textbooks
David G. Koontz
koontz at MasPar.COM
Wed Oct 25 10:44:15 PDT 1995
IP/FP and PC1 describe how to get from input bytes to L, R, C and D Blocks:
The Initial Permutation
The Initial Permuation (IP) is a description of how a byte wide interface is
connected to a 64 bit block comprised of two 32 bit blocks (L and R). Consider
a byte wide interface with the bits numbered 1-8. The event numbered bits
go to the L Block and the odd numbered bits go to the R block. Note that the
bit order is big endian, where bit 1 is most significant and bit 8 is least
least significant. The input block is typically loaded as 8 successive byte
loads:
Port MSB7 Input (LR) Left
Bit Bit Block (64 bits) Block (32 bits)
2------6-------58 50 42 34 26 18 10 2 1 2 3 4 5 6 7 8
4------4-------60 52 44 36 28 20 12 4 9 10 11 12 13 14 15 16
6------2-------62 54 46 38 30 22 14 6 17 18 19 20 21 22 23 24
8------0-------64 56 48 40 32 24 16 8 25 26 27 28 29 30 31 32
Right
Block (32 bits)
1------7-------57 49 41 33 25 17 9 1 1 2 3 4 5 6 7 8
3------5-------59 51 43 35 27 19 11 3 9 10 11 12 13 14 15 16
5------3-------61 53 45 37 29 21 13 5 17 18 19 20 21 22 23 24
7------1-------63 55 47 39 31 23 15 7 25 26 27 28 29 30 31 32
Input Byte 8 7 6 5 4 3 2 1
The Final Permutation
The Final Permutation (IP-1) provides the inverse, it standarizes the output
of the R16L16 output block to a byte wide interface. The Output block is
ordered Right then Left to allow complementary operation for subsequent
decryption. Were one to perform an IP followed by IP-1 without any intervening
round iteration operations, one would end up with odd and even bits swapped:
Right Output (R16L16) Standard Port
Block (32 bits) Block (64 bits) Bit Bit
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8---------6--------2
9 10 11 12 13 14 15 16 9 10 11 12 13 14 15 16---------4--------4
17 18 19 20 21 22 23 24 17 18 19 20 21 22 23 24---------2--------6
25 26 27 28 29 30 31 32 25 26 27 28 29 30 31 32---------0--------8
Left
Block (32 bits)
1 2 3 4 5 6 7 8 33 34 35 36 37 38 39 40---------7--------1
9 10 11 12 13 14 15 16 41 42 43 44 45 46 47 48---------5--------3
17 18 19 20 21 22 23 24 49 50 51 52 53 54 55 56---------3--------5
25 26 27 28 29 30 31 32 57 58 59 60 61 62 63 64---------1--------7
Output Byte 8 7 6 5 4 3 2 1
>From FIPS Pub 46-2:
Final Permuation IP-1:
Output Byte
40 8 48 16 56 24 64 32 1
39 7 47 15 55 23 63 31 2
38 6 46 14 54 22 62 30 3
37 5 45 13 53 21 61 29 4
36 4 44 12 52 20 60 28 5
35 3 43 11 51 19 59 27 6
34 2 42 10 50 18 58 26 7
33 1 41 9 49 17 57 25 8
1 2 3 4 5 6 7 8 Port Bit
7 6 5 4 3 2 1 0 MSB7 Bits
In the simplest hardware implementation of DES, the Left and Right blocks are
comprised in hardware of four 8 bit register each. Each 8 bit register can be
serially loaded (IP), serially unloaded (IP-1), or parallel output and parallel
loaded (round interation). DES is an encryption algorithm originally required
to be implemented in hardware, specified in 1977 - predating 16 or 32 bit
microprocessor peripherals.
Permuted Choice 1
PC1 performs a similar function loading the C and D 28 bit registers (comprised
of three 8 bit bidirectional shift register and 1 4 bit bidirectional shift
register, all with parallel outputs). The C and D registers can be serially
loaded (shifting right), or serially shifted left or right in a closed ring
for encryption or decryption.
Port MSB7
Bits Bits
Input (CD) C
Block, 64 bits Block (28 bits)
1--------7------57 49 41 33 25 17 9 1 MS 1 2 3 4 5 6 7 8
2--------6------58 50 42 34 26 18 10 2 9 10 11 12 13 14 15 16
3--------5------59 51 43 35 27 19 11 3 17 18 19 20 21 22 23 24
4--------4------60 52 44 36 ----------- (C(28)) 25 26 27 28
D
Block (28 bits)
7--------1------63 55 47 39 31 23 15 7 1 2 3 4 5 6 7 8
6--------2------62 54 46 38 30 22 14 6 9 10 11 12 13 14 15 16
5--------3------61 53 45 37 29 21 33 5 17 18 19 20 21 22 23 24
4-------(D(25)--------------28 20 12 4 25 26 27 28
8--------0------64 56 48 40 32 24 16 8 LS (parity)
Input Byte 8 7 6 5 4 3 2 1
Note that bit 4 is used as input for both C and D. This implies that C(28)
output is used as the serial input to D(25). The least significant bit is
used for odd parity.
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