release/src/router/tinc/src/ed25519/add_scalar.c

   1 #include "ed25519.h"
   2 #include "ge.h"
   3 #include "sc.h"
   4
   5
   6 /* see http://crypto.stackexchange.com/a/6215/4697 */
   7 void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar) {
   8     const unsigned char SC_1[32] = {1}; /* scalar with value 1 */
   9
  10     unsigned char n[32];
  11     ge_p3 nB;
  12     ge_p1p1 A_p1p1;
  13     ge_p3 A;
  14     ge_p3 public_key_unpacked;
  15     ge_cached T;
  16
  17     int i;
  18
  19     /* copy the scalar and clear highest bit */
  20     for (i = 0; i < 31; ++i) {
  21         n[i] = scalar[i];
  22     }
  23     n[31] = scalar[31] & 127;
  24
  25     /* private key: a = n + t */
  26     if (private_key) {
  27         sc_muladd(private_key, SC_1, n, private_key);
  28     }
  29
  30     /* public key: A = nB + T */
  31     if (public_key) {
  32         /* if we know the private key we don't need a point addition, which is faster */
  33         /* using a "timing attack" you could find out wether or not we know the private
  34            key, but this information seems rather useless - if this is important pass
  35            public_key and private_key seperately in 2 function calls */
  36         if (private_key) {
  37             ge_scalarmult_base(&A, private_key);
  38         } else {
  39             /* unpack public key into T */
  40             ge_frombytes_negate_vartime(&public_key_unpacked, public_key);
  41             fe_neg(public_key_unpacked.X, public_key_unpacked.X); // undo negate
  42             fe_neg(public_key_unpacked.T, public_key_unpacked.T); // undo negate
  43             ge_p3_to_cached(&T, &public_key_unpacked);
  44
  45             /* calculate n*B */
  46             ge_scalarmult_base(&nB, n);
  47
  48             /* A = n*B + T */
  49             ge_add(&A_p1p1, &nB, &T);
  50             ge_p1p1_to_p3(&A, &A_p1p1);
  51         }
  52
  53         /* pack public key */
  54         ge_p3_tobytes(public_key, &A);
  55     }
  56 }