a corner bracket, either of the pair of brackets 「」, serving as a quotation mark in Chinese and Japanese orthography

spare key

(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.

鉤虫: velvet worm

(idiomatic) to hold the key (to)

velvet worm

(cryptography) secret key

keyhole

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