[publysher]

No, in order to mess with your repo, an attacker would need your private key. Github only publishes public keys. For more information you can check out https://en.wikipedia.org/wiki/Public-key_cryptography

[karlshea]

GitHub doesn't even have your private key.

[yesco]

Oh yea, duh. Thanks for clearing that up for me, I get the two mixed up a lot. (which is bad)