diff --git a/api/net/sf/briar/api/protocol/Author.java b/api/net/sf/briar/api/protocol/Author.java
index 48eb32c50..0006cd64f 100644
--- a/api/net/sf/briar/api/protocol/Author.java
+++ b/api/net/sf/briar/api/protocol/Author.java
@@ -2,11 +2,24 @@ package net.sf.briar.api.protocol;
 
 import net.sf.briar.api.serial.Writable;
 
+/** A pseudonymous author of messages. */
 public interface Author extends Writable {
 
+	/** The maximum length of an author's name, in UTF-8 bytes. */
+	static final int MAX_NAME_LENGTH = 50;
+
+	/** The maximum length of an author's public key, in bytes. */
+	static final int MAX_PUBLIC_KEY_LENGTH = 1000;
+
+	/** Returns the author's unique identifier. */
 	AuthorId getId();
 
+	/** Returns the author's name. */
 	String getName();
 
+	/**
+	 * Returns the public key that is used to verify messages signed by the
+	 * author.
+	 */
 	byte[] getPublicKey();
 }
diff --git a/api/net/sf/briar/api/protocol/Group.java b/api/net/sf/briar/api/protocol/Group.java
index d25b69eed..7e33755c3 100644
--- a/api/net/sf/briar/api/protocol/Group.java
+++ b/api/net/sf/briar/api/protocol/Group.java
@@ -5,6 +5,12 @@ import net.sf.briar.api.serial.Writable;
 /** A group to which users may subscribe. */
 public interface Group extends Writable {
 
+	/** The maximum length of a group's name, in UTF-8 bytes. */
+	static final int MAX_NAME_LENGTH = 50;
+
+	/** The maximum length of a group's public key, in bytes. */
+	static final int MAX_PUBLIC_KEY_LENGTH = 1000;
+
 	/** Returns the group's unique identifier. */
 	GroupId getId();
 
diff --git a/api/net/sf/briar/api/protocol/Message.java b/api/net/sf/briar/api/protocol/Message.java
index 89e021245..3df4b5f46 100644
--- a/api/net/sf/briar/api/protocol/Message.java
+++ b/api/net/sf/briar/api/protocol/Message.java
@@ -2,8 +2,12 @@ package net.sf.briar.api.protocol;
 
 public interface Message {
 
+	/** The maximum size of a serialised message, in bytes. */
 	static final int MAX_SIZE = (1024 * 1024) - 200;
 
+	/** The maximum size of a signature, in bytes. */
+	static final int MAX_SIGNATURE_SIZE = 100;
+
 	/** Returns the message's unique identifier. */
 	MessageId getId();
 
diff --git a/api/net/sf/briar/api/serial/Reader.java b/api/net/sf/briar/api/serial/Reader.java
index c0a4f8f69..9b8960229 100644
--- a/api/net/sf/briar/api/serial/Reader.java
+++ b/api/net/sf/briar/api/serial/Reader.java
@@ -38,8 +38,11 @@ public interface Reader {
 
 	boolean hasString() throws IOException;
 	String readString() throws IOException;
+	String readString(int maxLength) throws IOException;
+
 	boolean hasBytes() throws IOException;
 	byte[] readBytes() throws IOException;
+	byte[] readBytes(int maxLength) throws IOException;
 
 	boolean hasList() throws IOException;
 	List<Object> readList() throws IOException;
diff --git a/components/net/sf/briar/protocol/AuthorReader.java b/components/net/sf/briar/protocol/AuthorReader.java
index 8a25ffc17..e0d73ed3b 100644
--- a/components/net/sf/briar/protocol/AuthorReader.java
+++ b/components/net/sf/briar/protocol/AuthorReader.java
@@ -31,8 +31,8 @@ class AuthorReader implements ObjectReader<Author> {
 		// Read and digest the data
 		r.addConsumer(digesting);
 		r.readUserDefinedTag(Tags.AUTHOR);
-		String name = r.readString();
-		byte[] publicKey = r.readBytes();
+		String name = r.readString(Author.MAX_NAME_LENGTH);
+		byte[] publicKey = r.readBytes(Author.MAX_PUBLIC_KEY_LENGTH);
 		r.removeConsumer(digesting);
 		// Build and return the author
 		AuthorId id = new AuthorId(messageDigest.digest());
diff --git a/components/net/sf/briar/protocol/BatchIdReader.java b/components/net/sf/briar/protocol/BatchIdReader.java
index 3daeda286..4542a0f5e 100644
--- a/components/net/sf/briar/protocol/BatchIdReader.java
+++ b/components/net/sf/briar/protocol/BatchIdReader.java
@@ -13,7 +13,7 @@ class BatchIdReader implements ObjectReader<BatchId> {
 
