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Update comments in sha.cpp

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Jeffrey Walton 2019-05-19 15:51:09 -04:00
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// sha.cpp - modified by Wei Dai from Steve Reid's public domain sha1.c
// Steve Reid implemented SHA-1. Wei Dai implemented SHA-2. Jeffrey Walton
// implemented Intel SHA extensions based on Intel articles and code by
// Sean Gulley. Jeffrey Walton implemented ARM SHA based on ARM code and
// code from Johannes Schneiders, Skip Hovsmith and Barry O'Rourke.
// All code is in the public domain.
// Steve Reid implemented SHA-1. Wei Dai implemented SHA-2. Jeffrey
// Walton implemented Intel SHA extensions based on Intel articles and code
// by Sean Gulley. Jeffrey Walton implemented ARM SHA based on ARM code and
// code from Johannes Schneiders, Skip Hovsmith and Barry O'Rourke. All
// code is in the public domain.
// In August 2017 JW reworked the internals to align all the implementations.
// Formerly all hashes were software based, IterHashBase handled endian conversions,
// and IterHashBase dispatched a single to block SHA{N}::Transform. SHA{N}::Transform
// then performed the single block hashing. It was repeated for multiple blocks.
// In August 2017 JW reworked the internals to align all the
// implementations. Formerly all hashes were software based, IterHashBase
// handled endian conversions, and IterHashBase dispatched a single to
// block SHA{N}::Transform. SHA{N}::Transform then performed the single
// block hashing. It was repeated for multiple blocks.
//
// The rework added SHA{N}::HashMultipleBlocks (class) and SHA{N}_HashMultipleBlocks
// (free standing). There are also hardware accelerated variations. Callers enter
// SHA{N}::HashMultipleBlocks (class), and the function calls SHA{N}_HashMultipleBlocks
// (free standing) or SHA{N}_HashBlock (free standing) as a fallback.
// The rework added SHA{N}::HashMultipleBlocks (class) and
// SHA{N}_HashMultipleBlocks (free standing). There are also hardware
// accelerated variations. Callers enter SHA{N}::HashMultipleBlocks (class)
// and the function calls SHA{N}_HashMultipleBlocks (free standing) or
// SHA{N}_HashBlock (free standing) as a fallback.
//
// An added wrinkle is hardware is little endian, C++ is big endian, and callers use
// big endian, so SHA{N}_HashMultipleBlock accepts a ByteOrder for the incoming data
// arrangement. Hardware based SHA{N}_HashMultipleBlock can often perform the endian
// swap much easier by setting an EPI mask. Endian swap incurs no penalty on Intel SHA,
// and 4-instruction penalty on ARM SHA. Under C++ the full software based swap penalty
// is incurred due to use of ReverseBytes().
// An added wrinkle is hardware is little endian, C++ is big endian, and
// callers use big endian, so SHA{N}_HashMultipleBlock accepts a ByteOrder
// for the incoming data arrangement. Hardware based SHA{N}_HashMultipleBlock
// can often perform the endian swap much easier by setting an EPI mask.
// Endian swap incurs no penalty on Intel SHA, and 4-instruction penalty on
// ARM SHA. Under C++ the full software based swap penalty is incurred due
// to use of ReverseBytes().
//
// The rework also removed the hacked-in pointers to implementations.
// In May 2019 JW added Cryptogams ARMv7 and NEON implementations for SHA1,
// SHA256 and SHA512. The Cryptogams code closed a performance gap on modern
// 32-bit ARM devices. Cryptogams is Andy Polyakov's project used to create
// high speed crypto algorithms and share them with other developers. Andy's
// code runs 30% to 50% faster than C/C++ code. The Cryptogams code can be
// disabled in config_asm.h. An example of integrating Andy's code is at
// https://wiki.openssl.org/index.php/Cryptogams_SHA.
// use "cl /EP /P /DCRYPTOPP_GENERATE_X64_MASM sha.cpp" to generate MASM code