Initial support for sender keys.

libaxolotl/protobuf/LocalStorageProtocol.proto

@@ -84,3 +84,29 @@ message IdentityKeyPairStructure {
     optional bytes publicKey  = 1;
     optional bytes privateKey = 2;
 }
+
+message SenderKeyStateStructure {
+    message SenderChainKey {
+        optional uint32 iteration = 1;
+        optional bytes  seed      = 2;
+    }
+
+    message SenderMessageKey {
+        optional uint32 iteration = 1;
+        optional bytes  seed      = 2;
+    }
+
+    message SenderSigningKey {
+        optional bytes public  = 1;
+        optional bytes private = 2;
+    }
+
+    optional uint32           senderKeyId       = 1;
+    optional SenderChainKey   senderChainKey    = 2;
+    optional SenderSigningKey senderSigningKey  = 3;
+    repeated SenderMessageKey senderMessageKeys = 4;
+}
+
+message SenderKeyRecordStructure {
+    repeated SenderKeyStateStructure senderKeyStates = 1;
+}

libaxolotl/protobuf/WhisperTextProtocol.proto

@@ -26,3 +26,16 @@ message KeyExchangeMessage {
   optional bytes  identityKey      = 4;
   optional bytes  baseKeySignature = 5;
 }
+
+message SenderKeyMessage {
+  optional uint32 id         = 1;
+  optional uint32 iteration  = 2;
+  optional bytes  ciphertext = 3;
+}
+
+message SenderKeyDistributionMessage {
+  optional uint32 id         = 1;
+  optional uint32 iteration  = 2;
+  optional bytes  chainKey   = 3;
+  optional bytes  signingKey = 4;
+}

libaxolotl/src/androidTest/java/org/whispersystems/test/groups/GroupCipherTest.java

