Optimal Forgeries Against Polynomial-Based MACs and GCM

Polynomial-based authentication algorithms, such as GCM and Poly1305, have seen widespread adoption in practice. Due to their importance, a significant amount of attention has been given to understanding and improving both proofs and attacks against such schemes. At EUROCRYPT 2005, Bernstein published the best known analysis of the schemes when instantiated with PRPs, thereby establishing the most lenient limits on the amount of data the schemes can process per key. A long line of work, initiated by Handschuh and Preneel at CRYPTO 2008, finds the best known attacks, advancing our understanding of the fragility of the schemes. Yet surprisingly, no known attacks perform as well as the predicted worst-case attacks allowed by Bernstein\u27s analysis, nor has there been any advancement in proofs improving Bernstein\u27s bounds, and the gap between attacks and analysis is significant. We settle the issue by finding a novel attack against polynomial-based authentication algorithms using PRPs, and combine it with new analysis, to show that Bernstein\u27s bound, and our attacks, are optimal

Nonce-Disrespecting Adversaries: Practical Forgery Attacks on GCM in TLS

We investigate nonce reuse issues with the GCM block cipher mode as used in TLS and focus in particular on AES-GCM, the most widely deployed variant. With an Internet-wide scan we identified 184 HTTPS servers repeating nonces, which fully breaks the authenticity of the connections. Affected servers include large corporations, financial institutions, and a credit card company. We present a proof of concept of our attack allowing to violate the authenticity of affected HTTPS connections which in turn can be utilized to inject seemingly valid content into encrypted sessions. Furthermore we discovered over 70,000 HTTPS servers using random nonces, which puts them at risk of nonce reuse if a large amount of data is sent over the same connection

MEGA: Malleable Encryption Goes Awry

MEGA is a leading cloud storage platform with more than 250 million users and 1000 Petabytes of stored data. MEGA claims to offer user-controlled, end-to-end security. This is achieved by having all data encryption and decryption operations done on MEGA clients, under the control of keys that are only available to those clients. This is intended to protect MEGA users from attacks by MEGA itself, or by adversaries who have taken control of MEGA’s infrastructure. We provide a detailed analysis of MEGA’s use of cryptography in such a malicious server setting. We present five distinct attacks against MEGA, which together allow for a full compromise of the confidentiality of user files. Additionally, the integrity of user data is damaged to the extent that an attacker can insert malicious files of their choice which pass all authenticity checks of the client. We built proof-of-concept versions of all the attacks. Four of the five attacks are eminently practical. They have all been responsibly disclosed to MEGA and remediation is underway. Taken together, our attacks highlight significant shortcomings in MEGA’s cryptographic architecture. We present immediately deployable countermeasures, as well as longer-term recommendations. We also provide a broader discussion of the challenges of cryptographic deployment at massive scale under strong threat models

Pseudorandom Black Swans: Cache Attacks on CTR_DRBG

Modern cryptography requires the ability to securely generate pseudorandom numbers. However, despite decades of work on side channel attacks, there is little discussion of their application to pseudorandom number generators (PRGs). In this work we set out to address this gap, empirically evaluating the side channel resistance of common PRG implementations. We find that hard-learned lessons about side channel leakage from encryption primitives have not been applied to PRGs, at all levels of abstraction. At the design level, the NIST-recommended CTR_DRBG design does not have forward security if an attacker is able to compromise the state via a side-channel attack. At the primitive level, popular implementations of CTR_DRBG such as OpenSSL\u27s FIPS module and NetBSD\u27s kernel use leaky T-table AES as their underlying block cipher, enabling cache side-channel attacks. Finally, we find that many implementations make parameter choices that enable an attacker to fully exploit the side-channel attack in a realistic scenario and recover secret keys from TLS connections. We empirically demonstrate our attack in two scenarios. In the first, we carry out an asynchronous cache attack that recovers the private state from vulnerable CTR_DRBG implementations under realistic conditions to recover long-term authentication keys when the attacker is a party in the TLS connection. In the second scenario, we show that an attacker can exploit the high temporal resolution provided by Intel SGX to carry out a blind attack to recover CTR\_DRBG\u27s state within three AES encryptions, without viewing output, and thus to decrypt passively collected TLS connections from the victim

EverCrypt: A Fast, Verified, Cross-Platform Cryptographic Provider

We present EverCrypt: a comprehensive collection of verified, high-performance cryptographic functionalities available via a carefully designed API. The API provably supports agility (choosing between multiple algorithms for the same functionality) and multiplexing (choosing between multiple implementations of the same algorithm). Through abstraction and zero-cost generic programming, we show how agility can simplify verification without sacrificing performance, and we demonstrate how C and assembly can be composed and verified against shared specifications. We substantiate the effectiveness of these techniques with new verified implementations (including hashes, Curve25519, and AES-GCM) whose performance matches or exceeds the best unverified implementations. We validate the API design with two high-performance verified case studies built atop EverCrypt, resulting in line-rate performance for a secure network protocol and a Merkle-tree library, used in a production blockchain, that supports 2.7 million insertions/sec. Altogether, EverCrypt consists of over 124K verified lines of specs, code, and proofs, and it produces over 29K lines of C and 14K lines of assembly code

Surreptitiously Weakening Cryptographic Systems

Revelations over the past couple of years highlight the importance of understanding malicious and surreptitious weakening of cryptographic systems. We provide an overview of this domain, using a number of historical examples to drive development of a weaknesses taxonomy. This allows comparing different approaches to sabotage. We categorize a broader set of potential avenues for weakening systems using this taxonomy, and discuss what future research is needed to provide sabotage-resilient cryptography

Attacking Deterministic Signature Schemes using Fault Attacks

Many digital signature schemes rely on random numbers that are unique and non-predictable per signature. Failures of random number generators may have catastrophic effects such as compromising private signature keys. In recent years, many widely-used cryptographic technologies adopted deterministic signature schemes because they are presumed to be safer to implement. In this paper, we analyze the security of deterministic ECDSA and EdDSA signature schemes and show that the elimination of random number generators in these schemes enables new kinds of fault attacks. We formalize these attacks and introduce practical attack scenarios against EdDSA using the Rowhammer fault attack. EdDSA is used in many widely used protocols such as TLS, SSH and IPSec, and we show that these protocols are not vulnerable to our attack. We formalize the necessary requirements of protocols using these deterministic signature schemes to be vulnerable, and discuss mitigation strategies and their effect on fault attacks against deterministic signature schemes

Cybersecurity and Quantum Computing: friends or foes?

L'abstract è presente nell'allegato / the abstract is in the attachmen