Debian – When Did Apt Start Supporting Last Two Releases for Migration?

aptdebianhistory

I was playing with my Debian install today and discovered an interesting thing. If I do for e.g. 

$ apt-key list
/etc/apt/trusted.gpg
--------------------
pub   rsa4096 2019-04-15 [SC] [expires: 2024-04-13]
      12D4 CD60 0C22 40A9 F4A8  2071 D7B0 B669 41D0 1538
uid           [ unknown] riot.im packages <packages@riot.im>
sub   rsa3072 2019-04-15 [S] [expires: 2021-04-14]

pub   rsa4096 2019-04-15 [SC] [expires: 2024-04-13]
      AAF9 AE84 3A75 84B5 A3E4  CD2B CF45 A512 DE2D A058
uid           [ unknown] matrix.org packages <packages@matrix.org>
sub   rsa3072 2019-04-15 [S] [expires: 2021-04-14]

pub   rsa4096 2017-05-22 [SC] [expires: 2025-05-20]
      E1CF 20DD FFE4 B89E 8026  58F1 E0B1 1894 F66A EC98
uid           [ unknown] Debian Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>
sub   rsa4096 2017-05-22 [S] [expires: 2025-05-20]

pub   rsa4096 2014-11-21 [SC] [expires: 2022-11-19]
      D211 6914 1CEC D440 F2EB  8DDA 9D6D 8F6B C857 C906
uid           [ unknown] Debian Security Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>

/etc/apt/trusted.gpg.d/debian-archive-buster-automatic.gpg
----------------------------------------------------------
pub   rsa4096 2019-04-14 [SC] [expires: 2027-04-12]
      80D1 5823 B7FD 1561 F9F7  BCDD DC30 D7C2 3CBB ABEE
uid           [ unknown] Debian Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>
sub   rsa4096 2019-04-14 [S] [expires: 2027-04-12]

/etc/apt/trusted.gpg.d/debian-archive-buster-security-automatic.gpg
-------------------------------------------------------------------
pub   rsa4096 2019-04-14 [SC] [expires: 2027-04-12]
      5E61 B217 265D A980 7A23  C5FF 4DFA B270 CAA9 6DFA
uid           [ unknown] Debian Security Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>
sub   rsa4096 2019-04-14 [S] [expires: 2027-04-12]

/etc/apt/trusted.gpg.d/debian-archive-buster-stable.gpg
-------------------------------------------------------
pub   rsa4096 2019-02-05 [SC] [expires: 2027-02-03]
      6D33 866E DD8F FA41 C014  3AED DCC9 EFBF 77E1 1517
uid           [ unknown] Debian Stable Release Key (10/buster) <debian-release@lists.debian.org>

I was curious as to why it has jessie and stretch keys when I am using debian-buster. In the above these are the two keys – 

pub   rsa4096 2017-05-22 [SC] [expires: 2025-05-20]
      E1CF 20DD FFE4 B89E 8026  58F1 E0B1 1894 F66A EC98
uid           [ unknown] Debian Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>
sub   rsa4096 2017-05-22 [S] [expires: 2025-05-20]

pub   rsa4096 2014-11-21 [SC] [expires: 2022-11-19]
      D211 6914 1CEC D440 F2EB  8DDA 9D6D 8F6B C857 C906
uid           [ unknown] Debian Security Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>

I removed the keys using – 

$ sudo apt-key del E1CF20DDFFE4B89E802658F1E0B11894F66AEC98

as well as – 

$ sudo apt-key del D21169141CECD440F2EB8DDA9D6D8F6BC857C906

when both the keys were deleted I got pub keys errors 

$ sudo apt update
Hit:1 http://cdn-fastly.deb.debian.org/debian buster InRelease                                               
Err:1 http://cdn-fastly.deb.debian.org/debian buster InRelease                           
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
Hit:2 http://cdn-fastly.deb.debian.org/debian-security buster/updates InRelease         
Err:2 http://cdn-fastly.deb.debian.org/debian-security buster/updates InRelease         
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY AA8E81B4331F7F50
Hit:3 http://cdn-fastly.deb.debian.org/debian unstable InRelease                         
Err:3 http://cdn-fastly.deb.debian.org/debian unstable InRelease                         
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
Hit:4 http://cdn-fastly.deb.debian.org/debian experimental InRelease                     
Err:4 http://cdn-fastly.deb.debian.org/debian experimental InRelease                     
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
Hit:5 http://debug.mirrors.debian.org/debian-debug buster-debug InRelease
Err:5 http://debug.mirrors.debian.org/debian-debug buster-debug InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
Hit:6 http://debug.mirrors.debian.org/debian-debug unstable-debug InRelease
Hit:7 http://debug.mirrors.debian.org/debian-debug experimental-debug InRelease
Err:6 http://debug.mirrors.debian.org/debian-debug unstable-debug InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
Err:7 http://debug.mirrors.debian.org/debian-debug experimental-debug InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
Hit:8 https://packages.riot.im/debian buster InRelease
Reading package lists... Done
Building dependency tree       
Reading state information... Done
3 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cdn-fastly.deb.debian.org/debian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cdn-fastly.deb.debian.org/debian-security buster/updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY AA8E81B4331F7F50
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cdn-fastly.deb.debian.org/debian unstable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cdn-fastly.deb.debian.org/debian experimental InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://debug.mirrors.debian.org/debian-debug buster-debug InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://debug.mirrors.debian.org/debian-debug unstable-debug InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://debug.mirrors.debian.org/debian-debug experimental-debug InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: Failed to fetch http://cdn-fastly.deb.debian.org/debian/dists/buster/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: Failed to fetch http://cdn-fastly.deb.debian.org/debian-security/dists/buster/updates/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY AA8E81B4331F7F50
W: Failed to fetch http://cdn-fastly.deb.debian.org/debian/dists/unstable/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: Failed to fetch http://cdn-fastly.deb.debian.org/debian/dists/experimental/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: Failed to fetch http://debug.mirrors.debian.org/debian-debug/dists/buster-debug/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: Failed to fetch http://debug.mirrors.debian.org/debian-debug/dists/unstable-debug/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453EC
W: Failed to fetch http://debug.mirrors.debian.org/debian-debug/dists/experimental-debug/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY 04EE7237B7D453E

