doc.go (3179B)
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. 62 package sha3 // import "golang.org/x/crypto/sha3"