I'm trying to understand how Kerberos works, in particular how the initial client sends a request to the Access Server. Some YouTube videos I've seen say the initial request is partially encrypted using the client's password. First, what is meant by "partially" encrypted? Second, considering a password is just an arbitrary string I didn't think it could be used as a symmetric key. Can any string be a symmetric key? I thought it's a special process to derive keys.

In another video I heard the message is not encrypted but a nonce is used to prevent a replay attack. If this is the case, how exactly does a nonce prevent a replay attack? The initial request must contain the password/hash, otherwise where's the actual security from any machine requesting a ticket?


1 Answer 1


Note that it's the Authentication Service, not access service.

Why wouldn't a password be capable of being a symmetric key? It just needs to be the right length. Since Kerberos doesn't require users passwords to be a certain length what actually happens is the password is passed through a key derivation function (KDF). This output is considered the user long term credential. Both the client and AS need to know this long term credential.

What happens is the client sends the AS-REQ and the AS responds saying the user needs to do pre-auth. The client will resend the original request, but then the client will encrypt the current time to the long term credential and stick it in the pa-data list. This is the partial encryption bit. Only the timestamp is encrypted where the rest of the message is cleartext.

  • "Why wouldn't a password be capable of being a symmetric key?" so anything can be a symmetric key?
    – northerner
    Commented Apr 22, 2021 at 10:54
  • Yes, as long as both parties agree to it. Whether something is a good symmetric key is something else entirely though.
    – Steve
    Commented Apr 22, 2021 at 15:13
  • This is only true for symmetric keys, right? Asymmetric keys must be created in a special way and can't be anything.
    – northerner
    Commented Apr 22, 2021 at 20:54
  • Right. Asymmetric keys exist as pairs and have a special relationship. Each key is generally based on random prime numbers, and the protection they provide is derived from computing a math problem that is very hard to reverse without knowledge of the primes. Ideal symmetric keys are just perfectly random numbers.
    – Steve
    Commented Apr 22, 2021 at 23:18
  • Any idea what is meant by the request being partially encrypted? Like how can something be partially encrypted. This is where in the video it is stated.
    – northerner
    Commented Apr 25, 2021 at 7:30

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