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Question:

I am developing an iPhone app. During development, I need to connect to a server that's using a self-signed SSL certificate. I'm pretty certain - (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition, NSURLCredential *))completionHandler is my opportunity to write some exception code to allow this. However, I can't find any resources that tell me how to do this. I can see the following error in the log:

NSURLConnection/CFURLConnection HTTP load failed (kCFStreamErrorDomainSSL, -9813)

In addition to this, when I NSLog(@"error = %@", error); from within the above delegate method I get:

Error Domain=NSURLErrorDomain Code=-1202 "The certificate for this server is invalid. You might be connecting to a server that is pretending to be “api.mydevelopmenturl.com” which could put your confidential information at risk." UserInfo=0x10cbdbcf0 {NSUnderlyingError=0x112ec9730 "The certificate for this server is invalid. You might be connecting to a server that is pretending to be “api.mydevelopmenturl.com” which could put your confidential information at risk.", NSErrorFailingURLStringKey=https://api.mydevelopmenturl.com/posts, NSErrorFailingURLKey=https://api.mydevelopmenturl.com/posts, NSLocalizedRecoverySuggestion=Would you like to connect to the server anyway?, NSURLErrorFailingURLPeerTrustErrorKey=, NSLocalizedDescription=The certificate for this server is invalid. You might be connecting to a server that is pretending to be “api.mydevelopmenturl.com” which could put your confidential information at risk.}

Any ideas on how to resolve this issue? Please post code as I've read the conceptual docs and I don't understand them. Here's an example of one that's beyond me: https://developer.apple.com/library/content/technotes/tn2232/_index.html


Best Answer:


Apple has a Technical Note 2232 which is quite informative and explains in detail HTTPS server trust evaluation.

In this case error -1202 in the NSURLErrorDomain domain is NSURLErrorServerCertificateUntrusted, which means that server trust evaluation has failed. You might also receive a variety of other errors; Appendix A: Common Server Trust Evaluation Errors lists the most common ones.

From the Technical Note:

In most cases the best way to resolve a server trust evaluation failure is to fix the server. This has two benefits: it offers the best security and it reduces the amount of code you have to write. The remainder of this technote describes how you can diagnose server trust evaluation failures and, if it's not possible to fix the server, how you can customize server trust evaluation to allow your connection to proceed without completely undermining the user's security.

The particular bit that is germane to this question is the section on NSURLSession server trust evaluation:

NSURLSession allows you to customize HTTPS server trust evaluation by implementing the -URLSession:didReceiveChallenge:completionHandler: delegate method. To customize HTTPS server trust evaluation, look for a challenge whose protection space has an authentication method of NSURLAuthenticationMethodServerTrust. For those challenges, resolve them as described below. For other challenges, the ones that you don't care about, call the completion handler block with the NSURLSessionAuthChallengePerformDefaultHandling disposition and a NULL credential.

When dealing with the NSURLAuthenticationMethodServerTrust authentication challenge, you can get the trust object from the challenge's protection space by calling the -serverTrust method. After using the trust object to do your own custom HTTPS server trust evaluation, you must resolve the challenge in one of two ways:

If you want to deny the connection, call the completion handler block with the NSURLSessionAuthChallengeCancelAuthenticationChallenge disposition and a NULL credential.

If you want to allow the connection, create a credential from your trust object (using +[NSURLCredential credentialForTrust:]) and call the completion handler block with that credential and the NSURLSessionAuthChallengeUseCredential disposition.

The upshot of all this is that if you implement the following delegate method, you can override server trust for a particular server:

- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition, NSURLCredential *))completionHandler
{
    if([challenge.protectionSpace.authenticationMethod
                           isEqualToString:NSURLAuthenticationMethodServerTrust])
    {
        if([challenge.protectionSpace.host
                           isEqualToString:@"domaintoverride.com"])
        {
            NSURLCredential *credential = 
                          [NSURLCredential credentialForTrust:
                                          challenge.protectionSpace.serverTrust];
            completionHandler(NSURLSessionAuthChallengeUseCredential,credential);
        }
        else
            completionHandler(NSURLSessionAuthChallengeCancelAuthenticationChallenge, nil);
    }
}

Note that you have to handle both the case of the host matching the one you want to override and all other cases. If you don't handle the "all other cases" part, the behavior result is undefined.




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