Line data    Source code

       1             : #include "fd_secp256k1_private.h"
       2             : 
       3             : /* Given the coordinate X and the odd-ness of the Y coordinate, recovers Y and
       4             :    returns the affine group element. Returns NULL if there is no valid pair. */
       5             : static inline fd_secp256k1_point_t *
       6        2256 : fd_secp256k1_recovery_y( fd_secp256k1_point_t *r, fd_secp256k1_fp_t const *x, int odd ) {
       7        2256 :   fd_secp256k1_fp_t x2[1], x3[1];
       8             : 
       9             :   /* x^3 + b */
      10        2256 :   fd_secp256k1_fp_sqr( x2, x );
      11        2256 :   fd_secp256k1_fp_mul( x3, x2, x );
      12        2256 :   fd_secp256k1_fp_add( x3, x3, fd_secp256k1_const_b_mont );
      13             : 
      14             :   /* y^2 = x^3 + b <=> y = sqrt(x^3 + b) */
      15        2256 :   if( FD_UNLIKELY( !fd_secp256k1_fp_sqrt( r->y, x3 ) ) ) {
      16          81 :     return NULL;
      17          81 :   }
      18             : 
      19        2175 :   if( fd_secp256k1_fp_is_odd( r->y ) != odd ) {
      20        1064 :     fd_secp256k1_fp_negate( r->y, r->y );
      21        1064 :   }
      22             : 
      23        2175 :   fd_secp256k1_fp_set( r->x, x );
      24        2175 :   fd_secp256k1_fp_set( r->z, fd_secp256k1_const_one_mont );
      25        2175 :   return r;
      26        2256 : }
      27             : 
      28             : uchar *
      29             : fd_secp256k1_recover( uchar        public_key[64],
      30             :                       uchar const  msg_hash[32],
      31             :                       uchar const  sig[64],
      32        2646 :                       int          recovery_id ) {
      33        2646 :   if( FD_UNLIKELY( !( recovery_id>=0 && recovery_id<=3 ) ) ) {
      34         183 :     return NULL;
      35         183 :   }
      36             : 
      37        2463 :   fd_secp256k1_scalar_t s[1];
      38        2463 :   fd_secp256k1_scalar_t rs[1];
      39        2463 :   if( FD_UNLIKELY( !fd_secp256k1_scalar_frombytes( rs, &sig[  0 ] ) ) ) {
      40           6 :     return NULL;
      41           6 :   }
      42        2457 :   if( FD_UNLIKELY( !fd_secp256k1_scalar_frombytes(  s, &sig[ 32 ] ) ) ) {
      43           2 :     return NULL;
      44           2 :   }
      45             : 
      46        2455 :   fd_secp256k1_fp_t r[1];
      47        2455 :   fd_secp256k1_fp_tomont( r, rs );
      48             : 
      49        2455 :   if( recovery_id & 2 ) {
      50             :     /* If rs >= p - n, return NULL. Otherwise, add the n to r.
      51             :        https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/modules/recovery/main_impl.h#L104-L109 */
      52         203 :     if( FD_UNLIKELY( fd_uint256_cmp( rs, fd_secp256k1_const_p_minus_n ) >= 0 ) ) {
      53         199 :       return NULL;
      54         199 :     }
      55             :     /* Note that *only* r is incremented, rs is left unchanged. */
      56           4 :     fd_secp256k1_fp_add( r, r, fd_secp256k1_const_n_mont );
      57           4 :   }
      58             : 
      59             :   /* Recover the full public key group element. */
      60        2256 :   fd_secp256k1_point_t a[1];
      61        2256 :   if( FD_UNLIKELY( !fd_secp256k1_recovery_y( a, r, recovery_id & 1 ) ) ) {
      62          81 :     return NULL;
      63          81 :   }
      64             : 
      65        2175 :   fd_uint256_t msg[1];
      66        2175 :   memcpy( msg, msg_hash, 32 );
      67        2175 :   fd_uint256_bswap( msg, msg );
      68             :   /* The message scalar is unconditionally reduced to the scalar field.
      69             :      https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/scalar_4x64_impl.h#L151 */
      70        2175 :   fd_secp256k1_scalar_reduce( msg, msg );
      71        2175 :   fd_secp256k1_scalar_tomont( msg, msg );
      72             : 
      73        2175 :   fd_secp256k1_scalar_t rn[1], u1[1], u2[1];
      74        2175 :   fd_secp256k1_point_t pubkey[1];
      75             : 
      76             :   /* We delay converting rs into montgomery domain since
      77             :      we may need to perform the comparison against p-n first. */
      78        2175 :   fd_secp256k1_scalar_tomont( s, s );
      79             : 
      80             :   /* Unfortunately s2n-bignum has no API for performing
      81             :      in-montgomery inversion, so we invert and then convert. */
      82        2175 :   fd_secp256k1_scalar_invert( rn, rs );
      83        2175 :   fd_secp256k1_scalar_tomont( rn, rn );
      84             : 
      85        2175 :   fd_secp256k1_scalar_mul   ( u1, rn, msg );
      86        2175 :   fd_secp256k1_scalar_negate( u1, u1      );
      87        2175 :   fd_secp256k1_scalar_mul   ( u2, rn, s   );
      88             : 
      89        2175 :   fd_secp256k1_scalar_demont( u2, u2 );
      90        2175 :   fd_secp256k1_scalar_demont( u1, u1 );
      91        2175 :   fd_secp256k1_double_base_mul( pubkey, u1, a, u2 );
      92             : 
      93             :   /* If the computed pubkey is the identity point, we return NULL
      94             :      https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/modules/recovery/main_impl.h#L120 */
      95        2175 :   if( FD_UNLIKELY( fd_secp256k1_point_is_identity( pubkey ) ) ) {
      96           2 :     return NULL;
      97           2 :   }
      98             : 
      99             :   /* Serialize the public key into an uncompressed form.
     100             :      The output does not have the recovery_id. */
     101        2173 :   fd_secp256k1_point_to_affine( pubkey, pubkey );
     102        2173 :   fd_secp256k1_fp_tobytes( &public_key[  0 ], pubkey->x );
     103        2173 :   fd_secp256k1_fp_tobytes( &public_key[ 32 ], pubkey->y );
     104        2173 :   return public_key;
     105        2175 : }