These packages are build for myself and a few friends and are completely unsupported by the Debian Maintainers!
Install if you like but don't blame me if your computer explodes ;-)
But at least these are only simple backports so there shouldn't arise any problems. Of course if there are any dependency problems or other kinds of backporting related bugs I will try to figure them out, just send me an email :)

Simply add the following to your sources.list:

deb https://slashproc.org/debian/dists/bookworm/ ./

Or if you prefer seperate lists in the /etc/apt/sources.list.d/ folder:

sudo wget -O /etc/apt/sources.list.d/slashproc.list https://slashproc.org/debian/dists/bookworm/slashproc.list

If you only want to install an explicit package from the repo you could also use:

deb [arch=amd64 signed-by=/etc/apt/trusted.gpg.d/slashproc.gpg] https://slashproc.org/debian/dists/bookworm/swtpm/binary-amd64 ./

Adding the gpg-key (Yes, I know that apt-key is deprecated):

wget -O- https://slashproc.org/debian/slashproc-debian-new.asc | sudo apt-key --keyring /etc/apt/trusted.gpg.d/slashproc.gpg add -

Now 'apt-key list 6CF8CF58' should output the following:

pub   rsa4096 2019-03-02 [SC] [expires: 2029-02-27]
      DE96 F25A 79A2 BE98 A621  58D3 91B6 B089 6CF8 CF58
uid           [ unknown] Manfred Schmitt (http://slashproc.org/debian/ Archive Key) (signing only) 

After executing 'sudo apt update' you will hopefully be able to install packages from the repository without any warnings from apt.

The Sources are also available in the repository (but not viewable via web), just use the same sources.list line but replace deb with deb-src and the arch with sources, e.g.:

deb-src https://slashproc.org/debian/dists/bookworm/swtpm/sources ./

Or if you want the deb-src for the complete repo use:

deb-src https://slashproc.org/debian/dists/bookworm ./

Current packages:
swtpm