gpg-serve-key

This script allows to transfer a public/private GPG key from a server to another device where communication is only possible over https. Note that in general this should not be a first choice. For example, if you have ssh access, a better way to transfer a key would be

ssh user@remote gpg2 --export-secret-key KEYID | gpg2 --import

However, transfer over https is usually a better choice than e.g. emailing an exported secret-key file to yourself. The one particular use case motivating this script was the import of a secret key into the Pass iOS app.

While transfer over https in principle makes it accessible to anyone, the script takes strong measures to protect the key:

• They key is directly read through a pipe from the gpg executable. The secret key is never written to disk

• The server encrypts the communication with SSL (that is, using the https protocol) by default. While this creates the additional overhead of requiring valid SSL certificates for the public hostname under which the server will be reached, it is essential to guarantee that the key cannot be sniffed in transit. For use within a trusted network, the encryption can be disabled, although you are strongly discouraged from doing so.

• The key is exposed at a url that contains a random token and using a random port number (by default), e.g. for the KEYID 57A6CAA6

  https://michaelgoerz.net:47409/v1f4Y7XixMQ/57A6CAA6-secret.key
  

• Brute-forcing the token is prevented through rate limiting, that is, by an exponentially increasing delay after an invalid request

• The server responds with HTTP headers that disable caching by the client.

• The server writes log messages about every served request. This allows to monitor for unexpected access and to detect if the key has been compromised (as a last resort)

Through the --serve-file option, files in addition to the GPG key may be served (e.g. a private SSH key)

Requirements

• Python >= 3.5
• click package
• A server that is accessible through a public hostname, with GPG installed and the private key for the KEYID that is to be exported in its keychain. Check available private keys with gpg2 -K.
• SSL certificates for the public hostname. It is recommended to use Let's Encrypt. You may use an existing certificate for a webserver running on the host. Since gpg-serve-key will run on a non-standard port, it will not be necessary to temporarily suspend the web server.

Usage

Run the script directly as e.g.

./gpg-serve-key \
    --cert-file=/etc/letsencrypt/live/michaelgoerz.net/cert.pem \
    --key-file=/etc/letsencrypt/live/michaelgoerz.net/privkey.pem \
    --host=michaelgoerz.net 57A6CAA6

See ./gpg-serve-key --help for more details. You may use either the short 8-digit key KEYID, or the full length KEYID as shown by gpg -K.

The command will start a temporary webserver at a random port and serve both the public and the private key at URLs such as

https://michaelgoerz.net:47409/v1f4Y7XixMQ/57A6CAA6-public.key
https://michaelgoerz.net:47409/v1f4Y7XixMQ/57A6CAA6-secret.key

If using a Cloudflare proxy for the domain, it must be temporarily disabled. Make sure any firewall running on the server is set up allow access to the port. On Ubuntu, to allow access to, e.g., port 47409, run

sudo ufw allow 47409

After importing the keys from the above URLs, stop the server by hitting ctrl+c.

If applicable, remove the firewall rule (sudo ufw delete allow 47409), and re-enable the Cloudflare proxy.