PreKey is being removed after the first message decryption

[gediminasiv]

Hello!

I am trying to implement your protocol to our own application and encountered an unexpected problem.

Lets imagine a standard situation. User A sends message to User B. Operation is standard - User A encrypts a message using User B prekeys, and then User B downloads that message from server, decrypts it with his prekeys.

The problem occurs when User B doesn't reply to User A message, and User A sends another message. Message type is still 3, so it has to be decrypted with decryptPreKeyWhisperMessage. Problem is that after first decryption of prekey message, local prekey of User B is deleted, so it cannot do any more decryption for User A messages.

If I leave removePreKey function in my storage class empty, everything works just fine.

The code is pretty basic, taken from your samples:

 try {
      if (message.type === 1) {
        plaintext = await cipher.decryptWhisperMessage(message.body!, "binary");
      } else if (message.type === 3) {
        plaintext = await cipher.decryptPreKeyWhisperMessage(
          message.body!,
          "binary"
        );
      }

      stringPlaintext = new TextDecoder().decode(new Uint8Array(plaintext));
    } catch (e) {
      console.log(
        `Decrypting unsuccessful. Message shouldn't be saved to server.`
      );
      throw e;
    }

Is this a bug, or am I missing something out?

[rolfeschmidt]

@gediminasiv - thanks for bringing this up. I replied on the libsignal-protocol-typescript issue. At first glance I think the SDK should delay removal of the pre-key, but want to confirm.