Prepare PDF to embed PAdES Digital Signature in Swift for iOS

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//
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//  Copyright © 2020-2025 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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/// The example here is similar to the `PrepareDocumentForContainedSignatureExample` example, only here we are preparing a
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/// different signature type that will be written into the document during the embed phase (`EmebedContainedPadesSignatureExample`).
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/// We are specifying the PDF advanced digital signature type "PAdES". This translates to the signature `SubFilter` value of
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/// "ETSI.CAdES.detached" in the PDF.
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///
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/// Currently, we cannot write this signature type in the PDF, so this type can only work with "Contained" signatures, where the signature field is
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/// prepared first and a valid PAdES signature is inserted into the `Contents` entry of the PDF signature dictionary.
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///
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/// Please refer to the ETSI standard for more information on PAdES signatures in PDF:
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/// https://www.etsi.org/deliver/etsi_ts/119100_119199/11914403/01.01.01_60/ts_11914403v010101p.pdf
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class PrepareDocumentForContainedPadesSignatureExample: Example {
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    override init() {
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        super.init()
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        title = "Prepare a document to embed a PAdES advanced digital signature afterwards"
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        category = .forms
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        priority = 22
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        // Load a sample document with an already created signature form field.
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        let document = AssetLoader.writableDocument(for: "Form.pdf", overrideIfExists: true)
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        // We know this document must have a file URL because we are loading it from a file.
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        let newURL = document.fileURL!
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        // Now we get the signature form widget.
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        let signatureFormElement = document.annotations(at: 0, type: SignatureFormElement.self).first!
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        // During document preparation, we are able to configure a custom digital signature appearance.
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        // We use an instance of a `PSPDFSignatureAppearance` to do that.
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        let signatureAppearance = PDFSignatureAppearance {
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            $0.appearanceMode = PDFSignatureAppearance.Mode.signatureAndDescription
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            $0.showSigningDate = false
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        }
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        // Configure a data sink as destination.
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        let dataSink = try! FileDataSink(fileURL: newURL, options: [])
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        // Now configure the signer that will prepare the signature field. Configuring the "signersName" property
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        // will customize the signer's name that will appear on the visual signature.
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        let signer = PDFSigner()
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        signer.signersName = "PSPDFKit GmbH"
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        // We also need to set the signature type to PAdES advanced for this example as the signature we
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        // use in the signing step is PAdES compatible.
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        signer.signatureType = .pades
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        var preparedDocument: Document?
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        // Typically, when preparing a document, we are not interested in what is inside the contents of the signature form field.
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        // Passing an instance of a `PSPDFBlankSignatureContents` will fill the contents with zeroes.
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        let signatureContents = BlankSignatureContents()
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        signer.prepare(signatureFormElement, toBeSignedWith: signatureAppearance, contents: signatureContents, writingTo: dataSink) { (_ success: Bool, _ document: DataSink?, _ err: Error?) in
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            let fileDataProvider = FileDataProvider(fileURL: newURL)
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            do {
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                try fileDataProvider.replaceContents(with: dataSink)
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            } catch {
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                print(error)
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            }
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            preparedDocument = Document(dataProviders: [fileDataProvider])
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        }
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        return PDFViewController(document: preparedDocument!)
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    }
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}
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/// This example takes the previously prepared "Form.pdf" in the `PrepareDocumentForContainedPadesSignatureExample` above,
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/// which contains space for a PAdES signature, and embeds a PAdES signature we have generated with external tools.
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///
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/// Note: the PAdES signature creation mechanism is in progress at Nutrient, so it is only possible to sign a PDF with PAdES
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/// signature using a pre-prepared PAdES signature binary outlined in the steps above.
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class EmebedContainedPadesSignatureExample: Example {
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    override init() {
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        super.init()
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        title = "Embed a PAdES advanced digital signature in an already prepared PDF document"
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        category = .forms
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        priority = 23
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        // Load a sample document with a signature form field that was already prepared for signing with a PAdES signature.
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        // See `PrepareDocumentForContainedPadesSignatureExample` to learn how to do that.
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        let document = AssetLoader.writableDocument(for: "Form for PAdES Signing.pdf", overrideIfExists: true)
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        let newURL = document.fileURL!
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        // Now we get the signature form widget.
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        let signatureFormElement = document.annotations(at: 0, type: SignatureFormElement.self).first!
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        // The prepared signature is a .bin file that contains a PAdES signature using our JohnAppleseed self-signed
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        // certificate.
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        // See comments above this example class for how we generated this PAdES signature.
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        let preparedSignatureSampleURL = AssetLoader.assetURL(for: "John Appleseed PAdES Signature.bin")
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        // We use the sample `PSPDFSignatureContents` implementation `BinaryFileSignatureContents` from the
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        // `ContainedDigitalSignaturesExample.swift` to load our pre-generated PAdES signature binary.
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        let signatureContents = BinaryFileSignatureContents(signatureContentsPath: preparedSignatureSampleURL)
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        let dataSink = try! FileDataSink(fileURL: newURL, options: [])
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        // Finally, create the signer and embed the PAdES signature we have already generated in this document.
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        let signer = PDFSigner()
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        var preparedDocument: Document?
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        signer.embedSignature(in: signatureFormElement, with: signatureContents, writingTo: dataSink) { (_ success: Bool, _ document: DataSink?, _ err: Error?) in
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            let fileDataProvider = FileDataProvider(fileURL: newURL)
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            do {
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                try fileDataProvider.replaceContents(with: dataSink)
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            } catch {
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                print(error)
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            }
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            // Optionally, here you could make sure that the signed document is valid.
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            // Read the `PSPDFSignatureValidator` documentation to learn how to do that.
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            preparedDocument = Document(dataProviders: [fileDataProvider])
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        }
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        return PDFViewController(document: preparedDocument!)
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    }
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}