Alamofire 5 alternative to sessionDidReceiveChallenge

Question

I have just shifted to Alamofire 5.

Earlier I used URLSession and Certificate Pinner and to handle auth challenge I used delegate method of URLSessionDelegate with hash values

func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge,
                completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {
    print("being challanged! for \(challenge.protectionSpace.host)")
    guard let trust = challenge.protectionSpace.serverTrust else {
        print("invalid trust!")
        completionHandler(.cancelAuthenticationChallenge, nil)
        return
    }

    let credential = URLCredential(trust: trust)

    let pinner = setupCertificatePinner(host: challenge.protectionSpace.host)

    if (!pinner.validateCertificateTrustChain(trust)) {
        print("failed: invalid certificate chain!")
        challenge.sender?.cancel(challenge)
    }

    if (pinner.validateTrustPublicKeys(trust)) {
        completionHandler(.useCredential, credential)
    } else {
        didPinningFailed = true
        print("couldn't validate trust for \(challenge.protectionSpace.host)")
        completionHandler(.cancelAuthenticationChallenge, nil)
    }

}

Having moved to Alamofire 5, there is no method sessionDidReceiveChallenge which was available in earlier version.

I tried:

private let session: Session = {
    let manager = ServerTrustManager(allHostsMustBeEvaluated: true, evaluators:
        ["devDomain.com": DisabledTrustEvaluator(),
         "prodDomain.com": PublicKeysTrustEvaluator()])
    let configuration = URLSessionConfiguration.af.default

    return Session(configuration: configuration, serverTrustManager: manager)
}()

But I get error:

Error Domain=Alamofire.AFError Code=11 "Server trust evaluation failed due to reason: No public keys were found or provided for evaluation."

Update: I'd still prefer a way to parse it using 256 fingerprint only, as we get domains and its hashes in first api call.

Answer 1

First you need a ServerTrustEvaluating that handle the certificate pinning a simple implement would be something similar to

public final class CertificatePinnerTrustEvaluator: ServerTrustEvaluating {

    public init() {}

    func setupCertificatePinner(host: String) -> CertificatePinner {

        //get the CertificatePinner
    }

    public func evaluate(_ trust: SecTrust, forHost host: String) throws {

        let pinner = setupCertificatePinner(host: host)

        if (!pinner.validateCertificateTrustChain(trust)) {
            print("failed: invalid certificate chain!")
            throw AFError.serverTrustEvaluationFailed(reason: .noCertificatesFound)
        }

        if (!pinner.validateTrustPublicKeys(trust)) {
            print ("couldn't validate trust for \(host)")

            throw AFError.serverTrustEvaluationFailed(reason: .noCertificatesFound)
        }
    }
}

To be able to use the same evaluator I would suggest to subclass ServerTrustManager to return the same evaluator I did it like this:

class CertificatePinnerServerTrustManager: ServerTrustManager {

    let evaluator = CertificatePinnerTrustEvaluator()

    init() {
        super.init(allHostsMustBeEvaluated: true, evaluators: [:])
    }

    open override func serverTrustEvaluator(forHost host: String) throws -> ServerTrustEvaluating? {

        return evaluator
    }
}

after that you should be ready to go by creating the session and passing the manager to it

private let session: Session = {

    let trustManager = CertificatePinnerServerTrustManager()

    return Session(serverTrustManager: trustManager)
}()

My reference was the method urlSession(_:task:didReceive:completionHandler:) in Alamofire source in SessionDelegate.swift at line 86 (Alamofire V5.2.1)