iOS certificate pinning with Swift and NSURLSession

Question

Howto add certificate pinning to a NSURLSession in Swift?

The OWASP website contains only an example for Objective-C and NSURLConnection.

Answer 1

Here's an updated version for Swift 3

import Foundation
import Security

class NSURLSessionPinningDelegate: NSObject, URLSessionDelegate {

    func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Swift.Void) {

        // Adapted from OWASP https://www.owasp.org/index.php/Certificate_and_Public_Key_Pinning#iOS

        if (challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodServerTrust) {
            if let serverTrust = challenge.protectionSpace.serverTrust {
                var secresult = SecTrustResultType.invalid
                let status = SecTrustEvaluate(serverTrust, &secresult)

                if(errSecSuccess == status) {
                    if let serverCertificate = SecTrustGetCertificateAtIndex(serverTrust, 0) {
                        let serverCertificateData = SecCertificateCopyData(serverCertificate)
                        let data = CFDataGetBytePtr(serverCertificateData);
                        let size = CFDataGetLength(serverCertificateData);
                        let cert1 = NSData(bytes: data, length: size)
                        let file_der = Bundle.main.path(forResource: "name-of-cert-file", ofType: "cer")

                        if let file = file_der {
                            if let cert2 = NSData(contentsOfFile: file) {
                                if cert1.isEqual(to: cert2 as Data) {
                                    completionHandler(URLSession.AuthChallengeDisposition.useCredential, URLCredential(trust:serverTrust))
                                    return
                                }
                            }
                        }
                    }
                }
            }
        }

        // Pinning failed
        completionHandler(URLSession.AuthChallengeDisposition.cancelAuthenticationChallenge, nil)
    }

}