ed25519/key_exchange.c

   1 #include "ed25519.h"
   2 #include "fe.h"
   3
   4 void ed25519_key_exchange(unsigned char *shared_secret, const unsigned char *public_key, const unsigned char *private_key) {
   5     unsigned char e[32];
   6     unsigned int i;
   7
   8     fe x1;
   9     fe x2;
  10     fe z2;
  11     fe x3;
  12     fe z3;
  13     fe tmp0;
  14     fe tmp1;
  15
  16     int pos;
  17     unsigned int swap;
  18     unsigned int b;
  19
  20     /* copy the private key and make sure it's valid */
  21     for (i = 0; i < 32; ++i) {
  22         e[i] = private_key[i];
  23     }
  24
  25     e[0] &= 248;
  26     e[31] &= 63;
  27     e[31] |= 64;
  28
  29     /* unpack the public key and convert edwards to montgomery */
  30     /* due to CodesInChaos: montgomeryX = (edwardsY + 1)*inverse(1 - edwardsY) mod p */
  31     fe_frombytes(x1, public_key);
  32     fe_1(tmp1);
  33     fe_add(tmp0, x1, tmp1);
  34     fe_sub(tmp1, tmp1, x1);
  35     fe_invert(tmp1, tmp1);
  36     fe_mul(x1, tmp0, tmp1);
  37
  38     fe_1(x2);
  39     fe_0(z2);
  40     fe_copy(x3, x1);
  41     fe_1(z3);
  42
  43     swap = 0;
  44     for (pos = 254; pos >= 0; --pos) {
  45         b = e[pos / 8] >> (pos & 7);
  46         b &= 1;
  47         swap ^= b;
  48         fe_cswap(x2, x3, swap);
  49         fe_cswap(z2, z3, swap);
  50         swap = b;
  51
  52         /* from montgomery.h */
  53         fe_sub(tmp0, x3, z3);
  54         fe_sub(tmp1, x2, z2);
  55         fe_add(x2, x2, z2);
  56         fe_add(z2, x3, z3);
  57         fe_mul(z3, tmp0, x2);
  58         fe_mul(z2, z2, tmp1);
  59         fe_sq(tmp0, tmp1);
  60         fe_sq(tmp1, x2);
  61         fe_add(x3, z3, z2);
  62         fe_sub(z2, z3, z2);
  63         fe_mul(x2, tmp1, tmp0);
  64         fe_sub(tmp1, tmp1, tmp0);
  65         fe_sq(z2, z2);
  66         fe_mul121666(z3, tmp1);
  67         fe_sq(x3, x3);
  68         fe_add(tmp0, tmp0, z3);
  69         fe_mul(z3, x1, z2);
  70         fe_mul(z2, tmp1, tmp0);
  71     }
  72
  73     fe_cswap(x2, x3, swap);
  74     fe_cswap(z2, z3, swap);
  75
  76     fe_invert(z2, z2);
  77     fe_mul(x2, x2, z2);
  78     fe_tobytes(shared_secret, x2);
  79 }