diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSource.java b/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSource.java
new file mode 100644
index 000000000..83a88a29f
--- /dev/null
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSource.java
@@ -0,0 +1,11 @@
+package org.briarproject.bramble.api.io;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.sync.MessageId;
+
+public interface BlockSource {
+
+	int getBlockCount(MessageId m) throws DbException;
+
+	byte[] getBlock(MessageId m, int blockNumber) throws DbException;
+}
diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/io/MessageInputStreamFactory.java b/bramble-api/src/main/java/org/briarproject/bramble/api/io/MessageInputStreamFactory.java
new file mode 100644
index 000000000..68e9f85e1
--- /dev/null
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/io/MessageInputStreamFactory.java
@@ -0,0 +1,17 @@
+package org.briarproject.bramble.api.io;
+
+import org.briarproject.bramble.api.sync.MessageId;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public interface MessageInputStreamFactory {
+
+	/**
+	 * Returns an {@link InputStream} for reading the given message from the
+	 * database. This method returns immediately. If the message is not in the
+	 * database or cannot be read, reading from the stream will throw an
+	 * {@link IOException};
+	 */
+	InputStream getMessageInputStream(MessageId m);
+}
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/io/BlockInputStream.java b/bramble-core/src/main/java/org/briarproject/bramble/io/BlockInputStream.java
new file mode 100644
index 000000000..e4ae53931
--- /dev/null
+++ b/bramble-core/src/main/java/org/briarproject/bramble/io/BlockInputStream.java
@@ -0,0 +1,155 @@
+package org.briarproject.bramble.io;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InterruptedIOException;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.lang.System.arraycopy;
+import static java.lang.Thread.currentThread;
+
+/**
+ * An {@link InputStream} that asynchronously fetches blocks of data on demand.
+ */
+@ThreadSafe
+@NotNullByDefault
+abstract class BlockInputStream extends InputStream {
+
+	private final int minBufferBytes;
+	private final BlockingQueue<Buffer> queue = new ArrayBlockingQueue<>(1);
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	@Nullable
+	private Buffer buffer = null;
+
+	@GuardedBy("lock")
+	private int offset = 0;
+
+	@GuardedBy("lock")
+	private boolean fetchingBlock = false;
+
+	abstract void fetchBlockAsync(int blockNumber);
+
+	BlockInputStream(int minBufferBytes) {
+		this.minBufferBytes = minBufferBytes;
+	}
+
+	@Override
+	public int read() throws IOException {
+		synchronized (lock) {
+			if (!prepareRead()) return -1;
+			if (buffer == null) throw new AssertionError();
+			return buffer.data[offset++] & 0xFF;
+		}
+	}
+
+	@Override
+	public int read(byte[] b) throws IOException {
+		return read(b, 0, b.length);
+	}
+
+	@Override
+	public int read(byte[] b, int off, int len) throws IOException {
+		if (off < 0 || len < 0 || off + len > b.length)
+			throw new IllegalArgumentException();
+		synchronized (lock) {
+			if (!prepareRead()) return -1;
+			if (buffer == null) throw new AssertionError();
+			len = Math.min(len, buffer.length - offset);
+			if (len < 0) throw new AssertionError();
+			arraycopy(buffer.data, offset, b, off, len);
+			offset += len;
+			return len;
+		}
+	}
+
+	private boolean prepareRead() throws IOException {
+		throwExceptionIfNecessary();
+		if (isEndOfStream()) return false;
+		if (shouldFetchBlock()) fetchBlockAsync();
+		waitForBlock();
+		if (buffer == null) throw new AssertionError();
+		return offset < buffer.length;
+	}
+
+	@GuardedBy("lock")
+	private void throwExceptionIfNecessary() throws IOException {
+		if (buffer != null && buffer.exception != null)
+			throw new IOException(buffer.exception);
+	}
+
+	@GuardedBy("lock")
+	private boolean isEndOfStream() {
+		return buffer != null && offset == buffer.length && !fetchingBlock;
+	}
+
+	@GuardedBy("lock")
+	private boolean shouldFetchBlock() {
+		if (fetchingBlock) return false;
+		if (buffer == null) return true;
+		if (buffer.length == 0) return false;
+		return buffer.length - offset < minBufferBytes;
+	}
+
+	@GuardedBy("lock")
+	private void fetchBlockAsync() {
+		if (buffer == null) fetchBlockAsync(0);
+		else fetchBlockAsync(buffer.blockNumber + 1);
+		fetchingBlock = true;
+	}
+
+	@GuardedBy("lock")
+	private void waitForBlock() throws IOException {
+		if (buffer != null && offset < buffer.length) return;
+		try {
+			buffer = queue.take();
+		} catch (InterruptedException e) {
+			currentThread().interrupt();
+			throw new InterruptedIOException();
+		}
+		fetchingBlock = false;
+		offset = 0;
+		throwExceptionIfNecessary();
+	}
+
+	void fetchSucceeded(int blockNumber, byte[] data, int length) {
+		queue.add(new Buffer(blockNumber, data, length));
+	}
+
+	void fetchFailed(int blockNumber, Exception exception) {
+		queue.add(new Buffer(blockNumber, exception));
+	}
+
+	private static class Buffer {
+
+		private final int blockNumber;
+		private final byte[] data;
+		private final int length;
+		@Nullable
+		private final Exception exception;
+
+		private Buffer(int blockNumber, byte[] data, int length) {
+			if (length < 0 || length > data.length)
+				throw new IllegalArgumentException();
+			this.blockNumber = blockNumber;
+			this.data = data;
+			this.length = length;
+			exception = null;
+		}
+
+		private Buffer(int blockNumber, Exception exception) {
+			this.blockNumber = blockNumber;
+			this.exception = exception;
+			data = new byte[0];
+			length = 0;
+		}
+	}
+}
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/io/BlockSourceInputStream.java b/bramble-core/src/main/java/org/briarproject/bramble/io/BlockSourceInputStream.java
new file mode 100644
index 000000000..959a79a47
--- /dev/null
+++ b/bramble-core/src/main/java/org/briarproject/bramble/io/BlockSourceInputStream.java
@@ -0,0 +1,53 @@
+package org.briarproject.bramble.io;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.io.BlockSource;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import java.util.concurrent.Executor;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A {@link BlockInputStream} that fetches data from a {@link BlockSource}.
