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 const 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 const unsigned char G[] = { 2 };
  32
  33 /*
  34  * Variables.
  35  */
  36 struct dh_ctx {
  37     Bignum x, e, p, q, qmask, g;
  38 };
  39
  40 /*
  41  * Common DH initialisation.
  42  */
  43 static void dh_init(struct dh_ctx *ctx)
  44 {
  45     ctx->q = bignum_rshift(ctx->p, 1);
  46     ctx->qmask = bignum_bitmask(ctx->q);
  47     ctx->x = ctx->e = NULL;
  48 }
  49
  50 /*
  51  * Initialise DH for the standard group1.
  52  */
  53 void *dh_setup_group1(void)
  54 {
  55     struct dh_ctx *ctx = snew(struct dh_ctx);
  56     ctx->p = bignum_from_bytes(P, sizeof(P));
  57     ctx->g = bignum_from_bytes(G, sizeof(G));
  58     dh_init(ctx);
  59     return ctx;
  60 }
  61
  62 /*
  63  * Initialise DH for an alternative group.
  64  */
  65 void *dh_setup_group(Bignum pval, Bignum gval)
  66 {
  67     struct dh_ctx *ctx = snew(struct dh_ctx);
  68     ctx->p = copybn(pval);
  69     ctx->g = copybn(gval);
  70     dh_init(ctx);
  71     return ctx;
  72 }
  73
  74 /*
  75  * Clean up and free a context.
  76  */
  77 void dh_cleanup(void *handle)
  78 {
  79     struct dh_ctx *ctx = (struct dh_ctx *)handle;
  80     freebn(ctx->x);
  81     freebn(ctx->e);
  82     freebn(ctx->p);
  83     freebn(ctx->g);
  84     freebn(ctx->q);
  85     freebn(ctx->qmask);
  86     sfree(ctx);
  87 }
  88
  89 /*
  90  * DH stage 1: invent a number x between 1 and q, and compute e =
  91  * g^x mod p. Return e.
  92  *
  93  * If `nbits' is greater than zero, it is used as an upper limit
  94  * for the number of bits in x. This is safe provided that (a) you
  95  * use twice as many bits in x as the number of bits you expect to
  96  * use in your session key, and (b) the DH group is a safe prime
  97  * (which SSH demands that it must be).
  98  *
  99  * P. C. van Oorschot, M. J. Wiener
 100  * "On Diffie-Hellman Key Agreement with Short Exponents".
 101  * Advances in Cryptology: Proceedings of Eurocrypt '96
 102  * Springer-Verlag, May 1996.
 103  */
 104 Bignum dh_create_e(void *handle, int nbits)
 105 {
 106     struct dh_ctx *ctx = (struct dh_ctx *)handle;
 107     int i;
 108
 109     int nbytes;
 110     unsigned char *buf;
 111
 112     nbytes = ssh1_bignum_length(ctx->qmask);
 113     buf = snewn(nbytes, unsigned char);
 114
 115     do {
 116         /*
 117          * Create a potential x, by ANDing a string of random bytes
 118          * with qmask.
 119          */
 120         if (ctx->x)
 121             freebn(ctx->x);
 122         if (nbits == 0 || nbits > bignum_bitcount(ctx->qmask)) {
 123             ssh1_write_bignum(buf, ctx->qmask);
 124             for (i = 2; i < nbytes; i++)
 125                 buf[i] &= random_byte();
 126             ssh1_read_bignum(buf, &ctx->x);
 127         } else {
 128             int b, nb;
 129             ctx->x = bn_power_2(nbits);
 130             b = nb = 0;
 131             for (i = 0; i < nbits; i++) {
 132                 if (nb == 0) {
 133                     nb = 8;
 134                     b = random_byte();
 135                 }
 136                 bignum_set_bit(ctx->x, i, b & 1);
 137                 b >>= 1;
 138                 nb--;
 139             }
 140         }
 141     } while (bignum_cmp(ctx->x, One) <= 0 || bignum_cmp(ctx->x, ctx->q) >= 0);
 142
 143     sfree(buf);
 144
 145     /*
 146      * Done. Now compute e = g^x mod p.
 147      */
 148     ctx->e = modpow(ctx->g, ctx->x, ctx->p);
 149
 150     return ctx->e;
 151 }
 152
 153 /*
 154  * DH stage 2: given a number f, compute K = f^x mod p.
 155  */
 156 Bignum dh_find_K(void *handle, Bignum f)
 157 {
 158     struct dh_ctx *ctx = (struct dh_ctx *)handle;
 159     Bignum ret;
 160     ret = modpow(f, ctx->x, ctx->p);
 161     return ret;
 162 }