6.6. パッケージ信頼性の確認

Falcot Corp の管理者にとって、セキュリティはとても重要です。つまり管理者は、パッケージが途中で改竄されていないこと、確かに Debian から提供されていることを確認し、確認のとれたパッケージだけをインストールしなければいけません。コンピュータクラッカーは、他の合法的なパッケージに悪意あるコードを追加しようと試みます。そのようなパッケージがインストールされた場合、クラッカーが設計したことは何でも、たとえばパスワードや機密情報の漏洩などでも、実行されます。この危険性を回避するために、Debian は不正改竄を防ぐ封印を提供し、インストール時にそのパッケージが公式メンテナから提供されたものと本当に同じこと、第三者によって変更されていないことを保証しています。

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/
debian-archive-bullseye-automatic.gpg
debian-archive-bullseye-security-automatic.gpg
debian-archive-bullseye-stable.gpg
debian-archive-buster-automatic.gpg
debian-archive-buster-security-automatic.gpg
debian-archive-buster-stable.gpg
debian-archive-stretch-automatic.gpg
debian-archive-stretch-security-automatic.gpg
debian-archive-stretch-stable.gpg

IN PRACTICE 信頼された鍵の追加

第三者のパッケージソースを sources.list に追加したら、APT に対してパッケージソースに対応する GPG 認証鍵を信頼することを伝える必要があります (そうでなければ、APT はそのパッケージソースから提供されるパッケージの信頼性を保証できないとエラーを出し続けます)。最初のステップはもちろん公開鍵を手に入れることです。大抵の場合、公開鍵は小さなテキストファイルの形で配布されています。以降の例ではこの公開鍵を key.asc と呼びます。

To add the key to APT's trusted keyring, the administrator can just put it in a *.gpg file in /etc/apt/trusted.gpg.d/ (*.asc files are discouraged and can cause problems). This is supported since Debian Stretch. With older releases, you had to run apt-key add < key.asc, which is considered obsolete already.

While this is very common behavior, it also creates a possible issue. APT does not know which key belongs to which repository. If one of the keys in APT's keyring matches the signature of an archive, APT considers the archive as authenticated. Once a key has been added to the trusted keyring, a malicious attacker could actually provide its own "version" of a Debian mirror signed by the same key. It might therefor actually be better to put the key as *.gpg file into /usr/share/keyrings/ and use the Signed-By field in sources.list for third party repositories as shown in the example below.

Imagine, that the Falcot Corp. runs their own public repository and you want to use it. They have signed it and provide a publicly available key(ring) as well to download. To restrict the key to the repository, you can simply do this:

$ sudo wget -O falcot-keyring.gpg -P /usr/share/keyrings https://packages.falcot.com/debian/repository.gpg

deb [ signed-by=/usr/share/keyrings/falcot-keyring.gpg ] https://packages.falcot.com/debian bullseye main

Beware that adding and using third party repositories always poses a security and stability risk!

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.

SPECIFIC CASE Unsigned repositories

While in general unsigned repositories shouldn't be used, some users might still require them, for example to provide some locally built packages. APT can be forced to accept such a repository by prepending the repository URL using allow-insecure=yes or trusted=yes. We will demonstrate it using the example from 第 15.3 節「APT 用のパッケージリポジトリの作成」:

deb [ trusted=yes ] http://packages.falcot.com/ updates/
deb [ trusted=yes ] http://packages.falcot.com/ internal/