Cannot add GPG key: General Error

You're probably getting this error because Ubuntu 19.04 "Disco Dingo" went out of support this past January.

However you can try to get the the key manually...

[1] Add the repository key...

    $ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys B7579F80E494ED3406A59DF9081525E2B4F1283B

[2] Verify the key was added. You will see the following line...

    pub rsa4096 2015-11-05 [SC]
          B757 9F80 E494 ED34 06A5 9DF9 0815 25E2 B4F1 283B
    uid [ unknown] Launchpad PPA for PJ Singh

[3] Make sure your /etc/apt/sources.list.d/cubic-wizard-ubuntu-release-impish.list file looks like this:

    deb http://ppa.launchpad.net/cubic-wizard/release/ubuntu/ impish main
    # deb-src http://ppa.launchpad.net/cubic-wizard/release/ubuntu/ impish main

    (It will be OK to use "impish" because the Cubic code-base is the same for all officially supported Ubuntu releases from 18.04 and above).

Thank you for your help, but the problem is still with me.

This time, I have dependency errors:

cubic:
 Depends: binwalk but it is not going to be installed
 Depends: isolinux but it is not going to be installed
  Depends: python3-apt (>=1.6.5) but 1.6.4 is to be installed
 Depends: python3-magic but it is not going to be installed
 Depends: python3-packaging but it is not going to be installed
 Depends: python3-pydbus but it is not going to be installed
 Depends: qemu-system-gui (>=1:4.2) but it is not installable or
  base-files (<=11) but 19.3.0 is to be installed
 Depends: qemu-system-x86 but it is not going to be installed
 Depends: systemd-container but it is not going to be installed
 Depends: xorriso but it is not going to be installed

All the missing packages you listed are in the universe repository.

[4]
The dependency errors may be fixed using:

    $ sudo apt-add-repository universe

Otherwise, you can manually add the universe repository by using nano to edit:

    /etc/apt/sources.list
    ...or...
    /etc/apt/sources.list.d/official-package-repositories.list

    (Note: I ~think~ Linux Mint uses official-package-repositories.list).

[5]
I ~think~ Linux Mint 19.x is based on Ubuntu 18.04 "Bionic Beaver".

Your sources should look something like this:

    deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
    deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
    deb http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
    deb http://security.ubuntu.com/ubuntu bionic-security universe

[6]
Update your packages cache:

    $ sudo apt update

[7]
Optionally, I suggest upgrading all your packages to the latest available versions.

    $ sudo apt upgrade

[8]
After you've resolved all of your package dependencies, there is one more issue you will run into on Linux Mint.

Cubic needs the base-files package version <=10.1ubuntu2.2 (for "Bionic" based systems) or the qemu-system-gui package version >=1:4.2.

In Linux Mint 19.x, this base-files package is labeled as version "19.3.1" (!!!), and this confuses the Cubic installer.

To work around this, we can temporarily install a "fake" qemu-system-gui package to circumvent the package dependency.

    $ sudo apt-get install equivs
    $ cd ~/Documents
    $ equivs-control qemu-system-gui
    $ sed -i "s|^Package.*|Package: qemu-system-gui|g" qemu-system-gui
    $ sed -i "s|^# Version.*|Version: 1:4.2|g" qemu-system-gui
    $ equivs-build qemu-system-gui
    $ sudo dpkg -i qemu-system-gui_4.2_all.deb

[9]
Install Cubic

    $ sudo apt install --no-install-recommends cubic

[10]
Finally, uninstall the "fake" qemu-system-gui package, while keeping Cubic.

    $ sudo dpkg -r --force depends qemu-system-gui

Thanks for the prompt reply.

However, I was so impatient that I tried another method: going to the
Classics repository and downloading another version from there.

That out of the way, I will keep your reply stored until I find
time/necessity to use the information.

Thanks!

On 2021-11-21 4:35 p.m., Cubic PPA wrote:
> Your question #699574 on Cubic changed:
> https://answers.launchpad.net/cubic/+question/699574
>
> Status: Open => Answered
>
> Cubic PPA proposed the following answer:
> All the missing packages you listed are in the universe repository.
>
> [4]
> The dependency errors may be fixed using:
>
> $ sudo apt-add-repository universe
>
> Otherwise, you can manually add the universe repository by using nano to
> edit:
>
> /etc/apt/sources.list
> ...or...
> /etc/apt/sources.list.d/official-package-repositories.list
>
> (Note: I ~think~ Linux Mint uses official-package-
> repositories.list).
>
> [5]
> I ~think~ Linux Mint 19.x is based on Ubuntu 18.04 "Bionic Beaver".
>
> Your sources should look something like this:
>
> deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
> deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
> deb http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
> deb http://security.ubuntu.com/ubuntu bionic-security universe
>
> [6]
> Update your packages cache:
>
> $ sudo apt update
>
> [7]
> Optionally, I suggest upgrading all your packages to the latest available versions.
>
> $ sudo apt upgrade
>
> [8]
> After you've resolved all of your package dependencies, there is one more issue you will run into on Linux Mint.
>
> Cubic needs the base-files package version <=10.1ubuntu2.2 (for "Bionic"
> based systems) or the qemu-system-gui package version >=1:4.2.
>
> In Linux Mint 19.x, this base-files package is labeled as version
> "19.3.1" (!!!), and this confuses the Cubic installer.
>
> To work around this, we can temporarily install a "fake" qemu-system-gui
> package to circumvent the package dependency.
>
> $ sudo apt-get install equivs
> $ cd ~/Documents
> $ equivs-control qemu-system-gui
> $ sed -i "s|^Package.*|Package: qemu-system-gui|g" qemu-system-gui
> $ sed -i "s|^# Version.*|Version: 1:4.2|g" qemu-system-gui
> $ equivs-build qemu-system-gui
> $ sudo dpkg -i qemu-system-gui_4.2_all.deb
>
> [9]
> Install Cubic
>
> $ sudo apt install --no-install-recommends cubic
>
> [10]
> Finally, uninstall the "fake" qemu-system-gui package, while keeping Cubic.
>
> $ sudo dpkg -r --force depends qemu-system-gui
>

Superseded this question with the following question/answer for installing the latest version of Cubic in Linux Mint 19.x...

https://answers.launchpad.net/cubic/+question/699671

Bug #1952680, "Cannot install Cubic in Linux Mint 19.x" has been opened to address the installation issue.

Fix released in version 2021.11-63.

To install Cubic...

    $ sudo apt-add-repository universe
    $ sudo apt-add-repository ppa:cubic-wizard/release
    $ sudo apt update
    $ sudo apt install --no-install-recommends cubic

Notes:

• This fix addresses the base-files package and qemu-system-gui package dependency issue when installing Cubic in Linux Mint. (See Comment # 3 above).

• This fix does not address the GPG key error. (See comment # 1 above for the solution to this issue).