Astrolabous: A Universally Composable Time Lock Encryption Scheme

Nikolaos Lamprou, Myrto Arapinis, Thomas Zacharias

Research output: Chapter in Book/Report/Conference proceedingConference contribution


In this work, we study the cryptographic primitive called time-lock encryption (TLE). The concept of TLE involves a party initiating the encryption of a message that one can only decrypt after a certain amount of time has elapsed. Following the universal composability (UC) paradigm introduced by Canetti [IEEE FOCS 2001], we formally abstract the concept of TLE into an ideal functionality in a flexible way. In addition, we provide a standalone definition for secure TLE schemes in a game-based style and we devise a hybrid protocol that relies on such a secure TLE scheme. We show that if the underlying TLE scheme satisfies the standalone game-based security definition, then our hybrid protocol UC realises the TLE functionality in the random oracle model. Finally, we present Astrolabous, a TLE construction that satisfies our security definition, leading to the first UC realization of the TLE functionality. Interestingly, it is hard to prove UC secure any of the TLE construction proposed in the literature. The reason behind this difficulty relates to the UC framework itself. Intuitively, to capture semantic security, no information should be leaked regarding the plaintext in the ideal world, thus the ciphertext should not contain any information relating to the message. On the other hand, all ciphertexts will eventually open, resulting in a trivial distinction of the real from the ideal world in the standard model. We overcome this limitation by extending any secure TLE construction adopting the techniques of Nielsen [CRYPTO 2002] in the random oracle model. Specifically, the description of the extended TLE algorithms includes calls to the random oracle, allowing our simulator to equivocate. This extension can be applied to any TLE algorithm that satisfies our standalone game-based security definition, and in particular to Astrolabous.
Original languageEnglish
Title of host publicationAdvances in Cryptology – ASIACRYPT 2021
Subtitle of host publication27th International Conference on the Theory and Application of Cryptology and Information Security, Singapore, December 6–10, 2021, Proceedings, Part II
EditorsMehdi Tibouchi, Huaxiong Wang
Place of PublicationCham
PublisherSpringer, Cham
Number of pages30
ISBN (Electronic)978-3-030-92075-3
ISBN (Print)978-3-030-92074-6
Publication statusPublished - 1 Dec 2021
Event27th Annual International Conference on the Theory and Applications of Cryptology and Information Security - , Singapore
Duration: 5 Dec 20219 Dec 2021

Publication series

ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference27th Annual International Conference on the Theory and Applications of Cryptology and Information Security
Abbreviated titleAsiacrypt 2021
Internet address


  • Time-lock encryption
  • Universal composability
  • Fairness


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