Downloading http://security.debian.org/dists/stretch/updates/InRelease...
gpgv:  RSA key ID C857C906
gpgv: Good signature from "Debian Security Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>"
gpgv:  RSA key ID 331F7F50
gpgv: Good signature from "Debian Security Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>"

Mirror [mirror19]: http://security.debian.org/ stretch/updates [src] successfully added.
You can run 'aptly mirror update mirror19' to download repository contents.