Count 7 bits at a time.

Add ImageActivity to show image attachment in full-screen

See merge request briar/briar!999

bramble-api/src/main/java/org/briarproject/bramble/util/ByteUtils.java

@@ -1,5 +1,7 @@
 package org.briarproject.bramble.util;
 
+import org.briarproject.bramble.api.FormatException;
+
 public class ByteUtils {
 
 	/**
@@ -12,15 +14,26 @@ public class ByteUtils {
 	 */
 	public static final long MAX_32_BIT_UNSIGNED = 4294967295L; // 2^32 - 1
 
-	/** The number of bytes needed to encode a 16-bit integer. */
+	/**
+	 * The number of bytes needed to encode a 16-bit integer.
+	 */
 	public static final int INT_16_BYTES = 2;
 
-	/** The number of bytes needed to encode a 32-bit integer. */
+	/**
+	 * The number of bytes needed to encode a 32-bit integer.
+	 */
 	public static final int INT_32_BYTES = 4;
 
-	/** The number of bytes needed to encode a 64-bit integer. */
+	/**
+	 * The number of bytes needed to encode a 64-bit integer.
+	 */
 	public static final int INT_64_BYTES = 8;
 
+	/**
+	 * The maximum number of bytes needed to encode a variable-length integer.
+	 */
+	public static final int MAX_VARINT_BYTES = 9;
+
 	public static void writeUint16(int src, byte[] dest, int offset) {
 		if (src < 0) throw new IllegalArgumentException();
 		if (src > MAX_16_BIT_UNSIGNED) throw new IllegalArgumentException();
@@ -55,6 +68,42 @@ public class ByteUtils {
 		dest[offset + 7] = (byte) (src & 0xFF);
 	}
 
+	/**
+	 * Returns the number of bytes needed to represent 'src' as a
+	 * variable-length integer.
+	 * <p>
+	 * 'src' must not be negative.
+	 */
+	public static int getVarIntBytes(long src) {
+		if (src < 0) throw new IllegalArgumentException();
+		int len = 1;
+		while ((src >>= 7) > 0) len++;
+		return len;
+	}
+
+	/**
+	 * Writes 'src' to 'dest' as a variable-length integer, starting at
+	 * 'offset', and returns the number of bytes written.
+	 * <p>
+	 * `src` must not be negative.
+	 */
+	public static int writeVarInt(long src, byte[] dest, int offset) {
+		if (src < 0) throw new IllegalArgumentException();
+		int len = getVarIntBytes(src);
+		if (dest.length < offset + len) throw new IllegalArgumentException();
+		// Work backwards from the end
+		int end = offset + len - 1;
+		for (int i = end; i >= offset; i--) {
+			// Encode 7 bits
+			dest[i] = (byte) (src & 0x7F);
+			// Raise the continuation flag, except for the last byte
+			if (i < end) dest[i] |= (byte) 0x80;
+			// Shift out the bits that were encoded
+			src >>= 7;
+		}
+		return len;
+	}
+
 	public static int readUint16(byte[] src, int offset) {
 		if (src.length < offset + INT_16_BYTES)
 			throw new IllegalArgumentException();
@@ -83,14 +132,46 @@ public class ByteUtils {
 				| (src[offset + 7] & 0xFFL);
 	}
 
-	public static int readUint(byte[] src, int bits) {
-		if (src.length << 3 < bits) throw new IllegalArgumentException();
-		int dest = 0;
-		for (int i = 0; i < bits; i++) {
-			if ((src[i >> 3] & 128 >> (i & 7)) != 0) dest |= 1 << bits - i - 1;
+	/**
+	 * Returns the length in bytes of a variable-length integer encoded in
+	 * 'src' starting at 'offset'.
+	 *
+	 * @throws FormatException if there is not a valid variable-length integer
+	 * at the specified position.
+	 */
+	public static int getVarIntBytes(byte[] src, int offset)
+			throws FormatException {
+		if (src.length < offset) throw new IllegalArgumentException();
+		for (int i = 0; i < MAX_VARINT_BYTES && offset + i < src.length; i++) {
+			// If the continuation flag is lowered, this is the last byte
+			if ((src[offset + i] & 0x80) == 0) return i + 1;
 		}
-		if (dest < 0) throw new AssertionError();
-		if (dest >= 1 << bits) throw new AssertionError();
-		return dest;
+		// We've read 9 bytes or reached the end of the input without finding
+		// the last byte
+		throw new FormatException();
+	}
+
+	/**
+	 * Reads a variable-length integer from 'src' starting at 'offset' and
+	 * returns it.
+	 *
+	 * @throws FormatException if there is not a valid variable-length integer
+	 * at the specified position.
+	 */
+	public static long readVarInt(byte[] src, int offset)
+			throws FormatException {
+		if (src.length < offset) throw new IllegalArgumentException();
+		long dest = 0;
+		for (int i = 0; i < MAX_VARINT_BYTES && offset + i < src.length; i++) {
+			// Decode 7 bits
+			dest |= src[offset + i] & 0x7F;
+			// If the continuation flag is lowered, this is the last byte
+			if ((src[offset + i] & 0x80) == 0) return dest;
+			// Make room for the next 7 bits
+			dest <<= 7;
+		}
+		// We've read 9 bytes or reached the end of the input without finding
+		// the last byte
+		throw new FormatException();
 	}
 }

bramble-core/src/test/java/org/briarproject/bramble/util/ByteUtilsTest.java

@@ -1,118 +1,135 @@
 package org.briarproject.bramble.util;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.test.BrambleTestCase;
 import org.junit.Test;
 
+import java.util.Random;
+
+import static java.util.Arrays.fill;
 import static org.briarproject.bramble.util.ByteUtils.MAX_16_BIT_UNSIGNED;
 import static org.briarproject.bramble.util.ByteUtils.MAX_32_BIT_UNSIGNED;
+import static org.briarproject.bramble.util.ByteUtils.MAX_VARINT_BYTES;
+import static org.briarproject.bramble.util.ByteUtils.getVarIntBytes;
+import static org.briarproject.bramble.util.ByteUtils.readUint16;
+import static org.briarproject.bramble.util.ByteUtils.readUint32;
+import static org.briarproject.bramble.util.ByteUtils.readUint64;
+import static org.briarproject.bramble.util.ByteUtils.readVarInt;
+import static org.briarproject.bramble.util.ByteUtils.writeUint16;
+import static org.briarproject.bramble.util.ByteUtils.writeUint32;
+import static org.briarproject.bramble.util.ByteUtils.writeUint64;
+import static org.briarproject.bramble.util.ByteUtils.writeVarInt;
+import static org.briarproject.bramble.util.StringUtils.fromHexString;
+import static org.briarproject.bramble.util.StringUtils.toHexString;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
+@SuppressWarnings("ResultOfMethodCallIgnored")
 public class ByteUtilsTest extends BrambleTestCase {
 
 	@Test
 	public void testReadUint16() {
-		byte[] b = StringUtils.fromHexString("00000000");
-		assertEquals(0, ByteUtils.readUint16(b, 1));
-		b = StringUtils.fromHexString("00000100");
-		assertEquals(1, ByteUtils.readUint16(b, 1));
-		b = StringUtils.fromHexString("007FFF00");
-		assertEquals(Short.MAX_VALUE, ByteUtils.readUint16(b, 1));
-		b = StringUtils.fromHexString("00FFFF00");
-		assertEquals(65535, ByteUtils.readUint16(b, 1));
+		byte[] b = fromHexString("00000000");
+		assertEquals(0, readUint16(b, 1));
+		b = fromHexString("00000100");
+		assertEquals(1, readUint16(b, 1));
+		b = fromHexString("007FFF00");
+		assertEquals(Short.MAX_VALUE, readUint16(b, 1));
+		b = fromHexString("00FFFF00");
+		assertEquals(65535, readUint16(b, 1));
 	}
 
