Difference between SSL and Kerberos authentication?

Solution 1:

SSL uses public key cryptography:

1. You (or your browser) has a public/private keypair
2. The server has a public/private key as well
3. You generate a symmetric session key
4. You encrypt with the server's public key and send this encrypted session key to the server.
5. The server decrypts the encrypted session key with its private key.
6. You and the server begin communicating using the symmetric session key (basically because symmetric keys are faster).

Kerberos does not use public key cryptography. It uses a trusted 3rd party. Here's a sketch:

1. You both (server and client) prove your identity to a trusted 3rd party (via a secret).
2. When you want to use the server, you check and see that the server is trustworthy. Meanwhile, the server checks to see that you are trustworthy. Now, mutually assured of each others' identity. You can communicate with the server. 2

Solution 2:

While Kerberos and SSL are both protocols, Kerberos is an authentication protocol, but SSL is an encryption protocol. Kerberos usually uses UDP, SSL uses (most of the time) TCP. SSL authentication is usually done by checking the server's and the client's RSA or ECDSA keys embedded in something called X.509 certificates. You're authenticated by your certificate and the corresponding key. With Kerberos, you can be authenticated by your password, or some other way. Windows uses Kerberos for example, when used in domain.

Keep in mind: Recent versions of SSL are called TLS for Transport Layer Security.

Solution 3:

To put simply, Kerberos is a protocol for establishing mutual identity trust, or authentication, for a client and a server, via a trusted third-party, whereas SSL ensures authentication of the server alone, and only if its public key has already been established as trustworthy via another channel. Both provides secure communication between the server and client.

More formally (but without getting into mathematical proofs), given a client C, server S, and a third-party T which both C and S trust:

After Kerbeos authentication, it is established that:

• C believes S is who it intended to contact
• S believes C is who it claims to be
• C believes that it has a secure connection to S
• C believes that S believes it has a secure connection to C
• S believes that it has a secure connection to C
• S believes that C believes it has a secure connection to S

SSL, on the other hand, only establishes that:

• C believes S is who it intended to contact
• C believes it has a secure connection to S
• S believes it has a secure connection to C

Clearly, Kerberos establishes a stronger, more complete trust relationship.

Additionally, to establish the identity of S over SSL, C needs prior knowledge about S, or an external way to confirm this trust. For most people's everyday use, this comes in the form of Root Certificates, and caching of S's certificate for cross-referencing in the future.

Without this prior knowledge, SSL is susceptible to man-in-the-middle attack, where a third-party is able to pretend to be S to C by relaying communication between them using 2 separate secure channels to C and S. To compromise a Kerberos authentication, the eavesdropper must masquerade as T to both S and C. Note, however, that the set of trusts is still unbroken according to the goal of Kerberos, as the end-state is still correct according to the precondition "C and S trusts T".

Finally, as it has been pointed out in a comment, Kerberos can be and has been extended to use SSL-like mechanism for establishing the initial secure connection between C and T.