ssh: Certificate authorities

• In increasing level of security:
  • Password = thing you know
  • Key = file (private key) you have
  • Certificate = key + time limited connection to one or more principals (any token string really)
• Problem with keys
  • We employ "trust on first use", TOFU
    • Weak because people don't manually verify host/server public key fingerprints with an secondary channel
  • We don't have expiry
    • Key is valid so long as it's in authorized_keys
• A certificate authority, CA, is just a key pair
• Use the CA to sign a given public key to produce a certificate
  • Associated the key + principals + other SSH config
  • Can have an expiry date
• You can sign host keys or user keys (different flag for host)
• Configure client and/or server to trust the CA public key so it does not need to have seen a particular key before (avoid