Authentication in Java

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example, Trudy's first hop router could be configured to only forward datagrams containing Trudy's IP source address However, this capability is not universally deployed or enforced Bob would thus be foolish to assume that Trudy's network manager (who might be Trudy herself!) had configured Trudy's first hop router to only forward appropriately-addressed datagrams
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Authentication protocol ap30
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One classical approach to authentication is to use a secret password We have PIN numbers to identify ourselves to automatic teller machines and login passwords for operating systems The password is a shared secret between the authenticator and the person being authenticated We saw in section 225 that HTTP uses a password-based authentication scheme Telnet and FTP use password authentication as well In protocol ap30, Alice thus sends her secret password to Bob, as shown in Figure 73-3
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Figure 73-3: Protocol ap30 and a failure scenario
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The security flaw here is clear If Trudy eavesdrops on Alice's communication, then she can learn Alice's password Lest you think this is unlikely, consider the fact that when one Telnet's to another machine and logs in, the login password is sent unencrypted to the Telnet server Someone connected to the Telnet client or server's LAN can possibly "sniff" (read and store) all packets transmitted on the LAN and thus steal the login password In fact, this is a well-known approach for stealing passwords (see, eg, [Jimenez 1997] Such a threat is obviously very real, so ap30 clearly won't do
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Authentication protocol ap31
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Having just studied the previous section on cryptography, our next idea for fixing ap30 is naturally to use encryption By encrypting the password, Trudy will not be able to learn Alice's password! If we assume that Alice and Bob share a symmetric secret key, KA-B, then Alice can encrypt the password, send her identification message, "I am Alice," and her encrypted password to Bob Bob then decrypts the password and, assuming the password is correct, authenticates Alice Bob feels comfortable in authenticating Alice since not only does Alice know the password, but she also knows the shared secret key value needed to encrypt the password Let's call this protocol ap31
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While it is true that ap31 prevents Trudy from learning Alice's password, the use of cryptography here does not solve the authentication problem! Bob is again subject to a so-called playback attack: Trudy needs only eavesdrop on Alice's communication, record the encrypted version of the password, and then later play back the encrypted version of the password to Bob to pretend that she is Alice The use of an encrypted password doesn't make the situation manifestly different from that in Figure 73-3
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Authentication protocol ap40
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The problem with ap31 is that the same password is used over and over again One way to solve this problem would be to use a different password each time Alice and Bob could agree on a sequence of passwords (or on an algorithm for generating passwords) and use each password only once, in sequence This idea is used in the S/KEY system [RFC 1760], adopting an approach due to Lamport [Lamport 81] for generating a sequence of passwords Rather than just stop here with this solution, however, let us consider a more general approach for combating the playback attack The failure scenario in Figure 73-3 resulted from the fact that Bob could not distinguish between the original authentication of Alice and the later playback of Alice's original authentication That is, Bob could not tell if Alice was "live" (ie, was currently really on the other end of the connection) or whether the messages he was receiving were a recorded playback of a previous authentication of Alice The very (very!) observant reader will recall that the 3-way TCP handshake protocol needed to address the same problem - the server side of a TCP connection did not want to accept a connection if the received SYN segment was an old copy (retransmission) of a SYN segment from an earlier connection How did the TCP server side solve the problem of determining if the client was really "live" It chose an initial sequence number (which had not been used in a very long time), sent that number to the client, and then waited for the client to respond back with an ACK segment containing that number We can adopt the same idea here for authentication purposes A nonce is a number that a protocol will only ever use once-in-a-lifetime That is, once a protocol uses a nonce, it will never use that number again Our ap40 protocol uses a nonce as follows: ap40:
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