sshdes.c

一个支持FTP,SFTP的客户端程序

#include <assert.h>
#include "ssh.h"


/* des.c - implementation of DES
 */

/*
 * Description of DES
 * ------------------
 *
 * Unlike the description in FIPS 46, I'm going to use _sensible_ indices:
 * bits in an n-bit word are numbered from 0 at the LSB to n-1 at the MSB.
 * And S-boxes are indexed by six consecutive bits, not by the outer two
 * followed by the middle four.
 *
 * The DES encryption routine requires a 64-bit input, and a key schedule K
 * containing 16 48-bit elements.
 *
 *   First the input is permuted by the initial permutation IP.
 *   Then the input is split into 32-bit words L and R. (L is the MSW.)
 *   Next, 16 rounds. In each round:
 *     (L, R) <- (R, L xor f(R, K[i]))
 *   Then the pre-output words L and R are swapped.
 *   Then L and R are glued back together into a 64-bit word. (L is the MSW,
 *     again, but since we just swapped them, the MSW is the R that came out
 *     of the last round.)
 *   The 64-bit output block is permuted by the inverse of IP and returned.
 *
 * Decryption is identical except that the elements of K are used in the
 * opposite order. (This wouldn't work if that word swap didn't happen.)
 *
 * The function f, used in each round, accepts a 32-bit word R and a
 * 48-bit key block K. It produces a 32-bit output.
 *
 *   First R is expanded to 48 bits using the bit-selection function E.
 *   The resulting 48-bit block is XORed with the key block K to produce
 *     a 48-bit block X.
 *   This block X is split into eight groups of 6 bits. Each group of 6
 *     bits is then looked up in one of the eight S-boxes to convert
 *     it to 4 bits. These eight groups of 4 bits are glued back
 *     together to produce a 32-bit preoutput block.
 *   The preoutput block is permuted using the permutation P and returned.
 *
 * Key setup maps a 64-bit key word into a 16x48-bit key schedule. Although
 * the approved input format for the key is a 64-bit word, eight of the
 * bits are discarded, so the actual quantity of key used is 56 bits.
 *
 *   First the input key is converted to two 28-bit words C and D using
 *     the bit-selection function PC1.
 *   Then 16 rounds of key setup occur. In each round, C and D are each
 *     rotated left by either 1 or 2 bits (depending on which round), and
 *     then converted into a key schedule element using the bit-selection
 *     function PC2.
 *
 * That's the actual algorithm. Now for the tedious details: all those
 * painful permutations and lookup tables.
 *
 * IP is a 64-to-64 bit permutation. Its output contains the following
 * bits of its input (listed in order MSB to LSB of output).
 *
 *    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
 *
 * E is a 32-to-48 bit selection function. Its output contains the following
 * bits of its input (listed in order MSB to LSB of output).
 *
 *    0 31 30 29 28 27 28 27 26 25 24 23 24 23 22 21 20 19 20 19 18 17 16 15
 *   16 15 14 13 12 11 12 11 10  9  8  7  8  7  6  5  4  3  4  3  2  1  0 31
 *
 * The S-boxes are arbitrary table-lookups each mapping a 6-bit input to a
