SmallMail is an anonymous/pseudonymous distributed mail system. It is designed with privacy in mind. This document describes the protocol between a SmallMailServer and a SmallMailClient that allows users of SmallMailClients to exchange messages among eachother.

Addressing
In SmallMail every user can create as many mailboxes as he likes. Each mailbox has a distinctive address. All messages in SmallMail are encrypted (public key cryptography), so each mailbox is associated with a (public, private) key pair.
Key management
At the moment the protocol does not provide for exchange of public keys. For now we suggest “sneakernet” exchange of (signed) public keys between users. In a future version there will be a way look up keys on the server.

Errors
Errors are signaled by sending
“ERR” <message> at any place where a command or reply is expected.
(NotImplemented, BadParam, BadCommand, ServerError, ProtocolError, NoMessage)

Client-Server Protocol
SSL-TCP connection. Server presents certificate; self-signed certificates are fine. Server and client should at least support SSLv3.
Commands and arguments should be terminated with a single line feed (’\n’) and every line should be sent as one TLS record.
Big (binary) arguments, like message bodies that have their size given with the preceding command and can be sent in multiple chunks; they are NOT terminated with a line feed

Create mailbox
Generate key pair
Mailbox name is assigned by server (public key fingerprint)
C->S: “CRID” <keysize>
C->S: <public key> [binary]
S->C: “IDCR” <mailbox> | “REJECT”
The key will be sent in binary PGP format
Note: no error recovery

Send message
C->S: “SEND” <mailbox> <size>
S->C: “ACCEPT” | “REJECT”
After ACCEPT:
C->S: “BODY” <size>
C->S: <message body> [binary]
S->C: “RCVD”
Note: no error recovery

Client identification handshake
The server will start an identification handshake when a client wants access to a mailbox. It is a challenge-response protocol. It is server policy how often an identification handshake is performed; this can be  for every command, once per session or the server might implement some time-out.
S->C: “CHAL” <challenge>
C->S: “RESP” <respsize>
C->S: <response body> [binary]
After a failed challenge the server will respond with “REJECT” and close the connection.

List messages
C->S: “LIST” <mailbox>
[[identification handshake]]
Per message in mailbox:
S->C: “MSG” <msgid> <size>
S->C: “LSTD”

Receive message
C->S: “RECV” <mailbox> <msgid>
[[identification handshake]]
S->C: “BODY” <size>
S->C: <message body> [binary]

Get part of a message
C->S: “RPRT” <mailbox> <msgid> <offset> (<part size> | “END”)
[[identification handshake]]
S->C: “PART” <offset> <part size>
S->C: <message part> [binary]

Delete message
C->S: “DELE” <mailbox> <msgid>
[[identification handshake]]
S->C: “DLTD”