How to renew an expired key of Ubuntu repository

How to renew an expired key of Ubuntu repository

On my Ubuntu 18 system we cannot update our ReaR snapshot software package anymore, becasue the release key has been expired:

$ sudo apt-key list | grep -A 1 expired
Warning: apt-key output should not be parsed (stdout is not a terminal)
pub   rsa2048 2020-01-17 [SC] [expired: 2022-03-27]
      318C 5FDA D663 3E09 8383  18D2 082C AA5E ADB2 E40A
uid        [ expired] Archiving:Backup:Rear:Snapshot OBS Project <Archiving:Backup:Rear:Snapshot@build.opensuse.org>

We could skip checking expired keys altogether, but that is a really bad idea by creating the following file:

$ sudo echo 'Acquire::Check-Valid-Until no;' > /etc/apt/apt.conf.d/99no-check-valid-until

However, the correct way to renew the expired key is by executing the following:

$ wget -q -O - https://download.opensuse.org/repositories/Archiving:/Backup:/Rear:/Snapshot/xUbuntu_18.04/Release.key | sudo apt-key add -
OK

Now, we can double check if the expired key has been renewed:

$ sudo apt-key list | grep -i ReaR
Warning: apt-key output should not be parsed (stdout is not a terminal)
uid        [ unknown] Archiving:Backup:Rear:Snapshot OBS Project <Archiving:Backup:Rear:Snapshot@build.opensuse.org>

If you have the pgpdump utility installed you can do the following neat thing to inspect the Release key of your distribution:

$ curl -L https://download.opensuse.org/repositories/Archiving:/Backup:/Rear:/Snapshot/xUbuntu_18.04/Release.key | pgpdump 
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1145  100  1145    0     0   4543      0 --:--:-- --:--:-- --:--:--  4543
Old: Public Key Packet(tag 6)(269 bytes)
	Ver 4 - new
	Public key creation time - Fri Jan 17 12:21:38 CET 2020
	Pub alg - RSA Encrypt or Sign(pub 1)
	RSA n(2048 bits) - ...
	RSA e(17 bits) - ...
Old: User ID Packet(tag 13)(94 bytes)
	User ID - Archiving:Backup:Rear:Snapshot OBS Project <Archiving:Backup:Rear:Snapshot@build.opensuse.org>
Old: Signature Packet(tag 2)(318 bytes)
	Ver 4 - new
	Sig type - Positive certification of a User ID and Public Key packet(0x13).
	Pub alg - RSA Encrypt or Sign(pub 1)
	Hash alg - SHA256(hash 8)
	Hashed Sub: signature creation time(sub 2)(4 bytes)
		Time - Thu Mar 17 16:05:47 CET 2022
	Hashed Sub: key flags(sub 27)(1 bytes)
		Flag - This key may be used to certify other keys
		Flag - This key may be used to sign data
	Hashed Sub: key expiration time(sub 9)(4 bytes)
		Time - Sat May 25 17:05:47 CEST 2024
	Hashed Sub: preferred symmetric algorithms(sub 11)(5 bytes)
		Sym alg - AES with 256-bit key(sym 9)
		Sym alg - AES with 192-bit key(sym 8)
		Sym alg - AES with 128-bit key(sym 7)
		Sym alg - CAST5(sym 3)
		Sym alg - Triple-DES(sym 2)
	Hashed Sub: preferred hash algorithms(sub 21)(5 bytes)
		Hash alg - SHA256(hash 8)
		Hash alg - SHA1(hash 2)
		Hash alg - SHA384(hash 9)
		Hash alg - SHA512(hash 10)
		Hash alg - SHA224(hash 11)
	Hashed Sub: preferred compression algorithms(sub 22)(3 bytes)
		Comp alg - ZLIB <RFC1950>(comp 2)
		Comp alg - BZip2(comp 3)
		Comp alg - ZIP <RFC1951>(comp 1)
	Hashed Sub: features(sub 30)(1 bytes)
		Flag - Modification detection (packets 18 and 19)
	Hashed Sub: key server preferences(sub 23)(1 bytes)
		Flag - No-modify
	Sub: issuer key ID(sub 16)(8 bytes)
		Key ID - 0x082CAA5EADB2E40A
	Hash left 2 bytes - 62 2e 
	RSA m^d mod n(2048 bits) - ...
		-> PKCS-1
Old: Signature Packet(tag 2)(70 bytes)
	Ver 4 - new
	Sig type - Positive certification of a User ID and Public Key packet(0x13).
	Pub alg - DSA Digital Signature Algorithm(pub 17)
	Hash alg - SHA1(hash 2)
	Hashed Sub: signature creation time(sub 2)(4 bytes)
		Time - Fri Jan 17 12:21:39 CET 2020
	Sub: issuer key ID(sub 16)(8 bytes)
		Key ID - 0x3B3011B76B9D6523
	Hash left 2 bytes - e9 26 
	DSA r(159 bits) - ...
	DSA s(158 bits) - ...
		-> hash(DSA q bits)

References

[1] Debian release key expired in OpenSuse repo

[2] osc signkey