International Association for Cryptologic Research

International Association
for Cryptologic Research


Virendra Kumar


Provable-Security Analysis of Authenticated Encryption in Kerberos
Alexandra Boldyreva Virendra Kumar
Kerberos is a widely-deployed network authentication protocol that is being considered for standardization. Many works have analyzed its security, identifying flaws and often suggesting fixes, thus helping the protocol's evolution. Several recent results present successful formal-methods-based verification of a significant portion of the current version 5, and some even imply security in the computational setting. For these results to hold, encryption in Kerberos should satisfy strong cryptographic security notions. However, neither currently deployed as part of Kerberos encryption schemes nor their proposed revisions are known to provably satisfy such notions. We take a close look at Kerberos' encryption and confirm that most of the options in the current version provably provide privacy and authenticity, some with slight modification that we suggest. Our results complement the formal-methods-based analysis of Kerberos that justifies its current design.
CompChall: Addressing Password Guessing Attacks
Even though passwords are the most convenient means of authentication, they bring along themselves the threat of dictionary attacks. Dictionary attacks may be of two kinds: online and offline. While offline dictionary attacks are possible only if the adversary is able to collect data for a successful protocol execution by eavesdropping on the communication channel and can be successfully countered using public key cryptography, online dictionary attacks can be performed by anyone and there is no satisfactory solution to counter them. This paper presents a new authentication protocol which is called CompChall (computational challenge). The proposed protocol uses only one way hash functions as the building blocks and attempts to eliminate online dictionary attacks by implementing a challenge-response system. This challenge-response system is designed in a fashion that it does not pose any difficulty to a genuine user but is time consuming and computationally intensive for an adversary trying to launch a large number of login requests per unit time as in the case of an online dictionary attack. The protocol is stateless and thus less vulnerable to DoS (Denial of Service) attacks.