Feature: Public method for decrypting ciphertext name.

src/main/java/org/cryptomator/cryptofs/CryptoFileSystem.java

@@ -1,5 +1,7 @@
 package org.cryptomator.cryptofs;
 
+import org.cryptomator.cryptofs.common.Constants;
+
 import java.io.IOException;
 import java.nio.file.FileSystem;
 import java.nio.file.Files;
@@ -41,6 +43,26 @@ public abstract class CryptoFileSystem extends FileSystem {
 	 */
 	public abstract Path getCiphertextPath(Path cleartextPath) throws IOException;
 
+	/**
+	 * Computes from a valid,encrypted node (file or folder) its cleartext name.
+	 * <p>
+	 * Due to the structure of a vault, an encrypted node is valid if:
+	 * <ul>
+	 *     <li>the path points into the vault (duh!)</li>
+	 *     <li>the "file" extension is {@value Constants#CRYPTOMATOR_FILE_SUFFIX} or {@value Constants#DEFLATED_FILE_SUFFIX}</li>
+	 *     <li>the node name is at least {@value Constants#MIN_CIPHER_NAME_LENGTH} characters long</li>
+	 *     <li>it is located at depth 4 from the vault storage root, i.e. d/AB/CDEFG...XYZ/validFile.c9r</li>
+	 * </ul>
+	 *
+	 * @param ciphertextNode path to the ciphertext file or directory
+	 * @return the cleartext name of the ciphertext file or directory
+	 * @throws java.nio.file.NoSuchFileException if the ciphertextFile does not exist
+	 * @throws IOException if an I/O error occurs reading the ciphertext files
+	 * @throws IllegalArgumentException if {@param ciphertextNode} is not a valid ciphertext content node of the vault
+	 * @throws UnsupportedOperationException if the directory containing the {@param ciphertextNode} does not have a {@value Constants#DIR_ID_BACKUP_FILE_NAME} file
+	 */
+	public abstract String getCleartextName(Path ciphertextNode) throws IOException, UnsupportedOperationException;
+
 	/**
 	 * Provides file system performance statistics.
 	 * 

src/main/java/org/cryptomator/cryptofs/CryptoFileSystemImpl.java

@@ -17,8 +17,8 @@
 import org.cryptomator.cryptofs.common.DeletingFileVisitor;
 import org.cryptomator.cryptofs.common.FinallyUtil;
 import org.cryptomator.cryptofs.dir.CiphertextDirectoryDeleter;
-import org.cryptomator.cryptofs.dir.DirectoryStreamFilters;
 import org.cryptomator.cryptofs.dir.DirectoryStreamFactory;
+import org.cryptomator.cryptofs.dir.DirectoryStreamFilters;
 import org.cryptomator.cryptofs.fh.OpenCryptoFiles;
 import org.cryptomator.cryptolib.api.Cryptor;
 
@@ -95,16 +95,17 @@ class CryptoFileSystemImpl extends CryptoFileSystem {
 
 	private final CryptoPath rootPath;
 	private final CryptoPath emptyPath;
+	private final FileNameDecryptor fileNameDecryptor;
 
 	private volatile boolean open = true;
 
 	@Inject
-	public CryptoFileSystemImpl(CryptoFileSystemProvider provider, CryptoFileSystems cryptoFileSystems, @PathToVault Path pathToVault, Cryptor cryptor,
-								CryptoFileStore fileStore, CryptoFileSystemStats stats, CryptoPathMapper cryptoPathMapper, CryptoPathFactory cryptoPathFactory,
-								PathMatcherFactory pathMatcherFactory, DirectoryStreamFactory directoryStreamFactory, DirectoryIdProvider dirIdProvider, DirectoryIdBackup dirIdBackup,
-								AttributeProvider fileAttributeProvider, AttributeByNameProvider fileAttributeByNameProvider, AttributeViewProvider fileAttributeViewProvider,
-								OpenCryptoFiles openCryptoFiles, Symlinks symlinks, FinallyUtil finallyUtil, CiphertextDirectoryDeleter ciphertextDirDeleter, ReadonlyFlag readonlyFlag,
-								CryptoFileSystemProperties fileSystemProperties) {
+	public CryptoFileSystemImpl(CryptoFileSystemProvider provider, CryptoFileSystems cryptoFileSystems, @PathToVault Path pathToVault, Cryptor cryptor, //
+								CryptoFileStore fileStore, CryptoFileSystemStats stats, CryptoPathMapper cryptoPathMapper, CryptoPathFactory cryptoPathFactory, //
+								PathMatcherFactory pathMatcherFactory, DirectoryStreamFactory directoryStreamFactory, DirectoryIdProvider dirIdProvider, DirectoryIdBackup dirIdBackup, //
+								AttributeProvider fileAttributeProvider, AttributeByNameProvider fileAttributeByNameProvider, AttributeViewProvider fileAttributeViewProvider, //
+								OpenCryptoFiles openCryptoFiles, Symlinks symlinks, FinallyUtil finallyUtil, CiphertextDirectoryDeleter ciphertextDirDeleter, ReadonlyFlag readonlyFlag, //
+								CryptoFileSystemProperties fileSystemProperties, FileNameDecryptor fileNameDecryptor) {
 		this.provider = provider;
 		this.cryptoFileSystems = cryptoFileSystems;
 		this.pathToVault = pathToVault;
@@ -129,6 +130,7 @@ public CryptoFileSystemImpl(CryptoFileSystemProvider provider, CryptoFileSystems
 
