Padded data should be at least 32 bytes

src/main/java/com/mountainminds/three4j/PaddedBuffer.java

@@ -0,0 +1,61 @@
+/*******************************************************************************
+ * Copyright (c) 2023 Mountainminds GmbH & Co. KG
+ * 
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * SPDX-License-Identifier: MIT
+ *******************************************************************************/
+package com.mountainminds.three4j;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.security.SecureRandom;
+import java.util.Random;
+
+/**
+ * Internal utility for PKCS#7 padding.
+ */
+class PaddedBuffer extends DataOutputStream {
+
+	private static ThreadLocal<Random> RANDOM = ThreadLocal.withInitial(SecureRandom::new);
+
+	private static final int MSG_LEN_MIN = 32;
+	private static final int PAD_LEN_MIN = 1;
+	private static final int PAD_LEN_MAX = 255;
+
+	private final Random random;
+
+	PaddedBuffer(Random random) {
+		super(new ByteArrayOutputStream());
+		this.random = random;
+	}
+
+	PaddedBuffer() {
+		this(RANDOM.get());
+	}
+
+	byte[] withPadding() throws IOException {
+		var buffer = (ByteArrayOutputStream) out;
+		int panLenMin = Math.max(PAD_LEN_MIN, MSG_LEN_MIN - buffer.size());
+		int padding = random.nextInt(PAD_LEN_MAX - panLenMin + 1) + panLenMin;
+		for (int i = 0; i < padding; i++) {
+			write(padding);
+		}
+		return buffer.toByteArray();
+	}
+
+	static DataInputStream removePadding(byte[] buffer) {
+		int len = 0xff & buffer[buffer.length - 1];
+		return new DataInputStream(new ByteArrayInputStream(buffer, 0, buffer.length - len));
+	}
+
+}

src/main/java/com/mountainminds/three4j/PlainMessage.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2022 Mountainminds GmbH & Co. KG
+ * Copyright (c) 2023 Mountainminds GmbH & Co. KG
  * 
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
@@ -15,8 +15,6 @@
 
 import static java.nio.charset.StandardCharsets.UTF_8;
 
-import java.io.ByteArrayInputStream;
-import java.io.ByteArrayOutputStream;
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
@@ -30,7 +28,6 @@
 
 import com.google.gson.Gson;
 
-import software.pando.crypto.nacl.Bytes;
 import software.pando.crypto.nacl.CryptoBox;
 
 /**
@@ -51,15 +48,10 @@ public EncryptedMessage encrypt(PrivateKey privateKey, PublicKey publicKey) {
 	}
 
 	private final byte[] encode() {
-		try (var buffer = new ByteArrayOutputStream(); var out = new DataOutputStream(buffer)) {
-			out.write(getType());
-			encode(out);
-			// add random padding of 1 - 255 bytes (PKCS#7 style)
-			int padding = Math.max(1, 0xFF & Bytes.secureRandom(1)[0]);
-			for (int i = 0; i < padding; i++) {
-				out.write(padding);
-			}
-			return buffer.toByteArray();
+		try (var buffer = new PaddedBuffer()) {
+			buffer.write(getType());
+			encode(buffer);
+			return buffer.withPadding();
 		} catch (IOException e) {
 			// Must not happen with ByteArrayOutputStream
 			throw new RuntimeException("Unexpected IOException", e);
@@ -75,8 +67,7 @@ private final byte[] encode() {
 	 * @return decoded message of the respective subtype
 	 */
 	public static PlainMessage decode(byte[] bytes) throws IllegalArgumentException {
-		int padding = 0xFF & bytes[bytes.length - 1];
-		try (var in = new DataInputStream(new ByteArrayInputStream(bytes, 0, bytes.length - padding))) {
+		try (var in = PaddedBuffer.removePadding(bytes)) {
 			int type = in.read();
 			switch (type) {
 			case Text.TYPE:

src/test/java/com/mountainminds/three4j/PaddedBufferTest.java

@@ -0,0 +1,105 @@
+/*******************************************************************************
+ * Copyright (c) 2021 Mountainminds GmbH & Co. KG
+ * 
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * SPDX-License-Identifier: MIT
+ *******************************************************************************/
+package com.mountainminds.three4j;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.Random;
+
+import org.junit.jupiter.api.Test;
+
+public class PaddedBufferTest {
+
+	private final Random minRandom = new Random() {
+		private static final long serialVersionUID = 1L;
+
+		@Override
+		public int nextInt(int bound) {
+			return 0;
+		}
+	};
+
+	private final Random maxRandom = new Random() {
+		private static final long serialVersionUID = 1L;
+
+		@Override
+		public int nextInt(int bound) {
+			return bound - 1;
+		}
+	};
+
+	@Test
+	void finish_should_expand_message_to_at_least_32_bytes() throws IOException {
+		try (var buffer = new PaddedBuffer(minRandom)) {
+			buffer.write(0x12);
+			buffer.write(0x15);
+			var msg = buffer.withPadding();
+			var expected = bytes(0x12, 0x15).nbytes(30, 30).toByteArray();
+			assertArrayEquals(expected, msg);
+		}
+	}
+
+	@Test
+	void finish_should_add_at_least_1_byte() throws IOException {
+		try (var buffer = new PaddedBuffer(minRandom)) {
+			buffer.write(bytes().nbytes(64, 42).toByteArray());
+			var msg = buffer.withPadding();
+			var expected = bytes().nbytes(64, 42).nbytes(1, 1).toByteArray();
+			assertArrayEquals(expected, msg);
+		}
+	}
+
+	@Test
+	void finish_should_add_at_most_255_bytes() throws IOException {
+		try (var buffer = new PaddedBuffer(maxRandom)) {
+			buffer.write(0x42);
+			buffer.write(0x43);
+			var msg = buffer.withPadding();
+			var expected = bytes(0x42, 0x43).nbytes(255, 255).toByteArray();
+			assertArrayEquals(expected, msg);
+		}
+	}
+
+	@Test
+	void removePadding_should_remove_padding_of_length_1() throws IOException {
+		var data = PaddedBuffer.removePadding(bytes(1, 2, 3).nbytes(1, 1).toByteArray());
+		assertArrayEquals(bytes(1, 2, 3).toByteArray(), data.readAllBytes());
+	}
+
+	@Test
+	void removePadding_should_remove_padding_of_length_255() throws IOException {
+		var data = PaddedBuffer.removePadding(bytes(1, 2, 3).nbytes(255, 255).toByteArray());
+		assertArrayEquals(bytes(1, 2, 3).toByteArray(), data.readAllBytes());
+	}
+
+	private static Bytes bytes(int... bytes) {
+		var out = new Bytes();
+		for (int b : bytes) {
+			out.write(0xff & b);
+		}
+		return out;
+	}
+
+	private static class Bytes extends ByteArrayOutputStream {
+		Bytes nbytes(int count, int value) {
+			for (int i = 0; i < count; i++) {
+				write(0xff & value);
+			}
+			return this;
+		}
+	}
+
+}