Authentication in .NET

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procedure is called HTTP digest authentication, since a digest (hash) is calculated over both user password, the challenge and part of the actual HTTP request. However, the HTTP digest requires two round trips, during which the user first requests for some information by sending an HTTP request. The server rejects original request, indicating its requirement for digest authentication and sends a challenge toward the user. In the second request, the user includes the digest that is calculated based on the user s credentials. Obviously, this would lead to doubling the authenticated part of web traffic, so one solution that has been suggested is for the server to include (after the first authentication) a challenge in its response to the user, so the challenge can be used during the next authenticated request. For the reasons mentioned above, HTTP over secure socket layer (SSL) (explained below) has gained more popularity than HTTP digest. 4. Anonymous key exchange: If the communication channel carrying the authentication credentials between the two parties is protected by added encryption and integrity protection, then many of the basic authentication mechanisms such as password in the clear can be used over this secure channel. However, establishing a secure channel that provides data encryption requires that encryption keys are somehow shared between the two parties beforehand. Diffie and Hellman provided a very clever method, called righteously the Diffie Hellman method , that allows two parties, that have no previous relationships with each, establish a shared key and thereby a secure channel between themselves. We will describe the Diffie Hellman method in 3, but for now, suffice to say that it requires the peers to send their public keys to each other. Based on the exchange public keys, the peers arrive at the shared secret. Note that the public keys and cryptographic methods are only required for initial key exchange and not for the actual authentication itself. The first disadvantage is that this method still requires the client to have the ability to perform public key cryptography. Furthermore, in a raw form of Diffie Hellman, the peers do not provide a proof of their identity to each other; when the identities are not verified, the key exchange is considered to be anonymous. Since any active attacker can hijack the message, including a peer s public key and insert her own public key instead and pretend to be the legitimate peer, the anonymous key exchange is said to be prone to MITM attacks. The attacker can later on even get a hold of the passwords that the user is sending to the server over the channel she believes to be secure. Secure shell (SSH) uses the anonymous key exchange mechanism, during which the server sends a raw public key (without additional information) to the client. The client caches the public key to guard itself against future MITMs, where the attacker could replace the server key with her own key. The caching, however, does not help if the initial key exchange is under an MITM attack. Many methods have been suggested to alleviate the MITM problem. However, without a proper identity verification, the MITM threat persists and since the MITM is able to also sniff the channel, all the added complexity of key exchange does not buy any added security over the initial password mechanism. A way to circumvent this problem is to have the public key of each party signed by a trusted authority. The result is called a public key certificate that includes both the public key and the signature from the trusted authority. We will explain certificates in detail in s 3 and 9. 5. Zero-Knowledge password proofs: These methods are designed very cleverly to avoid the problem of the authentication mechanisms requiring pre-shared keys, or password that can either be sniffed or attacked by an MITM. Zero-knowledge password proof methods enhance the Diffie Hellman methods by added encryption using user-generated passwords. However, the methodology is heavily patented and hence has not gained any wide
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