"Both Monero and Tor use Ed25519 elliptic curve cryptography. A Monero public spend key is mathematically identical to a Tor onion service public key. This means anyone can compute a deterministic Tor onion address directly from any Monero address.

Each MoneroDM instance runs a Tor onion service computed from the wallet's Monero spend/view keypairs. To send a message, MoneroDM computes the public onion address of the peer from the given Monero address."

https://monerodm.org/