Private Key

Techopedia explains Private Key

A private key is only shared with the key's initiator, which ensures security. For example, A and B are message sender and message recipient, respectively. They have their own public and private keys pair. A, the message initiator (sendor), sends B a message. A's message is encrypted with B’s public key, while B uses its own private key to decrypt A’s received message.

Digital signature (digital certificate) is used to make sure that A is the original message sender. B verifies that fact using the following steps:

• B uses A's public key to decrypt the digital signature or certificate, as A must have used its own private key to encrypt the digital signature or certificate.
• If readable, the digital signature or certificate is authenticated with Certification Authority (CA).

In short, sending messages for encryption involves that sender uses recipient's public key and for encryption of digital certificate it's own private key. At the recipient end for decrypting message, recipient uses its own private key, while for digital certificate decryption, sender's public key is used.