When Alice wants to send an encrypted message to Bob, she must use:
- Bob’s public key to encrypt a message for him;
- Her (Alice’s) private key to sign the message;
- Her public key to decrypt own messages (to have an opportunity to read her encrypted message later).
Then Bob must use:
- His private key to decrypt an encrypted message from Alice;
- Alice’s public key to verify her signature (that it’s really a message from Alice).