resolve TODO

Author: infeo

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

@@ -82,8 +82,11 @@ boolean hasCipherNodeExtension(Path p) {
 		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 isAtCipherNodeLevel(Path absolutPah) {
+		if (!absolutPah.isAbsolute()) {
+			throw new IllegalArgumentException("Path " + absolutPah + "must be absolute");
+		}
+		return absolutPah.subpath(vaultPath.getNameCount(), absolutPah.getNameCount()).getNameCount() == 4;
 	}
 
 	boolean hasMinimumFileNameLength(Path p) {

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

@@ -167,5 +167,10 @@ public void validateTooShort() {
 		}
 	}
 
+	@Test
+	public void testIsAtCipherNodeLevelRequiresAbsolutePath() {
+		Assertions.assertThrows(IllegalArgumentException.class, () -> testObj.isAtCipherNodeLevel(Path.of("relative/path")));
+	}
+
 
 }