 		this.rootPath = cryptoPathFactory.rootFor(this);
 		this.emptyPath = cryptoPathFactory.emptyFor(this);
+		this.fileNameDecryptor = fileNameDecryptor;
 	}
 
 	@Override
@@ -151,6 +153,11 @@ public Path getCiphertextPath(Path cleartextPath) throws IOException {
 		}
 	}
 
+	@Override
+	public String getCleartextName(Path ciphertextNode) throws IOException, UnsupportedOperationException {
+		return fileNameDecryptor.decryptFilename(ciphertextNode);
+	}
+
 	@Override
 	public CryptoFileSystemStats getStats() {
 		return stats;

src/main/java/org/cryptomator/cryptofs/FileNameDecryptor.java

@@ -0,0 +1,96 @@
+package org.cryptomator.cryptofs;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.io.BaseEncoding;
+import org.cryptomator.cryptofs.common.Constants;
+import org.cryptomator.cryptofs.common.StringUtils;
+import org.cryptomator.cryptolib.api.CryptoException;
+import org.cryptomator.cryptolib.api.Cryptor;
+import org.cryptomator.cryptolib.api.FileNameCryptor;
+
+import javax.inject.Inject;
+import java.io.IOException;
+import java.nio.file.FileSystemException;
+import java.nio.file.NoSuchFileException;
+import java.nio.file.Path;
+import java.util.stream.Stream;
+
+/**
+ * @see CryptoFileSystem#getCleartextName(Path)
+ */
+@CryptoFileSystemScoped
+class FileNameDecryptor {
+
+	private final DirectoryIdBackup dirIdBackup;
+	private final LongFileNameProvider longFileNameProvider;
+	private final Path vaultPath;
+	private final FileNameCryptor fileNameCryptor;
+
+	@Inject
+	public FileNameDecryptor(@PathToVault Path vaultPath, Cryptor cryptor, DirectoryIdBackup dirIdBackup, LongFileNameProvider longFileNameProvider) {
+		this.vaultPath = vaultPath;
+		this.fileNameCryptor = cryptor.fileNameCryptor();
+		this.dirIdBackup = dirIdBackup;
+		this.longFileNameProvider = longFileNameProvider;
+	}
+
+	public String decryptFilename(Path ciphertextNode) throws IOException, UnsupportedOperationException {
+		validatePath(ciphertextNode.toAbsolutePath());
+		return decryptFilenameInternal(ciphertextNode);
+	}
+
+	@VisibleForTesting
+	String decryptFilenameInternal(Path ciphertextNode) throws IOException, UnsupportedOperationException {
+		byte[] dirId = null;
+		try {
+			dirId = dirIdBackup.read(ciphertextNode);
+		} catch (NoSuchFileException e) {
+			throw new UnsupportedOperationException("Directory does not have a " + Constants.DIR_ID_BACKUP_FILE_NAME + " file.");
+		} catch (CryptoException | IllegalStateException e) {
+			throw new FileSystemException(ciphertextNode.toString(), null, "Decryption of dirId backup file failed:" + e);
+		}
+		var fullCipherNodeName = ciphertextNode.getFileName().toString();
+		var cipherNodeExtension = fullCipherNodeName.substring(fullCipherNodeName.length() - 4);
+
+		String actualEncryptedName = switch (cipherNodeExtension) {
+			case Constants.CRYPTOMATOR_FILE_SUFFIX -> StringUtils.removeEnd(fullCipherNodeName, Constants.CRYPTOMATOR_FILE_SUFFIX);
+			case Constants.DEFLATED_FILE_SUFFIX -> longFileNameProvider.inflate(ciphertextNode);
+			default -> throw new IllegalStateException("SHOULD NOT REACH HERE");
+		};
+		try {
+			return fileNameCryptor.decryptFilename(BaseEncoding.base64Url(), actualEncryptedName, dirId);
+		} catch (CryptoException e) {
+			throw new FileSystemException(ciphertextNode.toString(), null, "Filname decryption failed:" + e);
+		}
+	}
+
+	@VisibleForTesting
+	void validatePath(Path absolutePath) {
+		if (!belongsToVault(absolutePath)) {
+			throw new IllegalArgumentException("Node %s is not a part of vault %s".formatted(absolutePath, vaultPath));
+		}
+		if (!isAtCipherNodeLevel(absolutePath)) {
+			throw new IllegalArgumentException("Node %s is not located at depth 4 from vault storage root".formatted(absolutePath));
+		}
+		if (!(hasCipherNodeExtension(absolutePath) && hasMinimumFileNameLength(absolutePath))) {
+			throw new IllegalArgumentException("Node %s does not end with %s or %s or filename is shorter than %d characters.".formatted(absolutePath, Constants.CRYPTOMATOR_FILE_SUFFIX, Constants.DEFLATED_FILE_SUFFIX, Constants.MIN_CIPHER_NAME_LENGTH));
+		}
+	}
+
+	boolean hasCipherNodeExtension(Path p) {
+		var name = p.getFileName();
+		return name != null && Stream.of(Constants.CRYPTOMATOR_FILE_SUFFIX, Constants.DEFLATED_FILE_SUFFIX).anyMatch(name.toString()::endsWith);
+	}
+
+	boolean isAtCipherNodeLevel(Path p) {
+		return vaultPath.relativize(p).getNameCount() == 4; //TODO: relativize is defined for two relative Paths. For two absolute paths, the result depends on the OS
+	}
+
+	boolean hasMinimumFileNameLength(Path p) {
+		return p.getFileName().toString().length() >= Constants.MIN_CIPHER_NAME_LENGTH;
+	}
+
+	boolean belongsToVault(Path p) {
+		return p.startsWith(vaultPath);
+	}
+}

