chat_proto/specs/inbox.md

---
title: INBOX
name: Inbound message queues
category: Standards Track
status: raw
tags: chat
editor: Jazz Alyxzander<jazz@status.im>
contributors:
---
# Abstract


# Background / Rationale / Motivation
Clients must be able to receive frames before conversations can be initialized. While its possible to coordinate a content topic out of band, that becomes limiting factor to growth. 


# Theory / Semantics

Inboxes are inbound only conversation types, which allow a client to receive messages from contacts. 
An inbox does not have a defined set of participants, and is used to receive messages when there is not an established channel between contacts. 

An inbox does not have an inherent keypair or identity associated with it - it's an agreed upon location to recieve messages. 


## Inbox Identifiers

Inboxes are Identified by a 

Inboxes do not need to be "created", and there is no required initialization. 


## Content Topic

// TODO: Inbox Topics will be defined in ContactBundles, allowing for dynamic topic usage
All clients must listen for messages posted with the content topic `/inbox/<inbox_address>`

`lower_hex(hash(/inbox/<inbox_address>)`



## Accepted types



## Encryption

All Frames sent to the Inbox MUST be encrypted to maintain message confidentiality. 

This protocol uses a reversed variant of the [KN noise handshake](https://noiseexplorer.com/patterns/KN/) to secure inbound messages.

 ```noise
KNfallback:
  <- e, s
  ...
  -> e, ee, es	
 ```

In this case the responder provides both `s` and `e` out of band. 

The handshakes primary purpose is to provide sender confidentiality, with some forward secrecy. The handshake is similar to a one way N handshake with a recipient side ephemeral key.   

Note this channel does not not provide sender authentication, and should only be used to implement a confidential message delivery with some forward secrecy. This limitation is intentional to maintain O-RTT encryption. As this is an inbound pathway further messages to establish mutual authentication with identity hiding would be wasteful. 

### Ciphersuite

The noise handshake is implemented with the following functions:

**DH:** X25519
**cipher:** AEAD_CHACHA20_POLY1305 
**hash:** BLAKE2s 

The noise protocol name would then be `Noise_KNfallback_25519_ChaChaPoly_BLAKE2s`

This protocol opts for 32bit variants to optimize for mobile and resource constrained environments.

### Endianness
[TODO: The Noiseprotocol specification recommends BigEndian length fields - Need to define if this protocol will deviate]

## Framing 

[TODO: Is there benefit to using SDS in this case? If all messages are invites and communication occurs else where, is this just wasting bytes?]
```mermaid 
flowchart TD
    UmbraEnvelopeV1 <--> EncryptedPayload
    EncryptedPayload <--> D{En/Decrypt}
    D <--> ReliableBytes
    ReliableBytes --> InboxV1Frame

```

### EncryptedBytes

The EncryptedBytes message is a self-describing wrapper for all encrypted payloads. As the protocol grows it will include potentially different encryption mechanisms. This message type makes no assumptions about the encryption used an allows new conversation types to use the same messaging framework.




## Wire Format Specification / Syntax
The wire format is specified using protocol buffers v3.

```protobuf

message InboxV1Frame {
    string recipient = 1;
    oneof frame_type {
        ... supported invite types
    }
}

message EncryptedBytes {

    oneof encryption {
        bytes encrypted_bytes=1;
        Plaintext plaintext = 2;
		Ecies ecies = 3;
    }
   
    
    message Ecies {
        bytes encrypted_bytes=1;
        bytes ephemeral_pubkey = 2;
        bytes tag = 3;
        
    }

    message Plaintext {
        bytes payload=1;
    }
}

```

## Implementation Suggestions (optional)
An optional *implementation suggestions* section may provide suggestions on how to approach implementation details, and, 
if available, point to existing implementations for reference.


## (Further Optional Sections)


## Security/Privacy Considerations
The encryption scheme used does not provide any sender authentication. Messages sent over this pathway need to validate the sender before trusting any of the contents. 

## Copyright

Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).

## References

A list of references.