Digital certificates play an important role in the authentication of communicating parties for transport layer security. Recently, however, frequent incidents such as the illegal issuance of fake certificates by a compromised certificate authority have raised concerns about the legacy certificate system. Certificate Transparency (CT) mitigates such issues by employing a log server to audit issued certificates publicly, making the certificate issuance and verification processes transparent. Unfortunately, the legacy CT ecosystem suffers from log server compromises and user browsing information leakage. Furthermore, the data structure for the certificate management in the legacy CT system incurs computation overhead linear to the number of registered certificates in the log. In this paper, we propose a secure CT scheme by leveraging a shared value tree (SVT), a novel log structure specifically designed to address the log server compromise and browsing information leakage problems. The verification time of SVT remains constant regardless of the number of registered certificates in the log. We analyze our scheme on the legacy CT system to demonstrate its incremental deployability, guaranteeing a smooth transition toward a more secure web ecosystem.

Transport Layer Security (TLS) protocol is often vulnerable to version downgrade attacks, where a man-in-the-middle attacker interferes with the handshake protocol and leads the communicating parties to fall back from a higher version of TLS to lower ones, which are typically provided for backward compatibility.

HyperText Transfer Protocol (HTTP) cookies are widely used on the web to enhance communication efficiency between a client and a server by storing stateful information. However, cookies may contain private and sensitive information about users. Thus, in order to guarantee the security of cookies, most web browsers and servers support not only Transport Layer Security (TLS) but also other mechanisms such as HTTP Strict Transport Security and cookie flags. However, a recent study has shown that it is possible to circumvent cookie flags in HTTPS by exploiting a vulnerability in HTTP software that allows message truncation. In this paper, we propose a novel cookie hijacking attack called rotten cookie which deactivates cookie flags even if they are protected by TLS by exploiting a weakness in HTTP in terms of integrity checks. According to our investigation, all major browsers ignore uninterpretable sections of the header of HTTP response messages and accept incorrect formats without any rejection. We demonstrate that, when combined with TLS or application vulnerabilities, this form of attack can obtain private cookies by removing cookie flags. Thus, the attacker can impersonate a legitimate user in the eyes of the server when cookies are used as an authentication token. We prove the practicality of our attack by demonstrating that our attack can lead five major web browsers to accept a cookie without any cookie flags. We thus present a mitigation strategy for the transport layer to preserve cookie security against our attack.

Femtocells are being deployed for coverage extension and higher data rates. However, interference problem between macrocell and femtocells should be solved before massive deployment. In this paper, we analyze the possible cases of frequency resource allocation in the OFDMA femtocell systems using Fractional Frequency Reuse (FFR) in order to minimize the interference between both cells. Under the assumed pre-allocation of macrocell users with the FFR, the allocable cases of sub-bands for the femtocells are investigated. Efficient and practical guidelines for femtocell's resource allocation are suggested with the consideration of macrocell users having a priority over femtocell users and of enhancing overall throughputs. Simulation results show the enhanced throughputs of both macrocell users and combined total users through practical interference-avoidance guidelines.