Ubuntu 14.04で sudo apt-get update するとエラーが出てました
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://nginx.org trusty Release: The following signatures were invalid: KEYEXPIRED 1471427554
W: Failed to fetch http://nginx.org/packages/ubuntu/dists/trusty/Release
キーが古い？ということで
$ wget https://nginx.org/keys/nginx_signing.key -O - | sudo apt-key add -
これで直りました
