This is probably a bad idea, but I am not sure why: SSL pinning by comparing certificate data.

trivialCertValidator

- (id)initWithCertificatePath:(NSString *)path
{
  self = [super init];
  if (self) {
      _certData = [NSData dataWithContentsOfFile:path];
  }
  return self;
}

- (void)validateChallenge:(NSURLAuthenticationChallenge *)challenge {
    SecTrustRef trust = challenge.protectionSpace.serverTrust;
    SecCertificateRef serverCertificate = SecTrustGetCertificateAtIndex(trust, 0);
    NSData *serverCertificateData = (__bridge NSData *)(SecCertificateCopyData(serverCertificate));
    if ([serverCertificateData isEqualToData:_certData]) {
        [[challenge sender] useCredential:[NSURLCredential credentialForTrust:trust] forAuthenticationChallenge:challenge];
    }
    else {
        [[challenge sender] cancelAuthenticationChallenge:challenge];
    }
}