diff --git a/include/axolotl/message.hh b/include/axolotl/message.hh
index cfbb715..2b9bc99 100644
--- a/include/axolotl/message.hh
+++ b/include/axolotl/message.hh
@@ -37,6 +37,7 @@ struct MessageWriter {
 
 struct MessageReader {
     std::uint8_t version;
+    bool has_counter;
     std::uint32_t counter;
     std::uint8_t const * input; std::size_t input_length;
     std::uint8_t const * ratchet_key; std::size_t ratchet_key_length;
@@ -46,9 +47,8 @@ struct MessageReader {
 
 /**
  * Writes the message headers into the output buffer.
- * Returns a writer struct populated with pointers into the output buffer.
+ * Populates the writer struct with pointers into the output buffer.
  */
-
 void encode_message(
     MessageWriter & writer,
     std::uint8_t version,
@@ -62,13 +62,67 @@ void encode_message(
 /**
  * Reads the message headers from the input buffer.
  * Populates the reader struct with pointers into the input buffer.
- * On failure returns std::size_t(-1).
  */
-std::size_t decode_message(
+void decode_message(
     MessageReader & reader,
     std::uint8_t const * input, std::size_t input_length,
     std::size_t mac_length
 );
 
 
+struct PreKeyMessageWriter {
+    std::uint8_t * identity_key;
+    std::uint8_t * base_key;
+    std::uint8_t * message;
+};
+
+
+struct PreKeyMessageReader {
+    std::uint8_t version;
+    bool has_registration_id;
+    bool has_one_time_key_id;
+    std::uint32_t registration_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 * message; std::size_t message_length;
+};
+
+/**
+ * The length of the buffer needed to hold a message.
+ */
+std::size_t encode_one_time_key_message_length(
+    std::uint32_t registration_id,
+    std::uint32_t one_time_key_id,
+    std::size_t identity_key_length,
+    std::size_t base_key_length,
+    std::size_t message_length
+);
+
+/**
+ * Writes the message headers into the output buffer.
+ * Populates the writer struct with pointers into the output buffer.
+ */
+void encode_one_time_key_message(
+    PreKeyMessageWriter & writer,
+    std::uint8_t version,
+    std::uint32_t registration_id,
+    std::uint32_t one_time_key_id,
+    std::size_t identity_key_length,
+    std::size_t base_key_length,
+    std::size_t message_length,
+    std::uint8_t * output
+);
+
+
+/**
+ * Reads the message headers from the input buffer.
+ * Populates the reader struct with pointers into the input buffer.
+ */
+void decode_one_time_key_message(
+    PreKeyMessageReader & reader,
+    std::uint8_t const * input, std::size_t input_length
+);
+
+
 } // namespace axolotl
diff --git a/src/message.cpp b/src/message.cpp
index 46cadd8..fcedd07 100644
--- a/src/message.cpp
+++ b/src/message.cpp
@@ -81,6 +81,82 @@ static std::uint8_t const RATCHET_KEY_TAG = 012;
 static std::uint8_t const COUNTER_TAG = 020;
 static std::uint8_t const CIPHERTEXT_TAG = 042;
 
+std::uint8_t * encode(
+    std::uint8_t * pos,
+    std::uint8_t tag,
+    std::uint32_t value
+) {
+    *(pos++) = tag;
+    return varint_encode(pos, value);
+}
+
+std::uint8_t * encode(
+    std::uint8_t * pos,
+    std::uint8_t tag,
+    std::uint8_t * & value, std::size_t value_length
+) {
+    *(pos++) = tag;
+    pos = varint_encode(pos, value_length);
+    value = pos;
+    return pos + value_length;
+}
+
+std::uint8_t const * decode(
+    std::uint8_t const * pos, std::uint8_t const * end,
+    std::uint8_t tag,
+    std::uint32_t & value, bool & has_value
+) {
+    if (pos != end && *pos == tag) {
+        ++pos;
+        std::uint8_t const * value_start = pos;
+        pos = varint_skip(pos, end);
+        value = varint_decode<std::uint32_t>(value_start, pos);
+        has_value = true;
+    }
+    return pos;
+}
+
+
+std::uint8_t const * decode(
+    std::uint8_t const * pos, std::uint8_t const * end,
+    std::uint8_t tag,
+    std::uint8_t const * & value, std::size_t & value_length
+) {
+    if (pos != end && *pos == tag) {
+        ++pos;
+        std::uint8_t const * len_start = pos;
+        pos = varint_skip(pos, end);
+        std::size_t len = varint_decode<std::size_t>(len_start, pos);
+        if (len > end - pos) return end;
+        value = pos;
+        value_length = len;
+        pos += len;
+    }
+    return pos;
+}
+
+std::uint8_t const * skip_unknown(
+    std::uint8_t const * pos, std::uint8_t const * end
+) {
+    if (pos != end) {
+        uint8_t tag = *pos;
+        if (tag & 0x7 == 0) {
+            pos = varint_skip(pos, end);
+            pos = varint_skip(pos, end);
+        } else if (tag & 0x7 == 2) {
+            pos = varint_skip(pos, end);
+            std::uint8_t const * len_start = pos;
+            pos = varint_skip(pos, end);
+            std::size_t len = varint_decode<std::size_t>(len_start, pos);
+            if (len > end - pos) return end;
+            pos += len;
+        } else {
+            return end;
+        }
+    }
+    return pos;
+}
+
 } // namespace
 