+ */
+@ThreadSafe
+@NotNullByDefault
+class BlockSourceInputStream extends BlockInputStream {
+
+	private final Executor executor;
+	private final BlockSource blockSource;
+	private final MessageId messageId;
+
+	private volatile int blockCount = -1;
+
+	BlockSourceInputStream(int minBufferBytes, Executor executor,
+			BlockSource blockSource, MessageId messageId) {
+		super(minBufferBytes);
+		this.executor = executor;
+		this.blockSource = blockSource;
+		this.messageId = messageId;
+	}
+
+	@Override
+	void fetchBlockAsync(int blockNumber) {
+		executor.execute(() -> {
+			try {
+				if (blockCount == -1) {
+					blockCount = blockSource.getBlockCount(messageId);
+				}
+				if (blockNumber > blockCount) {
+					fetchFailed(blockNumber, new IllegalArgumentException());
+				} else if (blockNumber == blockCount) {
+					fetchSucceeded(blockNumber, new byte[0], 0); // EOF
+				} else {
+					byte[] block = blockSource.getBlock(messageId, blockNumber);
+					fetchSucceeded(blockNumber, block, block.length);
+				}
+			} catch (DbException e) {
+				fetchFailed(blockNumber, e);
+			}
+		});
+	}
+}
diff --git a/bramble-core/src/test/java/org/briarproject/bramble/io/BlockSourceInputStreamTest.java b/bramble-core/src/test/java/org/briarproject/bramble/io/BlockSourceInputStreamTest.java
new file mode 100644
index 000000000..5f3c7539a
--- /dev/null
+++ b/bramble-core/src/test/java/org/briarproject/bramble/io/BlockSourceInputStreamTest.java
@@ -0,0 +1,152 @@
+package org.briarproject.bramble.io;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.io.BlockSource;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.jmock.Expectations;
+import org.jmock.lib.concurrent.Synchroniser;
+import org.junit.Test;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Random;
+import java.util.concurrent.Executor;
+
+import static java.util.concurrent.Executors.newSingleThreadExecutor;
+import static org.briarproject.bramble.test.TestUtils.getRandomId;
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.spongycastle.util.Arrays.copyOfRange;
+
+public class BlockSourceInputStreamTest extends BrambleMockTestCase {
+
+	private static final int MAX_DATA_BYTES = 1_000_000;
+	private static final int READ_BUFFER_BYTES = 4 * 1024;
+	private static final int BLOCK_BYTES = 32 * 1024;
+	private static final int MIN_BUFFER_BYTES = 32 * 1024;
+
+	private final BlockSource blockSource;
+
+	private final Random random = new Random();
+	private final Executor executor = newSingleThreadExecutor();
+	private final MessageId messageId = new MessageId(getRandomId());
+
+	public BlockSourceInputStreamTest() {
+		context.setThreadingPolicy(new Synchroniser());
+		blockSource = context.mock(BlockSource.class);
+	}
+
+	@Test
+	public void testReadSingleBytes() throws IOException {
+		byte[] data = createRandomData();
+		BlockSource source = new ByteArrayBlockSource(data, BLOCK_BYTES);
+		InputStream in = new BlockSourceInputStream(MIN_BUFFER_BYTES, executor,
+				source, messageId);
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		//noinspection ForLoopReplaceableByForEach
+		for (int i = 0; i < data.length; i++) {
+			int read = in.read();
+			assertNotEquals(-1, read);
+			out.write(read);
+		}
+		assertEquals(-1, in.read());
+		in.close();
+		out.flush();
+		out.close();
+		assertArrayEquals(data, out.toByteArray());
+	}
+
+	@Test
+	public void testReadByteArrays() throws IOException {
+		byte[] data = createRandomData();
+		BlockSource source = new ByteArrayBlockSource(data, BLOCK_BYTES);
+		InputStream in = new BlockSourceInputStream(MIN_BUFFER_BYTES, executor,
+				source, messageId);
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		byte[] buf = new byte[READ_BUFFER_BYTES];
+		int dataOffset = 0;
