PADVA: A BlockchainBased TLS Notary Service The TLS protocol is a de facto standard of secure clientserver communication on the Internet. Unfortunately, the publickey infrastructure (PKI) deployed by TLS is a weakestlink system introducing hundreds of links (i.e., trusted entities). Consequently, an adversary compromising a single trusted entity can impersonate any website. Notary systems, based on multipath probing, were early and promising proposals to detect and prevent such attacks. Unfortunately, despite their benefits, they are not widely deployed, mainly due to their longstanding unresolved problems. In this paper, we present Persistent and Accountable Domain Validation (PADVA), which is a nextgeneration blockchainbased TLS notary service. PADVA keeps notaries auditable and accountable, introduces servicelevel agreements and mechanisms to enforce them, relaxes availability requirements for notaries, and works with the legacy TLS ecosystem. We implemented and evaluated PADVA, and our experiments indicate its efficiency and deployability.

KCRS: A BlockchainBased Key Compromise Resilient Signature System. Digital signatures are widely used to assure authenticity and integrity of messages (including blockchain transactions). This assurance is based on assumption that the private signing key is kept secret, which may be exposed or compromised without being detected in the real world. Many schemes have been proposed to mitigate this problem, but most schemes are not compatible with widely used digital signature standards and do not help detect private key exposures. In this paper, we propose a Key Compromise Resilient Signature (KCRS) system, which leverages blockchain to detect key compromises and mitigate the consequences. Our solution keeps a log of valid certificates and digital signatures that have been issued on the blockchain, which can deter the abuse of compromised private keys. Since the blockchain is an open system, KCRS also provides a privacy protection mechanism to prevent the public from learning the relationship between signatures. We present a theoretical framework for the security of the system and a provablysecure construction. We also implement a prototype of KCRS and conduct experiments to demonstrate its practicability.