@@ -0,0 +1,139 @@
+package org.whispersystems.test.groups;
+
+import android.test.AndroidTestCase;
+import android.util.Log;
+
+import org.whispersystems.libaxolotl.DuplicateMessageException;
+import org.whispersystems.libaxolotl.InvalidMessageException;
+import org.whispersystems.libaxolotl.LegacyMessageException;
+import org.whispersystems.libaxolotl.ecc.ECKeyPair;
+import org.whispersystems.libaxolotl.groups.GroupCipher;
+import org.whispersystems.libaxolotl.groups.GroupSessionBuilder;
+import org.whispersystems.libaxolotl.protocol.SenderKeyDistributionMessage;
+import org.whispersystems.libaxolotl.util.KeyHelper;
+
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+public class GroupCipherTest extends AndroidTestCase {
+
+  public void testBasicEncryptDecrypt()
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException
+  {
+    InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
+    InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
+
+    GroupSessionBuilder aliceSessionBuilder = new GroupSessionBuilder(aliceStore);
+    GroupSessionBuilder bobSessionBuilder   = new GroupSessionBuilder(bobStore);
+
+    GroupCipher aliceGroupCipher = new GroupCipher(aliceStore, "groupWithBobInIt");
+    GroupCipher bobGroupCipher   = new GroupCipher(bobStore, "groupWithBobInIt::aliceUserName");
+
+    byte[]    aliceSenderKey        = KeyHelper.generateSenderKey();
+    ECKeyPair aliceSenderSigningKey = KeyHelper.generateSenderSigningKey();
+    int       aliceSenderKeyId      = KeyHelper.generateSenderKeyId();
+
+    SenderKeyDistributionMessage aliceDistributionMessage =
+        aliceSessionBuilder.process("groupWithBobInIt", aliceSenderKeyId, 0,
+                                    aliceSenderKey, aliceSenderSigningKey);
+
+    bobSessionBuilder.process("groupWithBobInIt::aliceUserName", aliceDistributionMessage);
+
+    byte[] ciphertextFromAlice = aliceGroupCipher.encrypt("smert ze smert".getBytes());
+    byte[] plaintextFromAlice  = bobGroupCipher.decrypt(ciphertextFromAlice);
+
+    assertTrue(new String(plaintextFromAlice).equals("smert ze smert"));
+  }
+
+  public void testBasicRatchet()
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException
+  {
+    InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
+    InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
+
+    GroupSessionBuilder aliceSessionBuilder = new GroupSessionBuilder(aliceStore);
+    GroupSessionBuilder bobSessionBuilder   = new GroupSessionBuilder(bobStore);
+
+    GroupCipher aliceGroupCipher = new GroupCipher(aliceStore, "groupWithBobInIt");
+    GroupCipher bobGroupCipher   = new GroupCipher(bobStore, "groupWithBobInIt::aliceUserName");
+
+    byte[]    aliceSenderKey        = KeyHelper.generateSenderKey();
+    ECKeyPair aliceSenderSigningKey = KeyHelper.generateSenderSigningKey();
+    int       aliceSenderKeyId      = KeyHelper.generateSenderKeyId();
+
+    SenderKeyDistributionMessage aliceDistributionMessage =
+        aliceSessionBuilder.process("groupWithBobInIt", aliceSenderKeyId, 0,
+                                    aliceSenderKey, aliceSenderSigningKey);
+
+    bobSessionBuilder.process("groupWithBobInIt::aliceUserName", aliceDistributionMessage);
+
+    byte[] ciphertextFromAlice  = aliceGroupCipher.encrypt("smert ze smert".getBytes());
+    byte[] ciphertextFromAlice2 = aliceGroupCipher.encrypt("smert ze smert2".getBytes());
+    byte[] ciphertextFromAlice3 = aliceGroupCipher.encrypt("smert ze smert3".getBytes());
+
+    byte[] plaintextFromAlice   = bobGroupCipher.decrypt(ciphertextFromAlice);
+
+    try {
+      bobGroupCipher.decrypt(ciphertextFromAlice);
+      throw new AssertionError("Should have ratcheted forward!");
+    } catch (DuplicateMessageException dme) {
+      // good
+    }
+
+    byte[] plaintextFromAlice2  = bobGroupCipher.decrypt(ciphertextFromAlice2);
+    byte[] plaintextFromAlice3  = bobGroupCipher.decrypt(ciphertextFromAlice3);
+
+    assertTrue(new String(plaintextFromAlice).equals("smert ze smert"));
+    assertTrue(new String(plaintextFromAlice2).equals("smert ze smert2"));
+    assertTrue(new String(plaintextFromAlice3).equals("smert ze smert3"));
+  }
+
+  public void testOutOfOrder()
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException
+  {
+    InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
+    InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
+
+    GroupSessionBuilder aliceSessionBuilder = new GroupSessionBuilder(aliceStore);
+    GroupSessionBuilder bobSessionBuilder   = new GroupSessionBuilder(bobStore);
+
+    GroupCipher aliceGroupCipher = new GroupCipher(aliceStore, "groupWithBobInIt");
+    GroupCipher bobGroupCipher   = new GroupCipher(bobStore, "groupWithBobInIt::aliceUserName");
+
+    byte[]    aliceSenderKey        = KeyHelper.generateSenderKey();
+    ECKeyPair aliceSenderSigningKey = KeyHelper.generateSenderSigningKey();
+    int       aliceSenderKeyId      = KeyHelper.generateSenderKeyId();
+
+    SenderKeyDistributionMessage aliceDistributionMessage =
+        aliceSessionBuilder.process("groupWithBobInIt", aliceSenderKeyId, 0,
+                                    aliceSenderKey, aliceSenderSigningKey);
+
+    bobSessionBuilder.process("groupWithBobInIt::aliceUserName", aliceDistributionMessage);
+
+
+    ArrayList<byte[]> ciphertexts = new ArrayList<>(100);
+
+    for (int i=0;i<100;i++) {
+      ciphertexts.add(aliceGroupCipher.encrypt("up the punks".getBytes()));
+    }
+
+    while (ciphertexts.size() > 0) {
+      int    index      = randomInt() % ciphertexts.size();
+      byte[] ciphertext = ciphertexts.remove(index);
+      byte[] plaintext  = bobGroupCipher.decrypt(ciphertext);
+
+      assertTrue(new String(plaintext).equals("up the punks"));
+    }
+  }
+
+  private int randomInt() {
+    try {
+      return SecureRandom.getInstance("SHA1PRNG").nextInt(Integer.MAX_VALUE);
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    }
+  }
+}

libaxolotl/src/androidTest/java/org/whispersystems/test/groups/InMemorySenderKeyStore.java

@@ -0,0 +1,33 @@
+package org.whispersystems.test.groups;
+
+import org.whispersystems.libaxolotl.groups.state.SenderKeyRecord;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyStore;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+
+public class InMemorySenderKeyStore implements SenderKeyStore {
+
+  private final Map<String, SenderKeyRecord> store = new HashMap<>();
+
+  @Override
+  public void storeSenderKey(String senderKeyId, SenderKeyRecord record) {
+    store.put(senderKeyId, record);
+  }
+
+  @Override
+  public SenderKeyRecord loadSenderKey(String senderKeyId) {
+    try {
+      SenderKeyRecord record = store.get(senderKeyId);
+
+      if (record == null) {
+        return new SenderKeyRecord();
+      } else {
+        return new SenderKeyRecord(record.serialize());
+      }
+    } catch (IOException e) {
+      throw new AssertionError(e);
+    }
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/GroupCipher.java

