The first step to any of this is being able to understand messages sent through the network. To do this, you need to build a parser. Each message can be considered to have three segments: a header, metadata, and payload. The header is used to figure out the size of a message, as well as how to divide up its various packets. The metadata is used to assist propagation functions. And the payload is what the user on the other end receives.

A more formal definition would look like:

Size of message    - 4 (big-endian) bytes defining the size of the message
------------------------All below may be compressed------------------------
Size of packet 0   - 4 bytes defining the plaintext size of packet 0
Pathfinding header - [broadcast, whisper, renegotiate]
Size of packet 1   - 4 bytes defining the plaintext size of packet 1
Sender ID          - A base_58 SHA384-based ID for the sender
Size of packet 2   - 4 bytes defining the plaintext size of packet 2
Message ID         - A base_58 SHA384-based ID for the message packets
Size of packet 3   - 4 bytes defining the plaintext size of packet 3
Timestamp          - A base_58 unix UTC timestamp of initial broadcast
Size of packet 4   - 4 bytes defining the plaintext size of packet 4
Payload header     - [broadcast, compression, whisper, handshake, peers,
                      request, resend, response, store, retrieve]
------------------------------Payload Packets------------------------------
Size of packet n
Packet n     (payload packet n-4)

To understand this, let’s work from the bottom up. When a user wants to construct a message, they feed a list of packets. For this example, let’s say it’s ['broadcast', 'test message']. When this list is fed into a node, it adds the metadata section as outlined below.

The first element is the pathfinding header. This alerts nodes to how they should treat the message. If it is broadcast or waterfall they are to forward this message to their peers. If it is whisper they are not to do so. renegotiate is exclusivley used for connection management.

Next is the sender ID, used to identify a user in your routing table. So if a user wants to reply to a message, they look up this ID in their routing table. As will be discussed below, there are methods you can specifically request a user ID to connect to.

After this is a message ID and timestamp, used to filter out messages that a node has seen before. This can also be used as a checksum.

['broadcast', '6VnYj9LjoVLTvU3uPhy4nxm6yv2wEvhaRtGHeV9wwFngWGGqKAzuZ8jK6gFuvq737V', '72tG7phqoAnoeWRKtWoSmseurpCtYg2wHih1y5ZX1AmUvihcH7CPZHThtm9LGvKtj7', '3EfSDb', 'broadcast', 'test message']

One thing to notice is that the sender ID, message ID, and timestamp are all in a strange encoding. This is base_58, borrowed from Bitcoin. It’s a way to encode numbers that allows for sufficient density while still maintaining some human readability. This will get defined formally later in the paper.

All of this still leaves out the header. Constructing this goes as follows:

For each packet, compute its length and pack this into four bytes. So a message of length 6 would look like '\x00\x00\x00\x06'. Take the resulting string and prepend it to the packet. In this example, you would end up with: '\x00\x00\x00\tbroadcast\x00\x00\x00B6VnYj9LjoVLTvU3uPhy4nxm6yv2wEvhaRtGHeV9wwFngWGGqKAzuZ8jK6gFuvq737V\x00\x00\x00B72tG7phqoAnoeWRKtWoSmseurpCtYg2wHih1y5ZX1AmUvihcH7CPZHThtm9LGvKtj7\x00\x00\x00\x063EfSDb\x00\x00\x00\tbroadcast\x00\x00\x00\x0ctest message'.

After running this message through whatever compression algorith which has been negotiated with the node’s peer, it compute the message’s size, and pack this into four bytes: '\x00\x00\x00\xc0'. This results in a final message of:

'\x00\x00\x00\xc0\x00\x00\x00\tbroadcast\x00\x00\x00B6VnYj9LjoVLTvU3uPhy4nxm6yv2wEvhaRtGHeV9wwFngWGGqKAzuZ8jK6gFuvq737V\x00\x00\x00B72tG7phqoAnoeWRKtWoSmseurpCtYg2wHih1y5ZX1AmUvihcH7CPZHThtm9LGvKtj7\x00\x00\x00\x063EfSDb\x00\x00\x00\tbroadcast\x00\x00\x00\x0ctest message'