Implementation of a Two-way Authentication Protocol Using Shared Key with Hash CS265 Sec. 2 David Wang

Introduction Authentication is important in most network communications Before two parties can transfer secure information, they have to make sure that they are talk to the right person (or machine). It is also important that both parties in the authentication process can identify each other without leaking any secret information. Authentication on the network is challenging because the information being transferred on the insure link may be overheard, intercepted, modified, or replayed by other parties

Introduction (cont.) Many different protocols are used for network authentication with different concerns in mind. Some protocols focus on high security; some protocols focus on performance How to the right choices will depend on the application requirement –For example: –Online banking system requires higher security and integrity –Online chatting application requires lower security

Application requirement Need an authentication protocol for an online stock broker Need to be very secure when users login to the system Two-way authentication is needed because users don’t want to give out their stock account and password to wrong place, and broker don’t want hacker to steal money. Efficiency is also important when users making orders, but the protocol will focus on the authentication. Using shared key algorithm with Hash

Protocol AB I’m A; E (Ra, K) E ((Rb, Ra), K) H (passwd, Rb) Notations : A – UsersRa – Random number generated by A B – BrokerRb – Random number generated by B (m1, m2) – Message m1 and Message m2K – Key shared by A and B E (p, k) – Encrypt plaintext p using key k H (m1, m2) – Hash message m1 together with message m2

Protocol Analysis Three messages to archive two-way authentication First message: A encrypts a random Ra number using ‘K’ and sends it to B. Only B can decrypt it to get Ra. Second message: B generates Rb and and sends E((Rb, Ra), K) to A. Only A can decrypt it to get Rb and Ra. A uses Ra to verify B. Third message: A sends H(Password, Rb) to B. B verifies A by computing the same hash. Finally: Finishes authentication by clearing some states. Can also use (Ra xor Rb) as session key to do the communication.

Protocol Analysis (cont.) Replay of the first message will not succeed because B will send back the other encrypted random number. Without knowing the key, this will not make sense to the intruder. Replay the second message will not succeed because A will sends back H(passwd, Rb). Without knowing Rb, intruder will not be able to figure out passwd. Replay the third message will not succeed because B will finishing the authentication after it receive the first hashed passwd. Offline password guessing is not very possible because the intruder won’t be able to figure out the random numbers

Protocol Analysis (cont.) However, can not prevent the modification to the the message. This may be weak to denial of service attack. Also, have to protect the password file on the server. Can use HMAC with the message to prevent modification. Can encrypt the password file on the server.

Implementation Implemented on top of HTTP protocol Client side: GUI component using Java Swing, Crypto algorithm using TEA Java implementation with CBC mode, Hash using MD5 Java implementation. Server side: Using JSP on Tomcat, Crypto algorithm using TEA Java implementation with CBC mode, Hash using MD5 implementation.

Demo Login Field Password Field Messages between client and server

Conclusion This protocol can prevent replay and offline password guessing. However, we have to notice that the protocol it is weak to modification attack, and we also need to protect the password file on the server side.

Credit to MD5 implementation The MD5 source code is borrowed form