@@ -0,0 +1,144 @@
+package org.whispersystems.libaxolotl.groups;
+
+import android.util.Log;
+
+import org.whispersystems.libaxolotl.DuplicateMessageException;
+import org.whispersystems.libaxolotl.InvalidKeyIdException;
+import org.whispersystems.libaxolotl.InvalidMessageException;
+import org.whispersystems.libaxolotl.LegacyMessageException;
+import org.whispersystems.libaxolotl.groups.ratchet.SenderChainKey;
+import org.whispersystems.libaxolotl.groups.ratchet.SenderMessageKey;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyRecord;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyState;
+import org.whispersystems.libaxolotl.protocol.SenderKeyMessage;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyStore;
+
+
+import java.security.InvalidAlgorithmParameterException;
+import java.security.NoSuchAlgorithmException;
+
+import javax.crypto.BadPaddingException;
+import javax.crypto.Cipher;
+import javax.crypto.IllegalBlockSizeException;
+import javax.crypto.NoSuchPaddingException;
+import javax.crypto.spec.IvParameterSpec;
+import javax.crypto.spec.SecretKeySpec;
+
+public class GroupCipher {
+
+  static final Object LOCK = new Object();
+
+  private final SenderKeyStore senderKeyStore;
+  private final String         senderKeyId;
+
+  public GroupCipher(SenderKeyStore senderKeyStore, String senderKeyId) {
+    this.senderKeyStore = senderKeyStore;
+    this.senderKeyId    = senderKeyId;
+  }
+
+  public byte[] encrypt(byte[] paddedPlaintext) {
+    synchronized (LOCK) {
+      SenderKeyRecord  record         = senderKeyStore.loadSenderKey(senderKeyId);
+      SenderKeyState   senderKeyState = record.getSenderKeyState();
+      SenderMessageKey senderKey      = senderKeyState.getSenderChainKey().getSenderMessageKey();
+      byte[]           ciphertext     = getCipherText(senderKey.getIv(), senderKey.getCipherKey(), paddedPlaintext);
+
+      SenderKeyMessage senderKeyMessage = new SenderKeyMessage(senderKeyState.getKeyId(),
+                                                               senderKey.getIteration(),
+                                                               ciphertext,
+                                                               senderKeyState.getSigningKeyPrivate());
+
+      senderKeyState.setSenderChainKey(senderKeyState.getSenderChainKey().getNext());
+
+      senderKeyStore.storeSenderKey(senderKeyId, record);
+
+      return senderKeyMessage.serialize();
+    }
+  }
+
+  public byte[] decrypt(byte[] senderKeyMessageBytes)
+      throws LegacyMessageException, InvalidMessageException, DuplicateMessageException
+  {
+    synchronized (LOCK) {
+      try {
+        SenderKeyRecord  record           = senderKeyStore.loadSenderKey(senderKeyId);
+        SenderKeyMessage senderKeyMessage = new SenderKeyMessage(senderKeyMessageBytes);
+        SenderKeyState   senderKeyState   = record.getSenderKeyState(senderKeyMessage.getKeyId());
+
+        senderKeyMessage.verifySignature(senderKeyState.getSigningKeyPublic());
+
+        SenderMessageKey senderKey = getSenderKey(senderKeyState, senderKeyMessage.getIteration());
+
+        byte[] plaintext = getPlainText(senderKey.getIv(), senderKey.getCipherKey(), senderKeyMessage.getCipherText());
+
+        senderKeyStore.storeSenderKey(senderKeyId, record);
+
+        return plaintext;
+      } catch (org.whispersystems.libaxolotl.InvalidKeyException | InvalidKeyIdException e) {
+        throw new InvalidMessageException(e);
+      }
+    }
+  }
+
+  private SenderMessageKey getSenderKey(SenderKeyState senderKeyState, int iteration)
+      throws DuplicateMessageException, InvalidMessageException
+  {
+    SenderChainKey senderChainKey = senderKeyState.getSenderChainKey();
+
+    if (senderChainKey.getIteration() > iteration) {
+      if (senderKeyState.hasSenderMessageKey(iteration)) {
+        return senderKeyState.removeSenderMessageKey(iteration);
+      } else {
+        throw new DuplicateMessageException("Received message with old counter: " +
+                                            senderChainKey.getIteration() + " , " + iteration);
+      }
+    }
+
+    if (senderChainKey.getIteration() - iteration > 2000) {
+      throw new InvalidMessageException("Over 2000 messages into the future!");
+    }
+
+    while (senderChainKey.getIteration() < iteration) {
+      senderKeyState.addSenderMessageKey(senderChainKey.getSenderMessageKey());
+      senderChainKey = senderChainKey.getNext();
+    }
+
+    senderKeyState.setSenderChainKey(senderChainKey.getNext());
+    return senderChainKey.getSenderMessageKey();
+  }
+
+  private byte[] getPlainText(byte[] iv, byte[] key, byte[] ciphertext)
+      throws InvalidMessageException
+  {
+    try {
+      IvParameterSpec ivParameterSpec = new IvParameterSpec(iv);
+      Cipher          cipher          = Cipher.getInstance("AES/CBC/PKCS5Padding");
+
+      cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "AES"), ivParameterSpec);
+
+      return cipher.doFinal(ciphertext);
+    } catch (NoSuchAlgorithmException | NoSuchPaddingException | java.security.InvalidKeyException |
+             InvalidAlgorithmParameterException e)
+    {
+      throw new AssertionError(e);
+    } catch (IllegalBlockSizeException | BadPaddingException e) {
+      throw new InvalidMessageException(e);
+    }
+  }
+
+  private byte[] getCipherText(byte[] iv, byte[] key, byte[] plaintext) {
+    try {
+      IvParameterSpec ivParameterSpec = new IvParameterSpec(iv);
+      Cipher          cipher          = Cipher.getInstance("AES/CBC/PKCS5Padding");
+
+      cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(key, "AES"), ivParameterSpec);
+
+      return cipher.doFinal(plaintext);
+    } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidAlgorithmParameterException |
+             IllegalBlockSizeException | BadPaddingException | java.security.InvalidKeyException e)
+    {
+      throw new AssertionError(e);
+    }
+  }
+
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/GroupSessionBuilder.java

