Conditional oblivious transfer and timed-release encryption

Abstract

We consider the problem of sending messages “into the future." Previous constructions for this task were either based on heuristic-assumptions or did not provide anonymity to the sender of the message.-In the public-key setting, we present an effcient and secure timed-release-encryption scheme using a “time server" which inputs the current time-into the system. The server has to only interact with the receiver and-never learns the sender's identity. The scheme's computational and communicational cost per request are only logarithmic in the time parameter.-The construction of our scheme is based on a novel cryptographic primitive: a variant of oblivious transfer which we call conditional oblivious-transfer. We define this primitive (which may be of independent interest)-and show an effcient construction for an instance of this new primitive-based on the quadratic residuosity assumption.