Toxcore is the core library used by Tox clients, responsible for handling the end-to-end encrypted communication of the Tox protocol. The cryptographic library used by Toxcore is NaCl via libsodium. Tox employs curve25519 for its key exchanges, xsalsa20 for symmetric encryption, and poly1305 for MACs.
The codecs used for audio and video are Opus and VP8, respectively.
The current implementation of Toxcore used by most clients is c-toxcore.
See the instructions.