 	@Test(expected = IllegalArgumentException.class)
 	public void testReadUint16ValidatesArguments1() {
-		ByteUtils.readUint16(new byte[1], 0);
+		readUint16(new byte[1], 0);
 	}
 
 	@Test(expected = IllegalArgumentException.class)
 	public void testReadUint16ValidatesArguments2() {
-		ByteUtils.readUint16(new byte[2], 1);
+		readUint16(new byte[2], 1);
 	}
 
 	@Test
 	public void testReadUint32() {
-		byte[] b = StringUtils.fromHexString("000000000000");
-		assertEquals(0, ByteUtils.readUint32(b, 1));
-		b = StringUtils.fromHexString("000000000100");
-		assertEquals(1, ByteUtils.readUint32(b, 1));
-		b = StringUtils.fromHexString("007FFFFFFF00");
-		assertEquals(Integer.MAX_VALUE, ByteUtils.readUint32(b, 1));
-		b = StringUtils.fromHexString("00FFFFFFFF00");
-		assertEquals(4294967295L, ByteUtils.readUint32(b, 1));
+		byte[] b = fromHexString("000000000000");
+		assertEquals(0, readUint32(b, 1));
+		b = fromHexString("000000000100");
+		assertEquals(1, readUint32(b, 1));
+		b = fromHexString("007FFFFFFF00");
+		assertEquals(Integer.MAX_VALUE, readUint32(b, 1));
+		b = fromHexString("00FFFFFFFF00");
+		assertEquals(4294967295L, readUint32(b, 1));
 	}
 
 	@Test(expected = IllegalArgumentException.class)
 	public void testReadUint32ValidatesArguments1() {
-		ByteUtils.readUint32(new byte[3], 0);
+		readUint32(new byte[3], 0);
 	}
 
 	@Test(expected = IllegalArgumentException.class)
 	public void testReadUint32ValidatesArguments2() {
-		ByteUtils.readUint32(new byte[4], 1);
+		readUint32(new byte[4], 1);
 	}
 
 	@Test
 	public void testReadUint64() {
-		byte[] b = StringUtils.fromHexString("00000000000000000000");
-		assertEquals(0L, ByteUtils.readUint64(b, 1));
-		b = StringUtils.fromHexString("00000000000000000100");
-		assertEquals(1L, ByteUtils.readUint64(b, 1));
-		b = StringUtils.fromHexString("007FFFFFFFFFFFFFFF00");
-		assertEquals(Long.MAX_VALUE, ByteUtils.readUint64(b, 1));
-		b = StringUtils.fromHexString("00800000000000000000");
-		assertEquals(Long.MIN_VALUE, ByteUtils.readUint64(b, 1));
-		b = StringUtils.fromHexString("00FFFFFFFFFFFFFFFF00");
-		assertEquals(-1L, ByteUtils.readUint64(b, 1));
+		byte[] b = fromHexString("00000000000000000000");
+		assertEquals(0L, readUint64(b, 1));
+		b = fromHexString("00000000000000000100");
+		assertEquals(1L, readUint64(b, 1));
+		b = fromHexString("007FFFFFFFFFFFFFFF00");
+		assertEquals(Long.MAX_VALUE, readUint64(b, 1));
+		b = fromHexString("00800000000000000000");
+		assertEquals(Long.MIN_VALUE, readUint64(b, 1));
+		b = fromHexString("00FFFFFFFFFFFFFFFF00");
+		assertEquals(-1L, readUint64(b, 1));
 	}
 
 	@Test(expected = IllegalArgumentException.class)
 	public void testReadUint64ValidatesArguments1() {
-		ByteUtils.readUint64(new byte[7], 0);
+		readUint64(new byte[7], 0);
 	}
 
 	@Test(expected = IllegalArgumentException.class)
 	public void testReadUint64ValidatesArguments2() {
-		ByteUtils.readUint64(new byte[8], 1);
+		readUint64(new byte[8], 1);
 	}
 
 	@Test
 	public void testWriteUint16() {
 		byte[] b = new byte[4];
-		ByteUtils.writeUint16(0, b, 1);
-		assertEquals("00000000", StringUtils.toHexString(b));
-		ByteUtils.writeUint16(1, b, 1);
-		assertEquals("00000100", StringUtils.toHexString(b));
-		ByteUtils.writeUint16(Short.MAX_VALUE, b, 1);
-		assertEquals("007FFF00", StringUtils.toHexString(b));
-		ByteUtils.writeUint16(MAX_16_BIT_UNSIGNED, b, 1);
-		assertEquals("00FFFF00", StringUtils.toHexString(b));
+		writeUint16(0, b, 1);
+		assertEquals("00000000", toHexString(b));
+		writeUint16(1, b, 1);
+		assertEquals("00000100", toHexString(b));
+		writeUint16(Short.MAX_VALUE, b, 1);
+		assertEquals("007FFF00", toHexString(b));
+		writeUint16(MAX_16_BIT_UNSIGNED, b, 1);
+		assertEquals("00FFFF00", toHexString(b));
 	}
 
 	@Test
 	public void testWriteUint16ValidatesArguments() {
 		try {
-			ByteUtils.writeUint16(0, new byte[1], 0);
+			writeUint16(0, new byte[1], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint16(0, new byte[2], 1);
+			writeUint16(0, new byte[2], 1);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint16(-1, new byte[2], 0);
+			writeUint16(-1, new byte[2], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint16(MAX_16_BIT_UNSIGNED + 1, new byte[2], 0);
+			writeUint16(MAX_16_BIT_UNSIGNED + 1, new byte[2], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
@@ -122,38 +139,38 @@ public class ByteUtilsTest extends BrambleTestCase {
 	@Test
 	public void testWriteUint32() {
 		byte[] b = new byte[6];
-		ByteUtils.writeUint32(0, b, 1);
-		assertEquals("000000000000", StringUtils.toHexString(b));
-		ByteUtils.writeUint32(1, b, 1);
-		assertEquals("000000000100", StringUtils.toHexString(b));
-		ByteUtils.writeUint32(Integer.MAX_VALUE, b, 1);
-		assertEquals("007FFFFFFF00", StringUtils.toHexString(b));
-		ByteUtils.writeUint32(MAX_32_BIT_UNSIGNED, b, 1);
-		assertEquals("00FFFFFFFF00", StringUtils.toHexString(b));
+		writeUint32(0, b, 1);
+		assertEquals("000000000000", toHexString(b));
+		writeUint32(1, b, 1);
+		assertEquals("000000000100", toHexString(b));
+		writeUint32(Integer.MAX_VALUE, b, 1);
+		assertEquals("007FFFFFFF00", toHexString(b));
+		writeUint32(MAX_32_BIT_UNSIGNED, b, 1);
+		assertEquals("00FFFFFFFF00", toHexString(b));
 	}
 
 	@Test
 	public void testWriteUint32ValidatesArguments() {
 		try {
-			ByteUtils.writeUint32(0, new byte[3], 0);
+			writeUint32(0, new byte[3], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint32(0, new byte[4], 1);
+			writeUint32(0, new byte[4], 1);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint32(-1, new byte[4], 0);
+			writeUint32(-1, new byte[4], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint32(MAX_32_BIT_UNSIGNED + 1, new byte[4], 0);
+			writeUint32(MAX_32_BIT_UNSIGNED + 1, new byte[4], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
@@ -163,30 +180,30 @@ public class ByteUtilsTest extends BrambleTestCase {
 	@Test
 	public void testWriteUint64() {
 		byte[] b = new byte[10];
-		ByteUtils.writeUint64(0, b, 1);
-		assertEquals("00000000000000000000", StringUtils.toHexString(b));
-		ByteUtils.writeUint64(1, b, 1);
-		assertEquals("00000000000000000100", StringUtils.toHexString(b));
-		ByteUtils.writeUint64(Long.MAX_VALUE, b, 1);
-		assertEquals("007FFFFFFFFFFFFFFF00", StringUtils.toHexString(b));
+		writeUint64(0, b, 1);
+		assertEquals("00000000000000000000", toHexString(b));
+		writeUint64(1, b, 1);
+		assertEquals("00000000000000000100", toHexString(b));
+		writeUint64(Long.MAX_VALUE, b, 1);
+		assertEquals("007FFFFFFFFFFFFFFF00", toHexString(b));
 	}
 