@@ -0,0 +1,38 @@
+package org.whispersystems.libaxolotl.groups;
+
+import org.whispersystems.libaxolotl.ecc.ECKeyPair;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyRecord;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyStore;
+import org.whispersystems.libaxolotl.protocol.SenderKeyDistributionMessage;
+
+public class GroupSessionBuilder {
+
+  private final SenderKeyStore senderKeyStore;
+
+  public GroupSessionBuilder(SenderKeyStore senderKeyStore) {
+    this.senderKeyStore = senderKeyStore;
+  }
+
+  public void process(String sender, SenderKeyDistributionMessage senderKeyDistributionMessage) {
+    synchronized (GroupCipher.LOCK) {
+      SenderKeyRecord senderKeyRecord = senderKeyStore.loadSenderKey(sender);
+      senderKeyRecord.addSenderKeyState(senderKeyDistributionMessage.getId(),
+                                        senderKeyDistributionMessage.getIteration(),
+                                        senderKeyDistributionMessage.getChainKey(),
+                                        senderKeyDistributionMessage.getSignatureKey());
+      senderKeyStore.storeSenderKey(sender, senderKeyRecord);
+    }
+  }
+
+  public SenderKeyDistributionMessage process(String groupId, int keyId, int iteration,
+                                              byte[] chainKey, ECKeyPair signatureKey)
+  {
+    synchronized (GroupCipher.LOCK) {
+      SenderKeyRecord senderKeyRecord = senderKeyStore.loadSenderKey(groupId);
+      senderKeyRecord.setSenderKeyState(keyId, iteration, chainKey, signatureKey);
+      senderKeyStore.storeSenderKey(groupId, senderKeyRecord);
+
+      return new SenderKeyDistributionMessage(keyId, iteration, chainKey, signatureKey.getPublicKey());
+    }
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/ratchet/SenderChainKey.java

@@ -0,0 +1,49 @@
+package org.whispersystems.libaxolotl.groups.ratchet;
+
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
+
+import javax.crypto.Mac;
+import javax.crypto.spec.SecretKeySpec;
+
+public class SenderChainKey {
+
+  private static final byte[] MESSAGE_KEY_SEED = {0x01};
+  private static final byte[] CHAIN_KEY_SEED   = {0x02};
+
+  private final int    iteration;
+  private final byte[] chainKey;
+
+  public SenderChainKey(int iteration, byte[] chainKey) {
+    this.iteration = iteration;
+    this.chainKey  = chainKey;
+  }
+
+  public int getIteration() {
+    return iteration;
+  }
+
+  public SenderMessageKey getSenderMessageKey() {
+    return new SenderMessageKey(iteration, getDerivative(MESSAGE_KEY_SEED, chainKey));
+  }
+
+  public SenderChainKey getNext() {
+    return new SenderChainKey(iteration + 1, getDerivative(CHAIN_KEY_SEED, chainKey));
+  }
+
+  public byte[] getSeed() {
+    return chainKey;
+  }
+
+  private byte[] getDerivative(byte[] seed, byte[] key) {
+    try {
+      Mac mac = Mac.getInstance("HmacSHA256");
+      mac.init(new SecretKeySpec(key, "HmacSHA256"));
+
+      return mac.doFinal(seed);
+    } catch (NoSuchAlgorithmException | InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/ratchet/SenderMessageKey.java

