[Bug 2065932] [NEW] Only adds the weak key for PPAs dual-signed with both weak and strong keys

[Anders Kaseorg]

Public bug reported:

After running ‘add-apt-repository ppa:git-core/ppa’ on Ubuntu 24.04,
‘apt update’ gives this warning:

W: https://ppa.launchpadcontent.net/git-
core/ppa/ubuntu/dists/noble/InRelease: Signature by key
E1DD270288B4E6030699E45FA1715D88E1DF1F24 uses weak algorithm (rsa1024)

But this PPA is dual-signed by two keys, only one of which is weak.
add-apt-repository has chosen to install the rsa1024 key in
sources.list.d.  It should choose the rsa4096 key instead.

$ curl 'https://ppa.launchpadcontent.net/git-core/ppa/ubuntu/dists/noble/InRelease' | gpg
…
gpg: Signature made Thu 16 May 2024 05:22:18 AM PDT
gpg:                using RSA key F911AB184317630C59970973E363C90F8F1B6217
gpg: Good signature from "Launchpad PPA for Ubuntu Git Maintainers" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: F911 AB18 4317 630C 5997  0973 E363 C90F 8F1B 6217
gpg: Signature made Thu 16 May 2024 05:22:18 AM PDT
gpg:                using RSA key E1DD270288B4E6030699E45FA1715D88E1DF1F24
gpg: Good signature from "Launchpad PPA for Ubuntu Git Maintainers" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: E1DD 2702 88B4 E603 0699  E45F A171 5D88 E1DF 1F24
$ gpg --list-keys F911AB184317630C59970973E363C90F8F1B6217 E1DD270288B4E6030699E45FA1715D88E1DF1F24
pub   rsa1024 2009-01-22 [SC]
      E1DD270288B4E6030699E45FA1715D88E1DF1F24
uid           [ unknown] Launchpad PPA for Ubuntu Git Maintainers

pub   rsa4096 2024-04-24 [SC]
      F911AB184317630C59970973E363C90F8F1B6217
uid           [ unknown] Launchpad PPA for Ubuntu Git Maintainers

Context: https://discourse.ubuntu.com/t/new-requirements-for-apt-
repository-signing-in-24-04/42854

** Affects: software-properties (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2065932

Title:
  Only adds the weak key for PPAs dual-signed with both weak and strong
  keys

Status in software-properties package in Ubuntu:
  New

Bug description:
  After running ‘add-apt-repository ppa:git-core/ppa’ on Ubuntu 24.04,
  ‘apt update’ gives this warning:

  W: https://ppa.launchpadcontent.net/git-
  core/ppa/ubuntu/dists/noble/InRelease: Signature by key
  E1DD270288B4E6030699E45FA1715D88E1DF1F24 uses weak algorithm (rsa1024)

  But this PPA is dual-signed by two keys, only one of which is weak.
  add-apt-repository has chosen to install the rsa1024 key in
  sources.list.d.  It should choose the rsa4096 key instead.

  $ curl 'https://ppa.launchpadcontent.net/git-core/ppa/ubuntu/dists/noble/InRelease' | gpg
  …
  gpg: Signature made Thu 16 May 2024 05:22:18 AM PDT
  gpg:                using RSA key F911AB184317630C59970973E363C90F8F1B6217
  gpg: Good signature from "Launchpad PPA for Ubuntu Git Maintainers" [unknown]
  gpg: WARNING: This key is not certified with a trusted signature!
  gpg:          There is no indication that the signature belongs to the owner.
  Primary key fingerprint: F911 AB18 4317 630C 5997  0973 E363 C90F 8F1B 6217
  gpg: Signature made Thu 16 May 2024 05:22:18 AM PDT
  gpg:                using RSA key E1DD270288B4E6030699E45FA1715D88E1DF1F24
  gpg: Good signature from "Launchpad PPA for Ubuntu Git Maintainers" [unknown]
  gpg: WARNING: This key is not certified with a trusted signature!
  gpg:          There is no indication that the signature belongs to the owner.
  Primary key fingerprint: E1DD 2702 88B4 E603 0699  E45F A171 5D88 E1DF 1F24
  $ gpg --list-keys F911AB184317630C59970973E363C90F8F1B6217 E1DD270288B4E6030699E45FA1715D88E1DF1F24
  pub   rsa1024 2009-01-22 [SC]
        E1DD270288B4E6030699E45FA1715D88E1DF1F24
  uid           [ unknown] Launchpad PPA for Ubuntu Git Maintainers

  pub   rsa4096 2024-04-24 [SC]
        F911AB184317630C59970973E363C90F8F1B6217
  uid           [ unknown] Launchpad PPA for Ubuntu Git Maintainers

  Context: https://discourse.ubuntu.com/t/new-requirements-for-apt-
  repository-signing-in-24-04/42854

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