 	@Test
 	public void testWriteUint64ValidatesArguments() {
 		try {
-			ByteUtils.writeUint64(0, new byte[7], 0);
+			writeUint64(0, new byte[7], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint64(0, new byte[8], 1);
+			writeUint64(0, new byte[8], 1);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
 		}
 		try {
-			ByteUtils.writeUint64(-1, new byte[8], 0);
+			writeUint64(-1, new byte[8], 0);
 			fail();
 		} catch (IllegalArgumentException expected) {
 			// Expected
@@ -194,17 +211,170 @@ public class ByteUtilsTest extends BrambleTestCase {
 	}
 
 	@Test
-	public void testReadUint() {
-		byte[] b = new byte[1];
-		b[0] = (byte) 128;
-		for (int i = 0; i < 8; i++) {
-			assertEquals(1 << i, ByteUtils.readUint(b, i + 1));
-		}
-		b = new byte[2];
-		for (int i = 0; i < 65535; i++) {
-			ByteUtils.writeUint16(i, b, 0);
-			assertEquals(i, ByteUtils.readUint(b, 16));
-			assertEquals(i >> 1, ByteUtils.readUint(b, 15));
+	public void testGetVarIntBytesToWrite() {
+		assertEquals(1, getVarIntBytes(0));
+		assertEquals(1, getVarIntBytes(0x7F)); // Max 7-bit int
+		assertEquals(2, getVarIntBytes(0x7F + 1));
+		assertEquals(2, getVarIntBytes(0x3FFF)); // Max 14-bit int
+		assertEquals(3, getVarIntBytes(0x3FFF + 1));
+		assertEquals(3, getVarIntBytes(0x1FFFFF)); // Max 21-bit int
+		assertEquals(4, getVarIntBytes(0x1FFFFF + 1));
+		assertEquals(4, getVarIntBytes(0xFFFFFFF)); // Max 28-bit int
+		assertEquals(5, getVarIntBytes(0xFFFFFFF + 1));
+		assertEquals(5, getVarIntBytes(0x7FFFFFFFFL)); // Max 35-bit int
+		assertEquals(6, getVarIntBytes(0x7FFFFFFFFL + 1));
+		assertEquals(6, getVarIntBytes(0x3FFFFFFFFFFL)); // Max 42-bit int
+		assertEquals(7, getVarIntBytes(0x3FFFFFFFFFFL + 1));
+		assertEquals(7, getVarIntBytes(0x1FFFFFFFFFFFFL)); // Max 49-bit int
+		assertEquals(8, getVarIntBytes(0x1FFFFFFFFFFFFL + 1));
+		assertEquals(8, getVarIntBytes(0xFFFFFFFFFFFFFFL)); // Max 56-bit int
+		assertEquals(9, getVarIntBytes(0xFFFFFFFFFFFFFFL + 1));
+		assertEquals(9, getVarIntBytes(0x7FFFFFFFFFFFFFFFL)); // Max 63-bit int
+		assertEquals(MAX_VARINT_BYTES, getVarIntBytes(Long.MAX_VALUE)); // Same
+	}
+
+	@Test
+	public void testWriteVarInt() {
+		testWriteVarInt(0, 1, "00");
+		testWriteVarInt(1, 1, "01");
+		testWriteVarInt(0x7F, 1, "7F"); // Max 7-bit int
+		testWriteVarInt(0x7F + 1, 2, "8100");
+		testWriteVarInt(0x3FFF, 2, "FF7F"); // Max 14-bit int
+		testWriteVarInt(0x3FFF + 1, 3, "818000");
+		testWriteVarInt(0x1FFFFF, 3, "FFFF7F"); // Max 21-bit int
+		testWriteVarInt(0x1FFFFF + 1, 4, "81808000");
+		testWriteVarInt(0xFFFFFFF, 4, "FFFFFF7F"); // Max 28-bit int
+		testWriteVarInt(0xFFFFFFF + 1, 5, "8180808000");
+		testWriteVarInt(0x7FFFFFFFFL, 5, "FFFFFFFF7F"); // Max 35-bit int
+		testWriteVarInt(0x7FFFFFFFFL + 1, 6, "818080808000");
+		testWriteVarInt(0x3FFFFFFFFFFL, 6, "FFFFFFFFFF7F"); // Max 42-bit int
+		testWriteVarInt(0x3FFFFFFFFFFL + 1, 7, "81808080808000");
+		testWriteVarInt(0x1FFFFFFFFFFFFL, 7, "FFFFFFFFFFFF7F"); // Max 49
+		testWriteVarInt(0x1FFFFFFFFFFFFL + 1, 8, "8180808080808000");
+		testWriteVarInt(0xFFFFFFFFFFFFFFL, 8, "FFFFFFFFFFFFFF7F"); // Max 56
+		testWriteVarInt(0xFFFFFFFFFFFFFFL + 1, 9, "818080808080808000");
+		testWriteVarInt(0x7FFFFFFFFFFFFFFFL, 9, "FFFFFFFFFFFFFFFF7F"); // Max 63
+		testWriteVarInt(Long.MAX_VALUE, MAX_VARINT_BYTES, "FFFFFFFFFFFFFFFF7F");
+	}
+
+	private void testWriteVarInt(long src, int len, String destHex) {
+		byte[] dest = new byte[9];
+		assertEquals(len, writeVarInt(src, dest, 0));
+		assertEquals(destHex, toHexString(dest).substring(0, len * 2));
+	}
+
+	@Test
+	public void testGetVarIntBytesToRead() throws FormatException {
+		testGetVarIntBytesToRead(1, "00", 0);
+		testGetVarIntBytesToRead(1, "01", 0);
+		testGetVarIntBytesToRead(1, "7F", 0); // Max 7-bit int
+		testGetVarIntBytesToRead(2, "8100", 0);
+		testGetVarIntBytesToRead(2, "FF7F", 0); // Max 14-bit int
+		testGetVarIntBytesToRead(3, "818000", 0);
+		testGetVarIntBytesToRead(3, "FFFF7F", 0); // Max 21-bit int
+		testGetVarIntBytesToRead(4, "81808000", 0);
+		testGetVarIntBytesToRead(4, "FFFFFF7F", 0); // Max 28-bit int
+		testGetVarIntBytesToRead(5, "8180808000", 0);
+		testGetVarIntBytesToRead(5, "FFFFFFFF7F", 0); // Max 35-bit int
+		testGetVarIntBytesToRead(6, "818080808000", 0);
+		testGetVarIntBytesToRead(6, "FFFFFFFFFF7F", 0); // Max 42-bit int
+		testGetVarIntBytesToRead(7, "81808080808000", 0);
+		testGetVarIntBytesToRead(7, "FFFFFFFFFFFF7F", 0); // Max 49-bit int
+		testGetVarIntBytesToRead(8, "8180808080808000", 0);
+		testGetVarIntBytesToRead(8, "FFFFFFFFFFFFFF7F", 0); // Max 56-bit int
+		testGetVarIntBytesToRead(9, "818080808080808000", 0);
+		testGetVarIntBytesToRead(9, "FFFFFFFFFFFFFFFF7F", 0); // Max 63-bit int
+		// Start at offset, ignore trailing data
+		testGetVarIntBytesToRead(1, "FF0000", 1);
+		testGetVarIntBytesToRead(9, "00FFFFFFFFFFFFFFFF7F00", 1);
+	}
+
+	private void testGetVarIntBytesToRead(int len, String srcHex, int offset)
+			throws FormatException {
+		assertEquals(len, getVarIntBytes(fromHexString(srcHex), offset));
+	}
+
+	@Test(expected = FormatException.class)
+	public void testGetVarIntBytesToReadThrowsExceptionAtEndOfInput()
+			throws FormatException {
+		byte[] src = new byte[MAX_VARINT_BYTES - 1];
+		fill(src, (byte) 0xFF);
+		// Reaches end of input without finding lowered continuation flag
+		getVarIntBytes(src, 0);
+	}
+
+	@Test(expected = FormatException.class)
+	public void testGetVarIntBytesToReadThrowsExceptionAfterNineBytes()
+			throws FormatException {
+		byte[] src = new byte[MAX_VARINT_BYTES];
+		fill(src, (byte) 0xFF);
+		// Reaches max length without finding lowered continuation flag
+		getVarIntBytes(src, 0);
+	}
+
+	@Test
+	public void testReadVarInt() throws FormatException {
+		testReadVarInt(0, "00", 0);
+		testReadVarInt(1, "01", 0);
+		testReadVarInt(0x7F, "7F", 0); // Max 7-bit int
+		testReadVarInt(0x7F + 1, "8100", 0);
+		testReadVarInt(0x3FFF, "FF7F", 0); // Max 14-bit int
+		testReadVarInt(0x3FFF + 1, "818000", 0);
+		testReadVarInt(0x1FFFFF, "FFFF7F", 0); // Max 21-bit int
+		testReadVarInt(0x1FFFFF + 1, "81808000", 0);
+		testReadVarInt(0xFFFFFFF, "FFFFFF7F", 0); // Max 28-bit int
+		testReadVarInt(0xFFFFFFF + 1, "8180808000", 0);
+		testReadVarInt(0x7FFFFFFFFL, "FFFFFFFF7F", 0); // Max 35-bit int
+		testReadVarInt(0x7FFFFFFFFL + 1, "818080808000", 0);
+		testReadVarInt(0x3FFFFFFFFFFL, "FFFFFFFFFF7F", 0); // Max 42-bit int
+		testReadVarInt(0x3FFFFFFFFFFL + 1, "81808080808000", 0);
+		testReadVarInt(0x1FFFFFFFFFFFFL, "FFFFFFFFFFFF7F", 0); // Max 49-bit int
+		testReadVarInt(0x1FFFFFFFFFFFFL + 1, "8180808080808000", 0);
+		testReadVarInt(0xFFFFFFFFFFFFFFL, "FFFFFFFFFFFFFF7F", 0); // Max 56
+		testReadVarInt(0xFFFFFFFFFFFFFFL + 1, "818080808080808000", 0);
+		testReadVarInt(0x7FFFFFFFFFFFFFFFL, "FFFFFFFFFFFFFFFF7F", 0); // Max 63
+		testReadVarInt(Long.MAX_VALUE, "FFFFFFFFFFFFFFFF7F", 0);
+		// Start at offset, ignore trailing data
+		testReadVarInt(0, "FF0000", 1);
+		testReadVarInt(Long.MAX_VALUE, "00FFFFFFFFFFFFFFFF7F00", 1);
+	}
+
+	private void testReadVarInt(long dest, String srcHex, int offset)
+			throws FormatException {
+		assertEquals(dest, readVarInt(fromHexString(srcHex), offset));
+	}
+
+	@Test(expected = FormatException.class)
+	public void testReadVarIntThrowsExceptionAtEndOfInput()
+			throws FormatException {
+		byte[] src = new byte[MAX_VARINT_BYTES - 1];
+		fill(src, (byte) 0xFF);
+		// Reaches end of input without finding lowered continuation flag
+		readVarInt(src, 0);
+	}
+
+	@Test(expected = FormatException.class)
+	public void testReadVarIntThrowsExceptionAfterNineBytes()
+			throws FormatException {
+		byte[] src = new byte[MAX_VARINT_BYTES];
+		fill(src, (byte) 0xFF);
+		// Reaches max length without finding lowered continuation flag
+		readVarInt(src, 0);
+	}
+
+	@Test
+	public void testWriteAndReadVarInt() throws FormatException {
+		Random random = new Random();
+		int padding = 10;
+		byte[] buf = new byte[MAX_VARINT_BYTES + padding];
+		for (int i = 0; i < 1000; i++) {
+			long src = random.nextLong() & 0x7FFFFFFFFFFFFFFFL; // Non-negative
+			int offset = random.nextInt(padding);
+			int len = getVarIntBytes(src);
+			assertEquals(len, writeVarInt(src, buf, offset));
+			assertEquals(len, getVarIntBytes(buf, offset));
+			assertEquals(src, readVarInt(buf, offset));
+			fill(buf, (byte) 0);
 		}
 	}
 }

