apt-secure

This is a manual page describing APT's security model, not a command.
Read the man page:

$ man apt-secure

apt-secure is a manual page (not a command) that describes APT's archive authentication system. It explains how APT verifies package integrity using cryptographic signatures.
The document covers:
- How Release files are signed with GPG
- How Packages files are verified via checksums
- How to add trusted keys
- Security implications of untrusted repositories

/etc/apt/trusted.gpg.d/

Directory for trusted keyring files used to verify repository signatures.

Package sources, supporting signed-by option for per-repository keys.

Release.gpg

Detached GPG signature for Release file

Release file with inline signature

Stored in /etc/apt/trusted.gpg.d/

Modern method to specify per-repository keys

Understanding apt-secure is important for adding third-party repositories safely. Using allow-insecure or allow-unauthenticated bypasses security.

apt(8), apt-key(8), sources.list(5)