block.go

Documentation: crypto/des

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package des
     6  
     7  import "encoding/binary"
     8  
     9  func cryptBlock(subkeys []uint64, dst, src []byte, decrypt bool) {
    10  	b := binary.BigEndian.Uint64(src)
    11  	b = permuteInitialBlock(b)
    12  	left, right := uint32(b>>32), uint32(b)
    13  
    14  	left = (left << 1) | (left >> 31)
    15  	right = (right << 1) | (right >> 31)
    16  
    17  	if decrypt {
    18  		for i := 0; i < 8; i++ {
    19  			left, right = feistel(left, right, subkeys[15-2*i], subkeys[15-(2*i+1)])
    20  		}
    21  	} else {
    22  		for i := 0; i < 8; i++ {
    23  			left, right = feistel(left, right, subkeys[2*i], subkeys[2*i+1])
    24  		}
    25  	}
    26  
    27  	left = (left << 31) | (left >> 1)
    28  	right = (right << 31) | (right >> 1)
    29  
    30  	// switch left & right and perform final permutation
    31  	preOutput := (uint64(right) << 32) | uint64(left)
    32  	binary.BigEndian.PutUint64(dst, permuteFinalBlock(preOutput))
    33  }
    34  
    35  // Encrypt one block from src into dst, using the subkeys.
    36  func encryptBlock(subkeys []uint64, dst, src []byte) {
    37  	cryptBlock(subkeys, dst, src, false)
    38  }
    39  
    40  // Decrypt one block from src into dst, using the subkeys.
    41  func decryptBlock(subkeys []uint64, dst, src []byte) {
    42  	cryptBlock(subkeys, dst, src, true)
    43  }
    44  
    45  // DES Feistel function
    46  func feistel(l, r uint32, k0, k1 uint64) (lout, rout uint32) {
    47  	var t uint32
    48  
    49  	t = r ^ uint32(k0>>32)
    50  	l ^= feistelBox[7][t&0x3f] ^
    51  		feistelBox[5][(t>>8)&0x3f] ^
    52  		feistelBox[3][(t>>16)&0x3f] ^
    53  		feistelBox[1][(t>>24)&0x3f]
    54  
    55  	t = ((r << 28) | (r >> 4)) ^ uint32(k0)
    56  	l ^= feistelBox[6][(t)&0x3f] ^
    57  		feistelBox[4][(t>>8)&0x3f] ^
    58  		feistelBox[2][(t>>16)&0x3f] ^
    59  		feistelBox[0][(t>>24)&0x3f]
    60  
    61  	t = l ^ uint32(k1>>32)
    62  	r ^= feistelBox[7][t&0x3f] ^
    63  		feistelBox[5][(t>>8)&0x3f] ^
    64  		feistelBox[3][(t>>16)&0x3f] ^
    65  		feistelBox[1][(t>>24)&0x3f]
    66  
    67  	t = ((l << 28) | (l >> 4)) ^ uint32(k1)
    68  	r ^= feistelBox[6][(t)&0x3f] ^
    69  		feistelBox[4][(t>>8)&0x3f] ^
    70  		feistelBox[2][(t>>16)&0x3f] ^
    71  		feistelBox[0][(t>>24)&0x3f]
    72  
    73  	return l, r
    74  }
    75  
    76  // feistelBox[s][16*i+j] contains the output of permutationFunction
    77  // for sBoxes[s][i][j] << 4*(7-s)
    78  var feistelBox [8][64]uint32
    79  
    80  // general purpose function to perform DES block permutations
    81  func permuteBlock(src uint64, permutation []uint8) (block uint64) {
    82  	for position, n := range permutation {
    83  		bit := (src >> n) & 1
    84  		block |= bit << uint((len(permutation)-1)-position)
    85  	}
    86  	return
    87  }
    88  
    89  func init() {
    90  	for s := range sBoxes {
    91  		for i := 0; i < 4; i++ {
    92  			for j := 0; j < 16; j++ {
    93  				f := uint64(sBoxes[s][i][j]) << (4 * (7 - uint(s)))
    94  				f = permuteBlock(f, permutationFunction[:])
    95  
    96  				// Row is determined by the 1st and 6th bit.
    97  				// Column is the middle four bits.
    98  				row := uint8(((i & 2) << 4) | i&1)
    99  				col := uint8(j << 1)
   100  				t := row | col
   101  
   102  				// The rotation was performed in the feistel rounds, being factored out and now mixed into the feistelBox.
   103  				f = (f << 1) | (f >> 31)
   104  
   105  				feistelBox[s][t] = uint32(f)
   106  			}
   107  		}
   108  	}
   109  }
   110  
   111  // permuteInitialBlock is equivalent to the permutation defined
   112  // by initialPermutation.
   113  func permuteInitialBlock(block uint64) uint64 {
   114  	// block = b7 b6 b5 b4 b3 b2 b1 b0 (8 bytes)
   115  	b1 := block >> 48
   116  	b2 := block << 48
   117  	block ^= b1 ^ b2 ^ b1<<48 ^ b2>>48
   118  
   119  	// block = b1 b0 b5 b4 b3 b2 b7 b6
   120  	b1 = block >> 32 & 0xff00ff
   121  	b2 = (block & 0xff00ff00)
   122  	block ^= b1<<32 ^ b2 ^ b1<<8 ^ b2<<24 // exchange b0 b4 with b3 b7
   123  
   124  	// block is now b1 b3 b5 b7 b0 b2 b4 b7, the permutation:
   125  	//                  ...  8
   126  	//                  ... 24
   127  	//                  ... 40
   128  	//                  ... 56
   129  	//  7  6  5  4  3  2  1  0
   130  	// 23 22 21 20 19 18 17 16
   131  	//                  ... 32
   132  	//                  ... 48
   133  
   134  	// exchange 4,5,6,7 with 32,33,34,35 etc.
