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Rabbit is a high-speed stream cipher first presented^[1] in February 2003 at the 10th FSE workshop. In May 2005, it was submitted to the eSTREAM project of the ECRYPT network.

Rabbit was designed by Martin Boesgaard, Mette Vesterager, Thomas Pedersen, Jesper Christiansen and Ove Scavenius.

Rabbit uses a 128-bit key and a 64-bit initialization vector. The cipher was designed with high performance in software in mind, where fully optimized implementations achieve an encryption speed of up to 3.7 cycles per byte on a Pentium 3, and of 9.7 cycles per byte on an ARM7. However, the cipher also turns out to be very fast and compact in hardware.

The core component of the cipher is a bitstream generator which encrypts 128 message bits per iteration. The cipher's strength rests on a strong mixing of its inner state between two consecutive iterations. The mixing function is entirely based on arithmetical operations that are available on a modern processor, i.e., no S-boxes or lookup tables are required to implement the cipher.

The authors of the cipher have provided a full set of cryptanalytic white papers on the Cryptico home page^[2]. It is also described in RFC 4503. Cryptico has patented the algorithm and requires a license fee for commercial use of the cipher. The license fee is waived for non-commercial uses.

Rabbit claims 128-bit security against attackers whose target is one specific key. If, however, the attacker targets a large number of keys at once and does not really care which one he breaks, then the small IV size results in a reduced security level of 96 bit. This is due to generic TMD trade-off attacks^[3].

While a small bias in the output of Rabbit exists^[4], resulting in a distinguisher with 2²⁴⁷ complexity discovered by Jean-Philippe Aumasson in December 2006, it's not a threat to Rabbit's security because its complexity is significantly higher than the brute-force of the key space (2¹²⁸).

• Cryptico homepage
• Rabbit RFC
• eSTREAM page on Rabbit