Paper 2022/073 Original: eprint.iacr.org/2022/073 ↗

Forward-Secure Public Key Encryption without Key Update from Proof-of-Stake Blockchain

Abstract

A forward-secure public-key encryption (PKE) scheme prevents eavesdroppers from decrypting past ciphertexts in order to mitigate the damage caused by a potential secret key compromise. In prior works, forward security in a non-interactive setting, such as forward-secure PKE, is achieved by constantly updating (secret) keys. In this paper, we formalize the notion of blockchain-based forward-secure PKE and show the feasibility of constructing a forward-secure PKE scheme without key update (i.e. both the public key and the secret key are immutable), assuming the existence of a proof-of-stake blockchain with the distinguishable forking property introduced by Goyal, et al. (TCC 2017). Our construction uses the proof-of-stake blockchain as an immutable decryption log and witness encryption by Garg, et al. (STOC 2013) to ensure that the same ciphertext cannot be decrypted twice, thereby rendering a compromised secret key useless with respect to decryption of past ciphertext the legitimate user has already decrypted.