diff --git a/include/olm/account.hh b/include/olm/account.hh
index b51fc1e..98b6b56 100644
--- a/include/olm/account.hh
+++ b/include/olm/account.hh
@@ -100,7 +100,7 @@ struct Account {
     );
 
     OneTimeKey const * lookup_key(
-        std::uint32_t id
+        Curve25519PublicKey const & public_key
     );
 
     std::size_t remove_key(
diff --git a/include/olm/message.hh b/include/olm/message.hh
index fefdd20..5ce0a62 100644
--- a/include/olm/message.hh
+++ b/include/olm/message.hh
@@ -73,16 +73,16 @@ void decode_message(
 struct PreKeyMessageWriter {
     std::uint8_t * identity_key;
     std::uint8_t * base_key;
+    std::uint8_t * one_time_key;
     std::uint8_t * message;
 };
 
 
 struct PreKeyMessageReader {
     std::uint8_t version;
-    bool has_one_time_key_id;
-    std::uint32_t one_time_key_id;
     std::uint8_t const * identity_key; std::size_t identity_key_length;
     std::uint8_t const * base_key; std::size_t base_key_length;
+    std::uint8_t const * one_time_key; std::size_t one_time_key_length;
     std::uint8_t const * message; std::size_t message_length;
 };
 
@@ -91,9 +91,9 @@ struct PreKeyMessageReader {
  * The length of the buffer needed to hold a message.
  */
 std::size_t encode_one_time_key_message_length(
-    std::uint32_t one_time_key_id,
     std::size_t identity_key_length,
     std::size_t base_key_length,
+    std::size_t one_time_key_length,
     std::size_t message_length
 );
 
@@ -105,9 +105,9 @@ std::size_t encode_one_time_key_message_length(
 void encode_one_time_key_message(
     PreKeyMessageWriter & writer,
     std::uint8_t version,
-    std::uint32_t one_time_key_id,
     std::size_t identity_key_length,
     std::size_t base_key_length,
+    std::size_t one_time_key_length,
     std::size_t message_length,
     std::uint8_t * output
 );
diff --git a/include/olm/olm.hh b/include/olm/olm.hh
index ab71689..891f953 100644
--- a/include/olm/olm.hh
+++ b/include/olm/olm.hh
@@ -187,7 +187,6 @@ size_t olm_create_outbound_session(
     OlmSession * session,
     OlmAccount * account,
     void const * their_identity_key, size_t their_identity_key_length,
-    unsigned their_one_time_key_id,
     void const * their_one_time_key, size_t their_one_time_key_length,
     void const * random, size_t random_length
 );
diff --git a/include/olm/session.hh b/include/olm/session.hh
index a0aff08..4d674e5 100644
--- a/include/olm/session.hh
+++ b/include/olm/session.hh
@@ -44,9 +44,9 @@ struct Session {
 
     RemoteKey alice_identity_key;
     Curve25519PublicKey alice_base_key;
+    Curve25519PublicKey bob_one_time_key;
     std::uint32_t bob_one_time_key_id;
 
-
     std::size_t new_outbound_session_random_length();
 
     std::size_t new_outbound_session(
diff --git a/src/account.cpp b/src/account.cpp
index 54edf6e..297d2f4 100644
--- a/src/account.cpp
+++ b/src/account.cpp
@@ -18,10 +18,12 @@
 
 
 olm::OneTimeKey const * olm::Account::lookup_key(
-    std::uint32_t id
+    olm::Curve25519PublicKey const & public_key
 ) {
     for (olm::OneTimeKey const & key : one_time_keys) {
-        if (key.id == id) return &key;
+        if (0 == memcmp(key.key.public_key, public_key.public_key, 32)) {
+            return &key;
+        }
     }
     return 0;
 }
diff --git a/src/message.cpp b/src/message.cpp
index af43916..93473b9 100644
--- a/src/message.cpp
+++ b/src/message.cpp
@@ -232,7 +232,7 @@ void olm::decode_message(
 
 namespace {
 
-static std::uint8_t const ONE_TIME_KEY_ID_TAG = 010;
+static std::uint8_t const ONE_TIME_KEY_ID_TAG = 012;
 static std::uint8_t const BASE_KEY_TAG = 022;
 static std::uint8_t const IDENTITY_KEY_TAG = 032;
 static std::uint8_t const MESSAGE_TAG = 042;
@@ -241,13 +241,13 @@ static std::uint8_t const MESSAGE_TAG = 042;
 