@@ -0,0 +1,38 @@
+package org.whispersystems.libaxolotl.groups.ratchet;
+
+import org.whispersystems.libaxolotl.kdf.HKDFv3;
+import org.whispersystems.libaxolotl.util.ByteUtil;
+
+public class SenderMessageKey {
+
+  private final int    iteration;
+  private final byte[] iv;
+  private final byte[] cipherKey;
+  private final byte[] seed;
+
+  public SenderMessageKey(int iteration, byte[] seed) {
+    byte[] derivative = new HKDFv3().deriveSecrets(seed, "WhisperGroup".getBytes(), 48);
+    byte[][] parts    = ByteUtil.split(derivative, 16, 32);
+
+    this.iteration = iteration;
+    this.seed      = seed;
+    this.iv        = parts[0];
+    this.cipherKey = parts[1];
+  }
+
+  public int getIteration() {
+    return iteration;
+  }
+
+  public byte[] getIv() {
+    return iv;
+  }
+
+  public byte[] getCipherKey() {
+    return cipherKey;
+  }
+
+  public byte[] getSeed() {
+    return seed;
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/state/SenderKeyRecord.java

@@ -0,0 +1,61 @@
+package org.whispersystems.libaxolotl.groups.state;
+
+import org.whispersystems.libaxolotl.InvalidKeyIdException;
+import org.whispersystems.libaxolotl.ecc.ECKeyPair;
+import org.whispersystems.libaxolotl.ecc.ECPublicKey;
+import org.whispersystems.libaxolotl.groups.state.SenderKeyState;
+import org.whispersystems.libaxolotl.state.StorageProtos;
+
+import java.io.IOException;
+import java.util.LinkedList;
+import java.util.List;
+
+import static org.whispersystems.libaxolotl.state.StorageProtos.SenderKeyRecordStructure;
+
+public class SenderKeyRecord {
+
+  private List<SenderKeyState> senderKeyStates = new LinkedList<>();
+
+  public SenderKeyRecord() {}
+
+  public SenderKeyRecord(byte[] serialized) throws IOException {
+    SenderKeyRecordStructure senderKeyRecordStructure = SenderKeyRecordStructure.parseFrom(serialized);
+
+    for (StorageProtos.SenderKeyStateStructure structure : senderKeyRecordStructure.getSenderKeyStatesList()) {
+      this.senderKeyStates.add(new SenderKeyState(structure));
+    }
+  }
+
+  public SenderKeyState getSenderKeyState() {
+    return senderKeyStates.get(0);
+  }
+
+  public SenderKeyState getSenderKeyState(int keyId) throws InvalidKeyIdException {
+    for (SenderKeyState state : senderKeyStates) {
+      if (state.getKeyId() == keyId) {
+        return state;
+      }
+    }
+
+    throw new InvalidKeyIdException("No keys for: " + keyId);
+  }
+
+  public void addSenderKeyState(int id, int iteration, byte[] chainKey, ECPublicKey signatureKey) {
+    senderKeyStates.add(new SenderKeyState(id, iteration, chainKey, signatureKey));
+  }
+
+  public void setSenderKeyState(int id, int iteration, byte[] chainKey, ECKeyPair signatureKey) {
+    senderKeyStates.clear();
+    senderKeyStates.add(new SenderKeyState(id, iteration, chainKey, signatureKey));
+  }
+
+  public byte[] serialize() {
+    SenderKeyRecordStructure.Builder recordStructure = SenderKeyRecordStructure.newBuilder();
+
+    for (SenderKeyState senderKeyState : senderKeyStates) {
+      recordStructure.addSenderKeyStates(senderKeyState.getStructure());
+    }
+
+    return recordStructure.build().toByteArray();
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/state/SenderKeyState.java

