[BA] Analyzing the security of distributed authentication protocols

252 / BBB

2026-09-11

Jonas Eppard

10:00

Password-Based Threshold Authentication (PbTA) protocols distribute the identity
provider of a Single Sign-On (SSO) system across multiple servers to reduce the impact
of a server compromise while allowing users to log in using only a password. This
enables to mitigate the single point of failure at the Identity Provider (IdP) common
in SSO systems. Proactively Secure Distributed Single Sign-On (PESTO) is a protocol
used to distribute the IdP and still allowing the use of common token authentication
for Relying Partys (RPs).
This thesis analyses the security of PESTO and related distributed authentication
protocols. Since a security flaw in PESTO was found that provides the attacker a
way to check if two login attempt were done with the same password the focus of the
analysis is if this flaw is also present in other distributed authentication protocols.
For the attack an attacker needs to compromise one of the authentication servers and
observe one login attempt. Using an online brute-force attack the attacker is able to
check if the password is the same as in the observed login attempt.
After proposing the attack against PESTO a mitigation of the attack will be presented
that stops the attack without increasing the round complexity of the protocol.
It is shown that the attack proposed againt PESTO is also viable against multiple
other protocols. This indicates a fundamental error with the security models used by
PbTA protocols. Therefore, this thesis argues if future work should revise the used
security notions.