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        2253 : fd_secp256k1_recovery_y( fd_secp256k1_point_t *r, fd_secp256k1_fp_t const *x, int odd ) {
       7        2253 :   fd_secp256k1_fp_t x2[1], x3[1];
       8             : 
       9             :   /* x^3 + b */
      10        2253 :   fd_secp256k1_fp_sqr( x2, x );
      11        2253 :   fd_secp256k1_fp_mul( x3, x2, x );
      12        2253 :   fd_secp256k1_fp_add( x3, x3, fd_secp256k1_const_b_mont );
      13             : 
      14             :   /* y^2 = x^3 + b <=> y = sqrt(x^3 + b) */
      15        2253 :   if( FD_UNLIKELY( !fd_secp256k1_fp_sqrt( r->y, x3 ) ) ) {
      16          81 :     return NULL;
      17          81 :   }
      18             : 
      19        2172 :   if( fd_secp256k1_fp_is_odd( r->y ) != odd ) {
      20        1063 :     fd_secp256k1_fp_negate( r->y, r->y );
      21        1063 :   }
      22             : 
      23        2172 :   fd_secp256k1_fp_set( r->x, x );
      24        2172 :   fd_secp256k1_fp_set( r->z, fd_secp256k1_const_one_mont );
      25        2172 :   return r;
      26        2253 : }
      27             : 
      28             : uchar *
      29             : fd_secp256k1_recover( uchar        public_key[64],
      30             :                       uchar const  msg_hash[32],
      31             :                       uchar const  sig[64],
      32        2644 :                       int          recovery_id ) {
      33        2644 :   if( FD_UNLIKELY( !( recovery_id>=0 && recovery_id<=3 ) ) ) {
      34             :     /* COV: the callers do the same check */
      35         183 :     return NULL;
      36         183 :   }
      37             : 
      38        2461 :   fd_secp256k1_scalar_t s[1];
      39        2461 :   fd_secp256k1_scalar_t rs[1];
      40        2461 :   if( FD_UNLIKELY( !fd_secp256k1_scalar_frombytes( rs, &sig[  0 ] ) ) ) {
      41           6 :     return NULL;
      42           6 :   }
      43        2455 :   if( FD_UNLIKELY( !fd_secp256k1_scalar_frombytes(  s, &sig[ 32 ] ) ) ) {
      44           2 :     return NULL;
      45           2 :   }
      46             : 
      47        2453 :   fd_secp256k1_fp_t r[1];
      48        2453 :   fd_secp256k1_fp_tomont( r, rs );
      49             : 
      50        2453 :   if( recovery_id & 2 ) {
      51             :     /* If rs >= p - n, return NULL. Otherwise, add the n to r.
      52             :        https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/modules/recovery/main_impl.h#L104-L109 */
      53         203 :     if( FD_UNLIKELY( fd_uint256_cmp( rs, fd_secp256k1_const_p_minus_n ) >= 0 ) ) {
      54         199 :       return NULL;
      55         199 :     }
      56             :     /* Note that *only* r is incremented, rs is left unchanged. */
      57           4 :     fd_secp256k1_fp_add( r, r, fd_secp256k1_const_n_mont );
      58           4 :   }
      59             : 
      60             :   /* Recover the full public key group element. */
      61        2254 :   fd_secp256k1_point_t a[1];
      62        2254 :   if( FD_UNLIKELY( !fd_secp256k1_recovery_y( a, r, recovery_id & 1 ) ) ) {
      63          81 :     return NULL;
      64          81 :   }
      65             : 
      66        2173 :   fd_uint256_t msg[1];
      67        2173 :   memcpy( msg, msg_hash, 32 );
      68        2173 :   fd_uint256_bswap( msg, msg );
      69             :   /* The message scalar is unconditionally reduced to the scalar field.
      70             :      https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/scalar_4x64_impl.h#L151 */
      71        2173 :   fd_secp256k1_scalar_reduce( msg, msg );
      72        2173 :   fd_secp256k1_scalar_tomont( msg, msg );
      73             : 
      74        2173 :   fd_secp256k1_scalar_t rn[1], u1[1], u2[1];
      75        2173 :   fd_secp256k1_point_t pubkey[1];
      76             : 
      77             :   /* We delay converting rs into montgomery domain since
      78             :      we may need to perform the comparison against p-n first. */
      79        2173 :   fd_secp256k1_scalar_tomont( s, s );
      80             : 
      81             :   /* Unfortunately s2n-bignum has no API for performing
      82             :      in-montgomery inversion, so we invert and then convert. */
      83        2173 :   fd_secp256k1_scalar_invert( rn, rs );
      84        2173 :   fd_secp256k1_scalar_tomont( rn, rn );
      85             : 
      86        2173 :   fd_secp256k1_scalar_mul   ( u1, rn, msg );
      87        2173 :   fd_secp256k1_scalar_negate( u1, u1      );
      88        2173 :   fd_secp256k1_scalar_mul   ( u2, rn, s   );
      89             : 
      90        2173 :   fd_secp256k1_scalar_demont( u2, u2 );
      91        2173 :   fd_secp256k1_scalar_demont( u1, u1 );
      92        2173 :   fd_secp256k1_double_base_mul( pubkey, u1, a, u2 );
      93             : 
      94             :   /* If the computed pubkey is the identity point, we return NULL
      95             :      https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/modules/recovery/main_impl.h#L120 */
      96        2173 :   if( FD_UNLIKELY( fd_secp256k1_point_is_identity( pubkey ) ) ) {
      97           2 :     return NULL;
      98           2 :   }
      99             : 
     100             :   /* Serialize the public key into an uncompressed form.
     101             :      The output does not have the recovery_id. */
     102        2171 :   fd_secp256k1_point_to_affine( pubkey, pubkey );
     103        2171 :   fd_secp256k1_fp_tobytes( &public_key[  0 ], pubkey->x );
     104        2171 :   fd_secp256k1_fp_tobytes( &public_key[ 32 ], pubkey->y );
     105        2171 :   return public_key;
     106        2173 : }