nginx secure link

nginx_secure

# Nginx config example
```
location /asset {
    secure_link $arg_md5,$arg_expires;
    secure_link_md5 "secret$uri$arg_expires";

    if ($secure_link = "") {
        return 403;
    }

    if ($secure_link = "0") {
        return 410;
    }
}
```

# python md5 example
```
from hashlib import md5
import time


# sign url to nginx secure link
def sign_url(url, expire, secret):
    expire_time = str(int(time.time()+expire))
    signurl = md5(secret + url + expire_time).digest().encode('base64')[:-1].replace('+/', '-_').strip('=')
    return signurl, expire_time

```

# URL example
http://example.com/asset/cool.mp3?md5=1OULnuvGJVQvca6QnATewQ&expires=1461305812