part_of_ecc-eddsa.c

/* Encode (X,Y) using the EdDSA scheme.  MINLEN is the required length
   in bytes for the result.  If WITH_PREFIX is set the returned buffer
   is prefixed with a 0x40 byte.  On success 0 is returned and a
   malloced buffer with the encoded point is stored at R_BUFFER; the
   length of this buffer is stored at R_BUFLEN.  */
static gpg_err_code_t
eddsa_encode_x_y (gcry_mpi_t x, gcry_mpi_t y, unsigned int minlen,
                  int with_prefix,
                  unsigned char **r_buffer, unsigned int *r_buflen)
{
  unsigned char *rawmpi;
  unsigned int rawmpilen;
  int off = with_prefix? 1:0;

  rawmpi = _gcry_mpi_get_buffer_extra (y, minlen, off?-1:0, &rawmpilen, NULL);
  if (!rawmpi)
    return gpg_err_code_from_syserror ();
  if (mpi_test_bit (x, 0) && rawmpilen)
    rawmpi[off + rawmpilen - 1] |= 0x80;  /* Set sign bit.  */
  if (off)
    rawmpi[0] = 0x40;

  *r_buffer = rawmpi;
  *r_buflen = rawmpilen + off;
  return 0;
}