Joseph Jaeger

Mass-surveillance without the State: Strongly Undetectable Algorithm-Substitution Attacks

We present new algorithm-substitution attacks (ASAs) on symmetric encryption that improve over prior ones in two ways. First, while prior attacks only broke a sub-class of randomized schemes having a property called coin injectivity, our attacks break ALL randomized schemes. Second, while prior attacks are stateful, ours are stateless, achieving a notion of strong undetectability that we formalize. Together this shows that ASAs are an even more dangerous and powerful mass surveillance method than previously thought. Our work serves to increase awareness about what is possible with ASAs and to spur the search for deterrents and counter-measures.

Ratcheted Encryption and Key Exchange: The Security of Messaging

We aim to understand, formalize and provably achieve the goals underlying the core key-ratcheting technique of Borisov, Goldberg and Brewer, extensions of which are now used in secure messaging systems. We give syntax and security definitions for ratcheted encryption and key-exchange. We give a proven-secure protocol for ratcheted key exchange. We then show how to generically obtain ratcheted encryption from ratcheted key-exchange and standard encryption.

Better Than Advertised: Improved Collision-Resistance Guarantees for MD-Based Hash Functions

The MD transform that underlies the MD and SHA families iterates a compression function h to get a hash function H. The question we ask is, what property X of h guarantees collision resistance (CR) of H? The classical answer is that X itself be CR. We show that weaker conditions X, in particular forms of what we call constrained-CR, suffice. This reduces demands on compression functions, to the benefit of security, and also, forensically, explains why collision-finding attacks on compression functions have not, historically, lead to immediate breaks of the corresponding hash functions. We obtain our results via a definitional framework called RS security, and a parameterized treatment of MD, that also serve to unify prior work and variants of the transform.

Honey Encryption Beyond Message Recovery Security

Juels and Ristenpart introduced honey encryption HE and showed how to achieve message recovery security even in the face of attacks that can exhaustively try all likely keys. This is important in contexts like password-based encryption where keys are very low entropy, and HE schemes based on the JR construction were subsequently proposed for use in password management systems and even long-term protection of genetic data. But message recovery security is in this setting, like previous ones, a relatively weak property, and in particular does not prohibit an attacker from learning partial information about plaintexts or from usefully mauling ciphertexts. We show that one can build HE schemes that can hide partial information about plaintexts and that prevent mauling even in the face of exhaustive brute force attacks. To do so, we introduce target-distribution semantic-security and target-distribution non-malleability security notions. We prove that a slight variant of the JR HE construction can meet them. The proofs require new balls-and-bins type analyses significantly different from those used in prior work. Finally, we provide a formal proof of the folklore result that an unbounded adversary which obtains a limited number of encryptions of known plaintexts can always succeed at message recovery.

Optimal Channel Security Against Fine-Grained State Compromise: The Safety of Messaging

We aim to understand the best possible security of a (bidirectional) cryptographic channel against an adversary that may arbitrarily and repeatedly learn the secret state of either communicating party. We give a formal security definition and a proven-secure construction. This construction provides better security against state compromise than the Signal Double Ratchet Algorithm or any other known channel construction. To facilitate this we define and construct new forms of public-key encryption and digital signatures that update their keys over time.