Add a CheckpointManager class and a BuildCheckpoints tool that saves a set of checkpoints to disk. By default there is one every difficulty transition period (two weeks), which means a relatively small amount of RAM needed to hold them all. There are only 111 checkpoints so far and old ones can be thinned out if needed.

tools/src/main/java/com/google/bitcoin/tools/BuildCheckpoints.java

@@ -0,0 +1,86 @@
+package com.google.bitcoin.tools;
+
+import com.google.bitcoin.core.*;
+import com.google.bitcoin.store.BlockStore;
+import com.google.bitcoin.store.MemoryBlockStore;
+import com.google.bitcoin.store.SPVBlockStore;
+import com.google.bitcoin.utils.BriefLogFormatter;
+
+import java.io.*;
+import java.net.InetAddress;
+import java.nio.ByteBuffer;
+import java.security.DigestOutputStream;
+import java.security.MessageDigest;
+import java.util.Date;
+import java.util.TreeMap;
+
+import static com.google.common.base.Preconditions.checkState;
+
+/**
+ * Downloads and verifies a full chain from your local peer, emitting checkpoints at each difficulty transition period
+ * to a file which is then signed with your key.
+ */
+public class BuildCheckpoints {
+    public static void main(String[] args) throws Exception {
+        BriefLogFormatter.init();
+        final NetworkParameters params = NetworkParameters.prodNet();
+
+        // Sorted map of UNIX time of block to StoredBlock object.
+        final TreeMap<Integer, StoredBlock> checkpoints = new TreeMap<Integer, StoredBlock>();
+
+        final BlockStore store = new MemoryBlockStore(params);
+        final BlockChain chain = new BlockChain(params, store);
+        final PeerGroup peerGroup = new PeerGroup(params, chain);
+        peerGroup.addAddress(InetAddress.getLocalHost());
+        peerGroup.setFastCatchupTimeSecs(new Date().getTime() / 1000);
+
+        chain.addListener(new AbstractBlockChainListener() {
+            @Override
+            public void notifyNewBestBlock(StoredBlock block) throws VerificationException {
+                int height = block.getHeight();
+                if (height % params.interval == 0) {
+                    System.out.println(String.format("Checkpointing block %s at height %d",
+                            block.getHeader().getHash(), block.getHeight()));
+                    checkpoints.put(height, block);
+                }
+            }
+        });
+
+        peerGroup.startAndWait();
+        peerGroup.downloadBlockChain();
+
+        checkState(checkpoints.size() > 0);
+
+        // Write checkpoint data out.
+        final FileOutputStream fileOutputStream = new FileOutputStream("checkpoints", false);
+        MessageDigest digest = MessageDigest.getInstance("SHA-256");
+        final DigestOutputStream digestOutputStream = new DigestOutputStream(fileOutputStream, digest);
+        digestOutputStream.on(false);
+        final DataOutputStream dataOutputStream = new DataOutputStream(digestOutputStream);
+        dataOutputStream.writeBytes("CHECKPOINTS 1");
+        dataOutputStream.writeInt(0);  // Number of signatures to read. Do this later.
+        digestOutputStream.on(true);
+        dataOutputStream.writeInt(checkpoints.size());
+        ByteBuffer buffer = ByteBuffer.allocate(StoredBlock.COMPACT_SERIALIZED_SIZE);
+        for (StoredBlock block : checkpoints.values()) {
+            block.serializeCompact(buffer);
+            dataOutputStream.write(buffer.array());
+            buffer.position(0);
+        }
+        dataOutputStream.close();
+        Sha256Hash checkpointsHash = new Sha256Hash(digest.digest());
+        System.out.println("Hash of checkpoints data is " + checkpointsHash);
+        digestOutputStream.close();
+        fileOutputStream.close();
+
+        peerGroup.stopAndWait();
+        store.close();
+
+        // Sanity check the created file.
+        CheckpointManager manager = new CheckpointManager(params, new FileInputStream("checkpoints"));
+        checkState(manager.numCheckpoints() == checkpoints.size());
+        StoredBlock test = manager.getCheckpointBefore(1348310800);  // Just after block 200,000
+        checkState(test.getHeight() == 199584);
+        checkState(test.getHeader().getHashAsString().equals("000000000000002e00a243fe9aa49c78f573091d17372c2ae0ae5e0f24f55b52"));
+    }
+}