A challenge from a server requiring authentication from the client.


Most apps do not create authentication challenges themselves. However, you might need to create authentication challenge objects when adding support for custom networking protocols, as part of your custom NSURLProtocol subclasses.

Instead, your app receives authentication challenges in various NSURLSession, NSURLConnection, and NSURLDownload delegate methods, such as URLSession:task:didReceiveChallenge:completionHandler:. These objects provide the information you’ll need when deciding how to handle a server’s request for authentication. At the core of that authentication challenge is a protection space that defines the type of authentication being requested, the host and port number, the networking protocol, and (where applicable) the authentication realm (a group of related URLs on the same server that share a single set of credentials).

Your app responds to authentication challenges by providing an NSURLCredential object. The details depend on the API you are using and on the type of challenge.

At a high level, if you’re providing the user’s credentials to a server or proxy, the proposedCredential method provides a credential that matches the criteria specified in the protection space, retrieved from the NSURLCredentialStorage class handling the request (assuming such a credential exists).

If the previousFailureCount method returns 0 and the proposed credential exists, the proposed credential has not yet been tried, which means you should try it. If it returns a nonzero result, then the server has rejected the proposed credential, and you should use that credential to populate a password or certificate chooser dialog, then provide a new credential. You can create password-based credentials by calling the credentialWithUser:password:persistence: method or create certificate-based credentials with the credentialWithIdentity:certificates:persistence:.

If the authentication’s protection space uses the NSURLAuthenticationMethodServerTrust authentication method, the request is asking you to verify the server’s authenticity. In this case, the proposedCredential method provides a credential based on the certificates that the server provided as part of its initial TLS handshake. Most apps should request default handling for authentication challenges based on a server trust protection space, but if you need to override the default TLS validation behavior, you can do so as described in Overriding TLS Chain Validation Correctly.

For more information about how URL sessions handle the different types of authentication challenges, see NSURLSession and URL Session Programming Guide.


Creating an authentication challenge instance


Returns an initialized NSURLAuthenticationChallenge object copying the properties from challenge, and setting the authentication sender to sender.


Returns an initialized NSURLAuthenticationChallenge object for the specified protection space, credential, failure count, server response, error, and sender.

Getting authentication challenge properties


The error object representing the last authentication failure.


The URL response object representing the last authentication failure.


The receiver’s count of failed authentication attempts.


The proposed credential for this challenge.


The receiver’s protection space.


The receiver’s sender.


The NSURLAuthenticationChallengeSender protocol represents the interface that the sender of an authentication challenge must implement.


Inherits From

Conforms To

See Also

Authentication and Credentials

Handling an Authentication Challenge

Respond appropriately when a server demands authentication for a URL request.


An authentication credential consisting of authentication information specific to the type of credential and the type of persistent storage to use, if any.


An object that manages the credential storage.


A server or an area on a server, commonly referred to as a realm, that requires authentication.