Debian apt-get error There is no public key available for the following key IDs

Hit http://ftp.debian.org jessie/main amd64 Packages
Hit http://ftp.debian.org jessie/main Translation-en
Ign http://ftp.debian.org jessie-backports/main Translation-en_US
Ign http://ftp.debian.org jessie-backports/main Translation-en
Err http://ftp.debian.org jessie-backports/main amd64 Packages
404 Not Found [IP: 130.89.148.12 80]
Fetched 9,839 B in 3s (2,913 B/s)
W: There is no public key available for the following key IDs:
AA8E81B4331F7F50
W: GPG error: http://packages.dotdeb.org jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E9C74FEEA2098A6E
W: Failed to fetch https://packages.sury.org/php/dists/jessie/main/binary-amd64/Packages HttpError404

W: Failed to fetch http://ftp.debian.org/debian/dists/jessie-backports/main/binary-amd64/Packages 404 Not Found [IP: 130.89.148.12 80]

E: Some index files failed to download. They have been ignored, or old ones used instead.

Solution

$ apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E9C74FEEA2098A6E

Alternate Solution

$ sudo aptitude install debian-archive-keyring

Source: http://gnuru.org/article/1486/debian-public-keys-error-2

Also causing problem

https://www.jesusamieiro.com/failed-to-fetch-https-packages-sury-org-php-dists-jessie-inrelease/