   135  	b1 = block & 0x0f0f00000f0f0000
   136  	b2 = block & 0x0000f0f00000f0f0
   137  	block ^= b1 ^ b2 ^ b1>>12 ^ b2<<12
   138  
   139  	// block is the permutation:
   140  	//
   141  	//   [+8]         [+40]
   142  	//
   143  	//  7  6  5  4
   144  	// 23 22 21 20
   145  	//  3  2  1  0
   146  	// 19 18 17 16    [+32]
   147  
   148  	// exchange 0,1,4,5 with 18,19,22,23
   149  	b1 = block & 0x3300330033003300
   150  	b2 = block & 0x00cc00cc00cc00cc
   151  	block ^= b1 ^ b2 ^ b1>>6 ^ b2<<6
   152  
   153  	// block is the permutation:
   154  	// 15 14
   155  	// 13 12
   156  	// 11 10
   157  	//  9  8
   158  	//  7  6
   159  	//  5  4
   160  	//  3  2
   161  	//  1  0 [+16] [+32] [+64]
   162  
   163  	// exchange 0,2,4,6 with 9,11,13,15:
   164  	b1 = block & 0xaaaaaaaa55555555
   165  	block ^= b1 ^ b1>>33 ^ b1<<33
   166  
   167  	// block is the permutation:
   168  	// 6 14 22 30 38 46 54 62
   169  	// 4 12 20 28 36 44 52 60
   170  	// 2 10 18 26 34 42 50 58
   171  	// 0  8 16 24 32 40 48 56
   172  	// 7 15 23 31 39 47 55 63
   173  	// 5 13 21 29 37 45 53 61
   174  	// 3 11 19 27 35 43 51 59
   175  	// 1  9 17 25 33 41 49 57
   176  	return block
   177  }
   178  
   179  // permuteInitialBlock is equivalent to the permutation defined
   180  // by finalPermutation.
   181  func permuteFinalBlock(block uint64) uint64 {
   182  	// Perform the same bit exchanges as permuteInitialBlock
   183  	// but in reverse order.
   184  	b1 := block & 0xaaaaaaaa55555555
   185  	block ^= b1 ^ b1>>33 ^ b1<<33
   186  
   187  	b1 = block & 0x3300330033003300
   188  	b2 := block & 0x00cc00cc00cc00cc
   189  	block ^= b1 ^ b2 ^ b1>>6 ^ b2<<6
   190  
   191  	b1 = block & 0x0f0f00000f0f0000
   192  	b2 = block & 0x0000f0f00000f0f0
   193  	block ^= b1 ^ b2 ^ b1>>12 ^ b2<<12
   194  
   195  	b1 = block >> 32 & 0xff00ff
   196  	b2 = (block & 0xff00ff00)
   197  	block ^= b1<<32 ^ b2 ^ b1<<8 ^ b2<<24
   198  
   199  	b1 = block >> 48
   200  	b2 = block << 48
   201  	block ^= b1 ^ b2 ^ b1<<48 ^ b2>>48
   202  	return block
   203  }
   204  
   205  // creates 16 28-bit blocks rotated according
   206  // to the rotation schedule
   207  func ksRotate(in uint32) (out []uint32) {
   208  	out = make([]uint32, 16)
   209  	last := in
   210  	for i := 0; i < 16; i++ {
   211  		// 28-bit circular left shift
   212  		left := (last << (4 + ksRotations[i])) >> 4
   213  		right := (last << 4) >> (32 - ksRotations[i])
   214  		out[i] = left | right
   215  		last = out[i]
   216  	}
   217  	return
   218  }
   219  
   220  // creates 16 56-bit subkeys from the original key
   221  func (c *desCipher) generateSubkeys(keyBytes []byte) {
   222  	// apply PC1 permutation to key
   223  	key := binary.BigEndian.Uint64(keyBytes)
   224  	permutedKey := permuteBlock(key, permutedChoice1[:])
   225  
   226  	// rotate halves of permuted key according to the rotation schedule
   227  	leftRotations := ksRotate(uint32(permutedKey >> 28))
   228  	rightRotations := ksRotate(uint32(permutedKey<<4) >> 4)
   229  
   230  	// generate subkeys
   231  	for i := 0; i < 16; i++ {
   232  		// combine halves to form 56-bit input to PC2
   233  		pc2Input := uint64(leftRotations[i])<<28 | uint64(rightRotations[i])
   234  		// apply PC2 permutation to 7 byte input
   235  		c.subkeys[i] = unpack(permuteBlock(pc2Input, permutedChoice2[:]))
   236  	}
   237  }
   238  
   239  // Expand 48-bit input to 64-bit, with each 6-bit block padded by extra two bits at the top.
   240  // By doing so, we can have the input blocks (four bits each), and the key blocks (six bits each) well-aligned without
   241  // extra shifts/rotations for alignments.
   242  func unpack(x uint64) uint64 {
   243  	var result uint64
   244  
   245  	result = ((x>>(6*1))&0xff)<<(8*0) |
   246  		((x>>(6*3))&0xff)<<(8*1) |
   247  		((x>>(6*5))&0xff)<<(8*2) |
   248  		((x>>(6*7))&0xff)<<(8*3) |
   249  		((x>>(6*0))&0xff)<<(8*4) |
   250  		((x>>(6*2))&0xff)<<(8*5) |
   251  		((x>>(6*4))&0xff)<<(8*6) |
   252  		((x>>(6*6))&0xff)<<(8*7)
   253  
   254  	return result
   255  }
   256
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