Rabbit is a synchronous stream cipher designed for high-performance software encryption. It was designed by Martin Boesgaard, Mette Vesterager, Thomas Christensen, and Erik Zenner and was submitted to the eSTREAM project in 2003, where it became a Phase 3 finalist in the software profile. Rabbit uses a 128-bit key and 64-bit initialisation vector.
eSTREAM was a research project (2004–2008) run by the EU ECRYPT network to identify new stream ciphers suitable for widespread adoption. Rabbit was one of the finalists in the software category, alongside Salsa20 (the predecessor to ChaCha20).
For new applications, prefer AES-256-GCM or ChaCha20-Poly1305 as they provide authenticated encryption (AEAD). Rabbit does not include a MAC, so you would need to add authentication separately to prevent tampering.