@@ -0,0 +1,146 @@
+package org.whispersystems.libaxolotl.groups.state;
+
+import android.util.Log;
+
+import com.google.protobuf.ByteString;
+
+import org.whispersystems.libaxolotl.InvalidKeyException;
+import org.whispersystems.libaxolotl.ecc.Curve;
+import org.whispersystems.libaxolotl.ecc.ECKeyPair;
+import org.whispersystems.libaxolotl.ecc.ECPrivateKey;
+import org.whispersystems.libaxolotl.ecc.ECPublicKey;
+import org.whispersystems.libaxolotl.groups.ratchet.SenderChainKey;
+import org.whispersystems.libaxolotl.groups.ratchet.SenderMessageKey;
+import org.whispersystems.libaxolotl.util.guava.Optional;
+
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+
+import static org.whispersystems.libaxolotl.state.StorageProtos.SenderKeyStateStructure;
+
+public class SenderKeyState {
+
+  private SenderKeyStateStructure senderKeyStateStructure;
+
+  public SenderKeyState(int id, int iteration, byte[] chainKey, ECPublicKey signatureKey) {
+    this(id, iteration, chainKey, signatureKey, Optional.<ECPrivateKey>absent());
+  }
+
+  public SenderKeyState(int id, int iteration, byte[] chainKey, ECKeyPair signatureKey) {
+    this(id, iteration, chainKey, signatureKey.getPublicKey(), Optional.of(signatureKey.getPrivateKey()));
+  }
+
+  private SenderKeyState(int id, int iteration, byte[] chainKey,
+                        ECPublicKey signatureKeyPublic,
+                        Optional<ECPrivateKey> signatureKeyPrivate)
+  {
+    SenderKeyStateStructure.SenderChainKey senderChainKeyStructure =
+        SenderKeyStateStructure.SenderChainKey.newBuilder()
+                                              .setIteration(iteration)
+                                              .setSeed(ByteString.copyFrom(chainKey))
+                                              .build();
+
+    SenderKeyStateStructure.SenderSigningKey.Builder signingKeyStructure =
+        SenderKeyStateStructure.SenderSigningKey.newBuilder()
+                                                .setPublic(ByteString.copyFrom(signatureKeyPublic.serialize()));
+
+    if (signatureKeyPrivate.isPresent()) {
+      signingKeyStructure.setPrivate(ByteString.copyFrom(signatureKeyPrivate.get().serialize()));
+    }
+
+    this.senderKeyStateStructure = SenderKeyStateStructure.newBuilder()
+                                                          .setSenderKeyId(id)
+                                                          .setSenderChainKey(senderChainKeyStructure)
+                                                          .setSenderSigningKey(signingKeyStructure)
+                                                          .build();
+  }
+
+  public SenderKeyState(SenderKeyStateStructure senderKeyStateStructure) {
+    this.senderKeyStateStructure = senderKeyStateStructure;
+  }
+
+  public int getKeyId() {
+    return senderKeyStateStructure.getSenderKeyId();
+  }
+
+  public SenderChainKey getSenderChainKey() {
+    return new SenderChainKey(senderKeyStateStructure.getSenderChainKey().getIteration(),
+                              senderKeyStateStructure.getSenderChainKey().getSeed().toByteArray());
+  }
+
+  public void setSenderChainKey(SenderChainKey chainKey) {
+    SenderKeyStateStructure.SenderChainKey senderChainKeyStructure =
+        SenderKeyStateStructure.SenderChainKey.newBuilder()
+                                              .setIteration(chainKey.getIteration())
+                                              .setSeed(ByteString.copyFrom(chainKey.getSeed()))
+                                              .build();
+
+    this.senderKeyStateStructure = senderKeyStateStructure.toBuilder()
+                                                          .setSenderChainKey(senderChainKeyStructure)
+                                                          .build();
+  }
+
+  public ECPublicKey getSigningKeyPublic() throws InvalidKeyException {
+    return Curve.decodePoint(senderKeyStateStructure.getSenderSigningKey()
+                                                    .getPublic()
+                                                    .toByteArray(), 0);
+  }
+
+  public ECPrivateKey getSigningKeyPrivate() {
+    return Curve.decodePrivatePoint(senderKeyStateStructure.getSenderSigningKey()
+                                                           .getPrivate().toByteArray());
+  }
+
+  public boolean hasSenderMessageKey(int iteration) {
+    for (SenderKeyStateStructure.SenderMessageKey senderMessageKey : senderKeyStateStructure.getSenderMessageKeysList()) {
+      if (senderMessageKey.getIteration() == iteration) return true;
+    }
+
+    return false;
+  }
+
+  public void addSenderMessageKey(SenderMessageKey senderMessageKey) {
+    SenderKeyStateStructure.SenderMessageKey senderMessageKeyStructure =
+        SenderKeyStateStructure.SenderMessageKey.newBuilder()
+                                                .setIteration(senderMessageKey.getIteration())
+                                                .setSeed(ByteString.copyFrom(senderMessageKey.getSeed()))
+                                                .build();
+
+    this.senderKeyStateStructure = this.senderKeyStateStructure.toBuilder()
+                                                               .addSenderMessageKeys(senderMessageKeyStructure)
+                                                               .build();
+  }
+
+  public SenderMessageKey removeSenderMessageKey(int iteration) {
+    List<SenderKeyStateStructure.SenderMessageKey>     keys     = new LinkedList<>(senderKeyStateStructure.getSenderMessageKeysList());
+    Iterator<SenderKeyStateStructure.SenderMessageKey> iterator = keys.iterator();
+
+    SenderKeyStateStructure.SenderMessageKey result = null;
+
+    while (iterator.hasNext()) {
+      SenderKeyStateStructure.SenderMessageKey senderMessageKey = iterator.next();
+
+      if (senderMessageKey.getIteration() == iteration) {
+        result = senderMessageKey;
+        iterator.remove();
+        break;
+      }
+    }
+
+    this.senderKeyStateStructure = this.senderKeyStateStructure.toBuilder()
+                                                               .clearSenderMessageKeys()
+                                                               .addAllSenderMessageKeys(keys)
+                                                               .build();
+
+    if (result != null) {
+      return new SenderMessageKey(result.getIteration(), result.getSeed().toByteArray());
+    } else {
+      return null;
+    }
+  }
+
+  public SenderKeyStateStructure getStructure() {
+    return senderKeyStateStructure;
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/groups/state/SenderKeyStore.java

