error);
                                                    }
                                                }];

    [dataTask resume];
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler
{
    completionHandler(NSURLSessionAuthChallengeUseCredential, [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust]);
}

@end

Update: This is the class interface, the SessionTest class is the NSURLSessionDataDelegate, to start the data download you create a SessionTest object and call the startSession method.
@interface SessionTest : NSObject <NSURLSessionDelegate>

- (void) startSession;

@end

