Paper 2016/856

Spritz---a spongy RC4-like stream cipher and hash function.

Ronald L. Rivest and Jacob C. N. Schuldt

Abstract

This paper reconsiders the design of the stream cipher RC4, and proposes an improved variant, which we call ``Spritz'' (since the output comes in fine drops rather than big blocks.) Our work leverages the considerable cryptanalytic work done on the original RC4 and its proposed variants. It also uses simulations extensively to search for biases and to guide the selection of intermediate expressions. We estimate that Spritz can produce output with about 24 cycles/byte of computation. Furthermore, our statistical tests suggest that about $2^{81}$ bytes of output are needed before one can reasonably distinguish Spritz output from random output; this is a marked improvement over RC4. [Footnote: However, see Appendix F for references to more recent work that suggest that our estimates of the work required to break Spritz may be optimistic.] In addition, we formulate Spritz as a ``sponge (or sponge-like) function,'' (see Bertoni et al.), which can ``Absorb'' new data at any time, and from which one can ``Squeeze'' pseudorandom output sequences of arbitrary length. Spritz can thus be easily adapted for use as a cryptographic hash function, an encryption algorithm, or a message-authentication code generator. (However, in hash-function mode, Spritz is rather slow.)