 
 std::size_t olm::encode_one_time_key_message_length(
-    std::uint32_t one_time_key_id,
+    std::size_t one_time_key_length,
     std::size_t identity_key_length,
     std::size_t base_key_length,
     std::size_t message_length
 ) {
     std::size_t length = VERSION_LENGTH;
-    length += 1 + varint_length(one_time_key_id);
+    length += 1 + varstring_length(one_time_key_length);
     length += 1 + varstring_length(identity_key_length);
     length += 1 + varstring_length(base_key_length);
     length += 1 + varstring_length(message_length);
@@ -258,15 +258,15 @@ std::size_t olm::encode_one_time_key_message_length(
 void olm::encode_one_time_key_message(
     olm::PreKeyMessageWriter & writer,
     std::uint8_t version,
-    std::uint32_t one_time_key_id,
     std::size_t identity_key_length,
     std::size_t base_key_length,
+    std::size_t one_time_key_length,
     std::size_t message_length,
     std::uint8_t * output
 ) {
     std::uint8_t * pos = output;
     *(pos++) = version;
-    pos = encode(pos, ONE_TIME_KEY_ID_TAG, one_time_key_id);
+    pos = encode(pos, ONE_TIME_KEY_ID_TAG, writer.one_time_key, one_time_key_length);
     pos = encode(pos, BASE_KEY_TAG, writer.base_key, base_key_length);
     pos = encode(pos, IDENTITY_KEY_TAG, writer.identity_key, identity_key_length);
     pos = encode(pos, MESSAGE_TAG, writer.message, message_length);
@@ -283,7 +283,7 @@ void olm::decode_one_time_key_message(
 
     if (pos == end) return;
     reader.version = *(pos++);
-    reader.has_one_time_key_id = false;
+    reader.one_time_key = nullptr;
     reader.identity_key = nullptr;
     reader.base_key = nullptr;
     reader.message = nullptr;
@@ -291,7 +291,7 @@ void olm::decode_one_time_key_message(
     while (pos != end) {
         pos = decode(
             pos, end, ONE_TIME_KEY_ID_TAG,
-            reader.one_time_key_id, reader.has_one_time_key_id
+            reader.one_time_key, reader.one_time_key_length
         );
         pos = decode(
             pos, end, BASE_KEY_TAG,
diff --git a/src/olm.cpp b/src/olm.cpp
index a9735a5..c1bfd4d 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -425,7 +425,6 @@ size_t olm_create_outbound_session(
     OlmSession * session,
     OlmAccount * account,
     void const * their_identity_key, size_t their_identity_key_length,
-    unsigned their_one_time_key_id,
     void const * their_one_time_key, size_t their_one_time_key_length,
     void const * random, size_t random_length
 ) {
@@ -442,7 +441,6 @@ size_t olm_create_outbound_session(
         from_c(their_identity_key), their_identity_key_length,
         identity_key.public_key
     );
-    one_time_key.id = their_one_time_key_id;
     olm::decode_base64(
         from_c(their_one_time_key), their_one_time_key_length,
         one_time_key.key.public_key
diff --git a/src/session.cpp b/src/session.cpp
index 0457042..df78e50 100644
--- a/src/session.cpp
+++ b/src/session.cpp
@@ -77,7 +77,7 @@ std::size_t olm::Session::new_outbound_session(
     alice_identity_key.id = 0;
     alice_identity_key.key = local_account.identity_keys.curve25519_key;
     alice_base_key = base_key;
-    bob_one_time_key_id = one_time_key.id;
+    bob_one_time_key = one_time_key.key;
 
     std::uint8_t shared_secret[96];
 
@@ -112,7 +112,8 @@ bool check_message_fields(
     ok = ok && reader.message;
     ok = ok && reader.base_key;
     ok = ok && reader.base_key_length == KEY_LENGTH;
-    ok = ok && reader.has_one_time_key_id;
+    ok = ok && reader.one_time_key;
+    ok = ok && reader.one_time_key_length == KEY_LENGTH;
     return ok;
 }
 
@@ -145,15 +146,15 @@ std::size_t olm::Session::new_inbound_session(
 
     std::memcpy(alice_identity_key.key.public_key, reader.identity_key, 32);
     std::memcpy(alice_base_key.public_key, reader.base_key, 32);
-    bob_one_time_key_id = reader.one_time_key_id;
+    std::memcpy(bob_one_time_key.public_key, reader.one_time_key, 32);
     olm::Curve25519PublicKey ratchet_key;
     std::memcpy(ratchet_key.public_key, message_reader.ratchet_key, 32);
 
-    olm::OneTimeKey const * bob_one_time_key = local_account.lookup_key(
-        bob_one_time_key_id
+    olm::OneTimeKey const * our_one_time_key = local_account.lookup_key(
+        bob_one_time_key
     );
 
