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

Thanks to the example found in this site: https://www.bugsee.com/blog/ssl-certificate-pinning-in-mobile-applications/ I built a version that pins the public key and not the entire certificate (more convenient if you renew your certificate periodically).

Update: Removed the forced unwrapping and replaced SecTrustEvaluate.

import Foundation
import CommonCrypto

class SessionDelegate : NSObject, URLSessionDelegate {

private static let rsa2048Asn1Header:[UInt8] = [
    0x30, 0x82, 0x01, 0x22, 0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86,
    0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00, 0x03, 0x82, 0x01, 0x0f, 0x00
];

private static let google_com_pubkey = ["4xVxzbEegwDBoyoGoJlKcwGM7hyquoFg4l+9um5oPOI="];
private static let google_com_full = ["KjLxfxajzmBH0fTH1/oujb6R5fqBiLxl0zrl2xyFT2E="];

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

    guard let serverTrust = challenge.protectionSpace.serverTrust else {
        completionHandler(.cancelAuthenticationChallenge, nil);
        return;
    }

    // Set SSL policies for domain name check
    let policies = NSMutableArray();
    policies.add(SecPolicyCreateSSL(true, (challenge.protectionSpace.host as CFString)));
    SecTrustSetPolicies(serverTrust, policies);

    var isServerTrusted = SecTrustEvaluateWithError(serverTrust, nil);

    if(isServerTrusted && challenge.protectionSpace.host == "www.google.com") {
        let certificate = SecTrustGetCertificateAtIndex(serverTrust, 0);
        //Compare public key
        if #available(iOS 10.0, *) {
            let policy = SecPolicyCreateBasicX509();
            let cfCertificates = [certificate] as CFArray;

            var trust: SecTrust?
            SecTrustCreateWithCertificates(cfCertificates, policy, &trust);

            guard trust != nil, let pubKey = SecTrustCopyPublicKey(trust!) else {
                completionHandler(.cancelAuthenticationChallenge, nil);
                return;
            }

            var error:Unmanaged?
            if let pubKeyData = SecKeyCopyExternalRepresentation(pubKey, &error) {
                var keyWithHeader = Data(bytes: SessionDelegate.rsa2048Asn1Header);
                keyWithHeader.append(pubKeyData as Data);
                let sha256Key = sha256(keyWithHeader);
                if(!SessionDelegate.google_com_pubkey.contains(sha256Key)) {
                    isServerTrusted = false;
                }
            } else {
                isServerTrusted = false;
            }
        } else { //Compare full certificate
            let remoteCertificateData = SecCertificateCopyData(certificate!) as Data;
            let sha256Data = sha256(remoteCertificateData);
            if(!SessionDelegate.google_com_full.contains(sha256Data)) {
                isServerTrusted = false;
            }
        }
    }

    if(isServerTrusted) {
        let credential = URLCredential(trust: serverTrust);
        completionHandler(.useCredential, credential);
    } else {
        completionHandler(.cancelAuthenticationChallenge, nil);
    }

}

func sha256(_ data : Data) -> String {
    var hash = [UInt8](repeating: 0,  count: Int(CC_SHA256_DIGEST_LENGTH))
    data.withUnsafeBytes {
        _ = CC_SHA256($0, CC_LONG(data.count), &hash)
    }
    return Data(bytes: hash).base64EncodedString();
}

}