Bitfinex has made a desperate attempt to reach out to the hacker that buggered their bum and liberated almost 120k Bitcoins from their platform in August with a post on their blog detailing the channels opened for dialogue. One of the methods even goes as far to suggest blockchain spam as a viable communication protocol.
From the announcement:
We believe that a combination of Tor and an anonymous email service should suffice to protect your identity and location. Encrypting your message with our PGP key further guarantees privacy from prying
eyes, but to prove your authenticity to us, we ask that you provide the public key associated with 1QDBWKgfftwuraEasMGSUvj9PPrswZv19q and sign your message with the corresponding private key.
Instead of using e-mail, you can send the authenticating information via Bitmessage and Tor. Our Bitmessage address is BM-2cW79647sMFe3fJKKGKAwXWwTSS293meq8.
Alternatively, you can send us a message on the Blockchain using OP_RETURN. You can encrypt a message (containing your pub key) with our PGP key, split up the message into 80-byte chunks, and send
transactions to 19eT7KGKo1gFjgBhEF4957wVNugkc2cakK from any one of the 2072 addresses currently holding the bitcoins in question.
Despite not being registered in the WoT, Bitfinex also somehow managed to make a gpg key and post it as well. The post closed by stating that they were "very anxious to hear" from the attacker, and asked if there is perhaps a different way that they would prefer to communicate, as absconding with $75 million in Bitcoin evidently did not send a clear enough message. (archived)