-    if (!bob_one_time_key) {
+    if (!our_one_time_key) {
         last_error = olm::ErrorCode::BAD_MESSAGE_KEY_ID;
         return std::size_t(-1);
     }
@@ -161,14 +162,14 @@ std::size_t olm::Session::new_inbound_session(
     std::uint8_t shared_secret[96];
 
     olm::curve25519_shared_secret(
-        bob_one_time_key->key, alice_identity_key.key, shared_secret
+        our_one_time_key->key, alice_identity_key.key, shared_secret
     );
     olm::curve25519_shared_secret(
         local_account.identity_keys.curve25519_key,
         alice_base_key, shared_secret + 32
     );
     olm::curve25519_shared_secret(
-        bob_one_time_key->key, alice_base_key, shared_secret + 64
+        our_one_time_key->key, alice_base_key, shared_secret + 64
     );
 
     ratchet.initialise_as_bob(shared_secret, 96, ratchet_key);
@@ -194,7 +195,9 @@ bool olm::Session::matches_inbound_session(
     same = same && 0 == std::memcmp(
         reader.base_key, alice_base_key.public_key, KEY_LENGTH
     );
-    same = same && reader.one_time_key_id == bob_one_time_key_id;
+    same = same && 0 == std::memcmp(
+        reader.one_time_key, bob_one_time_key.public_key, KEY_LENGTH
+    );
     return same;
 }
 
@@ -220,7 +223,7 @@ std::size_t olm::Session::encrypt_message_length(
     }
 
     return encode_one_time_key_message_length(
-        bob_one_time_key_id,
+        KEY_LENGTH,
         KEY_LENGTH,
         KEY_LENGTH,
         message_length
@@ -254,12 +257,15 @@ std::size_t olm::Session::encrypt(
         encode_one_time_key_message(
             writer,
             PROTOCOL_VERSION,
-            bob_one_time_key_id,
+            KEY_LENGTH,
             KEY_LENGTH,
             KEY_LENGTH,
             message_body_length,
             message
         );
+        std::memcpy(
+            writer.one_time_key, bob_one_time_key.public_key, KEY_LENGTH
+        );
         std::memcpy(
             writer.identity_key, alice_identity_key.key.public_key, KEY_LENGTH
         );
@@ -358,6 +364,7 @@ std::size_t olm::pickle_length(
     length += olm::pickle_length(value.alice_identity_key.id);
     length += olm::pickle_length(value.alice_identity_key.key);
     length += olm::pickle_length(value.alice_base_key);
+    length += olm::pickle_length(value.bob_one_time_key);
     length += olm::pickle_length(value.bob_one_time_key_id);
     length += olm::pickle_length(value.ratchet);
     return length;
@@ -372,6 +379,7 @@ std::uint8_t * olm::pickle(
     pos = olm::pickle(pos, value.alice_identity_key.id);
     pos = olm::pickle(pos, value.alice_identity_key.key);
     pos = olm::pickle(pos, value.alice_base_key);
+    pos = olm::pickle(pos, value.bob_one_time_key);
     pos = olm::pickle(pos, value.bob_one_time_key_id);
     pos = olm::pickle(pos, value.ratchet);
     return pos;
@@ -386,6 +394,7 @@ std::uint8_t const * olm::unpickle(
     pos = olm::unpickle(pos, end, value.alice_identity_key.id);
     pos = olm::unpickle(pos, end, value.alice_identity_key.key);
     pos = olm::unpickle(pos, end, value.alice_base_key);
+    pos = olm::unpickle(pos, end, value.bob_one_time_key);
     pos = olm::unpickle(pos, end, value.bob_one_time_key_id);
     pos = olm::unpickle(pos, end, value.ratchet);
     return pos;
diff --git a/tests/test_olm.cpp b/tests/test_olm.cpp
index 8ea2462..7c3ec87 100644
--- a/tests/test_olm.cpp
+++ b/tests/test_olm.cpp
@@ -89,7 +89,7 @@ mock_random_a(a_rand, sizeof(a_rand));
 assert_not_equals(std::size_t(-1), ::olm_create_outbound_session(
     a_session, a_account,
     b_id_keys + 88, 43,
-    ::atol((char *)(b_ot_keys + 62)), b_ot_keys + 74, 43,
+    b_ot_keys + 74, 43,
     a_rand, sizeof(a_rand)
 ));
 
@@ -193,7 +193,7 @@ mock_random_a(a_rand, sizeof(a_rand));
 assert_not_equals(std::size_t(-1), ::olm_create_outbound_session(
     a_session, a_account,
     b_id_keys + 88, 43,
-    ::atol((char *)(b_ot_keys + 62)), b_ot_keys + 74, 43,
+    b_ot_keys + 74, 43,
     a_rand, sizeof(a_rand)
 ));