avian

A chat program over DNS. Designed for censorship circumvention, not convenience.

This program is designed for the case of Russian internet being censored. As of writing, it's not necessary, but things can change at any moment. (It is also likely functional under several other forms of restricted internet.)

Features

Communicates with the server using only standard DNS; remains functional even if all normal TCP and UDP traffic is blocked, and if lookups are relayed through any number of caching recursive resolvers
Supports UTF-8, and terminal escape codes for formatting
that's it. avian is designed to smuggle information into and out of a censored internet; any conveniences, like UTF-8 support, are accidental.

Limitations

Server installation requires domain name configuration, which may require significant technical expertise.
Unix-likes only. Tested on Linux and WSL; does not work natively on Windows (PRs welcome).
There are no channels, usernames, banhammers, online status, file upload, or any other modern chatting conveniences. You'll have to type your name manually for each message, and give access to trusted people only.
It's slow. You can read 8 bytes or send 4 bytes per roundtrip to the server, no more; this is usually around 16 bytes per second.

Installation

Create the_key.h, containing

#define THE_SERVER "avian.example.com"
#define THE_SERVER_DNS "\5avian\7example\3com"
#define THE_KEY { 0x01, 0x02, 0x03, 0x04, 0x01, 0x02, 0x03, 0x04, 0x01, 0x02, 0x03, 0x04, 0x01, 0x02, 0x03, 0x04 }

Compile with g++ -xc++ main.cpp -xc aes.c -o avian (works on Linux and WSL; does not work natively on Windows)
Run ./avian --server somewhere on the open internet
Assign a NS record to avian.example.com, pointing to the IP or normal domain name of your server
Run ./avian on a computer with restricted internet access (and on one on the open internet, so your restricted friends can communicate)

Protocol

The avian protocol consists of 16 byte messages.

A query consists of a 32 bit counter (Unix timestamp - avian is not designed for operation beyond 2038), 32 bits expected chat history size (avian does not support chat history bigger than four gigabytes), 4 bytes data to append (padded with 00s if nothing to send), and 32 bits of 00 00 00 00 as a checksum. If expected chat history size does not match actual, the data will not be appended.

The counter+histsize field may not be reused, but as long as either is bigger than last time, the other may remain unchanged.

These 16 bytes will be encrypted with AES-128 in ECB mode, hex encoded using the alphabet abcdefghijklmnop, appended with the server name, and queried over DNS.

The resulting AAAA response will be xor'd with the query's plaintext, then decrypted with the same AES key.

The response plaintext consists of a 32 bit actual chat history size, 8 bytes chat data (or 00s if nothing), and 32 bits of 11 11 11 11.

This means receiving a message can only be done at 8 bytes per query (but can be parallelized), and sending is 4 bytes per trip (which can not be parallelized - the server will reject unordered writes).

Logo

Any other Ukraine-colored bird is also acceptable as logo.

What's with the name?

It's named after another censorship-resistant and inconvenient communication protocol.

Okay but all of your tools have weird names

All the normal names were taken, boring, or both

Alcaro / avian