 
@@ -109,75 +185,138 @@ void axolotl::encode_message(
 ) {
     std::uint8_t * pos = output;
     *(pos++) = version;
-    *(pos++) = COUNTER_TAG;
-    pos = varint_encode(pos, counter);
-    *(pos++) = RATCHET_KEY_TAG;
-    pos = varint_encode(pos, ratchet_key_length);
-    writer.ratchet_key = pos;
-    pos += ratchet_key_length;
-    *(pos++) = CIPHERTEXT_TAG;
-    pos = varint_encode(pos, ciphertext_length);
-    writer.ciphertext = pos;
-    pos += ciphertext_length;
+    pos = encode(pos, RATCHET_KEY_TAG, writer.ratchet_key, ratchet_key_length);
+    pos = encode(pos, COUNTER_TAG, counter);
+    pos = encode(pos, CIPHERTEXT_TAG, writer.ciphertext, ciphertext_length);
 }
 
 
-std::size_t axolotl::decode_message(
+void axolotl::decode_message(
     axolotl::MessageReader & reader,
     std::uint8_t const * input, std::size_t input_length,
     std::size_t mac_length
 ) {
     std::uint8_t const * pos = input;
     std::uint8_t const * end = input + input_length - mac_length;
-    std::uint8_t flags = 0;
-    std::size_t result = std::size_t(-1);
-    if (pos == end) return result;
+    std::uint8_t const * unknown = NULL;
+
+    if (pos == end) return;
     reader.version = *(pos++);
+    reader.input = input;
+    reader.input_length = input_length;
+    reader.has_counter = false;
+    reader.ratchet_key = NULL;
+    reader.ciphertext = NULL;
+
     while (pos != end) {
-        uint8_t tag = *(pos);
-        if (tag == COUNTER_TAG) {
-            ++pos;
-            std::uint8_t const * counter_start = pos;
-            pos = varint_skip(pos, end);
-            reader.counter = varint_decode<std::uint32_t>(counter_start, pos);
-            flags |= 1;
-        } else if (tag == RATCHET_KEY_TAG) {
-            ++pos;
-            std::uint8_t const * len_start = pos;
-            pos = varint_skip(pos, end);
-            std::size_t len = varint_decode<std::size_t>(len_start, pos);
-            if (len > end - pos) return result;
-            reader.ratchet_key_length = len;
-            reader.ratchet_key = pos;
-            pos += len;
-            flags |= 2;
-        } else if (tag == CIPHERTEXT_TAG) {
-            ++pos;
-            std::uint8_t const * len_start = pos;
-            pos = varint_skip(pos, end);
-            std::size_t len = varint_decode<std::size_t>(len_start, pos);
-            if (len > end - pos) return result;
-            reader.ciphertext_length = len;
-            reader.ciphertext = pos;
-            pos += len;
-            flags |= 4;
-        } else if (tag & 0x7 == 0) {
-            pos = varint_skip(pos, end);
-            pos = varint_skip(pos, end);
-        } else if (tag & 0x7 == 2) {
-            std::uint8_t const * len_start = pos;
-            pos = varint_skip(pos, end);
-            std::size_t len = varint_decode<std::size_t>(len_start, pos);
-            if (len > end - pos) return result;
-            pos += len;
-        } else {
-            return std::size_t(-1);
+        pos = decode(
+            pos, end, RATCHET_KEY_TAG,
+            reader.ratchet_key, reader.ratchet_key_length
+        );
+        pos = decode(
+            pos, end, COUNTER_TAG,
+            reader.counter, reader.has_counter
+        );
+        pos = decode(
+            pos, end, CIPHERTEXT_TAG,
+            reader.ciphertext, reader.ciphertext_length
+        );
+        if (unknown == pos) {
+            pos == skip_unknown(pos, end);
         }
+        unknown = pos;
+    }
+}
+
+
+namespace {
+
+static std::uint8_t const REGISTRATION_ID_TAG = 050;
+static std::uint8_t const ONE_TIME_KEY_ID_TAG = 010;
+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;
+
+} // namespace
+
+
+std::size_t axolotl::encode_one_time_key_message_length(
+    std::uint32_t registration_id,
+    std::uint32_t one_time_key_id,
+    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(registration_id);
+    length += 1 + varint_length(one_time_key_id);
+    length += 1 + varstring_length(identity_key_length);
+    length += 1 + varstring_length(base_key_length);
+    length += 1 + varstring_length(message_length);
+    return length;
+}
+
+
+void axolotl::encode_one_time_key_message(
+    axolotl::PreKeyMessageWriter & writer,
+    std::uint8_t version,
+    std::uint32_t registration_id,
+    std::uint32_t one_time_key_id,
+    std::size_t identity_key_length,
+    std::size_t base_key_length,
+    std::size_t message_length,
+    std::uint8_t * output
+) {
+    std::uint8_t * pos = output;
+    *(pos++) = version;
+    pos = encode(pos, REGISTRATION_ID_TAG, registration_id);
+    pos = encode(pos, ONE_TIME_KEY_ID_TAG, one_time_key_id);
+    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);
+}
+
+
+void axolotl::decode_one_time_key_message(
+    PreKeyMessageReader & reader,
+    std::uint8_t const * input, std::size_t input_length
+) {
+    std::uint8_t const * pos = input;
+    std::uint8_t const * end = input + input_length;
+    std::uint8_t const * unknown = NULL;
+
+    if (pos == end) return;
+    reader.version = *(pos++);
+    reader.has_registration_id = false;
+    reader.has_one_time_key_id = false;
+    reader.identity_key = NULL;
+    reader.base_key = NULL;
+    reader.message = NULL;
+
+    while (pos != end) {
+        pos = decode(
+            pos, end, REGISTRATION_ID_TAG,
+            reader.registration_id, reader.has_registration_id
+        );
+        pos = decode(
+            pos, end, ONE_TIME_KEY_ID_TAG,
+            reader.one_time_key_id, reader.has_one_time_key_id
+        );
+        pos = decode(
+            pos, end, BASE_KEY_TAG,
+            reader.base_key, reader.base_key_length
+        );
+        pos = decode(
+            pos, end, IDENTITY_KEY_TAG,
+            reader.identity_key, reader.identity_key_length
+        );
+        pos = decode(
+            pos, end, MESSAGE_TAG,
+            reader.message, reader.message_length
+        );
+        if (unknown == pos) {
+            pos == skip_unknown(pos, end);
+        }
+        unknown = pos;
     }
-    if (flags == 0x7) {
-        reader.input = input;
-        reader.input_length = input_length;
-        return std::size_t(pos - input);
-    }
-    return result;
 }
diff --git a/src/ratchet.cpp b/src/ratchet.cpp
index cd4f8f7..cbafa83 100644
--- a/src/ratchet.cpp
+++ b/src/ratchet.cpp
@@ -444,7 +444,7 @@ std::size_t axolotl::Session::decrypt(
     }
 
