xor.go

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found src the LICENSE file.
     4  
     5  package chacha20
     6  
     7  import "runtime"
     8  
     9  // Platforms that have fast unaligned 32-bit little endian accesses.
    10  const unaligned = runtime.GOARCH == "386" ||
    11  	runtime.GOARCH == "amd64" ||
    12  	runtime.GOARCH == "arm64" ||
    13  	runtime.GOARCH == "ppc64le" ||
    14  	runtime.GOARCH == "s390x"
    15  
    16  // addXor reads a little endian uint32 from src, XORs it with (a + b) and
    17  // places the result in little endian byte order in dst.
    18  func addXor(dst, src []byte, a, b uint32) {
    19  	_, _ = src[3], dst[3] // bounds check elimination hint
    20  	if unaligned {
    21  		// The compiler should optimize this code into
    22  		// 32-bit unaligned little endian loads and stores.
    23  		// TODO: delete once the compiler does a reliably
    24  		// good job with the generic code below.
    25  		// See issue #25111 for more details.
    26  		v := uint32(src[0])
    27  		v |= uint32(src[1]) << 8
    28  		v |= uint32(src[2]) << 16
    29  		v |= uint32(src[3]) << 24
    30  		v ^= a + b
    31  		dst[0] = byte(v)
    32  		dst[1] = byte(v >> 8)
    33  		dst[2] = byte(v >> 16)
    34  		dst[3] = byte(v >> 24)
    35  	} else {
    36  		a += b
    37  		dst[0] = src[0] ^ byte(a)
    38  		dst[1] = src[1] ^ byte(a>>8)
    39  		dst[2] = src[2] ^ byte(a>>16)
    40  		dst[3] = src[3] ^ byte(a>>24)
    41  	}
    42  }
    43
View as plain text