Asynchronously Sign PDF in Swift for iOS

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//
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//  Copyright © 2018-2025 PSPDFKit GmbH. All rights reserved.
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//
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//  The Nutrient sample applications are licensed with a modified BSD license.
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//  Please see License for details. This notice may not be removed from this file.
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//
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import PSPDFKit
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import PSPDFKitUI
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@MainActor class AsynchronousExternalDigitalSignatureExample: Example {
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    override init() {
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        super.init()
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        title = "Signs a document asynchronously, simulating a PIN-entry screen."
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        contentDescription = "Password is 'test'"
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        category = .forms
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        priority = 25
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController? {
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        Task {
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            let presentingController = delegate.currentViewController!.navigationController!
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            // Present the alert asking for the password for the digital certificate.
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            guard let enteredPin = await presentPinEntryAlert(on: presentingController) else { return }
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            resetAndAddCertificatesToTrustStore()
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            do {
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                // Unlock the certificate chain for signing.
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                let (certificates, privateKey) = try unlockCertificateChain(withPassword: enteredPin)
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                // Create the destination URL for the signed document.
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                let fileName = "\(UUID().uuidString).pdf"
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                let url = URL(fileURLWithPath: NSTemporaryDirectory().appending(fileName))
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                // Access the unsigned document and signature form element.
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                let unsignedDocument = AssetLoader.document(for: "Form.pdf")
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                let signatureFormElement = unsignedDocument.annotations(at: 0, type: SignatureFormElement.self).first!
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                let signatureAppearance = PDFSignatureAppearance {
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                    $0.showSignerName = true
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                    $0.showSignatureReason = true
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                    $0.appearanceMode = .signatureAndDescription
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                }
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                let configuration = SigningConfiguration(dataSigner: privateKey, certificates: certificates, hashAlgorithm: .SHA512, appearance: signatureAppearance, reason: "I agree with the terms of the contract.")
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                try await unsignedDocument.sign(formElement: signatureFormElement, configuration: configuration, outputDataProvider: FileDataProvider(fileURL: url))
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                presentingController.pushViewController(PDFViewController(document: Document(url: url)), animated: true)
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            } catch {
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                presentingController.showAlert(withTitle: "Couldn't add signature", message: "\(error)")
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                print(error)
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            }
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        }
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        return nil
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    }
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    /// Retrieves the unlocked key and certifcates from the "John Appleseed" private key using the given password.
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    private func unlockCertificateChain(withPassword password: String) throws -> ([X509], PrivateKey) {
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        let p12URL = AssetLoader.assetURL(for: "John Appleseed Private Key.p12")
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        let pkcsBlob = try! Data(contentsOf: p12URL)
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        let pkcs12 = PKCS12(data: pkcsBlob)
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        return try pkcs12.unlockCertificateChain(withPassword: password)
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    }
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    /// Presents an alert controller asking for the password to unlock the private key of the "John Appleseed" signing certificate.
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    /// - Returns: Entered password string.
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    private func presentPinEntryAlert(on viewController: UIViewController) async -> String? {
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        let alert = UIAlertController(title: "Please enter the .p12 password to sign the document:", message: nil, preferredStyle: .alert)
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        alert.addTextField { textField in
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            textField.placeholder = "Enter the .p12 password here..."
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            textField.isSecureTextEntry = true
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        }
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        return await withCheckedContinuation { continuation in
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            alert.addAction(UIAlertAction(title: "Cancel", style: .cancel, handler: { _ in
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                continuation.resume(returning: nil)
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            }))
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            alert.addAction(UIAlertAction(title: "OK", style: .default, handler: { _ in
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                let password = alert.textFields?.first?.text
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                continuation.resume(returning: password)
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            }))
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            viewController.present(alert, animated: true)
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        }
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    }
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    /// Adds the demo "John Appleseed" public key to trusted certificate store of the `PDFSignatureManager` in Nutrient after reseting the store.
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    private func resetAndAddCertificatesToTrustStore() {
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        // Reset the trusted certificates.
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        let signatureManager = SDK.shared.signatureManager
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        signatureManager.clearTrustedCertificates()
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        // Add certs to trust store for the signature validation process
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        let certURL = AssetLoader.assetURL(for: "John Appleseed Public Key.p7c")
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        let certData = try? Data(contentsOf: certURL)
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        let certificates = try? X509.certificates(fromPKCS7Data: certData!)
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        for x509 in certificates! {
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            signatureManager.addTrustedCertificate(x509)
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        }
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    }
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}