ParagonIE_Sodium_Core_Ed25519::pk_to_curve25519( string $pk ): string

131    public static function pk_to_curve25519($pk)132    {133        if (self::small_order($pk)) {134            throw new SodiumException('Public key is on a small order');135        }136        $A = self::ge_frombytes_negate_vartime(self::substr($pk, 0, 32));137        if (!self::is_on_main_subgroup($A)) {138            throw new SodiumException('Public key is not on a member of the main subgroup');139        }140 141        # fe_1(one_minus_y);142        # fe_sub(one_minus_y, one_minus_y, A.Y);143        # fe_invert(one_minus_y, one_minus_y);144        $one_minux_y = self::fe_invert(145            self::fe_sub(146                self::fe_1(),147                $A->Y148            )149        );150 151        # fe_1(x);152        # fe_add(x, x, A.Y);153        # fe_mul(x, x, one_minus_y);154        $x = self::fe_mul(155            self::fe_add(self::fe_1(), $A->Y),156            $one_minux_y157        );158 159        # fe_tobytes(curve25519_pk, x);160        return self::fe_tobytes($x);161    }