+		while (dataOffset < data.length) {
+			int length = Math.min(random.nextInt(buf.length) + 1,
+					data.length - dataOffset);
+			int bufOffset = 0;
+			if (length < buf.length)
+				bufOffset = random.nextInt(buf.length - length);
+			int read = in.read(buf, bufOffset, length);
+			assertNotEquals(-1, read);
+			out.write(buf, bufOffset, read);
+			dataOffset += read;
+		}
+		assertEquals(-1, in.read(buf, 0, 0));
+		in.close();
+		out.flush();
+		out.close();
+		assertArrayEquals(data, out.toByteArray());
+	}
+
+	@Test(expected = IOException.class)
+	public void testDbExceptionFromGetBlockCountIsRethrown() throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(blockSource).getBlockCount(messageId);
+			will(throwException(new DbException()));
+		}});
+
+		InputStream in = new BlockSourceInputStream(MIN_BUFFER_BYTES, executor,
+				blockSource, messageId);
+		//noinspection ResultOfMethodCallIgnored
+		in.read();
+	}
+
+	@Test(expected = IOException.class)
+	public void testDbExceptionFromGetBlockIsRethrown() throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(blockSource).getBlockCount(messageId);
+			will(returnValue(1));
+			oneOf(blockSource).getBlock(messageId, 0);
+			will(throwException(new DbException()));
+		}});
+
+		InputStream in = new BlockSourceInputStream(MIN_BUFFER_BYTES, executor,
+				blockSource, messageId);
+		//noinspection ResultOfMethodCallIgnored
+		in.read();
+	}
+
+	@Test
+	public void testReadFullBlockAtEndOfMessage() throws Exception {
+		testReadBlockAtEndOfMessage(BLOCK_BYTES);
+	}
+
+	@Test
+	public void testReadPartialBlockAtEndOfMessage() throws Exception {
+		testReadBlockAtEndOfMessage(BLOCK_BYTES - 1);
+	}
+
+	private void testReadBlockAtEndOfMessage(int blockLength) throws Exception {
+		byte[] block = new byte[blockLength];
+		random.nextBytes(block);
+
+		context.checking(new Expectations() {{
+			oneOf(blockSource).getBlockCount(messageId);
+			will(returnValue(1));
+			oneOf(blockSource).getBlock(messageId, 0);
+			will(returnValue(block));
+		}});
+
+		InputStream in = new BlockSourceInputStream(MIN_BUFFER_BYTES, executor,
+				blockSource, messageId);
+		byte[] buf = new byte[BLOCK_BYTES * 2];
+		assertEquals(block.length, in.read(buf, 0, buf.length));
+		assertArrayEquals(block, copyOfRange(buf, 0, block.length));
+		assertEquals(-1, in.read(buf, 0, buf.length));
+	}
+
+	private byte[] createRandomData() {
+		int length = random.nextInt(MAX_DATA_BYTES) + 1;
+		byte[] data = new byte[length];
+		random.nextBytes(data);
+		return data;
+	}
+}
diff --git a/bramble-core/src/test/java/org/briarproject/bramble/io/ByteArrayBlockSource.java b/bramble-core/src/test/java/org/briarproject/bramble/io/ByteArrayBlockSource.java
new file mode 100644
index 000000000..e96e1baaa
--- /dev/null
+++ b/bramble-core/src/test/java/org/briarproject/bramble/io/ByteArrayBlockSource.java
@@ -0,0 +1,32 @@
+package org.briarproject.bramble.io;
+
+import org.briarproject.bramble.api.io.BlockSource;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import static java.lang.System.arraycopy;
+
+class ByteArrayBlockSource implements BlockSource {
+
+	private final byte[] data;
+	private final int blockBytes;
+
+	ByteArrayBlockSource(byte[] data, int blockBytes) {
+		this.data = data;
+		this.blockBytes = blockBytes;
+	}
+
+	@Override
+	public int getBlockCount(MessageId m) {
+		return (data.length + blockBytes - 1) / blockBytes;
+	}
+
+	@Override
+	public byte[] getBlock(MessageId m, int blockNumber) {
+		int offset = blockNumber * blockBytes;
+		if (offset >= data.length) throw new IllegalArgumentException();
+		int length = Math.min(blockBytes, data.length - offset);
+		byte[] block = new byte[length];
+		arraycopy(data, offset, block, 0, length);
+		return block;
+	}
+}