briar-android/build.gradle

@@ -123,6 +123,7 @@ dependencies {
 		exclude group: 'com.android.support'
 		exclude module: 'disklrucache' // when there's no disk cache, we can't accidentally use it
 	}
+	implementation 'com.github.chrisbanes:PhotoView:2.1.4' // later versions already use androidx
 
 	annotationProcessor 'com.google.dagger:dagger-compiler:2.19'
 	annotationProcessor "com.github.bumptech.glide:compiler:$glideVersion"

briar-android/src/main/AndroidManifest.xml

@@ -113,6 +113,15 @@
 				/>
 		</activity>
 
+		<activity
+			android:name=".android.conversation.ImageActivity"
+			android:parentActivityName="org.briarproject.briar.android.conversation.ConversationActivity"
+			android:theme="@style/BriarTheme.Transparent.NoActionBar">
+			<meta-data
+				android:name="android.support.PARENT_ACTIVITY"
+				android:value="org.briarproject.briar.android.conversation.ConversationActivity"/>
+		</activity>
+
 		<activity
 			android:name="org.briarproject.briar.android.privategroup.creation.CreateGroupActivity"
 			android:label="@string/groups_create_group_title"

briar-android/src/main/java/org/briarproject/briar/android/activity/ActivityComponent.java

@@ -15,10 +15,11 @@ import org.briarproject.briar.android.blog.ReblogFragment;
 import org.briarproject.briar.android.blog.RssFeedImportActivity;
 import org.briarproject.briar.android.blog.RssFeedManageActivity;
 import org.briarproject.briar.android.blog.WriteBlogPostActivity;
-import org.briarproject.briar.android.conversation.AliasDialogFragment;
 import org.briarproject.briar.android.contact.ContactListFragment;
 import org.briarproject.briar.android.contact.ContactModule;
+import org.briarproject.briar.android.conversation.AliasDialogFragment;
 import org.briarproject.briar.android.conversation.ConversationActivity;
+import org.briarproject.briar.android.conversation.ImageActivity;
 import org.briarproject.briar.android.forum.CreateForumActivity;
 import org.briarproject.briar.android.forum.ForumActivity;
 import org.briarproject.briar.android.forum.ForumListFragment;
@@ -110,6 +111,8 @@ public interface ActivityComponent {
 
 	void inject(ConversationActivity activity);
 
+	void inject(ImageActivity activity);
+
 	void inject(ForumInvitationActivity activity);
 
 	void inject(BlogInvitationActivity activity);

briar-android/src/main/java/org/briarproject/briar/android/activity/BriarActivity.java

@@ -2,12 +2,11 @@ package org.briarproject.briar.android.activity;
 
 import android.annotation.SuppressLint;
 import android.content.Intent;
+import android.support.annotation.RequiresApi;
 import android.support.v7.app.ActionBar;
 import android.support.v7.app.AlertDialog;
 import android.support.v7.widget.Toolbar;
-import android.transition.Slide;
 import android.transition.Transition;
-import android.view.Gravity;
 import android.view.Window;
 import android.widget.CheckBox;
 
