1. Download backports GPG key

The key ID is on the backports website. We will save it to a temporary keyring.

$ gpg --no-default-keyring \
      --keyring /tmp/backports.gpg \
      --keyserver hkp://subkeys.pgp.net \
      --recv-keys 16BA136C
gpg: keyring `/tmp/backports.gpg' created
gpg: requesting key 16BA136C from hkp server subkeys.pgp.net
gpg: key 16BA136C: public key "Backports.org Archive Key <ftp-master@backports.org>" imported
gpg: public key of ultimately trusted key EF6CC3C1 not found
gpg: public key of ultimately trusted key 78733EF8 not found
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   2  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 2u
gpg: Total number processed: 1
gpg:               imported: 1

2. Check the backports key's signatures

Check the signatures using the Debian developers keyring (part of the debian-keyring package).

$ gpg --no-default-keyring \
      --keyring /tmp/backports.gpg \
      --keyring /usr/share/keyrings/debian-keyring.gpg \
      --check-sigs 16BA136C
pub   1024D/16BA136C 2005-08-21
uid                  Backports.org Archive Key <ftp-master@backports.org>
sig!         7E7B8AC9 2005-11-20  Joerg Jaspert <joerg@debian.org>
sig!3        16BA136C 2005-08-21  Backports.org Archive Key <ftp-master@backports.org>
sig!3        16BA136C 2005-08-21  Backports.org Archive Key <ftp-master@backports.org>
sub   2048g/5B82CECE 2005-08-21
sig!         16BA136C 2005-08-21  Backports.org Archive Key <ftp-master@backports.org>

3 signatures not checked due to missing keys

You can see three self-signatures, and one valid signature by a Debian developer.

3. Add the backports key to apt's keyring

$ gpg --no-default-keyring \
      --keyring /tmp/backports.gpg \
      --export 16BA136C | 
      sudo apt-key add -
OK