 	public BatchId readObject(Reader r) throws IOException {
 		r.readUserDefinedTag(Tags.BATCH_ID);
-		byte[] b = r.readBytes();
+		byte[] b = r.readBytes(UniqueId.LENGTH);
 		if(b.length != UniqueId.LENGTH) throw new FormatException();
 		return new BatchId(b);
 	}
diff --git a/components/net/sf/briar/protocol/GroupIdReader.java b/components/net/sf/briar/protocol/GroupIdReader.java
index 76c13e616..07bf2233f 100644
--- a/components/net/sf/briar/protocol/GroupIdReader.java
+++ b/components/net/sf/briar/protocol/GroupIdReader.java
@@ -13,7 +13,7 @@ class GroupIdReader implements ObjectReader<GroupId> {
 
 	public GroupId readObject(Reader r) throws IOException {
 		r.readUserDefinedTag(Tags.GROUP_ID);
-		byte[] b = r.readBytes();
+		byte[] b = r.readBytes(UniqueId.LENGTH);
 		if(b.length != UniqueId.LENGTH) throw new FormatException();
 		return new GroupId(b);
 	}
diff --git a/components/net/sf/briar/protocol/GroupReader.java b/components/net/sf/briar/protocol/GroupReader.java
index ef2d4b9d7..dd61419a0 100644
--- a/components/net/sf/briar/protocol/GroupReader.java
+++ b/components/net/sf/briar/protocol/GroupReader.java
@@ -31,10 +31,10 @@ class GroupReader implements ObjectReader<Group> {
 		// Read and digest the data
 		r.addConsumer(digesting);
 		r.readUserDefinedTag(Tags.GROUP);
-		String name = r.readString();
+		String name = r.readString(Group.MAX_NAME_LENGTH);
 		byte[] publicKey = null;
 		if(r.hasNull()) r.readNull();
-		else publicKey = r.readBytes();
+		else publicKey = r.readBytes(Group.MAX_PUBLIC_KEY_LENGTH);
 		r.removeConsumer(digesting);
 		// Build and return the group
 		GroupId id = new GroupId(messageDigest.digest());
diff --git a/components/net/sf/briar/protocol/MessageIdReader.java b/components/net/sf/briar/protocol/MessageIdReader.java
index 118bf0ea4..eecf32649 100644
--- a/components/net/sf/briar/protocol/MessageIdReader.java
+++ b/components/net/sf/briar/protocol/MessageIdReader.java
@@ -13,7 +13,7 @@ class MessageIdReader implements ObjectReader<MessageId> {
 
 	public MessageId readObject(Reader r) throws IOException {
 		r.readUserDefinedTag(Tags.MESSAGE_ID);
-		byte[] b = r.readBytes();
+		byte[] b = r.readBytes(UniqueId.LENGTH);
 		if(b.length != UniqueId.LENGTH) throw new FormatException();
 		return new MessageId(b);
 	}
diff --git a/components/net/sf/briar/protocol/MessageReader.java b/components/net/sf/briar/protocol/MessageReader.java
index ef7183128..b5530068e 100644
--- a/components/net/sf/briar/protocol/MessageReader.java
+++ b/components/net/sf/briar/protocol/MessageReader.java
@@ -67,19 +67,19 @@ class MessageReader implements ObjectReader<Message> {
 		long timestamp = r.readInt64();
 		if(timestamp < 0L) throw new FormatException();
 		// Skip the message body
-		r.readBytes();
+		r.readBytes(Message.MAX_SIZE);
 		// Record the length of the data covered by the author's signature
 		int signedByAuthor = (int) counting.getCount();
 		// Read the author's signature, if there is one
 		byte[] authorSig = null;
 		if(author == null) r.readNull();
-		else authorSig = r.readBytes();
+		else authorSig = r.readBytes(Message.MAX_SIGNATURE_SIZE);
 		// Record the length of the data covered by the group's signature
 		int signedByGroup = (int) counting.getCount();
 		// Read the group's signature, if there is one
 		byte[] groupSig = null;
 		if(group.getPublicKey() == null) r.readNull();
-		else groupSig = r.readBytes();
+		else groupSig = r.readBytes(Message.MAX_SIGNATURE_SIZE);
 		// That's all, folks
 		r.removeConsumer(counting);
 		r.removeConsumer(copying);
diff --git a/components/net/sf/briar/protocol/RequestReader.java b/components/net/sf/briar/protocol/RequestReader.java
index 0fe33531f..567779007 100644
--- a/components/net/sf/briar/protocol/RequestReader.java
+++ b/components/net/sf/briar/protocol/RequestReader.java
@@ -26,7 +26,7 @@ class RequestReader implements ObjectReader<Request> {
 		// Read the data
 		r.addConsumer(counting);
 		r.readUserDefinedTag(Tags.REQUEST);
-		byte[] bitmap = r.readBytes();
+		byte[] bitmap = r.readBytes(Request.MAX_SIZE);
 		r.removeConsumer(counting);
 		// Convert the bitmap into a BitSet
 		BitSet b = new BitSet(bitmap.length * 8);
diff --git a/components/net/sf/briar/serial/ReaderImpl.java b/components/net/sf/briar/serial/ReaderImpl.java
index 29cbb0650..3c7f82940 100644
--- a/components/net/sf/briar/serial/ReaderImpl.java
+++ b/components/net/sf/briar/serial/ReaderImpl.java
@@ -268,18 +268,16 @@ class ReaderImpl implements Reader {
 	}
 
