EDIT : si si, ce sont des vraies clés.
Bah je ne comprends pas, je viens même de le tester de mon côté, sur un Debian Jessie.
root@TestSMF ~# gpg --recv-keys 9D6D8F6BC857C906
gpg: directory `/root/.gnupg' created
gpg: new configuration file `/root/.gnupg/gpg.conf' created
gpg: WARNING: options in `/root/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/root/.gnupg/secring.gpg' created
gpg: keyring `/root/.gnupg/pubring.gpg' created
gpg: no keyserver known (use option --keyserver)
gpg: keyserver receive failed: bad URI
Même erreur que toi. Donc j'installe debian-keyring debian-archive-keyring
root@TestSMF ~# apt-get install debian-keyring debian-archive-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
debian-archive-keyring is already the newest version.
The following NEW packages will be installed:
debian-keyring
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 52.4 MB of archives.
After this operation, 58.0 MB of additional disk space will be used.
Do you want to continue? [Y/n]
Get:1 http://http.debian.net/debian/ jessie/main debian-keyring all 2015.04.10 [52.4 MB]
Fetched 52.4 MB in 3s (14.7 MB/s)
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously unselected package debian-keyring.
(Reading database ... 25495 files and directories currently installed.)
Preparing to unpack .../debian-keyring_2015.04.10_all.deb ...
Unpacking debian-keyring (2015.04.10) ...
Setting up debian-keyring (2015.04.10) ...
Counting objects: 1556, done.
Delta compression using up to 12 threads.
Compressing objects: 100% (940/940), done.
Writing objects: 100% (1556/1556), done.
Total 1556 (delta 127), reused 1556 (delta 127)
Et là je peux bien obtenir les clés.
root@TestSMF ~# gpg --recv-keys 9D6D8F6BC857C906
gpg: requesting key C857C906 from hkp server keys.gnupg.net
gpg: key C857C906: public key "Debian Security Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
root@TestSMF ~# gpg --export 9D6D8F6BC857C906 | apt-key add -
OK
root@TestSMF ~# gpg --recv-keys CBF8D6FD518E17E1
gpg: requesting key 518E17E1 from hkp server keys.gnupg.net
gpg: key 518E17E1: public key "Jessie Stable Release Key <debian-release@lists.debian.org>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
root@TestSMF ~# gpg --export CBF8D6FD518E17E1 | apt-key add -
OK