AES Encrypted File
Open AES-encrypted PDFs using a custom DataProvider without decrypting to disk first.
/* * Copyright © 2017-2026 PSPDFKit GmbH. All rights reserved. * * The PSPDFKit Sample applications are licensed with a modified BSD license. * Please see License for details. This notice may not be removed from this file. */
package com.pspdfkit.catalog.examples.java;
import static com.pspdfkit.catalog.SdkExample.TAG;
import android.content.Context;import android.content.Intent;import android.os.Parcel;import android.os.Parcelable;import android.util.Base64;import android.util.Log;import androidx.annotation.NonNull;import androidx.annotation.Nullable;import com.pspdfkit.catalog.R;import com.pspdfkit.catalog.SdkExample;import com.pspdfkit.catalog.tasks.ExtractAssetTask;import com.pspdfkit.configuration.activity.PdfActivityConfiguration;import com.pspdfkit.document.providers.WritableDataProvider;import com.pspdfkit.ui.PdfActivityIntentBuilder;import java.io.Closeable;import java.io.DataInputStream;import java.io.File;import java.io.FileOutputStream;import java.io.IOException;import java.io.RandomAccessFile;import java.nio.channels.Channels;import java.security.GeneralSecurityException;import java.security.SecureRandom;import java.util.Arrays;import java.util.concurrent.ConcurrentHashMap;import javax.crypto.Cipher;import javax.crypto.CipherInputStream;import javax.crypto.CipherOutputStream;import javax.crypto.spec.IvParameterSpec;import javax.crypto.spec.SecretKeySpec;
public class AesEncryptedFileExample extends SdkExample {
private static final String ASSET_FILE_NAME = "A_encrypted.pdf";
/** * This is 256B AES encryption key stored encoded as BASE64. In production apps this should be * secured! * */ private static final String BASE64_ENCRYPTION_KEY = "EQQlw3SNbBwbxkSi1jwwib4B4XqesCVDZv9LftsmE1U=";
public AesEncryptedFileExample(@NonNull Context context) { super(context, R.string.aesExampleTitle, R.string.aesExampleDescription); }
@Override public void launchExample( @NonNull final Context context, @NonNull final PdfActivityConfiguration.Builder configuration) { ExtractAssetTask.extract(ASSET_FILE_NAME, getTitle(), context, documentFile -> { AesDataProvider provider = new AesDataProvider(documentFile.getAbsolutePath(), BASE64_ENCRYPTION_KEY); Intent intent = PdfActivityIntentBuilder.fromDataProvider(context, provider) .configuration(configuration.build()) .build(); context.startActivity(intent); }); }
/** * This is a DataProvider that will open and decrypt an AES256-CTR encrypted file on the fly. It * won't store decrypted blocks anywhere and allows random seeking to prevent large PDF files from * causing OutOfMemoryExceptions. * * <p>The file itself has a 16-byte IV (also called nonce) prepended before the actual encrypted * payload. This serves as the initialization vector for decrypting the first byte. Subsequent bytes * use an incremented IV, for example block n uses IV+n. */ public static class AesDataProvider implements WritableDataProvider, Parcelable { public static final Creator<AesDataProvider> CREATOR = new Creator<>() { @Override public AesDataProvider createFromParcel(Parcel in) { return new AesDataProvider(in); }
@Override public AesDataProvider[] newArray(int size) { return new AesDataProvider[size]; } }; private static final int AES_BLOCK_SIZE = 16; private static final int IV_SIZE = 16; private static final int FILE_SIZE_NOT_SET = -1;
@NonNull private final File encryptedFile;
@NonNull private final byte[] encryptedFileKey;
private long decryptedFileSize = FILE_SIZE_NOT_SET;
@Nullable private byte[] encryptedFileIv = null;
private File temporaryOutputFile; private FileOutputStream fos; private CipherOutputStream cos;
// Per Thread Data - When multi threaded rendering is activated read() can be called from // multiple threads at the // same time. In order to support this we store everything we need for reading on a per thread // basis, otherwise // conflicts might lead to an exception being thrown. // We don't use ThreadLocal here as we need to be able clean everything from a single thread. // This is open file information per thread. @NonNull private final ConcurrentHashMap<Thread, RandomAccessFile> openFileHandles = new ConcurrentHashMap<>(); // We also need a Cipher for every thread. @NonNull private final ConcurrentHashMap<Thread, Cipher> aesCipherMap = new ConcurrentHashMap<>();
public AesDataProvider(@NonNull String encryptedFilePath, @NonNull String base64Aes256Key) { encryptedFile = new File(encryptedFilePath); encryptedFileKey = Base64.decode(base64Aes256Key, Base64.DEFAULT); }
