Mitigating private key compromise

Cryptosystems rely on the assumption that the computer end-points can securely store and use cryptographic keys. Yet, this assumption is rather hard to justify in practice. New software vulnerabilities are discovered every day, and malware is pervasive on mobile devices and desktop PCs. This thesis provides research on how to mitigate private key compromise in three different cases. The first case considers compromised signing keys of certificate authorities in public key infrastructure. To address this problem, we analyse and evaluate existing prominent certificate management systems, and propose a new system called "Distributed and Transparent Key Infrastructure", which is secure even if all service providers collude together. The second case considers the key compromise in secure communication. We develop a simple approach that either guarantees the confidentiality of messages sent to a device even if the device was previously compromised, or allows the user to detect that confidentiality failed. We propose a multi-device messaging protocol that exploits our concept to allow users to detect unauthorised usage of their device keys. The third case considers the key compromise in secret distribution. We develop a self-healing system, which provides a proactive security guarantee: an attacker can learn a secret only if s/he can compromise all servers simultaneously in a short period.