Measuring small subgroup attacks against Diffie-Hellman

Several recent standards, including NIST SP 800- 56A and RFC 5114, advocate the use of “DSA” parameters for Diffie-Hellman key exchange. While it is possible to use such parameters securely, additional validation checks are necessary to prevent well-known and potentially devastating attacks. In this paper, we observe that many Diffie-Hellman implementations do not properly validate key exchange inputs. Combined with other protocol properties and implementation choices, this can radically decrease security. We measure the prevalence of these parameter choices in the wild for HTTPS, POP3S, SMTP with STARTTLS, SSH, IKEv1, and IKEv2, finding millions of hosts using DSA and other non-“safe” primes for Diffie-Hellman key exchange, many of them in combination with potentially vulnerable behaviors. We examine over 20 open-source cryptographic libraries and applications and observe that until January 2016, not a single one validated subgroup orders by default. We found feasible full or partial key recovery vulnerabilities in OpenSSL, the Exim mail server, the Unbound DNS client, and Amazon’s load balancer, as well as susceptibility to weaker attacks in many other applications

An Internet-Wide Analysis of Diffie-Hellman Key Exchange and X.509 Certificates in TLS

Transport Layer Security (TLS) is a mature cryptographic protocol, but has flexibility during implementation which can introduce exploitable flaws. New vulnerabilities are routinely discovered that affect the security of TLS implementations. We discovered that discrete logarithm implementations have poor parameter validation, and we mathematically constructed a deniable backdoor to exploit this flaw in the finite field Diffie-Hellman key exchange. We described attack vectors an attacker could use to position this backdoor, and outlined a man-in-the-middle attack that exploits the backdoor to force Diffie-Hellman use during the TLS connection. We conducted an Internet-wide survey of ephemeral finite field Diffie-Hellman (DHE) across TLS and STARTTLS, finding hundreds of potentially backdoored DHE parameters and partially recovering the private DHE key in some cases. Disclosures were made to companies using these parameters, resulting in a public security advisory and discussions with the CTO of a billion-dollar company. We conducted a second Internet-wide survey investigating X.509 certificate name mismatch errors, finding approximately 70 million websites invalidated by these errors and additionally discovering over 1000 websites made inaccessible due to a combination of forced HTTPS and mismatch errors. We determined that name mismatch errors occur largely due to certificate mismanagement by web hosting and content delivery network companies. Further research into TLS implementations is necessary to encourage the use of more secure parameters

Measuring And Securing Cryptographic Deployments

This dissertation examines security vulnerabilities that arise due to communication failures and incentive mismatches along the path from cryptographic algorithm design to eventual deployment. I present six case studies demonstrating vulnerabilities in real-world cryptographic deployments. I also provide a framework with which to analyze the root cause of cryptographic vulnerabilities by characterizing them as failures in four key stages of the deployment process: algorithm design and cryptanalysis, standardization, implementation, and endpoint deployment. Each stage of this process is error-prone and influenced by various external factors, the incentives of which are not always aligned with security. I validate the framework by applying it to the six presented case studies, tracing each vulnerability back to communication failures or incentive mismatches in the deployment process. To curate these case studies, I develop novel techniques to measure both existing and new cryptographic attacks, and demonstrate the widespread impact of these attacks on real-world systems through measurement and cryptanalysis. While I do not claim that all cryptographic vulnerabilities can be described with this framework, I present a non-trivial (in fact substantial) number of case studies demonstrating that this framework characterizes the root cause of failures in a diverse set of cryptographic deployments

Using Large-Scale Empirical Methods to Understand Fragile Cryptographic Ecosystems

Cryptography is a key component of the security of the Internet. Unfortunately, the process of using cryptography to secure the Internet is fraught with failure. Cryptography is often fragile, as a single mistake can have devastating consequences on security, and this fragility is further complicated by the diverse and distributed nature of the Internet. This dissertation shows how to use empirical methods in the form of Internet-wide scanning to study how cryptography is deployed on the Internet, and shows this methodology can discover vulnerabilities and gain insights into fragile cryptographic ecosystems that are not possible without an empirical approach. I introduce improvements to ZMap, the fast Internet-wide scanner, that allow it to fully utilize a 10 GigE connection, and then use Internet-wide scanning to measure cryptography on the Internet. First, I study how Diffie-Hellman is deployed, and show that implementations are fragile and not resilient to small subgroup attacks. Next, I measure the prevalence of ``export-grade'' cryptography. Although regulations limiting the strength of cryptography that could be exported from the United States were lifted in 1999, Internet-wide scanning shows that support for various forms of export cryptography remains widespread. I show how purposefully weakening TLS to comply with these export regulations led to the FREAK, Logjam, and DROWN vulnerabilities, each of which exploits obsolete export-grade cryptography to attack modern clients. I conclude by discussing how empirical cryptography improved protocol design, and I present further opportunities for empirical research in cryptography.PHDComputer Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149809/1/davadria_1.pd