@@ -0,0 +1,8 @@
+package org.whispersystems.libaxolotl.groups.state;
+
+import org.whispersystems.libaxolotl.groups.state.SenderKeyRecord;
+
+public interface SenderKeyStore {
+  public void storeSenderKey(String senderKeyId, SenderKeyRecord record);
+  public SenderKeyRecord loadSenderKey(String senderKeyId);
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/protocol/CiphertextMessage.java

@@ -21,8 +21,10 @@ public interface CiphertextMessage {
   public static final int UNSUPPORTED_VERSION = 1;
   public static final int CURRENT_VERSION     = 3;
 
-  public static final int WHISPER_TYPE = 2;
+  public static final int WHISPER_TYPE                = 2;
-  public static final int PREKEY_TYPE  = 3;
+  public static final int PREKEY_TYPE                 = 3;
+  public static final int SENDERKEY_TYPE              = 4;
+  public static final int SENDERKEY_DISTRIBUTION_TYPE = 5;
 
   // This should be the worst case (worse than V2).  So not always accurate, but good enough for padding.
   public static final int ENCRYPTED_MESSAGE_OVERHEAD = 53;

libaxolotl/src/main/java/org/whispersystems/libaxolotl/protocol/SenderKeyDistributionMessage.java

@@ -0,0 +1,56 @@
+package org.whispersystems.libaxolotl.protocol;
+
+import com.google.protobuf.ByteString;
+
+import org.whispersystems.libaxolotl.ecc.ECPublicKey;
+import org.whispersystems.libaxolotl.util.ByteUtil;
+
+public class SenderKeyDistributionMessage implements CiphertextMessage {
+
+  private final int         id;
+  private final int         iteration;
+  private final byte[]      chainKey;
+  private final ECPublicKey signatureKey;
+  private final byte[]      serialized;
+
+  public SenderKeyDistributionMessage(int id, int iteration, byte[] chainKey, ECPublicKey signatureKey) {
+    byte[] version = {ByteUtil.intsToByteHighAndLow(CURRENT_VERSION, CURRENT_VERSION)};
+
+    this.id           = id;
+    this.iteration    = iteration;
+    this.chainKey     = chainKey;
+    this.signatureKey = signatureKey;
+    this.serialized   = WhisperProtos.SenderKeyDistributionMessage.newBuilder()
+                                                                  .setId(id)
+                                                                  .setIteration(iteration)
+                                                                  .setChainKey(ByteString.copyFrom(chainKey))
+                                                                  .setSigningKey(ByteString.copyFrom(signatureKey.serialize()))
+                                                                  .build().toByteArray();
+  }
+
+  @Override
+  public byte[] serialize() {
+    return serialized;
+  }
+
+  @Override
+  public int getType() {
+    return SENDERKEY_DISTRIBUTION_TYPE;
+  }
+
+  public int getIteration() {
+    return iteration;
+  }
+
+  public byte[] getChainKey() {
+    return chainKey;
+  }
+
+  public ECPublicKey getSignatureKey() {
+    return signatureKey;
+  }
+
+  public int getId() {
+    return id;
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/protocol/SenderKeyMessage.java

