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One of the cornerstones in censorship circumvention is fully encrypted protocols, which encrypt every byte of the payload in an attempt to “look like nothing”. In early November 2021, the Great Firewall of China (GFW) deployed a new censorship technique that passively detects—and subsequently blocks— fully encrypted traffic in real time. The GFW’s new censorship capability affects a large set of popular censorship circum- vention protocols, including but not limited to Shadowsocks, VMess, and Obfs4. Although China had long actively probed such protocols, this was the first report of purely passive de- tection, leading the anti-censorship community to ask how detection was possible. In this paper, we measure and characterize the GFW’s new system for censoring fully encrypted traffic. We find that, in- stead of directly defining what fully encrypted traffic is, the censor applies crude but efficient heuristics to exempt traffic that is unlikely to be fully encrypted traffic; it then blocks the remaining non-exempted traffic. These heuristics are based on the fingerprints of common protocols, the fraction of set bits, and the number, fraction, and position of printable ASCII characters. Our Internet scans reveal what traffic and which IP addresses the GFW inspects. We simulate the inferred GFW’s detection algorithm on live traffic at a university network tap to evaluate its comprehensiveness and false positives. We show evidence that the rules we inferred have good coverage of what the GFW actually uses. We estimate that, if applied broadly, it could potentially block about 0.6% of normal In- ternet traffic as collateral damage. Our understanding of the GFW’s new censorship mecha- nism helps us derive several practical circumvention strategies. We responsibly disclosed our findings and suggestions to the developers of different anti-censorship tools, helping millions of users successfully evade this new form of blocking 

ShadowTLS is a new type of circumvention tool where the relay forwards traffic to a legitimate (unblocked) TLS server until the end of the handshake, and then connects the client to a hidden proxy server (e.g. Shadowsocks). In contrast to previous probe-resistant proxies, this design can evade SNI- based blocking, since to the censor it appears as a legitimate TLS connection to an unblocked domain. In this paper, we describe several attacks against Shad- owTLS which would allow a censor to identify if a suspected IP is hosting a ShadowTLS relay or not (and block it accord- ingly), distinguishing it from the legitimate TLS servers it mimics. Our attacks require only a few TCP connections to the suspected IP, a capability that censors including China have already demonstrated in order to block previous proxies. We evaluate these vulnerabilities by performing Internet- wide scans to discover potential ShadowTLS relays, and find over 15K of them. We also describe mitigations against this attack that ShadowTLS (and proxies like it) can implement, and work with the ShadowTLS developers to deploy these fixes. 

It is well known in the cryptographic literature that the most common digital signature schemes used in practice can fail catastrophically in the presence of faults during computation. We use passive and active network measurements to analyze organically-occuring faults in billions of digital signatures generated by tens of millions of hosts. We find that a persistent rate of apparent hardware faults in unprotected implementa- tions has resulted in compromised certificate RSA private keys for years. The faulty signatures we observed allowed us to compute private RSA keys associated with a top-10 Alexa site, several browser-trusted wildcard certificates for organiza- tions that used a popular VPN product, and a small sporadic population of other web sites and network devices. These measurements illustrate the fragility of RSA PKCS#1v1.5 signature padding and provide insight on the risks faced by unprotected implementations on hardware at Internet scale.