Line data    Source code

       1             : #include "fd_secp256k1_private.h"
       2             : 
       3             : 
       4             : /* Given the coordinate X and the odd-ness of the Y coordinate, recovers Y and
       5             :    returns the affine group element. Returns NULL if there is no valid pair. */
       6             : static inline fd_secp256k1_point_t *
       7         122 : fd_secp256k1_recovery_y( fd_secp256k1_point_t *r, fd_secp256k1_fp_t const *x, int odd ) {
       8         122 :   fd_secp256k1_fp_t x2[1], x3[1];
       9             : 
      10             :   /* x^3 + b */
      11         122 :   fd_secp256k1_fp_sqr( x2, x );
      12         122 :   fd_secp256k1_fp_mul( x3, x2, x );
      13         122 :   fd_secp256k1_fp_add( x3, x3, fd_secp256k1_const_b_mont );
      14             : 
      15             :   /* y^2 = x^3 + b <=> y = sqrt(x^3 + b) */
      16         122 :   if( FD_UNLIKELY( !fd_secp256k1_fp_sqrt( r->y, x3 ) ) ) {
      17           2 :     return NULL;
      18           2 :   }
      19             : 
      20         120 :   if( fd_secp256k1_fp_is_odd( r->y ) != odd ) {
      21           6 :     fd_secp256k1_fp_negate( r->y, r->y );
      22           6 :   }
      23             : 
      24         120 :   fd_secp256k1_fp_set( r->x, x );
      25         120 :   fd_secp256k1_fp_set( r->z, fd_secp256k1_const_one_mont );
      26         120 :   return r;
      27         122 : }
      28             : 
      29             : uchar *
      30             : fd_secp256k1_recover( uchar        public_key[64],
      31             :                       uchar const  msg_hash[32],
      32             :                       uchar const  sig[64],
      33         128 :                       int          recovery_id ) {
      34         128 :   if( FD_UNLIKELY( !( recovery_id>=0 && recovery_id<=3 ) ) ) {
      35             :     /* COV: the callers do the same check */
      36           5 :     return NULL;
      37           5 :   }
      38             : 
      39         123 :   fd_secp256k1_scalar_t s[1];
      40         123 :   fd_secp256k1_scalar_t rs[1];
      41         123 :   if( FD_UNLIKELY( !fd_secp256k1_scalar_frombytes( rs, &sig[  0 ] ) ) ) {
      42           1 :     return NULL;
      43           1 :   }
      44         122 :   if( FD_UNLIKELY( !fd_secp256k1_scalar_frombytes(  s, &sig[ 32 ] ) ) ) {
      45           0 :     return NULL;
      46           0 :   }
      47             : 
      48         122 :   fd_secp256k1_fp_t r[1];
      49         122 :   bignum_tomont_p256k1( r->limbs, rs->limbs );
      50             : 
      51         122 :   if( recovery_id & 2 ) {
      52             :     /* If rs >= p - n, return NULL. Otherwise, add the n to r.
      53             :        https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/modules/recovery/main_impl.h#L104-L109 */
      54           0 :     if( FD_UNLIKELY( fd_uint256_cmp( rs, fd_secp256k1_const_p_minus_n ) >= 0 ) ) {
      55           0 :       return NULL;
      56           0 :     }
      57             :     /* Note that *only* r is incremented, rs is left unchanged. */
      58           0 :     fd_secp256k1_fp_add( r, r, fd_secp256k1_const_n_mont );
      59           0 :   }
      60             : 
      61             :   /* Recover the full public key group element. */
      62         122 :   fd_secp256k1_point_t a[1];
      63         122 :   if( FD_UNLIKELY( !fd_secp256k1_recovery_y( a, r, recovery_id & 1 ) ) ) {
      64           2 :     return NULL;
      65           2 :   }
      66             : 
      67         120 :   fd_uint256_t msg[1];
      68         120 :   memcpy( msg, msg_hash, 32 );
      69         120 :   fd_uint256_bswap( msg, msg );
      70             :   /* The message scalar is unconditionally reduced to the scalar field.
      71             :      https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/scalar_4x64_impl.h#L151 */
      72         120 :   bignum_mod_n256k1_4( msg->limbs, (ulong *)msg->limbs );
      73         120 :   fd_secp256k1_scalar_tomont( msg, msg );
      74             : 
      75         120 :   fd_secp256k1_scalar_t rn[1], u1[1], u2[1];
      76         120 :   fd_secp256k1_point_t pubkey[1];
      77             : 
      78             :   /* We delay converting rs into montgomery domain since
      79             :      we may need to perform the comparison against p-n first. */
      80         120 :   fd_secp256k1_scalar_tomont( s, s );
      81             : 
      82             :   /* Unfortunately s2n-bignum has no API for performing
      83             :      in-montgomery inversion, so we invert and then convert. */
      84         120 :   fd_secp256k1_scalar_invert( rn, rs );
      85         120 :   fd_secp256k1_scalar_tomont( rn, rn );
      86             : 
      87         120 :   fd_secp256k1_scalar_mul   ( u1, rn, msg );
      88         120 :   fd_secp256k1_scalar_negate( u1, u1      );
      89         120 :   fd_secp256k1_scalar_mul   ( u2, rn, s   );
      90             : 
      91         120 :   fd_secp256k1_scalar_demont( u2, u2 );
      92         120 :   fd_secp256k1_scalar_demont( u1, u1 );
      93         120 :   fd_secp256k1_double_base_mul( pubkey, u1, a, u2 );
      94             : 
      95             :   /* If the computed pubkey is the identity point, we return NULL
      96             :      https://github.com/bitcoin-core/secp256k1/blob/v0.7.1/src/modules/recovery/main_impl.h#L120 */
      97         120 :   if( FD_UNLIKELY( fd_secp256k1_point_is_identity( pubkey ) ) ) {
      98           0 :     return NULL;
      99           0 :   }
     100             : 
     101             :   /* Serialize the public key into an uncompressed form.
     102             :      The output does not have the recovery_id. */
     103         120 :   fd_secp256k1_point_to_affine( pubkey, pubkey );
     104         120 :   fd_secp256k1_fp_tobytes( &public_key[  0 ], pubkey->x );
     105         120 :   fd_secp256k1_fp_tobytes( &public_key[ 32 ], pubkey->y );
     106         120 :   return public_key;
     107         120 : }