@@ -33,6 +32,7 @@ import static android.os.Build.VERSION.SDK_INT;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_DOZE_WHITELISTING;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_PASSWORD;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_UNLOCK;
+import static org.briarproject.briar.android.util.UiUtils.excludeSystemUi;
 import static org.briarproject.briar.android.util.UiUtils.getDozeWhitelistingIntent;
 import static org.briarproject.briar.android.util.UiUtils.isSamsung7;
 
@@ -111,21 +111,28 @@ public abstract class BriarActivity extends BaseActivity {
 		lockManager.onActivityStop();
 	}
 
-	public void setSceneTransitionAnimation() {
-		if (SDK_INT < 21) return;
+	/**
+	 * Sets the transition animations.
+	 * @param enterTransition used to move views into initial positions
+	 * @param exitTransition used to move views out when starting a <b>new</b> activity.
+	 * @param returnTransition used when window is closing, because the activity is finishing.
+	 */
+	@RequiresApi(api = 21)
+	public void setSceneTransitionAnimation(
+			@Nullable Transition enterTransition,
+			@Nullable Transition exitTransition,
+			@Nullable Transition returnTransition) {
 		// workaround for #1007
 		if (isSamsung7()) {
 			return;
 		}
-		Transition slide = new Slide(Gravity.RIGHT);
-		slide.excludeTarget(android.R.id.statusBarBackground, true);
-		slide.excludeTarget(android.R.id.navigationBarBackground, true);
+		if (enterTransition != null) excludeSystemUi(enterTransition);
+		if (exitTransition != null) excludeSystemUi(exitTransition);
+		if (returnTransition != null) excludeSystemUi(returnTransition);
 		Window window = getWindow();
-		window.requestFeature(Window.FEATURE_CONTENT_TRANSITIONS);
-		window.setExitTransition(slide);
-		window.setEnterTransition(slide);
-		window.setTransitionBackgroundFadeDuration(getResources()
-				.getInteger(android.R.integer.config_longAnimTime));
+		window.setEnterTransition(enterTransition);
+		window.setExitTransition(exitTransition);
+		window.setReturnTransition(returnTransition);
 	}
 
 	/**

briar-android/src/main/java/org/briarproject/briar/android/blog/ReblogActivity.java

@@ -18,7 +18,6 @@ public class ReblogActivity extends BriarActivity implements
 
 	@Override
 	public void onCreate(Bundle savedInstanceState) {
-		setSceneTransitionAnimation();
 		super.onCreate(savedInstanceState);
 
 		Intent intent = getIntent();

briar-android/src/main/java/org/briarproject/briar/android/conversation/AttachmentItem.java

@@ -1,5 +1,8 @@
 package org.briarproject.briar.android.conversation;
 
+import android.os.Parcel;
+import android.os.Parcelable;
+
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.MessageId;
 
@@ -7,13 +10,26 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public class AttachmentItem {
+public class AttachmentItem implements Parcelable {
 
 	private final MessageId messageId;
 	private final int width, height;
 	private final int thumbnailWidth, thumbnailHeight;
 	private final boolean hasError;
 
+	public static final Creator<AttachmentItem> CREATOR =
+			new Creator<AttachmentItem>() {
+				@Override
+				public AttachmentItem createFromParcel(Parcel in) {
+					return new AttachmentItem(in);
+				}
+
+				@Override
+				public AttachmentItem[] newArray(int size) {
+					return new AttachmentItem[size];
+				}
+			};
+
 	AttachmentItem(MessageId messageId, int width, int height,
 			int thumbnailWidth, int thumbnailHeight, boolean hasError) {
 		this.messageId = messageId;
@@ -24,6 +40,17 @@ public class AttachmentItem {
 		this.hasError = hasError;
 	}
 
+	protected AttachmentItem(Parcel in) {
+		byte[] messageIdByte = new byte[MessageId.LENGTH];
+		in.readByteArray(messageIdByte);
+		messageId = new MessageId(messageIdByte);
+		width = in.readInt();
+		height = in.readInt();
+		thumbnailWidth = in.readInt();
+		thumbnailHeight = in.readInt();
+		hasError = in.readByte() != 0;
+	}
+
 	public MessageId getMessageId() {
 		return messageId;
 	}
@@ -48,4 +75,24 @@ public class AttachmentItem {
 		return hasError;
 	}
 
+	// TODO use counter instead, because in theory one attachment can appear in more than one messages
+	String getTransitionName() {
+		return String.valueOf(messageId.hashCode());
+	}
+
+	@Override
+	public int describeContents() {
+		return 0;
+	}
+
+	@Override
+	public void writeToParcel(Parcel dest, int flags) {
+		dest.writeByteArray(messageId.getBytes());
+		dest.writeInt(width);
+		dest.writeInt(height);
+		dest.writeInt(thumbnailWidth);
+		dest.writeInt(thumbnailHeight);
+		dest.writeByte((byte) (hasError ? 1 : 0));
+	}
+
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationActivity.java

@@ -9,11 +9,15 @@ import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.support.annotation.UiThread;
 import android.support.design.widget.Snackbar;
+import android.support.v4.app.ActivityCompat;
+import android.support.v4.app.ActivityOptionsCompat;
 import android.support.v4.content.ContextCompat;
 import android.support.v7.app.AlertDialog;
 import android.support.v7.widget.ActionMenuView;
 import android.support.v7.widget.LinearLayoutManager;
 import android.support.v7.widget.Toolbar;
+import android.transition.Slide;
+import android.transition.Transition;
 import android.util.SparseArray;
 import android.view.Menu;
 import android.view.MenuInflater;
@@ -96,8 +100,11 @@ import im.delight.android.identicons.IdenticonDrawable;
 import uk.co.samuelwall.materialtaptargetprompt.MaterialTapTargetPrompt;
 import uk.co.samuelwall.materialtaptargetprompt.MaterialTapTargetPrompt.PromptStateChangeListener;
 
+import static android.os.Build.VERSION.SDK_INT;
+import static android.support.v4.app.ActivityOptionsCompat.makeSceneTransitionAnimation;
 import static android.support.v4.view.ViewCompat.setTransitionName;
 import static android.support.v7.util.SortedList.INVALID_POSITION;
+import static android.view.Gravity.END;
 import static android.widget.Toast.LENGTH_SHORT;
 import static java.util.Collections.emptyList;
 import static java.util.Collections.sort;
@@ -108,6 +115,9 @@ import static org.briarproject.bramble.util.LogUtils.logDuration;
 import static org.briarproject.bramble.util.LogUtils.logException;
 import static org.briarproject.bramble.util.LogUtils.now;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_INTRODUCTION;
+import static org.briarproject.briar.android.conversation.ImageActivity.ATTACHMENT;
+import static org.briarproject.briar.android.conversation.ImageActivity.DATE;
+import static org.briarproject.briar.android.conversation.ImageActivity.NAME;
 import static org.briarproject.briar.android.settings.SettingsFragment.SETTINGS_NAMESPACE;
 import static org.briarproject.briar.android.util.UiUtils.getAvatarTransitionName;
 import static org.briarproject.briar.android.util.UiUtils.getBulbTransitionName;
@@ -186,7 +196,10 @@ public class ConversationActivity extends BriarActivity
 
 	@Override
 	public void onCreate(@Nullable Bundle state) {
-		setSceneTransitionAnimation();
+		if (SDK_INT >= 21) {
+			Transition slide = new Slide(END);
+			setSceneTransitionAnimation(slide, null, slide);
+		}
 		super.onCreate(state);
 
 		Intent i = getIntent();
@@ -802,6 +815,31 @@ public class ConversationActivity extends BriarActivity
 		startActivity(i);
 	}
 
