How to use the public key in Ubuntu 12.04

Hi, all friends!
   I want to install graph_tool in Ubuntu 12.04.
   I add the source to my sources.list and update the source.
   It replys "W: GPG error: oneiric Release: The
following signatures couldn't be verified because the public key is
not available: NO_PUBKEY 697EC21F15F95514"
   I don't know how to add the public key.
   I tried to save
keyfile and "sudo apt-key add keyfile" .
   But it says"gpg: no valid OpenPGP data found."
   What is this problem? Thank you for your suggestion.

2012/6/26 <graph-tool-request(a)>

attachment.html (12.6 KB)

This is because the keyserver is not working very reliably
at the moment, and you probably downloaded a 404 page or something
similar. I've just restarted the server, and it should be working
now. But if you have difficulties again, you can try any other key server, e.g.,