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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
/*
Implementation adapted from Needham and Wheeler's paper:
http://www.cix.co.uk/~klockstone/xtea.pdf
A precalculated look up table is used during encryption/decryption for values that are based purely on the key.
*/
package xtea
// XTEA is based on 64 rounds.
const numRounds = 64
// blockToUint32 reads an 8 byte slice into two uint32s.
// The block is treated as big endian.
func blockToUint32(src []byte) (uint32, uint32) {
r0 := uint32(src[0])<<24 | uint32(src[1])<<16 | uint32(src[2])<<8 | uint32(src[3])
r1 := uint32(src[4])<<24 | uint32(src[5])<<16 | uint32(src[6])<<8 | uint32(src[7])
return r0, r1
}
// uint32ToBlock writes two uint32s into an 8 byte data block.
// Values are written as big endian.
func uint32ToBlock(v0, v1 uint32, dst []byte) {
dst[0] = byte(v0 >> 24)
dst[1] = byte(v0 >> 16)
dst[2] = byte(v0 >> 8)
dst[3] = byte(v0)
dst[4] = byte(v1 >> 24)
dst[5] = byte(v1 >> 16)
dst[6] = byte(v1 >> 8)
dst[7] = byte(v1 >> 0)
}
// encryptBlock encrypts a single 8 byte block using XTEA.
func encryptBlock(c *Cipher, dst, src []byte) {
v0, v1 := blockToUint32(src)
// Two rounds of XTEA applied per loop
for i := 0; i < numRounds; {
v0 += ((v1<<4 ^ v1>>5) + v1) ^ c.table[i]
i++
v1 += ((v0<<4 ^ v0>>5) + v0) ^ c.table[i]
i++
}
uint32ToBlock(v0, v1, dst)
}
// decryptBlock decrypt a single 8 byte block using XTEA.
func decryptBlock(c *Cipher, dst, src []byte) {
v0, v1 := blockToUint32(src)
// Two rounds of XTEA applied per loop
for i := numRounds; i > 0; {
i--
v1 -= ((v0<<4 ^ v0>>5) + v0) ^ c.table[i]
i--
v0 -= ((v1<<4 ^ v1>>5) + v1) ^ c.table[i]
}
uint32ToBlock(v0, v1, dst)
}
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