diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSink.java b/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSink.java
index 388311ec9..4324b87e3 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSink.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSink.java
@@ -1,7 +1,6 @@
 package org.briarproject.bramble.api.io;
 
 import org.briarproject.bramble.api.db.DbException;
-import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.bramble.api.sync.tree.TreeHash;
 
 import java.util.List;
@@ -18,10 +17,4 @@ public interface BlockSink {
 	 */
 	void setPath(HashingId h, int blockNumber, List<TreeHash> path)
 			throws DbException;
-
-	/**
-	 * Sets the permanent ID of the message with the given temporary ID. The
-	 * temporary ID is no longer valid once this method has been called.
-	 */
-	void setMessageId(HashingId h, MessageId m) throws DbException;
 }
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/LeafNode.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/LeafNode.java
index 68d81d1a2..f7ee5b3e4 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/LeafNode.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/LeafNode.java
@@ -1,8 +1,21 @@
 package org.briarproject.bramble.api.sync.tree;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public class LeafNode extends TreeNode {
 
 	public LeafNode(TreeHash hash, int blockNumber) {
 		super(hash, 0, blockNumber, blockNumber);
 	}
+
+	@Override
+	public TreeNode getLeftChild() {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public TreeNode getRightChild() {
+		throw new UnsupportedOperationException();
+	}
 }
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/ParentNode.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/ParentNode.java
index 8d529bb9e..6e2973b26 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/ParentNode.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/ParentNode.java
@@ -1,9 +1,26 @@
 package org.briarproject.bramble.api.sync.tree;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
 public class ParentNode extends TreeNode {
 
+	private final TreeNode left, right;
+
 	public ParentNode(TreeHash hash, TreeNode left, TreeNode right) {
 		super(hash, Math.max(left.getHeight(), right.getHeight()) + 1,
 				left.getFirstBlockNumber(), right.getLastBlockNumber());
+		this.left = left;
+		this.right = right;
+	}
+
+	@Override
+	public TreeNode getLeftChild() {
+		return left;
+	}
+
+	@Override
+	public TreeNode getRightChild() {
+		return right;
 	}
 }
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/StreamHasher.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/StreamHasher.java
index 9e72cb4ff..4b9212238 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/StreamHasher.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/StreamHasher.java
@@ -4,8 +4,6 @@ import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.io.BlockSink;
 import org.briarproject.bramble.api.io.HashingId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.GroupId;
-import org.briarproject.bramble.api.sync.MessageId;
 
 import java.io.IOException;
 import java.io.InputStream;
@@ -16,8 +14,8 @@ public interface StreamHasher {
 	/**
 	 * Reads the given input stream, divides the data into blocks, stores
 	 * the blocks and the resulting hash tree using the given block sink and
-	 * temporary ID, and returns the message ID.
+	 * temporary ID, and returns the hash tree.
 	 */
-	MessageId hash(InputStream in, BlockSink sink, HashingId h, GroupId g,
-			long timestamp) throws IOException, DbException;
+	TreeNode hash(InputStream in, BlockSink sink, HashingId h)
+			throws IOException, DbException;
 }
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeNode.java b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeNode.java
index d5104f645..8d6a9f4d6 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeNode.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeNode.java
@@ -1,6 +1,9 @@
 package org.briarproject.bramble.api.sync.tree;
 
-public class TreeNode {
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public abstract class TreeNode {
 
 	private final TreeHash hash;
 	private final int height, firstBlockNumber, lastBlockNumber;
@@ -28,4 +31,8 @@ public class TreeNode {
 	public int getLastBlockNumber() {
 		return lastBlockNumber;
 	}
+
+	public abstract TreeNode getLeftChild();
+
+	public abstract TreeNode getRightChild();
 }
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/sync/tree/StreamHasherImpl.java b/bramble-core/src/main/java/org/briarproject/bramble/sync/tree/StreamHasherImpl.java
index d30545bee..6a6aa2f94 100644
--- a/bramble-core/src/main/java/org/briarproject/bramble/sync/tree/StreamHasherImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/sync/tree/StreamHasherImpl.java
@@ -4,16 +4,14 @@ import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.io.BlockSink;
 import org.briarproject.bramble.api.io.HashingId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.GroupId;
-import org.briarproject.bramble.api.sync.MessageFactory;
-import org.briarproject.bramble.api.sync.MessageId;
-import org.briarproject.bramble.api.sync.tree.LeafNode;
 import org.briarproject.bramble.api.sync.tree.StreamHasher;
 import org.briarproject.bramble.api.sync.tree.TreeHash;
 import org.briarproject.bramble.api.sync.tree.TreeHasher;
+import org.briarproject.bramble.api.sync.tree.TreeNode;
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.LinkedList;
 
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
@@ -27,21 +25,19 @@ import static org.briarproject.bramble.api.sync.SyncConstants.MAX_BLOCK_LENGTH;
 class StreamHasherImpl implements StreamHasher {
 
 	private final TreeHasher treeHasher;
-	private final MessageFactory messageFactory;
 	private final Provider<HashTree> hashTreeProvider;
 
 	@Inject
-	StreamHasherImpl(TreeHasher treeHasher, MessageFactory messageFactory,
+	StreamHasherImpl(TreeHasher treeHasher,
 			Provider<HashTree> hashTreeProvider) {
 		this.treeHasher = treeHasher;
-		this.messageFactory = messageFactory;
 		this.hashTreeProvider = hashTreeProvider;
 	}
 
 	@Override
-	public MessageId hash(InputStream in, BlockSink sink, HashingId h,
-			GroupId g, long timestamp) throws IOException, DbException {
-		HashTree hashTree = hashTreeProvider.get();
+	public TreeNode hash(InputStream in, BlockSink sink, HashingId h)
+			throws IOException, DbException {
+		HashTree tree = hashTreeProvider.get();
 		byte[] block = new byte[MAX_BLOCK_LENGTH];
 		int read;
 		for (int blockNumber = 0; (read = read(in, block)) > 0; blockNumber++) {
@@ -49,14 +45,11 @@ class StreamHasherImpl implements StreamHasher {
 			if (read == block.length) data = block;
 			else data = copyOfRange(block, 0, read);
 			sink.putBlock(h, blockNumber, data);
-			LeafNode leaf = treeHasher.hashBlock(blockNumber, data);
-			hashTree.addLeaf(leaf);
+			tree.addLeaf(treeHasher.hashBlock(blockNumber, data));
 		}
-		// TODO: Set paths on block sink
-		TreeHash rootHash = hashTree.getRoot().getHash();
-		MessageId m = messageFactory.getMessageId(g, timestamp, rootHash);
-		sink.setMessageId(h, m);
-		return m;
+		TreeNode root = tree.getRoot();
+		setPaths(sink, h, root, new LinkedList<>());
+		return root;
 	}
 
 	/**
@@ -73,4 +66,20 @@ class StreamHasherImpl implements StreamHasher {
 		}
 		return offset;
 	}
+
+	private void setPaths(BlockSink sink, HashingId h, TreeNode node,
+			LinkedList<TreeHash> path) throws DbException {
+		if (node.getHeight() == 0) {
+			// We've reached a leaf - store the path
+			sink.setPath(h, node.getFirstBlockNumber(), path);
+		} else {
+			// Add the right child's hash to the path and traverse the left
+			path.addFirst(node.getRightChild().getHash());
+			setPaths(sink, h, node.getLeftChild(), path);
+			// Add the left child's hash to the path and traverse the right
+			path.removeFirst();
+			path.addFirst(node.getLeftChild().getHash());
+			setPaths(sink, h, node.getRightChild(), path);
+		}
+	}
 }