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6.6. Checking Package Authenticity

Security is very important for Falcot Corp administrators. Accordingly, they need to ensure that they only install packages which are guaranteed to come from Debian with no tampering on the way. A computer cracker could try to add malicious code to an otherwise legitimate package. Such a package, if installed, could do anything the cracker designed it to do, including for instance disclosing passwords or confidential information. To circumvent this risk, Debian provides a tamper-proof seal to guarantee — at install time — that a package really comes from its official maintainer and hasn't been modified by a third party.
The seal works with a chain of cryptographic hashes and a signature and is explained in detail in apt-secure(8). Starting with Debian 10 Buster the signed file is the InRelease file, provided by the Debian mirrors. There is also a legacy file called Release. Both contain a list of the Packages files (including their compressed forms, Packages.gz and Packages.xz, and the incremental versions), along with their SHA256 hashes, which ensures that the files haven't been tampered with. These Packages files contain a list of the Debian packages available on the mirror, along with their hashes, which ensures in turn that the contents of the packages themselves haven't been altered either. The difference between InRelease and Release is that the former is cryptographically signed in-line, whereas the latter provides a detached signature in the form of the file Release.gpg.
APT needs a set of trusted GnuPG public keys to verify signatures in the InRelease and Release.gpg files available on the mirrors. It gets them from files in /etc/apt/trusted.gpg.d/ and from the /etc/apt/trusted.gpg keyring (managed by the apt-key command). The official Debian keys are provided and kept up-to-date by the debian-archive-keyring package which puts them in /etc/apt/trusted.gpg.d/:
# ls /etc/apt/trusted.gpg.d/
Once the appropriate keys are in the keyring, APT will check the signatures before any risky operation, so that frontends will display a warning if asked to install a package whose authenticity can't be ascertained.
Note, that binary packages are usually not signed. The integrity of a package can only be confirmed by checking its hashsums against a trusted (and possibly signed) hashsum source.