(computing) key / (music) key

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

key

(cryptography) secret key

(cryptography) public key

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

(idiomatic) to hold the key (to)

(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.

black key (of a piano, etc.)