     axolotl::MessageReader reader;
-    std::size_t body_length = axolotl::decode_message(
+    axolotl::decode_message(
         reader, input, input_length, ratchet_cipher.mac_length()
     );
 
@@ -453,7 +453,12 @@ std::size_t axolotl::Session::decrypt(
         return std::size_t(-1);
     }
 
-    if (body_length == size_t(-1) || reader.ratchet_key_length != KEY_LENGTH) {
+    if (!reader.has_counter || !reader.ratchet_key || !reader.ciphertext) {
+        last_error = axolotl::ErrorCode::BAD_MESSAGE_FORMAT;
+        return std::size_t(-1);
+    }
+
+    if (reader.ratchet_key_length != KEY_LENGTH) {
         last_error = axolotl::ErrorCode::BAD_MESSAGE_FORMAT;
         return std::size_t(-1);
     }
diff --git a/tests/test_message.cpp b/tests/test_message.cpp
index 9c0ab4a..b46b15a 100644
--- a/tests/test_message.cpp
+++ b/tests/test_message.cpp
@@ -17,8 +17,8 @@
 
 int main() {
 
-std::uint8_t message1[36] = "\x03\n\nratchetkey\x10\x01\"\nciphertexthmacsha2";
-std::uint8_t message2[36] = "\x03\x10\x01\n\nratchetkey\"\nciphertexthmacsha2";
+std::uint8_t message1[36] = "\x03\x10\x01\n\nratchetkey\"\nciphertexthmacsha2";
+std::uint8_t message2[36] = "\x03\n\nratchetkey\x10\x01\"\nciphertexthmacsha2";
 std::uint8_t ratchetkey[11] = "ratchetkey";
 std::uint8_t ciphertext[11] = "ciphertext";
 std::uint8_t hmacsha2[9] = "hmacsha2";
@@ -31,6 +31,7 @@ axolotl::MessageReader reader;
 axolotl::decode_message(reader, message1, 35, 8);
 
 assert_equals(std::uint8_t(3), reader.version);
+assert_equals(true, reader.has_counter);
 assert_equals(std::uint32_t(1), reader.counter);
 assert_equals(std::size_t(10), reader.ratchet_key_length);
 assert_equals(std::size_t(10), reader.ciphertext_length);