Various technologies pertaining to instant messaging are described herein. The control circuit portion determines, in consecutive sub detection periods, the capacitance detection signal change amount of the each detecting electrode in two groups from which the capacitance detection signals are detected in the consecutive sub detection periods, based on the capacitance detection signals consecutively detected from at least one same detecting electrode. At least one pressure sensor for measuring pneumatic pressure in a compressed-air bore of the valve-housing bottom is placed in or on the valve-housing bottom. By analyzing various behavioral characteristics associated with legitimate or fraudulent multi-factor authentication attempts, the BCA system adds another layer of security to online transactions.