+	@Override
+	public void onAttachmentClicked(View view,
+			ConversationMessageItem messageItem, AttachmentItem item) {
+		String name;
+		if (messageItem.isIncoming()) {
+			// must be available when items are being displayed
+			name = viewModel.getContactDisplayName().getValue();
+		} else {
+			name = getString(R.string.you);
+		}
+		Intent i = new Intent(this, ImageActivity.class);
+		i.putExtra(ATTACHMENT, item);
+		i.putExtra(NAME, name);
+		i.putExtra(DATE, messageItem.getTime());
+		if (SDK_INT >= 23) {
+			String transitionName = item.getTransitionName();
+			ActivityOptionsCompat options =
+					makeSceneTransitionAnimation(this, view, transitionName);
+			ActivityCompat.startActivity(this, i, options.toBundle());
+		} else {
+			// work-around for android bug #224270
+			startActivity(i);
+		}
+	}
+
 	@DatabaseExecutor
 	private void respondToIntroductionRequest(SessionId sessionId,
 			boolean accept, long time) throws DbException {
@@ -845,4 +883,5 @@ public class ConversationActivity extends BriarActivity
 		}
 		return attachments;
 	}
+
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationListener.java

@@ -1,6 +1,7 @@
 package org.briarproject.briar.android.conversation;
 
 import android.support.annotation.UiThread;
+import android.view.View;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
@@ -14,4 +15,7 @@ interface ConversationListener {
 
 	void openRequestedShareable(ConversationRequestItem item);
 
+	void onAttachmentClicked(View view, ConversationMessageItem messageItem,
+			AttachmentItem attachmentItem);
+
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationMessageViewHolder.java

@@ -85,7 +85,7 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 		if (item.getAttachments().isEmpty()) {
 			bindTextItem();
 		} else {
-			bindImageItem(item);
+			bindImageItem(item, listener);
 		}
 	}
 
@@ -98,7 +98,8 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 		textConstraints.applyTo(layout);
 	}
 
-	private void bindImageItem(ConversationMessageItem item) {
+	private void bindImageItem(ConversationMessageItem item,
+			ConversationListener listener) {
 		// TODO show more than just the first image
 		AttachmentItem attachment = item.getAttachments().get(0);
 
@@ -127,17 +128,18 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 			clearImage();
 			imageView.setImageResource(ERROR_RES);
 		} else {
-			loadImage(item, attachment);
+			loadImage(item, attachment, listener);
 		}
 	}
 
 	private void clearImage() {
 		GlideApp.with(imageView)
 				.clear(imageView);
+		imageView.setOnClickListener(null);
 	}
 
 	private void loadImage(ConversationMessageItem item,
-			AttachmentItem attachment) {
+			AttachmentItem attachment, ConversationListener listener) {
 		boolean leftCornerSmall =
 				(isIncoming() && !isRtl) || (!isIncoming() && isRtl);
 		boolean bottomRound = item.getText() == null;
@@ -152,6 +154,8 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 				.transition(withCrossFade())
 				.into(imageView)
 				.waitForLayout();
+		imageView.setOnClickListener(
+				view -> listener.onAttachmentClicked(view, item, attachment));
 	}
 
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ImageActivity.java

@@ -0,0 +1,233 @@
+package org.briarproject.briar.android.conversation;
+
+import android.graphics.drawable.Animatable;
+import android.graphics.drawable.Drawable;
+import android.os.Bundle;
+import android.support.annotation.Nullable;
+import android.support.annotation.RequiresApi;
+import android.support.design.widget.AppBarLayout;
+import android.support.v7.widget.Toolbar;
+import android.transition.Fade;
+import android.transition.Transition;
+import android.view.MenuItem;
+import android.view.View;
+import android.view.Window;
+import android.widget.TextView;
+
+import com.bumptech.glide.load.DataSource;
+import com.bumptech.glide.load.engine.GlideException;
+import com.bumptech.glide.request.RequestListener;
+import com.bumptech.glide.request.target.Target;
+import com.github.chrisbanes.photoview.PhotoView;
+
+import org.briarproject.briar.R;
+import org.briarproject.briar.android.activity.ActivityComponent;
+import org.briarproject.briar.android.activity.BriarActivity;
+import org.briarproject.briar.android.conversation.glide.GlideApp;
+import org.briarproject.briar.android.view.PullDownLayout;
+
+import static android.graphics.Color.TRANSPARENT;
+import static android.os.Build.VERSION.SDK_INT;
+import static android.view.View.GONE;
+import static android.view.View.SYSTEM_UI_FLAG_FULLSCREEN;
+import static android.view.View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN;
+import static android.view.View.SYSTEM_UI_FLAG_LAYOUT_STABLE;
+import static android.view.View.VISIBLE;
+import static android.view.WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS;
+import static android.widget.ImageView.ScaleType.FIT_START;
+import static com.bumptech.glide.load.engine.DiskCacheStrategy.NONE;
+import static java.util.Objects.requireNonNull;
+import static org.briarproject.briar.android.util.UiUtils.formatDateAbsolute;
+
+public class ImageActivity extends BriarActivity
+		implements PullDownLayout.Callback {
+
+	final static String ATTACHMENT = "attachment";
+	final static String NAME = "name";
+	final static String DATE = "date";
+
+	private PullDownLayout layout;
+	private AppBarLayout appBarLayout;
+	private PhotoView photoView;
+
+	@Override
+	public void injectActivity(ActivityComponent component) {
+		component.inject(this);
+	}
+
+	@Override
+	public void onCreate(@Nullable Bundle state) {
+		super.onCreate(state);
+
+		// Transitions
+		supportPostponeEnterTransition();
+		Window window = getWindow();
+		if (SDK_INT >= 21) {
+			Transition transition = new Fade();
+			setSceneTransitionAnimation(transition, null, transition);
+		}
+
+		// inflate layout
+		setContentView(R.layout.activity_image);
+		layout = findViewById(R.id.layout);
+		layout.getBackground().setAlpha(255);
+		layout.setCallback(this);
+
+		// Status Bar
+		if (SDK_INT >= 21) {
+			window.setStatusBarColor(TRANSPARENT);
+		} else if (SDK_INT >= 19) {
+			// we can't make the status bar transparent, but translucent
+			window.addFlags(FLAG_TRANSLUCENT_STATUS);
+		}
+
+		// Toolbar
+		appBarLayout = findViewById(R.id.appBarLayout);
+		Toolbar toolbar = requireNonNull(setUpCustomToolbar(true));
+		TextView contactName = toolbar.findViewById(R.id.contactName);
+		TextView dateView = toolbar.findViewById(R.id.dateView);
+
+		// Intent Extras
+		AttachmentItem attachment = getIntent().getParcelableExtra(ATTACHMENT);
+		String name = getIntent().getStringExtra(NAME);
+		long time = getIntent().getLongExtra(DATE, 0);
+		String date = formatDateAbsolute(this, time);
+		contactName.setText(name);
+		dateView.setText(date);
+
+		// Image View
+		photoView = findViewById(R.id.photoView);
+		if (SDK_INT >= 16) {
+			photoView.setOnClickListener(view -> toggleSystemUi());
+			window.getDecorView().setSystemUiVisibility(
+					SYSTEM_UI_FLAG_LAYOUT_STABLE |
+							SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN);
+		}
+
+		// Request Listener
+		RequestListener<Drawable> listener = new RequestListener<Drawable>() {
+			@Override
+			public boolean onLoadFailed(@Nullable GlideException e,
+					Object model, Target<Drawable> target,
+					boolean isFirstResource) {
+				supportStartPostponedEnterTransition();
+				return false;
+			}
+
+			@Override
+			public boolean onResourceReady(Drawable resource, Object model,
+					Target<Drawable> target, DataSource dataSource,
+					boolean isFirstResource) {
+				if (SDK_INT >= 21 && !(resource instanceof Animatable)) {
+					// set transition name only when not animatable,
+					// because the animation won't start otherwise
+					photoView.setTransitionName(
+							attachment.getTransitionName());
+				}
+				// Move image to the top if overlapping toolbar
+				if (isOverlappingToolbar(resource)) {
+					photoView.setScaleType(FIT_START);
+				}
+				supportStartPostponedEnterTransition();
+				return false;
+			}
+		};
+
+		// Load Image
+		GlideApp.with(this)
+				.load(attachment)
+				.diskCacheStrategy(NONE)
+				.error(R.drawable.ic_image_broken)
+				.dontTransform()
+				.addListener(listener)
+				.into(photoView);
+	}
+
+	@Override
+	public boolean onOptionsItemSelected(MenuItem item) {
+		switch (item.getItemId()) {
+			case android.R.id.home:
+				onBackPressed();
+				return true;
+			default:
+				return super.onOptionsItemSelected(item);
+		}
+	}
+
+	@Override
+	public void onPullStart() {
+		appBarLayout.animate()
+				.alpha(0f)
+				.start();
+	}
+
+	@Override
+	public void onPull(float progress) {
+		layout.getBackground().setAlpha(Math.round((1 - progress) * 255));
+	}
+
+	@Override
+	public void onPullCancel() {
+		appBarLayout.animate()
+				.alpha(1f)
+				.start();
+	}
+
+	@Override
+	public void onPullComplete() {
+		supportFinishAfterTransition();
+	}
+
+	@RequiresApi(api = 16)
+	private void toggleSystemUi() {
+		View decorView = getWindow().getDecorView();
+		if (appBarLayout.getVisibility() == VISIBLE) {
+			hideSystemUi(decorView);
+		} else {
+			showSystemUi(decorView);
+		}
+	}
+
+	@RequiresApi(api = 16)
+	private void hideSystemUi(View decorView) {
+		decorView.setSystemUiVisibility(SYSTEM_UI_FLAG_LAYOUT_STABLE
+				| SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+				| SYSTEM_UI_FLAG_FULLSCREEN
+		);
+		appBarLayout.animate()
+				.translationYBy(-1 * appBarLayout.getHeight())
+				.alpha(0f)
+				.withEndAction(() -> appBarLayout.setVisibility(GONE))
+				.start();
+	}
+
+	@RequiresApi(api = 16)
+	private void showSystemUi(View decorView) {
+		decorView.setSystemUiVisibility(
+				SYSTEM_UI_FLAG_LAYOUT_STABLE
+						| SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+		);
+		appBarLayout.animate()
+				.translationYBy(appBarLayout.getHeight())
+				.alpha(1f)
+				.withStartAction(() -> appBarLayout.setVisibility(VISIBLE))
+				.start();
+	}
+
+	private boolean isOverlappingToolbar(Drawable drawable) {
+		int width = drawable.getIntrinsicWidth();
+		int height = drawable.getIntrinsicHeight();
+		float widthPercentage = photoView.getWidth() / (float) width;
+		float heightPercentage = photoView.getHeight() / (float) height;
+		float scaleFactor = Math.min(widthPercentage, heightPercentage);
+		int realWidth = (int) (width * scaleFactor);
+		int realHeight = (int) (height * scaleFactor);
+		// return if photo doesn't use the full width,
+		// because it will be moved to the right otherwise
+		if (realWidth < photoView.getWidth()) return false;
+		int drawableTop = (photoView.getHeight() - realHeight) / 2;
+		return drawableTop < appBarLayout.getBottom() &&
+				drawableTop != appBarLayout.getTop();
+	}
+
+}