 * 4-bit output. In other words, each S-box is an array[64] of 4-bit numbers.
 * The S-boxes are listed below. The first S-box listed is applied to the
 * most significant six bits of the block X; the last one is applied to the
 * least significant.
 *
 *   14  0  4 15 13  7  1  4  2 14 15  2 11 13  8  1
 *    3 10 10  6  6 12 12 11  5  9  9  5  0  3  7  8
 *    4 15  1 12 14  8  8  2 13  4  6  9  2  1 11  7
 *   15  5 12 11  9  3  7 14  3 10 10  0  5  6  0 13
 *
 *   15  3  1 13  8  4 14  7  6 15 11  2  3  8  4 14
 *    9 12  7  0  2  1 13 10 12  6  0  9  5 11 10  5
 *    0 13 14  8  7 10 11  1 10  3  4 15 13  4  1  2
 *    5 11  8  6 12  7  6 12  9  0  3  5  2 14 15  9
 *
 *   10 13  0  7  9  0 14  9  6  3  3  4 15  6  5 10
 *    1  2 13  8 12  5  7 14 11 12  4 11  2 15  8  1
 *   13  1  6 10  4 13  9  0  8  6 15  9  3  8  0  7
 *   11  4  1 15  2 14 12  3  5 11 10  5 14  2  7 12
 *
 *    7 13 13  8 14 11  3  5  0  6  6 15  9  0 10  3
 *    1  4  2  7  8  2  5 12 11  1 12 10  4 14 15  9
 *   10  3  6 15  9  0  0  6 12 10 11  1  7 13 13  8
 *   15  9  1  4  3  5 14 11  5 12  2  7  8  2  4 14
 *
 *    2 14 12 11  4  2  1 12  7  4 10  7 11 13  6  1
 *    8  5  5  0  3 15 15 10 13  3  0  9 14  8  9  6
 *    4 11  2  8  1 12 11  7 10  1 13 14  7  2  8 13
 *   15  6  9 15 12  0  5  9  6 10  3  4  0  5 14  3
 *
 *   12 10  1 15 10  4 15  2  9  7  2 12  6  9  8  5
 *    0  6 13  1  3 13  4 14 14  0  7 11  5  3 11  8
 *    9  4 14  3 15  2  5 12  2  9  8  5 12 15  3 10
 *    7 11  0 14  4  1 10  7  1  6 13  0 11  8  6 13
 *
 *    4 13 11  0  2 11 14  7 15  4  0  9  8  1 13 10
 *    3 14 12  3  9  5  7 12  5  2 10 15  6  8  1  6
 *    1  6  4 11 11 13 13  8 12  1  3  4  7 10 14  7
 *   10  9 15  5  6  0  8 15  0 14  5  2  9  3  2 12
 *
 *   13  1  2 15  8 13  4  8  6 10 15  3 11  7  1  4
 *   10 12  9  5  3  6 14 11  5  0  0 14 12  9  7  2
 *    7  2 11  1  4 14  1  7  9  4 12 10 14  8  2 13
 *    0 15  6 12 10  9 13  0 15  3  3  5  5  6  8 11
 *
 * P is a 32-to-32 bit permutation. Its output contains the following
 * bits of its input (listed in order MSB to LSB of output).
 *
 *   16 25 12 11  3 20  4 15 31 17  9  6 27 14  1 22
 *   30 24  8 18  0  5 29 23 13 19  2 26 10 21 28  7
 *
 * PC1 is a 64-to-56 bit selection function. Its output is in two words,
 * C and D. The word C contains the following bits of its input (listed
 * in order MSB to LSB of output).
 *
 *    7 15 23 31 39 47 55 63  6 14 22 30 38 46
 *   54 62  5 13 21 29 37 45 53 61  4 12 20 28
 *
 * And the word D contains these bits.
 *
 *    1  9 17 25 33 41 49 57  2 10 18 26 34 42
 *   50 58  3 11 19 27 35 43 51 59 36 44 52 60
 *
 * PC2 is a 56-to-48 bit selection function. Its input is in two words,
 * C and D. These are treated as one 56-bit word (with C more significant,
 * so that bits 55 to 28 of the word are bits 27 to 0 of C, and bits 27 to
 * 0 of the word are bits 27 to 0 of D). The output contains the following
 * bits of this 56-bit input word (listed in order MSB to LSB of output).
 *
 *   42 39 45 32 55 51 53 28 41 50 35 46 33 37 44 52 30 48 40 49 29 36 43 54
 *   15  4 25 19  9  1 26 16  5 11 23  8 12  7 17  0 22  3 10 14  6 20 27 24
 */

