NSURLAuthenticationChallenge encapsulates a challenge from a server requiring authentication from the client.
- iOS 2.0+
- macOS 10.2+
- tvOS 9.0+
- watchOS 2.0+
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
Instead, your app receives authentication challenges in various
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).
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
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.