briar-android/src/main/java/org/briarproject/briar/android/util/UiUtils.java

@@ -16,6 +16,7 @@ import android.support.annotation.ColorInt;
 import android.support.annotation.ColorRes;
 import android.support.annotation.MainThread;
 import android.support.annotation.Nullable;
+import android.support.annotation.RequiresApi;
 import android.support.design.widget.TextInputLayout;
 import android.support.v4.app.FragmentManager;
 import android.support.v4.content.ContextCompat;
@@ -31,6 +32,7 @@ import android.text.method.LinkMovementMethod;
 import android.text.style.ClickableSpan;
 import android.text.style.ForegroundColorSpan;
 import android.text.style.URLSpan;
+import android.transition.Transition;
 import android.util.TypedValue;
 import android.view.KeyEvent;
 import android.view.View;
@@ -60,12 +62,16 @@ import static android.support.v7.app.AppCompatDelegate.MODE_NIGHT_NO;
 import static android.support.v7.app.AppCompatDelegate.MODE_NIGHT_YES;
 import static android.support.v7.app.AppCompatDelegate.setDefaultNightMode;
 import static android.text.format.DateUtils.DAY_IN_MILLIS;
+import static android.text.format.DateUtils.FORMAT_ABBREV_ALL;
 import static android.text.format.DateUtils.FORMAT_ABBREV_MONTH;
 import static android.text.format.DateUtils.FORMAT_ABBREV_RELATIVE;
 import static android.text.format.DateUtils.FORMAT_ABBREV_TIME;
 import static android.text.format.DateUtils.FORMAT_SHOW_DATE;
+import static android.text.format.DateUtils.FORMAT_SHOW_TIME;
+import static android.text.format.DateUtils.FORMAT_SHOW_YEAR;
 import static android.text.format.DateUtils.MINUTE_IN_MILLIS;
 import static android.text.format.DateUtils.WEEK_IN_MILLIS;
+import static android.text.format.DateUtils.YEAR_IN_MILLIS;
 import static android.view.KeyEvent.ACTION_DOWN;
 import static android.view.KeyEvent.KEYCODE_ENTER;
 import static android.view.inputmethod.EditorInfo.IME_NULL;
@@ -117,6 +123,13 @@ public class UiUtils {
 				MIN_DATE_RESOLUTION, flags).toString();
 	}
 
+	public static String formatDateAbsolute(Context ctx, long time) {
+		int flags = FORMAT_SHOW_TIME | FORMAT_SHOW_DATE | FORMAT_ABBREV_ALL;
+		long diff = System.currentTimeMillis() - time;
+		if (diff >= YEAR_IN_MILLIS) flags |= FORMAT_SHOW_YEAR;
+		return DateUtils.formatDateTime(ctx, time, flags);
+	}
+
 	public static int getDaysUntilExpiry() {
 		long now = System.currentTimeMillis();
 		long daysBeforeExpiry = (EXPIRY_DATE - now) / 1000 / 60 / 60 / 24;
@@ -318,6 +331,12 @@ public class UiUtils {
 				keyEvent.getKeyCode() == KEYCODE_ENTER;
 	}
 
+	@RequiresApi(api = 21)
+	public static void excludeSystemUi(Transition transition) {
+		transition.excludeTarget(android.R.id.statusBarBackground, true);
+		transition.excludeTarget(android.R.id.navigationBarBackground, true);
+	}
+
 	/**
 	 * Observes the given {@link LiveData} until the first change.
 	 * If the LiveData's value is available, the {@link Observer} will be

briar-android/src/main/java/org/briarproject/briar/android/view/PullDownLayout.java