/*
 * Implementation details
 * ----------------------
 * 
 * If you look at the code in this module, you'll find it looks
 * nothing _like_ the above algorithm. Here I explain the
 * differences...
 *
 * Key setup has not been heavily optimised here. We are not
 * concerned with key agility: we aren't codebreakers. We don't
 * mind a little delay (and it really is a little one; it may be a
 * factor of five or so slower than it could be but it's still not
 * an appreciable length of time) while setting up. The only tweaks
 * in the key setup are ones which change the format of the key
 * schedule to speed up the actual encryption. I'll describe those
 * below.
 *
 * The first and most obvious optimisation is the S-boxes. Since
 * each S-box always targets the same four bits in the final 32-bit
 * word, so the output from (for example) S-box 0 must always be
 * shifted left 28 bits, we can store the already-shifted outputs
 * in the lookup tables. This reduces lookup-and-shift to lookup,
 * so the S-box step is now just a question of ORing together eight
 * table lookups.
 *
 * The permutation P is just a bit order change; it's invariant
 * with respect to OR, in that P(x)|P(y) = P(x|y). Therefore, we
 * can apply P to every entry of the S-box tables and then we don't
 * have to do it in the code of f(). This yields a set of tables
 * which might be called SP-boxes.
 *
 * The bit-selection function E is our next target. Note that E is
 * immediately followed by the operation of splitting into 6-bit
 * chunks. Examining the 6-bit chunks coming out of E we notice
 * they're all contiguous within the word (speaking cyclically -
 * the end two wrap round); so we can extract those bit strings
 * individually rather than explicitly running E. This would yield
 * code such as
 *
 *     y |= SPboxes[0][ (rotl(R, 5) ^  top6bitsofK) & 0x3F ];
 *     t |= SPboxes[1][ (rotl(R,11) ^ next6bitsofK) & 0x3F ];
 *
 * and so on; and the key schedule preparation would have to
 * provide each 6-bit chunk separately.
 *
 * Really we'd like to XOR in the key schedule element before
 * looking up bit strings in R. This we can't do, naively, because
 * the 6-bit strings we want overlap. But look at the strings:
 *
 *       3322222222221111111111
 * bit   10987654321098765432109876543210
 * 
 * box0  XXXXX                          X
 * box1     XXXXXX
 * box2         XXXXXX
 * box3             XXXXXX
 * box4                 XXXXXX
 * box5                     XXXXXX
 * box6                         XXXXXX
 * box7  X                          XXXXX
 *
 * The bit strings we need to XOR in for boxes 0, 2, 4 and 6 don't
 * overlap with each other. Neither do the ones for boxes 1, 3, 5
 * and 7. So we could provide the key schedule in the form of two
 * words that we can separately XOR into R, and then every S-box
 * index is available as a (cyclically) contiguous 6-bit substring
 * of one or the other of the results.
 *
 * The comments in Eric Young's libdes implementation point out
 * that two of these bit strings require a rotation (rather than a
 * simple shift) to extract. It's unavoidable that at least _one_
 * must do; but we can actually run the whole inner algorithm (all
 * 16 rounds) rotated one bit to the left, so that what the `real'
 * DES description sees as L=0x80000001 we see as L=0x00000003.
 * This requires rotating all our SP-box entries one bit to the
 * left, and rotating each word of the key schedule elements one to
 * the left, and rotating L and R one bit left just after IP and
 * one bit right again just before FP. And in each round we convert
 * a rotate into a shift, so we've saved a few per cent.
 *
 * That's about it for the inner loop; the SP-box tables as listed
 * below are what I've described here (the original S value,
 * shifted to its final place in the input to P, run through P, and
 * then rotated one bit left). All that remains is to optimise the
 * initial permutation IP.
 *
 * IP is not an arbitrary permutation. It has the nice property
 * that if you take any bit number, write it in binary (6 bits),
 * permute those 6 bits and invert some of them, you get the final
 * position of that bit. Specifically, the bit whose initial
 * position is given (in binary) as fedcba ends up in position
 * AcbFED (where a capital letter denotes the inverse of a bit).
 *
 * We have the 64-bit data in two 32-bit words L and R, where bits
 * in L are those with f=1 and bits in R are those with f=0. We
 * note that we can do a simple transformation: suppose we exchange
 * the bits with f=1,c=0 and the bits with f=0,c=1. This will cause
 * the bit fedcba to be in position cedfba - we've `swapped' bits c
 * and f in the position of each bit!
 * 
 * Better still, this transformation is easy. In the example above,
 * bits in L with c=0 are bits 0x0F0F0F0F, and those in R with c=1
 * are 0xF0F0F0F0. So we can do
 *
 *     difference = ((R >> 4) ^ L) & 0x0F0F0F0F
 *     R ^= (difference << 4)
 *     L ^= difference
 *
 * to perform the swap. Let's denote this by bitswap(4,0x0F0F0F0F).
 * Also, we can invert the bit at the top just by exchanging L and
 * R. So in a few swaps and a few of these bit operations we can
 * do:
 * 
 * Initially the position of bit fedcba is     fedcba
 * Swap L with R to make it                    Fedcba
 * Perform bitswap( 4,0x0F0F0F0F) to make it   cedFba
 * Perform bitswap(16,0x0000FFFF) to make it   ecdFba
 * Swap L with R to make it                    EcdFba
 * Perform bitswap( 2,0x33333333) to make it   bcdFEa
 * Perform bitswap( 8,0x00FF00FF) to make it   dcbFEa
 * Swap L with R to make it                    DcbFEa
 * Perform bitswap( 1,0x55555555) to make it   acbFED
 * Swap L with R to make it                    AcbFED
 *
 * (In the actual code the four swaps are implicit: R and L are
 * simply used the other way round in the first, second and last
 * bitswap operations.)
 *
 * The final permutation is just the inverse of IP, so it can be
 * performed by a similar set of operations.
 */

