Consider storing certificate and key in OS X Keychain

[calmh]

On OS X at least, the system keychain is a secure way to store secrets. Secure in that it's encrypted at rest but transparently accessible to the application when the user has logged in and authenticated.

(Probably Windows has some equivalent.)

[canton7]

Make sure we consider multiple syncthing instances on the same machine, and instances installed on portable media.

[Avamander]

On most Linux distros there is some kind of keychain where the secrets could be stored.

[Ferroin]

Other things to consider:

1. Windows has such an interface too.
2. On at least Linux, accessing the key-ring is dependent on running in a user session, not just under that particular username. Mandating this for storage would make it impossible to run Syncthing as a daemon at system startup on Linux.
3. Both Windows and Linux prompt the user for credentials to access things on their key-ring.