apt update does not work

[Martin <martin94@cryptolab.net>]

I have installed old distribution
# deb cdrom:[Debian GNU/Linux 9.1.0 _Stretch_ - Official amd64 DVD Binary-1 20170722-11:29]/ stretch contrib main non-free
(because that was newest distribution I had on DVD with me)

I configured network and now I am trying to update the system to sid.
I commented everything in /etc/apt/sources.list and added only one line:

deb http://httpredir.debian.org/debian/ sid main contrib non-free non-free-firmware

I got error that that public key is not available:

root@redmoon:~# apt update
Get:1 http://httpredir.debian.org/debian sid InRelease [198 kB]
Err:1 http://httpredir.debian.org/debian sid InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 0E98404D386FA1D9 NO_PUBKEY 6ED0E7B82643E131
Reading package lists... Done
W: GPG error: http://httpredir.debian.org/debian sid InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 0E98404D386FA1D9 NO_PUBKEY 6ED0E7B82643E131
E: The repository 'http://httpredir.debian.org/debian sid InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Where do I get those needed keys and where do I put them?

Martin