Kerberized fedmsg? Probably not.

Feb 25, 2014 | categories: fedmsg, kerberos, fedora View Comments

Last week, someone contacted me about deploying fedmsg in a kerberos-enabled environment. They wanted to know if they could re-used their krb infrastructure to determine the authenticity of messages. I'm writing this blog post to help me think it through.

First, how fedmsg works

fedmsg doesn't encrypt its communications at all. Messages are either signed using X509 cert/key pairs and verified against against a Certificate Authority cert or signed and verified with GPG keys. Those keys/certs (as of now) need to be pre-shared.

Anyone can read the bus at any point. Anyone can write to "the bus" at any point. Only some messages are trusted. Read more in the fedmsg.crypto docs.

How I think Kerberos works

(... but I'm probably wrong. Have mercy on me).

Kerberos has two central servers that are used to manage trust between clients and servers:

  • The Authentication Server
  • The Ticket Granting Server

When a client wants to connect to a server...

  • It first contacts the Authentication Server. The Authentication Server responds with two things:
    • A "Client <-> Ticket-Granting Server" session key that is encrypted with the client's secret key.
    • A "Ticket Granting" Ticket that is encrypted with the Ticket Granting server's secret key.
  • The client then does two things:
    • The client decrypts the first part (the session key) which it can then use to contact the ticket granting server (it sends an "Authenticator"; it has a session with it now).
    • It sends the second part -- the encrypted Ticket Granting Ticket -- to the Ticket Granting server plus the ID of the service it ultimately wants to connect to.
  • The Ticket Granting Server then does two things:
    • The Ticket Granting server decrypts the Ticket Granting Ticket (with its own secret key)
    • It also uses the session key to decrypt the Authenticator.
  • Now the Ticket Granting Server is assured that the client is who they say they are. The Ticket Granting server then does two more things:
    • The Ticket Granting Server sends a "Client <-> The Service It Wanted To Connect To" ticket back to the client which is encrypted with The Service It Wanted To Connect To's secret key.
    • It also sends a session key for the client <-> server session back to the client.
  • Dutifully, the client sends this new ticket to the server with which it ultimately wants to be in a authenticated relationship.
  • That server then decrypts all this. It increments a timestamp and re-encrypts it with their new session key. It sends this back to the client.

It looks like when you issue kinit on a machine, that completes step 1. Steps 2-6 are carried out independantly by programs.

Kerberized fedmsg?

The advantage of pulling off a kerberized fedmsg would be that you wouldn't have to set up a separate public key infrastructure just for fedmsg -- you could re-use your kerberos infrastructure. You wouldn't have to generate a CA cert. You wouldn't have to generate certs for each sending service, sign them with the CA cert, and distribute them, manage them...

Is it possible?

Any fedmsg publishing process would have to first assume that the Ticket Granting Ticket is already set up via kinit. What happens next? Usually that TGT is used to request another ticket specifically for communication between the requesting program and another specific service. fedmsg doesn't work that way... Messages are published to anyone/everyone -- an unspecifed set of recipients. It is confusing because the client/server relationship is all twisted.

In step 2, there is no "target service ID" to request.

In step 4, there is no "target service" with a secret key to encrypt anything for.

Even if we faked something out with a 'virtual service' with its on secret key, to whom would the fedmsg publisher send the encrypted ticket in step 5?

The prospects don't look good for a kerberized fedmsg, but I'm open to ideas if you have them. Hit me up in #fedora-apps on freenode or on the messaging-sig if you want to talk more about it.

UPDATE - 2014-02-25, Simo Sorce tells me that kx509 is a thing that could be used to acquire a short-lived certificate as long as you have a TGT. Apparently it needs some work first, though.

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