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63 lines
3.1 KiB
Go
63 lines
3.1 KiB
Go
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package sha3 implements the SHA-3 fixed-output-length hash functions and
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// the SHAKE variable-output-length hash functions defined by FIPS-202.
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//
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// Both types of hash function use the "sponge" construction and the Keccak
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// permutation. For a detailed specification see http://keccak.noekeon.org/
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//
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// # Guidance
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//
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// If you aren't sure what function you need, use SHAKE256 with at least 64
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// bytes of output. The SHAKE instances are faster than the SHA3 instances;
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// the latter have to allocate memory to conform to the hash.Hash interface.
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//
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// If you need a secret-key MAC (message authentication code), prepend the
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// secret key to the input, hash with SHAKE256 and read at least 32 bytes of
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// output.
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//
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// # Security strengths
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//
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// The SHA3-x (x equals 224, 256, 384, or 512) functions have a security
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// strength against preimage attacks of x bits. Since they only produce "x"
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// bits of output, their collision-resistance is only "x/2" bits.
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//
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// The SHAKE-256 and -128 functions have a generic security strength of 256 and
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// 128 bits against all attacks, provided that at least 2x bits of their output
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// is used. Requesting more than 64 or 32 bytes of output, respectively, does
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// not increase the collision-resistance of the SHAKE functions.
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//
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// # The sponge construction
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//
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// A sponge builds a pseudo-random function from a public pseudo-random
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// permutation, by applying the permutation to a state of "rate + capacity"
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// bytes, but hiding "capacity" of the bytes.
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//
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// A sponge starts out with a zero state. To hash an input using a sponge, up
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// to "rate" bytes of the input are XORed into the sponge's state. The sponge
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// is then "full" and the permutation is applied to "empty" it. This process is
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// repeated until all the input has been "absorbed". The input is then padded.
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// The digest is "squeezed" from the sponge in the same way, except that output
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// is copied out instead of input being XORed in.
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//
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// A sponge is parameterized by its generic security strength, which is equal
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// to half its capacity; capacity + rate is equal to the permutation's width.
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// Since the KeccakF-1600 permutation is 1600 bits (200 bytes) wide, this means
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// that the security strength of a sponge instance is equal to (1600 - bitrate) / 2.
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//
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// # Recommendations
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//
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// The SHAKE functions are recommended for most new uses. They can produce
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// output of arbitrary length. SHAKE256, with an output length of at least
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// 64 bytes, provides 256-bit security against all attacks. The Keccak team
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// recommends it for most applications upgrading from SHA2-512. (NIST chose a
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// much stronger, but much slower, sponge instance for SHA3-512.)
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//
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// The SHA-3 functions are "drop-in" replacements for the SHA-2 functions.
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// They produce output of the same length, with the same security strengths
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// against all attacks. This means, in particular, that SHA3-256 only has
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// 128-bit collision resistance, because its output length is 32 bytes.
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package sha3
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