Private key issue

[Kilim Choi]

Hi

I was just wondering what we can pass for the private keys. I've tried running your app on heroku and I keep getting an error message upon signing in. When it hits /session, I'm getting EC_KEY_check_key: passed a null parameter which I assume is caused by https://github.com/nov/json-jwt/blob/301d718124ede1f850400f405f08f082e373e837/lib/json/jws.rb#L73

But I passed in prime256v1 as private key. But ended up with the error message above.
Then I tried different curves from OpenSSL::PKey::EC.builtin_curves but they didn't work.
I'm just wondering what I'm supposed to send.