@@ -0,0 +1,161 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2015 XiNGRZ <xxx@oxo.ooo>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * 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.
+ */
+
+package org.briarproject.briar.android.view;
+
+import android.content.Context;
+import android.support.annotation.Nullable;
+import android.support.v4.view.ViewCompat;
+import android.support.v4.widget.ViewDragHelper;
+import android.util.AttributeSet;
+import android.view.MotionEvent;
+import android.view.View;
+import android.view.ViewConfiguration;
+import android.widget.FrameLayout;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class PullDownLayout extends FrameLayout {
+
+	private final ViewDragHelper dragger;
+
+	private final int minimumFlingVelocity;
+
+	@Nullable
+	private Callback callback;
+
+	public PullDownLayout(Context context) {
+		this(context, null);
+	}
+
+	public PullDownLayout(Context context, @Nullable AttributeSet attrs) {
+		this(context, attrs, 0);
+	}
+
+	public PullDownLayout(Context context, @Nullable AttributeSet attrs,
+			int defStyleAttr) {
+		super(context, attrs, defStyleAttr);
+		dragger = ViewDragHelper.create(this, 1f / 8f, new ViewDragCallback());
+		minimumFlingVelocity =
+				ViewConfiguration.get(context).getScaledMinimumFlingVelocity();
+	}
+
+	public void setCallback(@Nullable Callback callback) {
+		this.callback = callback;
+	}
+
+	@Override
+	public boolean onInterceptTouchEvent(MotionEvent ev) {
+		return dragger.shouldInterceptTouchEvent(ev);
+	}
+
+	@Override
+	public boolean onTouchEvent(MotionEvent event) {
+		dragger.processTouchEvent(event);
+		return true;
+	}
+
+	@Override
+	public void computeScroll() {
+		if (dragger.continueSettling(true)) {
+			ViewCompat.postInvalidateOnAnimation(this);
+		}
+	}
+
+	public interface Callback {
+
+		void onPullStart();
+
+		void onPull(float progress);
+
+		void onPullCancel();
+
+		void onPullComplete();
+
+	}
+
+	private class ViewDragCallback extends ViewDragHelper.Callback {
+
+		@Override
+		public boolean tryCaptureView(View child, int pointerId) {
+			return true;
+		}
+
+		@Override
+		public int clampViewPositionHorizontal(View child, int left, int dx) {
+			return 0;
+		}
+
+		@Override
+		public int clampViewPositionVertical(View child, int top, int dy) {
+			return Math.max(0, top);
+		}
+
+		@Override
+		public int getViewHorizontalDragRange(View child) {
+			return 0;
+		}
+
+		@Override
+		public int getViewVerticalDragRange(View child) {
+			return getHeight();
+		}
+
+		@Override
+		public void onViewCaptured(View capturedChild, int activePointerId) {
+			if (callback != null) {
+				callback.onPullStart();
+			}
+		}
+
+		@Override
+		public void onViewPositionChanged(View changedView, int left, int top,
+				int dx, int dy) {
+			if (callback != null) {
+				callback.onPull((float) top / (float) getHeight());
+			}
+		}
+
+		@Override
+		public void onViewReleased(View releasedChild, float xvel, float yvel) {
+			int slop = yvel > minimumFlingVelocity ? getHeight() / 6 :
+					getHeight() / 3;
+			if (releasedChild.getTop() > slop) {
+				if (callback != null) {
+					callback.onPullComplete();
+				}
+			} else {
+				if (callback != null) {
+					callback.onPullCancel();
+				}
+
+				dragger.settleCapturedViewAt(0, 0);
+				invalidate();
+			}
+		}
+
+	}
+
+}

briar-android/src/main/res/layout/activity_image.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0" encoding="utf-8"?>
+<org.briarproject.briar.android.view.PullDownLayout
+	android:id="@+id/layout"
+	xmlns:android="http://schemas.android.com/apk/res/android"
+	xmlns:tools="http://schemas.android.com/tools"
+	android:layout_width="match_parent"
+	android:layout_height="match_parent"
+	android:background="@color/briar_black"
+	tools:context=".android.conversation.ImageActivity">
+
+	<com.github.chrisbanes.photoview.PhotoView
+		android:id="@+id/photoView"
+		android:layout_width="match_parent"
+		android:layout_height="match_parent"
+		tools:ignore="ContentDescription"
+		tools:srcCompat="@tools:sample/backgrounds/scenic"/>
+
+	<android.support.design.widget.AppBarLayout
+		android:id="@+id/appBarLayout"
+		android:layout_width="match_parent"
+		android:layout_height="wrap_content"
+		android:background="@color/msg_status_bubble_background">
+
+		<android.support.v7.widget.Toolbar
+			android:id="@+id/toolbar"
+			style="@style/BriarToolbar"
+			android:layout_width="match_parent"
+			android:layout_height="wrap_content"
+			android:background="@color/msg_status_bubble_background"
+			android:fitsSystemWindows="true">
+
+			<LinearLayout
+				android:layout_width="wrap_content"
+				android:layout_height="match_parent"
+				android:orientation="vertical">
+
+				<com.vanniktech.emoji.EmojiTextView
+					android:id="@+id/contactName"
+					style="@style/TextAppearance.AppCompat.Widget.ActionBar.Title.Inverse"
+					android:layout_width="wrap_content"
+					android:layout_height="match_parent"
+					android:ellipsize="end"
+					android:maxLines="1"
+					android:textColor="@color/action_bar_text"
+					tools:text="Contact Name of someone who chose a long name"/>
+
+				<TextView
+					android:id="@+id/dateView"
+					android:layout_width="wrap_content"
+					android:layout_height="wrap_content"
+					android:textColor="@color/action_bar_text"
+					tools:text="date"/>
+
+			</LinearLayout>
+
+		</android.support.v7.widget.Toolbar>
+
+	</android.support.design.widget.AppBarLayout>
+
+</org.briarproject.briar.android.view.PullDownLayout>

briar-android/src/main/res/values/strings.xml

@@ -134,6 +134,8 @@
 	<string name="dialog_title_delete_contact">Confirm Contact Deletion</string>
 	<string name="dialog_message_delete_contact">Are you sure that you want to remove this contact and all messages exchanged with this contact?</string>
 	<string name="contact_deleted_toast">Contact deleted</string>
+	<!-- This is shown in the action bar when opening an image in fullscreen that the user sent -->
+	<string name="you">You</string>
 
 	<!-- Adding Contacts -->
 	<string name="add_contact_title">Add a Contact</string>

briar-android/src/main/res/values/themes.xml

@@ -18,6 +18,12 @@
 		<item name="toolbarStyle">@style/BriarToolbar</item>
 	</style>
 
+	<style name="BriarTheme.Transparent.NoActionBar" parent="BriarTheme.NoActionBar">
+		<item name="android:windowBackground">@android:color/transparent</item>
+		<item name="android:windowIsTranslucent">true</item>
+		<item name="android:windowActionBarOverlay">true</item>
+	</style>
+
 	<style name="ActivityAnimation" parent="@android:style/Animation.Activity">
 		<item name="android:activityOpenEnterAnimation">@anim/screen_new_in</item>
 		<item name="android:activityOpenExitAnimation">@anim/screen_old_out</item>

briar-android/witness.gradle

@@ -89,6 +89,7 @@ dependencyVerification {
         'com.github.bumptech.glide:compiler:4.8.0:compiler-4.8.0.jar:1fa93dd0cf7ef0b8b98a59a67a1ee84915416c2d677d83a771ea3e32ad15e6bf',
         'com.github.bumptech.glide:gifdecoder:4.8.0:gifdecoder-4.8.0.aar:b00c5454a023a9488ea49603930d9c25e09192e5ceaadf64977aa52946b3c1b4',
         'com.github.bumptech.glide:glide:4.8.0:glide-4.8.0.aar:5ddf08b12cc43332e812988f16c2c39e7fce49d1c4d94b7948dcde7f00bf49d6',
+        'com.github.chrisbanes:PhotoView:2.1.4:PhotoView-2.1.4.aar:04cb397fcb3df0757c8aed6927ebdd247930b5c78ee9acc59cd07dccdaaf3460',
         'com.google.android.apps.common.testing.accessibility.framework:accessibility-test-framework:2.0:accessibility-test-framework-2.0.jar:cdf16ef8f5b8023d003ce3cc1b0d51bda737762e2dab2fedf43d1c4292353f7f',
         'com.google.android.apps.common.testing.accessibility.framework:accessibility-test-framework:2.1:accessibility-test-framework-2.1.jar:7b0aa6ed7553597ce0610684a9f7eca8021eee218f2e2f427c04a7fbf5f920bd',
         'com.google.code.findbugs:jsr305:1.3.9:jsr305-1.3.9.jar:905721a0eea90a81534abb7ee6ef4ea2e5e645fa1def0a5cd88402df1b46c9ed',

build.gradle

@@ -5,6 +5,7 @@ allprojects {
 		jcenter()
 		mavenLocal()
 		google()
+		maven { url "https://jitpack.io" }
 	}
 	afterEvaluate {
 		tasks.withType(Test) {