Digital signature when only one person has public/private key

Question

Hello I have a scenarios where a client and a bank must communicate in a secure way and they must use electronic signature but only the bank has public/private key. How will the communication occur? Meaning how will they authenticate each other who will give the key to who how will the encryption occur?

Answer 1

It can't. The bank must have a pair of key (private and public) representing the bank, and the client must also have a pair of keys representing the client. If only the bank has a pair of key only the banks identity can be verified. Example: the bank sends a public message to all clients and signs it with its private key. All the clients can by using the banks public key indeed verify that the message comes from the bank. However a client can also send an encrypted message to the bank using the banks public key, and only the bank can indeed read this message by using its private key, but the bank can not determine the identity of the client. The clients identity can of course be determined by other means for instance by a login credentials.