 	public String readString() throws IOException {
-		if(!hasString()) throw new FormatException();
-		consumeLookahead();
-		if(next == Tag.STRING) {
-			return readString(readLength());
-		} else {
-			int length = 0xFF & next ^ Tag.SHORT_STRING;
-			return readString(length);
-		}
+		return readString(Integer.MAX_VALUE);
 	}
 
-	private String readString(int length) throws IOException {
-		assert length >= 0;
+	public String readString(int maxLength) throws IOException {
+		if(!hasString()) throw new FormatException();
+		consumeLookahead();
+		int length;
+		if(next == Tag.STRING) length = readLength();
+		else length = 0xFF & next ^ Tag.SHORT_STRING;
+		if(length > maxLength) throw new FormatException();
 		if(length == 0) return "";
 		readIntoBuffer(length);
 		return new String(buf, 0, length, "UTF-8");
@@ -308,18 +306,16 @@ class ReaderImpl implements Reader {
 	}
 
 	public byte[] readBytes() throws IOException {
-		if(!hasBytes()) throw new FormatException();
-		consumeLookahead();
-		if(next == Tag.BYTES) {
-			return readBytes(readLength());
-		} else {
-			int length = 0xFF & next ^ Tag.SHORT_BYTES;
-			return readBytes(length);
-		}
+		return readBytes(Integer.MAX_VALUE);
 	}
 
-	private byte[] readBytes(int length) throws IOException {
-		assert length >= 0;
+	public byte[] readBytes(int maxLength) throws IOException {
+		if(!hasBytes()) throw new FormatException();
+		consumeLookahead();
+		int length;
+		if(next == Tag.BYTES) length = readLength();
+		else length = 0xFF & next ^ Tag.SHORT_BYTES;
+		if(length > maxLength) throw new FormatException();
 		if(length == 0) return EMPTY_BUFFER;
 		byte[] b = new byte[length];
 		readIntoBuffer(b, length);
diff --git a/test/net/sf/briar/serial/ReaderImplTest.java b/test/net/sf/briar/serial/ReaderImplTest.java
index 8baaf127b..4c27b963d 100644
--- a/test/net/sf/briar/serial/ReaderImplTest.java
+++ b/test/net/sf/briar/serial/ReaderImplTest.java
@@ -132,6 +132,16 @@ public class ReaderImplTest extends TestCase {
 		assertTrue(r.eof());
 	}
 
+	@Test
+	public void testReadStringMaxLength() throws Exception {
+		setContents("83666F6F" + "83666F6F");
+		assertEquals("foo", r.readString(3));
+		try {
+			r.readString(2);
+			assertTrue(false);
+		} catch(FormatException expected) {}
+	}
+
 	@Test
 	public void testReadBytes() throws Exception {
 		setContents("F603010203" + "93010203" + "F600" + "90");
@@ -142,6 +152,16 @@ public class ReaderImplTest extends TestCase {
 		assertTrue(r.eof());
 	}
 
+	@Test
+	public void testReadBytesMaxLength() throws Exception {
+		setContents("93010203" + "93010203");
+		assertTrue(Arrays.equals(new byte[] {1, 2, 3}, r.readBytes(3)));
+		try {
+			r.readBytes(2);
+			assertTrue(false);
+		} catch(FormatException expected) {}
+	}
+
 	@Test
 	public void testReadShortList() throws Exception {
 		setContents("A" + "3" + "01" + "83666F6F" + "FC0080");