@@ -0,0 +1,121 @@
+package org.whispersystems.libaxolotl.protocol;
+
+import com.google.protobuf.ByteString;
+import com.google.protobuf.InvalidProtocolBufferException;
+
+import org.whispersystems.libaxolotl.InvalidKeyException;
+import org.whispersystems.libaxolotl.InvalidMessageException;
+import org.whispersystems.libaxolotl.LegacyMessageException;
+import org.whispersystems.libaxolotl.ecc.Curve;
+import org.whispersystems.libaxolotl.ecc.ECPrivateKey;
+import org.whispersystems.libaxolotl.ecc.ECPublicKey;
+import org.whispersystems.libaxolotl.util.ByteUtil;
+
+import java.text.ParseException;
+
+public class SenderKeyMessage implements CiphertextMessage {
+
+  private static final int SIGNATURE_LENGTH = 64;
+
+  private final int         messageVersion;
+  private final int         keyId;
+  private final int         iteration;
+  private final byte[]      ciphertext;
+  private final byte[]      serialized;
+
+  public SenderKeyMessage(byte[] serialized) throws InvalidMessageException, LegacyMessageException {
+    try {
+      byte[][] messageParts = ByteUtil.split(serialized, 1, serialized.length - 1 - SIGNATURE_LENGTH, SIGNATURE_LENGTH);
+      byte     version      = messageParts[0][0];
+      byte[]   message      = messageParts[1];
+      byte[]   signature    = messageParts[2];
+
+      if (ByteUtil.highBitsToInt(version) < 3) {
+        throw new LegacyMessageException("Legacy message: " + ByteUtil.highBitsToInt(version));
+      }
+
+      if (ByteUtil.highBitsToInt(version) > CURRENT_VERSION) {
+        throw new InvalidMessageException("Unknown version: " + ByteUtil.highBitsToInt(version));
+      }
+
+      WhisperProtos.SenderKeyMessage senderKeyMessage = WhisperProtos.SenderKeyMessage.parseFrom(message);
+
+      if (!senderKeyMessage.hasId() ||
+          !senderKeyMessage.hasIteration() ||
+          !senderKeyMessage.hasCiphertext())
+      {
+        throw new InvalidMessageException("Incomplete message.");
+      }
+
+      this.serialized     = serialized;
+      this.messageVersion = ByteUtil.highBitsToInt(version);
+      this.keyId          = senderKeyMessage.getId();
+      this.iteration      = senderKeyMessage.getIteration();
+      this.ciphertext     = senderKeyMessage.getCiphertext().toByteArray();
+    } catch (InvalidProtocolBufferException | ParseException e) {
+      throw new InvalidMessageException(e);
+    }
+  }
+
+  public SenderKeyMessage(int keyId, int iteration, byte[] ciphertext, ECPrivateKey signatureKey) {
+    byte[] version = {ByteUtil.intsToByteHighAndLow(CURRENT_VERSION, CURRENT_VERSION)};
+    byte[] message = WhisperProtos.SenderKeyMessage.newBuilder()
+                                                   .setId(keyId)
+                                                   .setIteration(iteration)
+                                                   .setCiphertext(ByteString.copyFrom(ciphertext))
+                                                   .build().toByteArray();
+
+    byte[] signature = getSignature(signatureKey, ByteUtil.combine(version, message));
+
+    this.serialized       = ByteUtil.combine(version, message, signature);
+    this.messageVersion   = CURRENT_VERSION;
+    this.keyId            = keyId;
+    this.iteration        = iteration;
+    this.ciphertext       = ciphertext;
+  }
+
+  public int getKeyId() {
+    return keyId;
+  }
+
+  public int getIteration() {
+    return iteration;
+  }
+
+  public byte[] getCipherText() {
+    return ciphertext;
+  }
+
+  public void verifySignature(ECPublicKey signatureKey)
+      throws InvalidMessageException
+  {
+    try {
+      byte[][] parts    = ByteUtil.split(serialized, serialized.length - SIGNATURE_LENGTH, SIGNATURE_LENGTH);
+
+      if (!Curve.verifySignature(signatureKey, parts[0], parts[1])) {
+        throw new InvalidMessageException("Invalid signature!");
+      }
+
+    } catch (InvalidKeyException e) {
+      throw new InvalidMessageException(e);
+    }
+  }
+
+  private byte[] getSignature(ECPrivateKey signatureKey, byte[] serialized) {
+    try {
+      return Curve.calculateSignature(signatureKey, serialized);
+    } catch (InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+  @Override
+  public byte[] serialize() {
+    return serialized;
+  }
+
+  @Override
+  public int getType() {
+    return CiphertextMessage.SENDERKEY_TYPE;
+  }
+}

libaxolotl/src/main/java/org/whispersystems/libaxolotl/util/KeyHelper.java

@@ -116,4 +116,28 @@ public class KeyHelper {
     return new SignedPreKeyRecord(signedPreKeyId, System.currentTimeMillis(), keyPair, signature);
   }
 
+
+  public static ECKeyPair generateSenderSigningKey() {
+    return Curve.generateKeyPair();
+  }
+
+  public static byte[] generateSenderKey() {
+    try {
+      byte[] key = new byte[32];
+      SecureRandom.getInstance("SHA1PRNG").nextBytes(key);
+
+      return key;
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+  public static int generateSenderKeyId() {
+    try {
+      return SecureRandom.getInstance("SHA1PRNG").nextInt(Integer.MAX_VALUE);
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    }
+  }
+
 }