git.distorted.org.uk Git - u/mdw/putty/blob - sshdh.c

   1 #include "ssh.h"
   2
   3 const struct ssh_kex ssh_diffiehellman = {
   4     "diffie-hellman-group1-sha1"
   5 };
   6
   7 const struct ssh_kex ssh_diffiehellman_gex = {
   8     "diffie-hellman-group-exchange-sha1"
   9 };
  10
  11 /*
  12  * The prime p used in the key exchange.
  13  */
  14 static unsigned char P[] = {
  15     0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xC9, 0x0F, 0xDA, 0xA2,
  16     0x21, 0x68, 0xC2, 0x34, 0xC4, 0xC6, 0x62, 0x8B, 0x80, 0xDC, 0x1C, 0xD1,
  17     0x29, 0x02, 0x4E, 0x08, 0x8A, 0x67, 0xCC, 0x74, 0x02, 0x0B, 0xBE, 0xA6,
  18     0x3B, 0x13, 0x9B, 0x22, 0x51, 0x4A, 0x08, 0x79, 0x8E, 0x34, 0x04, 0xDD,
  19     0xEF, 0x95, 0x19, 0xB3, 0xCD, 0x3A, 0x43, 0x1B, 0x30, 0x2B, 0x0A, 0x6D,
  20     0xF2, 0x5F, 0x14, 0x37, 0x4F, 0xE1, 0x35, 0x6D, 0x6D, 0x51, 0xC2, 0x45,
  21     0xE4, 0x85, 0xB5, 0x76, 0x62, 0x5E, 0x7E, 0xC6, 0xF4, 0x4C, 0x42, 0xE9,
  22     0xA6, 0x37, 0xED, 0x6B, 0x0B, 0xFF, 0x5C, 0xB6, 0xF4, 0x06, 0xB7, 0xED,
  23     0xEE, 0x38, 0x6B, 0xFB, 0x5A, 0x89, 0x9F, 0xA5, 0xAE, 0x9F, 0x24, 0x11,
  24     0x7C, 0x4B, 0x1F, 0xE6, 0x49, 0x28, 0x66, 0x51, 0xEC, 0xE6, 0x53, 0x81,
  25     0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
  26 };
  27
  28 /*
  29  * The generator g = 2.
  30  */
  31 static unsigned char G[] = { 2 };
  32
  33 /*
  34  * Variables.
  35  */
  36 static Bignum x, e, p, q, qmask, g;
  37 static int need_to_free_pg;
  38
  39 /*
  40  * Common DH initialisation.
  41  */
  42 static void dh_init(void) {
  43     q = bignum_rshift(p, 1);
  44     qmask = bignum_bitmask(q);
  45 }
  46
  47 /*
  48  * Initialise DH for the standard group1.
  49  */
  50 void dh_setup_group1(void) {
  51     p = bignum_from_bytes(P, sizeof(P));
  52     g = bignum_from_bytes(G, sizeof(G));
  53     dh_init();
  54 }
  55
  56 /*
  57  * Initialise DH for an alternative group.
  58  */
  59 void dh_setup_group(Bignum pval, Bignum gval) {
  60     p = copybn(pval);
  61     g = copybn(gval);
  62     dh_init();
  63 }
  64
  65 /*
  66  * Clean up.
  67  */
  68 void dh_cleanup(void) {
  69     freebn(p);
  70     freebn(g);
  71     freebn(q);
  72     freebn(qmask);
  73 }
  74
  75 /*
  76  * DH stage 1: invent a number x between 1 and q, and compute e =
  77  * g^x mod p. Return e.
  78  *
  79  * If `nbits' is greater than zero, it is used as an upper limit
  80  * for the number of bits in x. This is safe provided that (a) you
  81  * use twice as many bits in x as the number of bits you expect to
  82  * use in your session key, and (b) the DH group is a safe prime
  83  * (which SSH demands that it must be).
  84  *
  85  * P. C. van Oorschot, M. J. Wiener
  86  * "On Diffie-Hellman Key Agreement with Short Exponents".
  87  * Advances in Cryptology: Proceedings of Eurocrypt '96
  88  * Springer-Verlag, May 1996.
  89  */
  90 Bignum dh_create_e(int nbits) {
  91     int i;
  92
  93     int nbytes;
  94     unsigned char *buf;
  95
  96     nbytes = ssh1_bignum_length(qmask);
  97     buf = smalloc(nbytes);
  98
  99     do {
 100         /*
 101          * Create a potential x, by ANDing a string of random bytes
 102          * with qmask.
 103          */
 104         if (x) freebn(x);
 105         if (nbits == 0 || nbits > ssh1_bignum_bitcount(qmask)) {
 106             ssh1_write_bignum(buf, qmask);
 107             for (i = 2; i < nbytes; i++)
 108                 buf[i] &= random_byte();
 109             ssh1_read_bignum(buf, &x);
 110         } else {
 111             int b, nb;
 112             x = bn_power_2(nbits);
 113             nb = 0;
 114             for (i = 0; i < nbits; i++) {
 115                 if (nb == 0) {
 116                     nb = 8;
 117                     b = random_byte();
 118                 }
 119                 bignum_set_bit(x, i, b & 1);
 120                 b >>= 1;
 121                 nb--;
 122             }
 123         }
 124     } while (bignum_cmp(x, One) <= 0 || bignum_cmp(x, q) >= 0);
 125
 126     /*
 127      * Done. Now compute e = g^x mod p.
 128      */
 129     e = modpow(g, x, p);
 130
 131     return e;
 132 }
 133
 134 /*
 135  * DH stage 2: given a number f, compute K = f^x mod p.
 136  */
 137 Bignum dh_find_K(Bignum f) {
 138     Bignum ret;
 139     ret = modpow(f, x, p);
 140     return ret;
 141 }