typedef struct {
    word32 k0246[16], k1357[16];
    word32 iv0, iv1;
} DESContext;

#define rotl(x, c) ( (x << c) | (x >> (32-c)) )
#define rotl28(x, c) ( ( (x << c) | (x >> (28-c)) ) & 0x0FFFFFFF)

static word32 bitsel(word32 * input, const int *bitnums, int size)
{
    word32 ret = 0;
    while (size--) {
	int bitpos = *bitnums++;
	ret <<= 1;
	if (bitpos >= 0)
	    ret |= 1 & (input[bitpos / 32] >> (bitpos % 32));
    }
    return ret;
}

static void des_key_setup(word32 key_msw, word32 key_lsw, DESContext * sched)
{

    static const int PC1_Cbits[] = {
	7, 15, 23, 31, 39, 47, 55, 63, 6, 14, 22, 30, 38, 46,
	54, 62, 5, 13, 21, 29, 37, 45, 53, 61, 4, 12, 20, 28
    };
    static const int PC1_Dbits[] = {
	1, 9, 17, 25, 33, 41, 49, 57, 2, 10, 18, 26, 34, 42,
	50, 58, 3, 11, 19, 27, 35, 43, 51, 59, 36, 44, 52, 60
    };
    /*
     * The bit numbers in the two lists below don't correspond to
     * the ones in the above description of PC2, because in the
     * above description C and D are concatenated so `bit 28' means
     * bit 0 of C. In this implementation we're using the standard
     * `bitsel' function above and C is in the second word, so bit
     * 0 of C is addressed by writing `32' here.
     */
    static const int PC2_0246[] = {
	49, 36, 59, 55, -1, -1, 37, 41, 48, 56, 34, 52, -1, -1, 15, 4,
	25, 19, 9, 1, -1, -1, 12, 7, 17, 0, 22, 3, -1, -1, 46, 43
    };
    static const int PC2_1357[] = {
	-1, -1, 57, 32, 45, 54, 39, 50, -1, -1, 44, 53, 33, 40, 47, 58,
	-1, -1, 26, 16, 5, 11, 23, 8, -1, -1, 10, 14, 6, 20, 27, 24
    };
    static const int leftshifts[] =
	{ 1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1 };

    word32 C, D;
    word32 buf[2];
    int i;

    buf[0] = key_lsw;
    buf[1] = key_msw;

    C = bitsel(buf, PC1_Cbits, 28);
    D = bitsel(buf, PC1_Dbits, 28);

    for (i = 0; i < 16; i++) {