/** * When parcelling, we only store the filepath and encryption key - everything else gets * restored from file. * * <p>Note: This will hand over the encryption key to the operating system! In order to keep the * key secret, you should consider persisting/retrieving it from a reliable source. */ private AesDataProvider(Parcel in) { encryptedFile = new File(in.readString()); encryptedFileKey = in.createByteArray(); }
/** * In AES-CTR mode, each AES block is encrypted with a key and IV. IV is incremented by number 1 * for each next block, so to figure out the IV for block N, we need to add N to the initial IV. * Only lower four bytes of IV can change like this. */ private static byte[] getIvForBlock(byte[] originalIv, long block) { long counter = ((long) originalIv[12] << 24 & 0xFF000000) | ((long) originalIv[13] << 16) & 0xFF0000 | ((long) originalIv[14] << 8) & 0xFF00 | ((long) originalIv[15] & 0xFF); counter += block;
byte[] iv = Arrays.copyOf(originalIv, 16); iv[12] = (byte) ((counter >> 24) & 0xFF); iv[13] = (byte) ((counter >> 16) & 0xFF); iv[14] = (byte) ((counter >> 8) & 0xFF); iv[15] = (byte) (counter & 0xFF); return iv; }
/** * This opens the file and reads the IV if it wasn't read yet. Every thread calling this gets * its own RandomAccessFile. */ private RandomAccessFile openFile() throws IOException { if (openFileHandles.containsKey(Thread.currentThread())) { return openFileHandles.get(Thread.currentThread()); } else { RandomAccessFile file = new RandomAccessFile(encryptedFile, "r");
if (encryptedFileIv == null) { synchronized (this) { if (encryptedFileIv == null) { // Encrypted file IV is stored at the beginning of the file, read it. encryptedFileIv = new byte[16]; file.read(encryptedFileIv, 0, 16);
decryptedFileSize = file.length() - IV_SIZE; // Don't take saved IV into account. } } }
Log.i(TAG, "Opened encrypted file " + encryptedFile.getAbsolutePath() + " size " + decryptedFileSize); openFileHandles.put(Thread.currentThread(), file); return file; } }
/** Creates a Cipher set up for decrypting. Every thread calling this gets its own Cipher. */ private Cipher getCipher() throws IOException { if (aesCipherMap.containsKey(Thread.currentThread())) { return aesCipherMap.get(Thread.currentThread()); } else { try { Cipher aesCipher = Cipher.getInstance("AES/CTR/NoPadding"); aesCipherMap.put(Thread.currentThread(), aesCipher); return aesCipher; } catch (GeneralSecurityException e) { throw new IOException("This device does not support AES-CTR!"); } } }
@NonNull @Override public byte[] read(long size, long offset) { try { // Grab the thread specific data needed for reading. RandomAccessFile file = openFile(); Cipher aesCipher = getCipher();
// AES is encrypted in 16B blocks which are the minimum we can read. So here we need to // figure out which block the offset falls in and start // decryption there. long block = offset / AES_BLOCK_SIZE;
// Each block has different IV, so we need to calculate the IV of the first block to // start decrypting. IvParameterSpec ivParameterSpec = new IvParameterSpec(getIvForBlock(encryptedFileIv, block)); aesCipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(encryptedFileKey, "AES"), ivParameterSpec);
// Figure out the location of the encrypted block inside the file - we need to add 16 to // offset because we stored IV at the beginning of the file. long seekPos = (block * AES_BLOCK_SIZE) + IV_SIZE; file.seek(seekPos);
// Initialize cipher stream from the set file location. final CipherInputStream cis = new CipherInputStream(Channels.newInputStream(file.getChannel()), aesCipher);
// On some devices (primarily Samsung) CipherInputStream implementations suffer from a // bug that stops reading early once an internal buffer is hit. // Wrapping the cipher stream into a DataInputStream allows to reliably read the entire // chunk that was requested. DataInputStream input = new DataInputStream(cis);
// Since we had to start on an AES block boundary, skip bytes which may not align with // it. int toSkip = (int) (offset % AES_BLOCK_SIZE); while (toSkip > 0) { toSkip -= input.skip(toSkip); }
// Read and decrypt data into the byte array. byte[] decryptedData = new byte[(int) size]; input.readFully(decryptedData); return decryptedData; } catch (GeneralSecurityException | IOException e) { Log.e(TAG, "Crypto exception: " + e.getMessage(), e); return new byte[0]; } catch (Exception e) { Log.e(TAG, "Exception: " + e.getMessage(), e); return new byte[0]; } }
