mdw@git.distorted.org.uk Git - secnet/blob - dh.c

   1 #include <stdio.h>
   2 #include <gmp.h>
   3 #include <limits.h>
   4
   5 #include "secnet.h"
   6 #include "util.h"
   7
   8 struct dh {
   9     closure_t cl;
  10     struct dh_if ops;
  11     struct cloc loc;
  12     MP_INT p,g; /* prime modulus and generator */
  13 };
  14
  15 static string_t dh_makepublic(void *sst, uint8_t *secret, int32_t secretlen)
  16 {
  17     struct dh *st=sst;
  18     string_t r;
  19     MP_INT a, b; /* a is secret key, b is public key */
  20
  21     mpz_init(&a);
  22     mpz_init(&b);
  23
  24     read_mpbin(&a, secret, secretlen);
  25
  26     mpz_powm(&b, &st->g, &a, &st->p);
  27
  28     r=write_mpstring(&b);
  29
  30     mpz_clear(&a);
  31     mpz_clear(&b);
  32     return r;
  33 }
  34
  35 static dh_makeshared_fn dh_makeshared;
  36 static void dh_makeshared(void *sst, uint8_t *secret, int32_t secretlen,
  37                           cstring_t rempublic, uint8_t *sharedsecret,
  38                           int32_t buflen)
  39 {
  40     struct dh *st=sst;
  41     MP_INT a, b, c;
  42
  43     mpz_init(&a);
  44     mpz_init(&b);
  45     mpz_init(&c);
  46
  47     read_mpbin(&a, secret, secretlen);
  48     mpz_set_str(&b, rempublic, 16);
  49
  50     mpz_powm(&c, &b, &a, &st->p);
  51
  52     write_mpbin(&c,sharedsecret,buflen);
  53
  54     mpz_clear(&a);
  55     mpz_clear(&b);
  56     mpz_clear(&c);
  57 }
  58
  59 static list_t *dh_apply(closure_t *self, struct cloc loc, dict_t *context,
  60                         list_t *args)
  61 {
  62     struct dh *st;
  63     string_t p,g;
  64     item_t *i;
  65
  66     st=safe_malloc(sizeof(*st),"dh_apply");
  67     st->cl.description="dh";
  68     st->cl.type=CL_DH;
  69     st->cl.apply=NULL;
  70     st->cl.interface=&st->ops;
  71     st->ops.st=st;
  72     st->ops.makepublic=dh_makepublic;
  73     st->ops.makeshared=dh_makeshared;
  74     st->loc=loc;
  75     /* We have two string arguments: the first is the modulus, and the
  76        second is the generator. Both are in hex. */
  77     i=list_elem(args,0);
  78     if (i) {
  79         if (i->type!=t_string) {
  80             cfgfatal(i->loc,"diffie-hellman","first argument must be a "
  81                      "string\n");
  82         }
  83         p=i->data.string;
  84         if (mpz_init_set_str(&st->p,p,16)!=0) {
  85             cfgfatal(i->loc,"diffie-hellman","\"%s\" is not a hex number "
  86                      "string\n",p);
  87         }
  88     } else {
  89         cfgfatal(loc,"diffie-hellman","you must provide a prime modulus\n");
  90     }
  91
  92     i=list_elem(args,1);
  93     if (i) {
  94         if (i->type!=t_string) {
  95             cfgfatal(i->loc,"diffie-hellman","second argument must be a "
  96                      "string\n");
  97         }
  98         g=i->data.string;
  99         if (mpz_init_set_str(&st->g,g,16)!=0) {
 100             cfgfatal(i->loc,"diffie-hellman","\"%s\" is not a hex number "
 101                      "string\n",g);
 102         }
 103     } else {
 104         cfgfatal(loc,"diffie-hellman","you must provide a generator\n");
 105     }
 106
 107     i=list_elem(args,2);
 108     if (i && i->type==t_bool && i->data.bool==False) {
 109         Message(M_INFO,"diffie-hellman (%s:%d): skipping modulus "
 110                 "primality check\n",loc.file,loc.line);
 111     } else {
 112         /* Test that the modulus is really prime */
 113         if (mpz_probab_prime_p(&st->p,5)==0) {
 114             cfgfatal(loc,"diffie-hellman","modulus must be a prime\n");
 115         }
 116     }
 117
 118     size_t sz=mpz_sizeinbase(&st->p,2)/8;
 119     if (sz>INT_MAX) {
 120         cfgfatal(loc,"diffie-hellman","modulus far too large\n");
 121     }
 122     if (mpz_cmp(&st->g,&st->p) >= 0) {
 123         cfgfatal(loc,"diffie-hellman","generator must be less than modulus\n");
 124     }
 125
 126     st->ops.len=sz;
 127
 128     st->ops.ceil_len=(mpz_sizeinbase(&st->p,2)+7)/8;
 129     /* According to the docs, mpz_sizeinbase(,256) is allowed to return
 130      * an answer which is 1 too large.  But mpz_sizeinbase(,2) isn't. */
 131
 132     return new_closure(&st->cl);
 133 }
 134
 135 void dh_module(dict_t *dict)
 136 {
 137     add_closure(dict,"diffie-hellman",dh_apply);
 138 }