key (tool for opening or locking something) / key; clue (important fact for solving something) / lock

(computing) key / (music) key

key

(cryptography) public key

(cryptography) secret key

(cryptography) key exchange - an exchange of peers' keys at initialization phase of connection.

black key (of a piano, etc.)

white key (of a piano, etc.)

(Internet slang) protected account (on social media, especially Twitter and Instagram)

to lock (a door, etc.) / (Internet slang) (of a SNS account) to change privacy setting to limited people and posts and/or profiles can be seen by limited people.