Coffre-fort temporel

There are two natural approaches to implementing timed-release crypto :

1. Use "time-lock puzzles": computational problems that cannot be solved without running a computer continuously for at least a certain amount of time.
2. Use trusted agents who promise not to reveal certain information until a specified date.

Abstract: The notion of "sending a secret message to the future" has been around for over a decade. Despite this, no solution to this problem is in common use, or even attained widespread acceptance as a fundamental cryptographic primitive. We name, construct and specify an implementation for this new cryptographic primitive, "Time-Lapse Cryptography", with which a sender can encrypt a message so that it is guaranteed to be revealed at an exact moment in the future, even if this revelation turns out to be undesirable to the sender. Our solution combines new ideas with Pedersen distributed key generation, Feldman verifiable threshold secret sharing, and El Gamal encryption, all of which rest upon the single, broadly accepted Decisional Diffie-Hellman assumption. We develop a Time-Lapse Cryptography Service ("the Service") based on a network of parties who jointly perform the service. The protocol is practical and secure: at a given time T the Service publishes a public key so that anyone can use it, even anonymously. Senders encrypt their messages with this public key whose private key is not known to anyone - not even a trusted third party - until a predefined and specific future time T + at which point the private key is constructed and published. At or after that time, anyone can decrypt the ciphertext using this private key. The Service is envisioned as a public utility publishing a continuous stream of encryption keys and subsequent corresponding time-lapse decryption keys. We complement our theoretical foundation with descriptions of specific attacks and defenses, and describe important applications of our service in sealed bid auctions, insider stock sales, clinical trials, and electronic voting.

Abstract. Timed-release encryption (TRE) is a two-factor encryption scheme combining public key encryption and time-dependent encryption- decryption requires a trapdoor which is kept confidential by a timeserver until at an appointed time. This paper revisits two recent results. In ESORICS 2007, Chalkias et al. proposed an efficient anonymous TRE scheme. Unfortunately, we show the security threats of their scheme in the presence of a curious time-server and an impatient recipient. Recently, Chow et al. proposed an encryption scheme in the standard model which can be used as TRE. Nevertheless, only confidentiality is guaranteed. We demonstrate how to support pre-open capability, which is often desirable in applications of TRE. Our extension also enables only the recipient to know the release-time from the ciphertext. This feature is not considered in previous notion of release-time confidentiality.

Résumé

Le voyage dans le temps a toujours été un sujet fascinant, que ce soit en littérature ou en physique. De même, en cryptographie, nous nous demandons comment conserver des données confidentielles pendant un certain temps. Dans cette thèse, nous étudions comment faire voyager des informations privées vers le futur. Ce travail est divisé en trois parties :

1. le chiffrement temporisé (timed-release encryption),
2. le chiffrement de témoin (witness encryption)
3. et l'auto-chiffrement (self-encryption).

(et j'ai coupé le résumé, trop long).

Abstract: Time-lock encryption is a method to encrypt a message such that it can only be decrypted after a certain deadline has passed. We propose a novel time-lock encryption scheme, whose main advantage over prior constructions is that even receivers with relatively weak computational resources should immediately be able to decrypt after the deadline, without any interaction with the sender, other receivers, or a trusted third party.

We build our time-lock encryption on top of the new concept of computational reference clocks and an extractable witness encryption scheme.

We explain how to construct a computational reference clock based on Bitcoin.

We show how to achieve constant level of multilinearity for witness encryption by using SNARKs.

We propose a new construction of a witness encryption scheme which is of independent interest : our scheme, based on Subset- Sum, achieves extractable security without relying on obfuscation. The scheme employs multilinear maps of arbitrary order and is independent of the implementations of multilinear maps.

We introduce a new primitive called computational reference clocks as an extension of the standard computational model, which provide a novel and very realistic method to "emulate" real-world time in a computational model.

We show that the widely-used cryptocurrency Bitcoin provides a practical example of such a reference clock, which shows that the assumption that these objects exist in practice is reasonable.

Abstract: Time-release cryptography is a special encryption technique that allows a message to be hidden for some time. The previous schemes have shortcomings in that the encryptor should predict the decryptor's computing power precisely or the trusted agent should be always available. In this paper, we propose a new, practical time-release blockchain, and find the key to decrypt the content after a certain time.

In order to verify the effectiveness of the blockchain system automatically, which uses the proof-of-work (PoW) and the consensus algorithm in the the proposed technique, we have implemented a prototype version of our blockchain system using Python.

The proposed method has the following advantages.

• First, the decryption time is automatically adjusted, even if the miner's computing power changes over time.
• Second, unlike previous time-lock puzzle schemes, our algorithm does not require additional computation work for solving the puzzle.
• Third, our scheme does not need any trusted agents (third parties).
• Fourth, the proposed method uses standard cryptographic algorithms.

We study the problem of building a verifiable delay function (VDF). A VDF requires a specified number of sequential steps to evaluate, yet produces a unique output that can be efficiently and publicly verified.

VDFs have many applications in decentralized systems, including public randomness beacons, leader election in consensus protocols, and proofs of replication.

We formalize the requirements for VDFs and present new candidate constructions that are the first to achieve an exponential gap between evaluation and verification time.

This problem is even harder in adversarial systems like blockchain. Nodes in the network can't trust an external source of time or any timestamp that appears in a message. Hashgraph for example, solves this problem with a "median" timestamp.

Each message that is seen by the network is signed and timestamped by a supermajority of the network. The median timestamp for the message is what Hashgraph calls "fair" ordering.

Each message has to travel to the supermajority of the nodes in the system, then after the message collects enough signatures, the entire set needs to be propagated to the entire network. As you can imagine, this is really slow.

• Solane: proof of history une sorte de preuve d'enchaînement d'évènements dans le temps pour les blockchains.