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// Copyright 2011 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.
package des
import (
"encoding/binary"
)
func cryptBlock(subkeys []uint64, dst, src []byte, decrypt bool) {
b := binary.BigEndian.Uint64(src)
b = permuteBlock(b, initialPermutation[:])
left, right := uint32(b>>32), uint32(b)
var subkey uint64
for i := 0; i < 16; i++ {
if decrypt {
subkey = subkeys[15-i]
} else {
subkey = subkeys[i]
}
left, right = right, left^feistel(right, subkey)
}
// switch left & right and perform final permutation
preOutput := (uint64(right) << 32) | uint64(left)
binary.BigEndian.PutUint64(dst, permuteBlock(preOutput, finalPermutation[:]))
}
// Encrypt one block from src into dst, using the subkeys.
func encryptBlock(subkeys []uint64, dst, src []byte) {
cryptBlock(subkeys, dst, src, false)
}
// Decrypt one block from src into dst, using the subkeys.
func decryptBlock(subkeys []uint64, dst, src []byte) {
cryptBlock(subkeys, dst, src, true)
}
// DES Feistel function
func feistel(right uint32, key uint64) (result uint32) {
sBoxLocations := key ^ permuteBlock(uint64(right), expansionFunction[:])
var sBoxResult uint32
for i := uint8(0); i < 8; i++ {
sBoxLocation := uint8(sBoxLocations>>42) & 0x3f
sBoxLocations <<= 6
// row determined by 1st and 6th bit
row := (sBoxLocation & 0x1) | ((sBoxLocation & 0x20) >> 4)
// column is middle four bits
column := (sBoxLocation >> 1) & 0xf
sBoxResult |= uint32(sBoxes[i][row][column]) << (4 * (7 - i))
}
return uint32(permuteBlock(uint64(sBoxResult), permutationFunction[:]))
}
// general purpose function to perform DES block permutations
func permuteBlock(src uint64, permutation []uint8) (block uint64) {
for position, n := range permutation {
bit := (src >> n) & 1
block |= bit << uint((len(permutation)-1)-position)
}
return
}
// creates 16 28-bit blocks rotated according
// to the rotation schedule
func ksRotate(in uint32) (out []uint32) {
out = make([]uint32, 16)
last := in
for i := 0; i < 16; i++ {
// 28-bit circular left shift
left := (last << (4 + ksRotations[i])) >> 4
right := (last << 4) >> (32 - ksRotations[i])
out[i] = left | right
last = out[i]
}
return
}
// creates 16 56-bit subkeys from the original key
func (c *desCipher) generateSubkeys(keyBytes []byte) {
// apply PC1 permutation to key
key := binary.BigEndian.Uint64(keyBytes)
permutedKey := permuteBlock(key, permutedChoice1[:])
// rotate halves of permuted key according to the rotation schedule
leftRotations := ksRotate(uint32(permutedKey >> 28))
rightRotations := ksRotate(uint32(permutedKey<<4) >> 4)
// generate subkeys
for i := 0; i < 16; i++ {
// combine halves to form 56-bit input to PC2
pc2Input := uint64(leftRotations[i])<<28 | uint64(rightRotations[i])
// apply PC2 permutation to 7 byte input
c.subkeys[i] = permuteBlock(pc2Input, permutedChoice2[:])
}
}
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