/** Nutrient expects size of decrypted PDF here. */ @Override public long getSize() { if (decryptedFileSize == FILE_SIZE_NOT_SET) { // Initialize the size the first time it's needed. // We do it as late as possible since right after saving the size sometimes isn't // updated yet. try { openFile(); } catch (IOException e) { decryptedFileSize = encryptedFile.length() - IV_SIZE; } } return decryptedFileSize; }
@NonNull @Override public String getUid() { return encryptedFile.getAbsolutePath(); }
@Nullable @Override public String getTitle() { return null; }
/** We need to close opened streams here. * */ @Override public void release() { try { Log.e(TAG, "Closing file " + encryptedFile.getAbsolutePath()); closeFiles(); } catch (IOException ignored) { } }
private void closeFiles() throws IOException { for (RandomAccessFile file : openFileHandles.values()) { file.close(); } openFileHandles.clear(); aesCipherMap.clear(); }
@Override protected void finalize() throws Throwable { super.finalize(); release(); }
@Override public boolean canWrite() { return true; }
@Override public boolean startWrite(@NonNull WriteMode writeMode) { // We return "supportsAppending()" as "false" so this shouldn't happen. if (writeMode == WriteMode.APPEND_TO_FILE) throw new IllegalArgumentException("Appending isn't supported by this provider.");
// We need to save information into a temporary file since input file will probably be read // as saving is in progress. temporaryOutputFile = new File(encryptedFile.getParent(), "tmp-write.pdf");
// To keep writing secure we must regenerate IV from random source for each file. byte[] outputAesIv = new byte[16]; SecureRandom rnd = new SecureRandom(); rnd.nextBytes(outputAesIv);
try { // Write IV at the start of file just like we had at the input. fos = new FileOutputStream(temporaryOutputFile); fos.write(outputAesIv);
// Setup encryption - use same key as for the input. Cipher c = Cipher.getInstance("AES/CTR/NoPadding"); IvParameterSpec ivSpec = new IvParameterSpec(outputAesIv); SecretKeySpec outputKey = new SecretKeySpec(encryptedFileKey, "AES"); c.init(Cipher.ENCRYPT_MODE, outputKey, ivSpec); cos = new CipherOutputStream(fos, c); } catch (IOException | GeneralSecurityException e) { Log.e(TAG, "Failed to open file for writing - " + e.getMessage(), e); return false; }
Log.i( TAG, "Writing changes to " + encryptedFile.getName() + " to temporary file " + temporaryOutputFile.getAbsolutePath()); return true; }
@Override public boolean write(@NonNull byte[] data) { try { cos.write(data); } catch (IOException e) { Log.e(TAG, "Failed to write encrypted file - " + e.getMessage(), e); return false; }
return true; }
@Override public boolean finishWrite() { // File has finished writing. Now we need to close the input file and replace it with the // freshly written temporary file. // Any reads after that will expect the new file already.
Throwable[] closeErrors = new Throwable[3]; boolean success = true; closeErrors[0] = safelyClose(cos); closeErrors[1] = safelyClose(fos);
try { closeFiles(); } catch (Throwable ex) { closeErrors[2] = ex; }
for (Throwable e : closeErrors) { if (e != null) { Log.e(TAG, "Error while closing output streams - " + e.getMessage(), e); success = false; } }
// Delete original file if (!encryptedFile.delete()) success = false; if (!temporaryOutputFile.renameTo(encryptedFile)) success = false;
if (!success) { return false; }
temporaryOutputFile = null; cos = null; fos = null; // We need to reset the iv after writing since we always pick a new one when saving. encryptedFileIv = null;
// We need to update the stored file size now as well. decryptedFileSize = encryptedFile.length() - IV_SIZE; Log.i(TAG, "Writing complete, replaced original file with new file of size " + decryptedFileSize); return true; }
@Override public boolean supportsAppending() { // Even though AES-CTR mode allows us to actually append as well, this example will keep it // simple and always rewrite the full file on save. return false; }
@Nullable private Throwable safelyClose(Closeable closeable) { try { closeable.close(); return null; } catch (Throwable closeError) { return closeError; } }
@Override public int describeContents() { return 0; }
@Override public void writeToParcel(Parcel dest, int flags) { dest.writeString(encryptedFile.getAbsolutePath()); dest.writeByteArray(encryptedFileKey); } }}This code sample is an example that illustrates how to use our SDK. Please adapt it to your specific use case.