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