In asymmetric (public-key) cryptography, what is a key pair, and which key does each party use?

Everyone generates a pair — a private/secret key they keep, and a public key they share. To send a secret you encrypt with the recipient's public key; they decrypt with their own private key.

• Private key (secret key, sk) — kept secret, never shared. Used to decrypt messages sent to you.
• Public key (pk) — published openly. Others use it to encrypt messages to you.

The two keys are mathematically linked: what one key locks, only the other key in the pair can unlock. Crucially, knowing the public key does not let you derive the private key.

Tip: Think of the public key as an open padlock you hand out freely; anyone can snap it shut on a box, but only your private key opens it.