src/test/java/org/cryptomator/cryptofs/CryptoFileSystemImplTest.java

@@ -105,6 +105,7 @@ public class CryptoFileSystemImplTest {
 	private final CiphertextDirectoryDeleter ciphertextDirDeleter = mock(CiphertextDirectoryDeleter.class);
 	private final ReadonlyFlag readonlyFlag = mock(ReadonlyFlag.class);
 	private final CryptoFileSystemProperties fileSystemProperties = mock(CryptoFileSystemProperties.class);
+	private final FileNameDecryptor filenameDecryptor = mock(FileNameDecryptor.class);
 
 	private final CryptoPath root = mock(CryptoPath.class);
 	private final CryptoPath empty = mock(CryptoPath.class);
@@ -127,7 +128,7 @@ public void setup() {
 				pathMatcherFactory, directoryStreamFactory, dirIdProvider, dirIdBackup, //
 				fileAttributeProvider, fileAttributeByNameProvider, fileAttributeViewProvider, //
 				openCryptoFiles, symlinks, finallyUtil, ciphertextDirDeleter, readonlyFlag, //
-				fileSystemProperties);
+				fileSystemProperties, filenameDecryptor);
 	}
 
 	@Test

src/test/java/org/cryptomator/cryptofs/DirectoryIdBackupTest.java

@@ -1,6 +1,8 @@
 package org.cryptomator.cryptofs;
 
 import org.cryptomator.cryptofs.common.Constants;
+import org.cryptomator.cryptofs.health.dirid.OrphanContentDirTest;
+import org.cryptomator.cryptofs.util.TestCryptoException;
 import org.cryptomator.cryptolib.api.CryptoException;
 import org.cryptomator.cryptolib.api.Cryptor;
 import org.cryptomator.cryptolib.common.DecryptingReadableByteChannel;
@@ -103,17 +105,13 @@ public void contentLongerThan36Chars() throws IOException {
 		@DisplayName("If the backup file cannot be decrypted, a CryptoException is thrown")
 		public void invalidEncryptionThrowsCryptoException() throws IOException {
 			var dirIdBackupSpy = spy(dirIdBackup);
-			var expectedException = new MyCryptoException();
+			var expectedException = new TestCryptoException();
 			Mockito.when(dirIdBackupSpy.wrapDecryptionAround(Mockito.any(), Mockito.eq(cryptor))).thenReturn(decChannel);
 			Mockito.when(decChannel.read(Mockito.any())).thenThrow(expectedException);
 			var actual = Assertions.assertThrows(CryptoException.class, () -> dirIdBackupSpy.read(contentPath));
 			Assertions.assertEquals(expectedException, actual);
 		}
 
-		static class MyCryptoException extends CryptoException {
-
-		}
-
 		@Test
 		@DisplayName("IOException accessing the file is rethrown")
 		public void ioException() throws IOException {