1// Copyright 2014 The Go Authors. All rights reserved.
 2// Use of this source code is governed by a BSD-style
 3// license that can be found in the LICENSE file.
 4
 5// Package sha3 implements the SHA-3 fixed-output-length hash functions and
 6// the SHAKE variable-output-length hash functions defined by FIPS-202.
 7//
 8// Both types of hash function use the "sponge" construction and the Keccak
 9// permutation. For a detailed specification see http://keccak.noekeon.org/
10//
11// # Guidance
12//
13// If you aren't sure what function you need, use SHAKE256 with at least 64
14// bytes of output. The SHAKE instances are faster than the SHA3 instances;
15// the latter have to allocate memory to conform to the hash.Hash interface.
16//
17// If you need a secret-key MAC (message authentication code), prepend the
18// secret key to the input, hash with SHAKE256 and read at least 32 bytes of
19// output.
20//
21// # Security strengths
22//
23// The SHA3-x (x equals 224, 256, 384, or 512) functions have a security
24// strength against preimage attacks of x bits. Since they only produce "x"
25// bits of output, their collision-resistance is only "x/2" bits.
26//
27// The SHAKE-256 and -128 functions have a generic security strength of 256 and
28// 128 bits against all attacks, provided that at least 2x bits of their output
29// is used.  Requesting more than 64 or 32 bytes of output, respectively, does
30// not increase the collision-resistance of the SHAKE functions.
31//
32// # The sponge construction
33//
34// A sponge builds a pseudo-random function from a public pseudo-random
35// permutation, by applying the permutation to a state of "rate + capacity"
36// bytes, but hiding "capacity" of the bytes.
37//
38// A sponge starts out with a zero state. To hash an input using a sponge, up
39// to "rate" bytes of the input are XORed into the sponge's state. The sponge
40// is then "full" and the permutation is applied to "empty" it. This process is
41// repeated until all the input has been "absorbed". The input is then padded.
42// The digest is "squeezed" from the sponge in the same way, except that output
43// is copied out instead of input being XORed in.
44//
45// A sponge is parameterized by its generic security strength, which is equal
46// to half its capacity; capacity + rate is equal to the permutation's width.
47// Since the KeccakF-1600 permutation is 1600 bits (200 bytes) wide, this means
48// that the security strength of a sponge instance is equal to (1600 - bitrate) / 2.
49//
50// # Recommendations
51//
52// The SHAKE functions are recommended for most new uses. They can produce
53// output of arbitrary length. SHAKE256, with an output length of at least
54// 64 bytes, provides 256-bit security against all attacks.  The Keccak team
55// recommends it for most applications upgrading from SHA2-512. (NIST chose a
56// much stronger, but much slower, sponge instance for SHA3-512.)
57//
58// The SHA-3 functions are "drop-in" replacements for the SHA-2 functions.
59// They produce output of the same length, with the same security strengths
60// against all attacks. This means, in particular, that SHA3-256 only has
61// 128-bit collision resistance, because its output length is 32 bytes.
62package sha3