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
  38 /*
  39  * Common DH initialisation.
  40  */
  41 static void dh_init(void)
  42 {
  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 {
  52     p = bignum_from_bytes(P, sizeof(P));
  53     g = bignum_from_bytes(G, sizeof(G));
  54     dh_init();
  55 }
  56
  57 /*
  58  * Initialise DH for an alternative group.
  59  */
  60 void dh_setup_group(Bignum pval, Bignum gval)
  61 {
  62     p = copybn(pval);
  63     g = copybn(gval);
  64     dh_init();
  65 }
  66
  67 /*
  68  * Clean up.
  69  */
  70 void dh_cleanup(void)
  71 {
  72     freebn(p);
  73     freebn(g);
  74     freebn(q);
  75     freebn(qmask);
  76 }
  77
  78 /*
  79  * DH stage 1: invent a number x between 1 and q, and compute e =
  80  * g^x mod p. Return e.
  81  *
  82  * If `nbits' is greater than zero, it is used as an upper limit
  83  * for the number of bits in x. This is safe provided that (a) you
  84  * use twice as many bits in x as the number of bits you expect to
  85  * use in your session key, and (b) the DH group is a safe prime
  86  * (which SSH demands that it must be).
  87  *
  88  * P. C. van Oorschot, M. J. Wiener
  89  * "On Diffie-Hellman Key Agreement with Short Exponents".
  90  * Advances in Cryptology: Proceedings of Eurocrypt '96
  91  * Springer-Verlag, May 1996.
  92  */
  93 Bignum dh_create_e(int nbits)
  94 {
  95     int i;
  96
  97     int nbytes;
  98     unsigned char *buf;
  99
 100     nbytes = ssh1_bignum_length(qmask);
 101     buf = smalloc(nbytes);
 102
 103     do {
 104         /*
 105          * Create a potential x, by ANDing a string of random bytes
 106          * with qmask.
 107          */
 108         if (x)
 109             freebn(x);
 110         if (nbits == 0 || nbits > bignum_bitcount(qmask)) {
 111             ssh1_write_bignum(buf, qmask);
 112             for (i = 2; i < nbytes; i++)
 113                 buf[i] &= random_byte();
 114             ssh1_read_bignum(buf, &x);
 115         } else {
 116             int b, nb;
 117             x = bn_power_2(nbits);
 118             b = nb = 0;
 119             for (i = 0; i < nbits; i++) {
 120                 if (nb == 0) {
 121                     nb = 8;
 122                     b = random_byte();
 123                 }
 124                 bignum_set_bit(x, i, b & 1);
 125                 b >>= 1;
 126                 nb--;
 127             }
 128         }
 129     } while (bignum_cmp(x, One) <= 0 || bignum_cmp(x, q) >= 0);
 130
 131     /*
 132      * Done. Now compute e = g^x mod p.
 133      */
 134     e = modpow(g, x, p);
 135
 136     return e;
 137 }
 138
 139 /*
 140  * DH stage 2: given a number f, compute K = f^x mod p.
 141  */
 142 Bignum dh_find_K(Bignum f)
 143 {
 144     Bignum ret;
 145     ret = modpow(f, x, p);
 146     return ret;
 147 }