Source: block.go in package crypto/des


package des

import (
	"encoding/binary"
	"sync"
)

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. feistelBox must be initialized via feistelBoxOnce.Do(initFeistelBox) first.

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

var feistelBoxOnce sync.Once

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 initFeistelBox() {
	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.

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 b6, 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.

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

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) {
	feistelBoxOnce.Do(initFeistelBox)

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

combine halves to form 56-bit input to PC2

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