electronic key
Refers to a physical device or card used to unlock doors, start cars, etc., often via radio signal or chip.
You enter this apartment building with an electronic key.
I lost my car's electronic key.
electronic cipher; digital encryption key
Used in cryptography and IT contexts for a digital key that encrypts or decrypts data. Less common than sense 1 in everyday language.
The communication is encrypted with an electronic cipher.
Compound of 電子 (electronic) and キー (key), a straightforward modern loanword-based term.