Indiscreet Logs: Persistent Diffie-Hellman Backdoors in TLS

Software implementations of discrete logarithm based cryptosystems over finite fields typically make the assumption that any domain parameters they are presented with are trustworthy, i.e., the parameters implement cyclic groups where the discrete logarithm problem is assumed to be hard. An informal and widespread justification for this seemingly exists that says validating parameters at run time is too computationally expensive relative to the perceived risk of a server sabotaging the privacy of its own connection. In this paper we explore this trust assumption and examine situations where it may not always be justified. We conducted an investigation of discrete logarithm domain parameters in use across the Internet and discovered evidence of a multitude of potentially backdoored moduli of unknown order in TLS and STARTTLS spanning numerous countries, organizations, and protocols. Although our disclosures resulted in a number of organizations taking down suspicious parameters, we argue the potential for TLS backdoors is systematic and will persist until either until better parameter hygiene is taken up by the community, or finite field based cryptography is eliminated altogether

Lessons Learned From Previous SSL/TLS Attacks - A Brief Chronology Of Attacks And Weaknesses

Since its introduction in 1994 the Secure Socket Layer (SSL) protocol (later renamed to Transport Layer Security (TLS)) evolved to the de facto standard for securing the transport layer. SSL/TLS can be used for ensuring data confidentiality, integrity and authenticity during transport. A main feature of the protocol is its flexibility. Modes of operation and security aims can easily be configured through different cipher suites. During its evolutionary development process several flaws were found. However, the flexible architecture of SSL/TLS allowed efficient fixes in order to counter the issues. This paper presents an overview on theoretical and practical attacks of the last 15 years, in chronological order and four categories: Attacks on the TLS Handshake protocol, on the TLS Record and Application Data Protocols, on the PKI infrastructure of TLS, and on various other attacks. We try to give a short ”Lessons Learned” at the end of each paragraph

Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice

International audienceWe investigate the security of Diffie-Hellman key exchange as used in popular Internet protocols and find it to be less secure than widely believed. First, we present Logjam, a novel flaw in TLS that lets a man-in-the-middle downgrade connections to " export-grade " Diffie-Hellman. To carry out this attack, we implement the number field sieve discrete log algorithm. After a week-long precomputation for a specified 512-bit group, we can compute arbitrary discrete logs in that group in about a minute. We find that 82% of vulnerable servers use a single 512-bit group, allowing us to compromise connections to 7% of Alexa Top Million HTTPS sites. In response, major browsers are being changed to reject short groups. We go on to consider Diffie-Hellman with 768-and 1024-bit groups. A small number of fixed or standardized groups are in use by millions of servers. Performing precomputations for just ten of these groups would allow a passive eavesdropper to decrypt traffic to up to 66% of IPsec VPN servers, 26% of SSH servers, 24% of popular HTTPS sites, or 16% of SMTP servers. In the 1024-bit case, we estimate that such computations are plausible given nation-state resources, and a close reading of published NSA leaks shows that the agency's attacks on VPNs are consistent with having achieved such a break. We conclude that moving to stronger key exchange methods should be a priority for the Internet community

TLS/PKI Challenges and certificate pinning techniques for IoT and M2M secure communications

Transport Layer Security is becoming the de facto standard to provide end-to-end security in the current Internet. IoT and M2M scenarios are not an exception since TLS is also being adopted there. The ability of TLS for negotiating any security parameter, its flexibility and extensibility are responsible for its wide adoption but also for several attacks. Moreover, as it relies on Public Key Infrastructure (PKI) for authentication, it is also affected by PKI problems. Considering the advent of IoT/M2M scenarios and their particularities, it is necessary to have a closer look at TLS history to evaluate the potential challenges of using TLS and PKI in these scenarios. According to this, the article provides a deep revision of several security aspects of TLS and PKI, with a particular focus on current Certificate Pinning solutions in order to illustrate the potential problems that should be addressed

Comparse: Provably Secure Formats for Cryptographic Protocols

Data formats used for cryptographic inputs have historically been the source of many attacks on cryptographic protocols, but their security guarantees remain poorly studied. One reason is that, due to their low-level nature, formats often fall outside of the security model. Another reason is that studying all of the uses of all of the formats within one protocol is too difficult to do by hand, and requires a comprehensive, automated framework. We propose a new framework, “Comparse”, that specifically tackles the security analysis of data formats in cryptographic protocols. Comparse forces the protocol analyst to systematically think about data formats, formalize them precisely, and show that they enjoy strong enough properties to guarantee the security of the protocol. Our methodology is developed in three steps. First, we introduce a high-level cryptographic API that lifts the traditional game-based cryptographic assumptions over bitstrings to work over high-level messages, using formats. This allows us to derive the conditions that secure formats must obey in order for their usage to be secure. Second, equipped with these security criteria, we implement a framework for specifying and verifying secure formats in the F* proof assistant. Our approach is based on format combinators, which enable compositional and modular proofs. In many cases, we relieve the user of having to write those combinators by hand, using compile-time term synthesis via Meta-F*. Finally, we show that our F* implementation can replace the symbolic notion of message formats previously implemented in the DY* protocol analysis framework. Our newer, bit-level precise accounting of formats closes the modeling gap, and allows DY* to reason about concrete messages and identify protocol flaws that it was previously oblivious to. We evaluate Comparse over several classic and real-world protocols. Our largest case studies use Comparse to formalize and provide security proofs for the formats used in TLS 1.3, as well as upcoming protocols like MLS and Compact TLS 1.3 (cTLS), providing confidence and feedback in the design of these protocols