My /etc/apt/sources.list is – 

$ cat /etc/apt/sources.list

                               #### Debian buster #########
        deb http://cdn-fastly.deb.debian.org/debian/ buster main contrib non-free
    deb-src http://cdn-fastly.deb.debian.org/debian buster main contrib non-free
        deb http://cdn-fastly.deb.debian.org/debian-security buster/updates main
    deb-src http://cdn-fastly.deb.debian.org/debian-security buster/updates main


                              #### Debian unstable #########
        deb http://cdn-fastly.deb.debian.org/debian unstable main contrib non-free
    deb-src http://cdn-fastly.deb.debian.org/debian unstable main contrib non-free


                           #### Debian experimental #########
       deb http://cdn-fastly.deb.debian.org/debian experimental main contrib
   deb-src http://cdn-fastly.deb.debian.org/debian experimental main contrib

                         ##### Debian Debug packages #######
       deb http://debug.mirrors.debian.org/debian-debug/ buster-debug main
       deb http://debug.mirrors.debian.org/debian-debug/ unstable-debug main
       deb http://debug.mirrors.debian.org/debian-debug/ experimental-debug main


                    ######## Third party repos #######
      deb https://riot.im/packages/debian/ buster main

and had to import them using gpg ( I know it's insecure but had to do -)

# gpg --recv-keys AA8E81B4331F7F50
gpg: key EDA0D2388AE22BA9: 11 signatures not checked due to missing keys
gpg: key EDA0D2388AE22BA9: public key "Debian Security Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1

and then – 

# gpg --export AA8E81B4331F7F50 | apt-key add
OK

I am curious to know when did apt start supporting signatures of old releases alongwith the newest keys ? Is it somewhat of a recent phenomena or a really old phenomena ? I mean whether it was done in Etch (4.0) or later ?

Update – It started as an Strech Install i.e 9 and now has buster.

As far as the other question is concerned /etc/apt/trusted.gpg.d/ has – 

/etc/apt/trusted.gpg.d/debian-archive-buster-automatic.gpg
----------------------------------------------------------
pub   rsa4096 2019-04-14 [SC] [expires: 2027-04-12]
      80D1 5823 B7FD 1561 F9F7  BCDD DC30 D7C2 3CBB ABEE
uid           [ unknown] Debian Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>
sub   rsa4096 2019-04-14 [S] [expires: 2027-04-12]

/etc/apt/trusted.gpg.d/debian-archive-buster-security-automatic.gpg
-------------------------------------------------------------------
pub   rsa4096 2019-04-14 [SC] [expires: 2027-04-12]
      5E61 B217 265D A980 7A23  C5FF 4DFA B270 CAA9 6DFA
uid           [ unknown] Debian Security Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>
sub   rsa4096 2019-04-14 [S] [expires: 2027-04-12]

/etc/apt/trusted.gpg.d/debian-archive-buster-stable.gpg
-------------------------------------------------------
pub   rsa4096 2019-02-05 [SC] [expires: 2027-02-03]
      6D33 866E DD8F FA41 C014  3AED DCC9 EFBF 77E1 1517
uid           [ unknown] Debian Stable Release Key (10/buster) <debian-release@lists.debian.org>

Best Answer

The signing once per release by ftp-master is described at ftp-master.debian.org:

Which release should be signed with which key?

Stable releases are signed by both the ftp-master automatic archive signing key in use at the time of the release, and a per-release stable key. Release files for other releases (proposed-updates, testing, testing-proposed-updates, unstable and experimental) are signed only by the ftp-master automatic key.

The security archive is signed by the ftp-master key only.

The current procedure is that there is one ftp-master key per release (former procedure introduced a new key once per year).

As far as archive.org can go back (2009), this has been the case, including the etch release:

The current procedure is that there is one ftp-master key per release (former procedure introduced a new key once per year).

Archive Keys

Active Signing Keys

The current (2007/etch) key can be downloaded here

It seems related to the creation of the package debian-archive-keyring for etch, which includes the files you deleted, on which apt depends upon since etch:

apt (0.6.46.2) unstable; urgency=low

* debian/control:
- depend on debian-archive-keyring to offer clean upgrade path
(closes:#386800)
[...]

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