23 December 2016

# NIST SP 800-185 officially released

NIST released the SP 800-185 standard with useful new functions based on Keccak: cSHAKE, KMAC, TupleHash and ParallelHash.

Yesterday, NIST published the SP 800-185 standard [PDF]. It contains the following new functions based on Keccak:

• cSHAKE is a family of two extendable-output functions (cSHAKE128 and cSHAKE256) that generalize upon SHAKE128 and SHAKE256. It is used as a building block for KMAC, TupleHash and ParallelHash. The main difference with the SHAKEs lies in an explicit domain separation mechanism. In addition to the usual input string, the user can supply a function name and a customization string. The former is standardized by NIST to separate different standard functions, while the user is free to supply anything in the latter.
• KMAC is a keyed hash function or pseudo-random function (PRF) that can be used, e.g., to compute a message authentication code (MAC) or to derive a session key from a master key. It is more efficient than HMAC by removing the need for HMAC's nested construction.
• TupleHash is a hash function whose input domain is any number of input strings. The output depends on the exact sequence of strings, not just their concatenation. For instance, TupleHash("a", "b", "c"), TupleHash("a", "bc"), TupleHash("abc") and TupleHash("abc", "") all give unrelated outputs.
• ParallelHash is a hash function that can exploit the parallelism in modern processors by way of a tree hash mode. This significantly speeds up the hashing of long inputs. E.g., we reported speeds of 2.73 cycles/byte and 2.31 c/b on Haswell and Skylake processors, respectively. (These figures are based on the draft specifications, but we do not expect them to differ significantly for the final specifications.)

These new functions all support the 128-bit and 256-bit security strengths. They all consistently support domain separation through the user-chosen customization string input. And they all support variable ouput length in a natural way.