Verified Models and Reference Implementations for the TLS 1.3 Standard Candidate

International audienceTLS 1.3 is the next version of the Transport Layer Security (TLS) protocol. Its clean-slate design is a reaction both to the increasing demand for low-latency HTTPS connections and to a series of recent high-profile attacks on TLS. The hope is that a fresh protocol with modern cryptography will prevent legacy problems; the danger is that it will expose new kinds of attacks, or reintroduce old flaws that were fixed in previous versions of TLS. After 18 drafts, the protocol is nearing completion, and the working group has appealed to researchers to analyze the protocol before publication. This paper responds by presenting a comprehensive analysis of the TLS 1.3 Draft-18 protocol. We seek to answer three questions that have not been fully addressed in previous work on TLS 1.3: (1) Does TLS 1.3 prevent well-known attacks on TLS 1.2, such as Logjam or the Triple Handshake, even if it is run in parallel with TLS 1.2? (2) Can we mechanically verify the computational security of TLS 1.3 under standard (strong) assumptions on its cryptographic primitives? (3) How can we extend the guarantees of the TLS 1.3 protocol to the details of its implementations? To answer these questions, we propose a methodology for developing verified symbolic and computational models of TLS 1.3 hand-in-hand with a high-assurance reference implementation of the protocol. We present symbolic ProVerif models for various intermediate versions of TLS 1.3 and evaluate them against a rich class of attacks to reconstruct both known and previously unpublished vulnerabilities that influenced the current design of the protocol. We present a computational CryptoVerif model for TLS 1.3 Draft-18 and prove its security. We present RefTLS, an interoperable implementation of TLS 1.0-1.3 and automatically analyze its protocol core by extracting a ProVerif model from its typed JavaScript code

RSA, DH, and DSA in the Wild

This book chapter outlines techniques for breaking cryptography by taking advantage of implementation mistakes made in practice, with a focus on those that exploit the mathematical structure of the most widely used public-key primitives

Secure Authentication in the Grid: A Formal Analysis of DNP3 SAv5

Most of the world's power grids are controlled remotely. Their control messages are sent over potentially insecure channels, driving the need for an authentication mechanism. The main communication mechanism for power grids and other utilities is defined by an IEEE standard, referred to as DNP3; this includes the Secure Authentication v5 (SAv5) protocol, which aims to ensure that messages are authenticated. We provide the first security analysis of the complete DNP3: SAv5 protocol. Previous work has considered the message-passing sub-protocol of SAv5 in isolation, and considered some aspects of the intended security properties. In contrast, we formally model and analyse the complex composition of the protocol's sub-protocols. In doing so, we consider the full state machine, the protocol's asymmetric mode, and the possibility of cross-protocol attacks. Furthermore, we model fine-grained security properties that closely match the standard's intended security properties. For our analysis, we leverage the Tamarin prover for the symbolic analysis of security protocols. Our analysis shows that the core DNP3: SAv5 design meets its intended security properties. Notably, we show that a previously reported attack does not apply to the standard. However, our analysis also leads to several concrete recommendations for improving future versions of the standard

On the Security of the TLS Protocol: A Systematic Analysis

TLS is the most widely-used cryptographic protocol on the Internet. It comprises the TLS Handshake Protocol, responsible for authentication and key establishment, and the TLS Record Protocol, which takes care of subsequent use of those keys to protect bulk data. TLS has proved remarkably stubborn to analysis using the tools of modern cryptography. This is due in part to its complexity and its flexibility. In this paper, we present the most complete analysis to date of the TLS Handshake protocol and its application to data encryption (in the Record Protocol). We show how to extract a key-encapsulation mechanism (KEM) from the TLS Handshake Protocol, and how the security of the entire TLS protocol follows from security properties of this KEM when composed with a secure authenticated encryption scheme in the Record Protocol. The security notion we achieve is a variant of the ACCE notion recently introduced by Jager et al. (Crypto ’12). Our approach enables us to analyse multiple different key establishment methods in a modular fashion, including the first proof of the most common deployment mode that is based on RSA PKCS #1v1.5 encryption, as well as Diffie-Hellman modes. Our results can be applied to settings where mutual authentication is provided and to the more common situation where only server authentication is applied