private static void HandleNormalTextMessage(MetaMessage metaMessage)
{
// TODO actually do database stuff
string senderUuid = metaMessage.OtherUuid;
// TODO make much better!
ChatState state = GlobalKeyStore.Instance.ChatStates[senderUuid];
NormalTextMessage message = JsonConvert.DeserializeObject <NormalTextMessage>(metaMessage.Payload);
MessageHeader header = new MessageHeader
{
DhRatchetKey = new Curve25519KeyPair(Convert.FromBase64String(message.DhKeyBase64), false, false),
PreviousCount = message.PreviousCount,
MessageNumber = message.MessageNumber
};
byte[] associatedData = null;
if (!(message.AssociatedDataBase64 is null))
{
associatedData = Convert.FromBase64String(message.AssociatedDataBase64);
}
byte[] decryptedData = RatchetDecrypt(state, header, Convert.FromBase64String(message.EncryptedTextBase64),
associatedData);
string decryptedText = Encoding.UTF8.GetString(decryptedData);
Device.BeginInvokeOnMainThread(async() =>
{
await Application.Current.MainPage.DisplayAlert("Message received!", $"From: {senderUuid}\nContent: {decryptedText}", "Ok");
});
}
private static (byte[], MessageHeader) RatchetEncrypt(ChatState state, byte[] plainText, byte[] associatedData = null)
{
byte[] messageKey, derivedAssociatedData;
(state.SendingChainKey, messageKey, derivedAssociatedData) =
CryptoUtils.RatchetChainKey(state.SendingChainKey);
MessageHeader header = new MessageHeader
{
PreviousCount = state.PreviousCount,
MessageNumber = state.CountSent,
DhRatchetKey = state.DhSendingKeyPair
};
state.CountSent++;
return(
CryptoUtils.EncryptWithMessageKey(messageKey, plainText,
CombineAdAndHeader(associatedData ?? derivedAssociatedData, header)), header);
}
private static byte[] RatchetDecrypt(ChatState state, MessageHeader header, byte[] cipherText,
byte[] associatedData = null)
{
// TODO do ratcheting with copy before it's verified to have worked
string missedMessagesKey = Convert.ToBase64String(header.DhRatchetKey.XPublicKey) + header.MessageNumber;
if (state.MissedMessages.TryGetValue(missedMessagesKey, out (byte[], byte[])skippedMessageTuple))
{
state.MissedMessages.Remove(missedMessagesKey);
(byte[] skippedMessageKey, byte[] skippedAssociatedData) = skippedMessageTuple;
return(CryptoUtils.DecryptWithMessageKey(skippedMessageKey, cipherText, CombineAdAndHeader(associatedData ?? skippedAssociatedData, header)));
}
if (state.DhReceivingKey is null || !header.DhRatchetKey.XPublicKey.SequenceEqual(state.DhReceivingKey.XPublicKey))
{
SkipMessageKeys(state, header.PreviousCount);
// Ratchet the chat state
state.PreviousCount = state.CountSent;
state.CountSent = 0;
state.CountReceived = 0;
state.DhReceivingKey = header.DhRatchetKey;
(state.RootKey, state.ReceivingChainKey) = CryptoUtils.RatchetRootKey(state.RootKey,
state.DhSendingKeyPair.CalculateSharedSecret(state.DhReceivingKey));
state.DhSendingKeyPair = new Curve25519KeyPair();
(state.RootKey, state.SendingChainKey) = CryptoUtils.RatchetRootKey(state.RootKey,
state.DhSendingKeyPair.CalculateSharedSecret(state.DhReceivingKey));
}
SkipMessageKeys(state, header.MessageNumber);
byte[] messageKey, derivedAssociatedData;
(state.ReceivingChainKey, messageKey, derivedAssociatedData) =
CryptoUtils.RatchetChainKey(state.ReceivingChainKey);
state.CountReceived++;
return(CryptoUtils.DecryptWithMessageKey(messageKey, cipherText, CombineAdAndHeader(associatedData ?? derivedAssociatedData, header)));
}
private static void SkipMessageKeys(ChatState state, uint upTo)
{
if (state.CountReceived + 50 < upTo)
{
throw new Exception("MAX_SKIP exceeded");
}
if (state.ReceivingChainKey is null)
{
return;
}
while (state.CountReceived < upTo)
{
byte[] messageKey, associatedData;
(state.ReceivingChainKey, messageKey, associatedData) =
CryptoUtils.RatchetChainKey(state.ReceivingChainKey);
string missedMessagesKey = Convert.ToBase64String(state.DhReceivingKey.XPublicKey) + state.CountReceived;
state.MissedMessages[missedMessagesKey] = (messageKey, associatedData);
state.CountReceived++;
}
}
