Calculating digital signature using a key from iOS Secure Enclave

KeychainSE.swift

    private func sign(algorithm: SecKeyAlgorithm, data: Data) {
        
        guard SecKeyIsAlgorithmSupported(key!, .sign, algorithm) else {
            UIAlertController.showSimple(title: "Can't sign",
                                         text: "Algorith not supported",
                                         from: self)
            return
        }
        
        // SecKeyCreateSignature call is blocking when the used key
        // is protected by biometry authentication. If that's not the case,
        // dispatching to a background thread isn't necessary.
        DispatchQueue.global().async {
            var error: Unmanaged<CFError>?
            let signature = SecKeyCreateSignature(self.key!, algorithm,
                                              data as CFData,
                                              &error) as Data?
            DispatchQueue.main.async {
                self.signature = signature
                guard signature != nil else {
                    UIAlertController.showSimple(title: "Can't sign",
                                                 text: (error!.takeRetainedValue() as Error).localizedDescription,
                                                 from: self)
                    return
                }
                // signature is a Data instance containing the digital signature value
                // ...
            }
        }
    }