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# Source file src/crypto/des/block.go

## Documentation: crypto/des

```  // Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style

package des

import "encoding/binary"

func cryptBlock(subkeys []uint64, dst, src []byte, decrypt bool) {
b := binary.BigEndian.Uint64(src)
b = permuteInitialBlock(b)
left, right := uint32(b>>32), uint32(b)

left = (left << 1) | (left >> 31)
right = (right << 1) | (right >> 31)

if decrypt {
for i := 0; i < 8; i++ {
left, right = feistel(left, right, subkeys[15-2*i], subkeys[15-(2*i+1)])
}
} else {
for i := 0; i < 8; i++ {
left, right = feistel(left, right, subkeys[2*i], subkeys[2*i+1])
}
}

left = (left << 31) | (left >> 1)
right = (right << 31) | (right >> 1)

// switch left & right and perform final permutation
preOutput := (uint64(right) << 32) | uint64(left)
binary.BigEndian.PutUint64(dst, permuteFinalBlock(preOutput))
}

// 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(l, r uint32, k0, k1 uint64) (lout, rout uint32) {
var t uint32

t = r ^ uint32(k0>>32)
l ^= feistelBox[7][t&0x3f] ^
feistelBox[5][(t>>8)&0x3f] ^
feistelBox[3][(t>>16)&0x3f] ^
feistelBox[1][(t>>24)&0x3f]

t = ((r << 28) | (r >> 4)) ^ uint32(k0)
l ^= feistelBox[6][(t)&0x3f] ^
feistelBox[4][(t>>8)&0x3f] ^
feistelBox[2][(t>>16)&0x3f] ^
feistelBox[0][(t>>24)&0x3f]

t = l ^ uint32(k1>>32)
r ^= feistelBox[7][t&0x3f] ^
feistelBox[5][(t>>8)&0x3f] ^
feistelBox[3][(t>>16)&0x3f] ^
feistelBox[1][(t>>24)&0x3f]

t = ((l << 28) | (l >> 4)) ^ uint32(k1)
r ^= feistelBox[6][(t)&0x3f] ^
feistelBox[4][(t>>8)&0x3f] ^
feistelBox[2][(t>>16)&0x3f] ^
feistelBox[0][(t>>24)&0x3f]

return l, r
}

// feistelBox[s][16*i+j] contains the output of permutationFunction
// for sBoxes[s][i][j] << 4*(7-s)
var feistelBox [8][64]uint32

// 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
}

func init() {
for s := range sBoxes {
for i := 0; i < 4; i++ {
for j := 0; j < 16; j++ {
f := uint64(sBoxes[s][i][j]) << (4 * (7 - uint(s)))
f = permuteBlock(f, permutationFunction[:])

// Row is determined by the 1st and 6th bit.
// Column is the middle four bits.
row := uint8(((i & 2) << 4) | i&1)
col := uint8(j << 1)
t := row | col

// The rotation was performed in the feistel rounds, being factored out and now mixed into the feistelBox.
f = (f << 1) | (f >> 31)

feistelBox[s][t] = uint32(f)
}
}
}
}

// permuteInitialBlock is equivalent to the permutation defined
// by initialPermutation.
func permuteInitialBlock(block uint64) uint64 {
// block = b7 b6 b5 b4 b3 b2 b1 b0 (8 bytes)
b1 := block >> 48
b2 := block << 48
block ^= b1 ^ b2 ^ b1<<48 ^ b2>>48

// block = b1 b0 b5 b4 b3 b2 b7 b6
b1 = block >> 32 & 0xff00ff
b2 = (block & 0xff00ff00)
block ^= b1<<32 ^ b2 ^ b1<<8 ^ b2<<24 // exchange b0 b4 with b3 b7

// block is now b1 b3 b5 b7 b0 b2 b4 b7, the permutation:
//                  ...  8
//                  ... 24
//                  ... 40
//                  ... 56
//  7  6  5  4  3  2  1  0
// 23 22 21 20 19 18 17 16
//                  ... 32
//                  ... 48

// exchange 4,5,6,7 with 32,33,34,35 etc.
b1 = block & 0x0f0f00000f0f0000
b2 = block & 0x0000f0f00000f0f0
block ^= b1 ^ b2 ^ b1>>12 ^ b2<<12

// block is the permutation:
//
//   [+8]         [+40]
//
//  7  6  5  4
// 23 22 21 20
//  3  2  1  0
// 19 18 17 16    [+32]

// exchange 0,1,4,5 with 18,19,22,23
b1 = block & 0x3300330033003300
b2 = block & 0x00cc00cc00cc00cc
block ^= b1 ^ b2 ^ b1>>6 ^ b2<<6

// block is the permutation:
// 15 14
// 13 12
// 11 10
//  9  8
//  7  6
//  5  4
//  3  2
//  1  0 [+16] [+32] [+64]

// exchange 0,2,4,6 with 9,11,13,15:
b1 = block & 0xaaaaaaaa55555555
block ^= b1 ^ b1>>33 ^ b1<<33

// block is the permutation:
// 6 14 22 30 38 46 54 62
// 4 12 20 28 36 44 52 60
// 2 10 18 26 34 42 50 58
// 0  8 16 24 32 40 48 56
// 7 15 23 31 39 47 55 63
// 5 13 21 29 37 45 53 61
// 3 11 19 27 35 43 51 59
// 1  9 17 25 33 41 49 57
return block
}

// permuteInitialBlock is equivalent to the permutation defined
// by finalPermutation.
func permuteFinalBlock(block uint64) uint64 {
// Perform the same bit exchanges as permuteInitialBlock
// but in reverse order.
b1 := block & 0xaaaaaaaa55555555
block ^= b1 ^ b1>>33 ^ b1<<33

b1 = block & 0x3300330033003300
b2 := block & 0x00cc00cc00cc00cc
block ^= b1 ^ b2 ^ b1>>6 ^ b2<<6

b1 = block & 0x0f0f00000f0f0000
b2 = block & 0x0000f0f00000f0f0
block ^= b1 ^ b2 ^ b1>>12 ^ b2<<12

b1 = block >> 32 & 0xff00ff
b2 = (block & 0xff00ff00)
block ^= b1<<32 ^ b2 ^ b1<<8 ^ b2<<24

b1 = block >> 48
b2 = block << 48
block ^= b1 ^ b2 ^ b1<<48 ^ b2>>48
return block
}

// 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] = unpack(permuteBlock(pc2Input, permutedChoice2[:]))
}
}

// Expand 48-bit input to 64-bit, with each 6-bit block padded by extra two bits at the top.
// By doing so, we can have the input blocks (four bits each), and the key blocks (six bits each) well-aligned without
// extra shifts/rotations for alignments.
func unpack(x uint64) uint64 {
var result uint64

result = ((x>>(6*1))&0xff)<<(8*0) |
((x>>(6*3))&0xff)<<(8*1) |
((x>>(6*5))&0xff)<<(8*2) |
((x>>(6*7))&0xff)<<(8*3) |
((x>>(6*0))&0xff)<<(8*4) |
((x>>(6*2))&0xff)<<(8*5) |
((x>>(6*4))&0xff)<<(8*6) |
((x>>(6*6))&0xff)<<(8*7)

return result
}

```

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