Add digital signature to PDF in Kotlin for Android

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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 PSPDFKit 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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package com.pspdfkit.catalog.examples.kotlin
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import android.content.Context
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import android.net.Uri
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import android.widget.Toast
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import com.pspdfkit.catalog.R
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import com.pspdfkit.catalog.SdkExample
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import com.pspdfkit.configuration.activity.PdfActivityConfiguration
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import com.pspdfkit.document.DocumentSource
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import com.pspdfkit.document.PdfDocumentLoader
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import com.pspdfkit.document.providers.AssetDataProvider
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import com.pspdfkit.signatures.SignerOptions
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import com.pspdfkit.signatures.SigningManager
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import com.pspdfkit.signatures.getPrivateKeyFromFile
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import com.pspdfkit.signatures.loadCertificateFromStream
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import com.pspdfkit.ui.PdfActivityIntentBuilder
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import com.pspdfkit.utils.PdfLog
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import java.io.File
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import java.security.PrivateKey
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import java.security.cert.X509Certificate
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/**
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 * An example that shows how to digitally sign a PDF document using [SigningManager].
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 * This is a Simple implementation where user provides Private key in [SignerOptions].
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 */
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class DigitalSignatureExample(context: Context) : SdkExample(context, R.string.digitalSignatureExampleTitle, R.string.digitalSignatureExampleDescription) {
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    override fun launchExample(context: Context, configuration: PdfActivityConfiguration.Builder) {
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        val assetName = "Form_example.pdf"
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        val unsignedDocument = PdfDocumentLoader.openDocument(context, DocumentSource(AssetDataProvider(assetName)))
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        val privateKey = getPrivateKey(context)
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        val certificate = getCertificate(context)
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        val signatureFormFields = unsignedDocument.documentSignatureInfo.signatureFormFields
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        val outputFile = File(context.filesDir, "signedDocument.pdf")
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        outputFile.delete() // make sure output is deleted from previous runs.
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        val signerOptions = SignerOptions.Builder(signatureFormFields[0], Uri.fromFile(outputFile))
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            .setPrivateKey(privateKey)
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            .setCertificates(listOf(certificate))
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            .setType(digitalSignatureType)
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            .build()
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        /** [SignerOptions] contains all the required configuration for [SigningManager]*/
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        SigningManager.signDocument(
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            context = context,
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            signerOptions = signerOptions,
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            onFailure = { e ->
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                Toast.makeText(context, "Error launching example. See logcat for details.", Toast.LENGTH_SHORT).show()
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                PdfLog.e("DigitalSignatureExample", e, "Error while launching example.")
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            }
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        ) {
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            val signedDocument = Uri.fromFile(outputFile)
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            // Load and show the signed document.
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            val intent = PdfActivityIntentBuilder.fromUri(context, signedDocument)
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                .configuration(configuration.build())
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                .build()
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            context.startActivity(intent)
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        }
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    }
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    private fun getPrivateKey(context: Context): PrivateKey {
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        val privateKeyFile = context.assets.open("digital-signatures/self-signed/demo.pkcs8")
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        return getPrivateKeyFromFile(privateKeyFile)
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    }
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    private fun getCertificate(context: Context): X509Certificate {
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        val certificateFile = context.assets.open("digital-signatures/self-signed/demo.cer")
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        return loadCertificateFromStream(certificateFile)
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    }
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}