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// Copyright (C) 2019 ProtonTech AG
// This file contains necessary tools for the aex and ocb packages.
//
// These functions SHOULD NOT be used elsewhere, since they are optimized for
// specific input nature in the EAX and OCB modes of operation.
package byteutil
// GfnDouble computes 2 * input in the field of 2^n elements.
// The irreducible polynomial in the finite field for n=128 is
// x^128 + x^7 + x^2 + x + 1 (equals 0x87)
// Constant-time execution in order to avoid side-channel attacks
func GfnDouble(input []byte) []byte {
if len(input) != 16 {
panic("Doubling in GFn only implemented for n = 128")
}
// If the first bit is zero, return 2L = L << 1
// Else return (L << 1) xor 0^120 10000111
shifted := ShiftBytesLeft(input)
shifted[15] ^= ((input[0] >> 7) * 0x87)
return shifted
}
// ShiftBytesLeft outputs the byte array corresponding to x << 1 in binary.
func ShiftBytesLeft(x []byte) []byte {
l := len(x)
dst := make([]byte, l)
for i := 0; i < l-1; i++ {
dst[i] = (x[i] << 1) | (x[i+1] >> 7)
}
dst[l-1] = x[l-1] << 1
return dst
}
// ShiftNBytesLeft puts in dst the byte array corresponding to x << n in binary.
func ShiftNBytesLeft(dst, x []byte, n int) {
// Erase first n / 8 bytes
copy(dst, x[n/8:])
// Shift the remaining n % 8 bits
bits := uint(n % 8)
l := len(dst)
for i := 0; i < l-1; i++ {
dst[i] = (dst[i] << bits) | (dst[i+1] >> uint(8-bits))
}
dst[l-1] = dst[l-1] << bits
// Append trailing zeroes
dst = append(dst, make([]byte, n/8)...)
}
// XorBytesMut assumes equal input length, replaces X with X XOR Y
func XorBytesMut(X, Y []byte) {
for i := 0; i < len(X); i++ {
X[i] ^= Y[i]
}
}
// XorBytes assumes equal input length, puts X XOR Y into Z
func XorBytes(Z, X, Y []byte) {
for i := 0; i < len(X); i++ {
Z[i] = X[i] ^ Y[i]
}
}
// RightXor XORs smaller input (assumed Y) at the right of the larger input (assumed X)
func RightXor(X, Y []byte) []byte {
offset := len(X) - len(Y)
xored := make([]byte, len(X))
copy(xored, X)
for i := 0; i < len(Y); i++ {
xored[offset+i] ^= Y[i]
}
return xored
}
// SliceForAppend takes a slice and a requested number of bytes. It returns a
// slice with the contents of the given slice followed by that many bytes and a
// second slice that aliases into it and contains only the extra bytes. If the
// original slice has sufficient capacity then no allocation is performed.
func SliceForAppend(in []byte, n int) (head, tail []byte) {
if total := len(in) + n; cap(in) >= total {
head = in[:total]
} else {
head = make([]byte, total)
copy(head, in)
}
tail = head[len(in):]
return
}
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