Unformatted text preview: icator. Finally, he compares the information in the authenticator with the information in the ticket, the client’s network address with the address the request was sent from, and the timestamp with the current time. If everything matches, he allows the request to proceed. Checking timestamps assumes that all machines have synchronized clocks, at least to within several minutes. If the time in the request is too far in the future or the past, the TGS treats the request as an attempt to replay a previous request. The TGS should also keep track of all live authenticators, because past requests can have timestamps that are still valid. Another request with the same ticket and timestamp as one already received can be ignored. The TGS responds to a valid request by returning a valid ticket for the client to present to the server. The TGS also creates a new session key for the client and the server, encrypted with the session key shared by the client and the TGS. Both of these messages are then sent back to the client. The client decrypts the message and extracts the session key. Requesting a Service
Now the client is ready to authenticate herself to the server. She creates a message very similar to the one sent to the TGS (which makes sense, since the TGS is a service). The client creates an authenticator, consisting of her name and network address, and a timestamp, encrypted with the session key for her and the server that the TGS generated. The request consists of the ticket received from Kerberos (already encrypted with the server’s secret key) and the encrypted authenticator. The server decrypts and checks the ticket and the authenticator, as discussed previously, and also checks the client’s address and the timestamp. If everything checks out, the server knows that, according to Kerberos, the client is who she says she is. For applications that require mutual authentication, the server sends the client back a message consisting of the timestamp, encrypted with the session key. This proves that the serve...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips