Paper 2002/077

Key-Insulated Public-Key Cryptosystems

Yevgeniy Dodis, Jonathan Katz, Shouhuai Xu, and Moti Yung

Abstract

Cryptographic computations (decryption, signature generation, etc.) are often performed on a relatively insecure device (e.g., a mobile device or an Internet-connected host) which cannot be trusted to maintain secrecy of the private key. We propose and investigate the notion of \emph{key-insulated security} whose goal is to minimize the damage caused by secret-key exposures. In our model, the secret key(s) stored on the insecure device are refreshed at discrete time periods via interaction with a physically-secure --- but computationally-limited --- device which stores a ``master key''. All cryptographic computations are still done on the insecure device, and the public key remains unchanged. In a (t, N)-key-insulated scheme, an adversary who compromises the insecure device and obtains secret keys for up to t periods of his choice is unable to violate the security of the cryptosystem for \emph{any} of the remaining N-t periods. Furthermore, the scheme remains secure (for \emph{all} time periods) against an adversary who compromises \emph{only} the physically-secure device. We notice that key-insulated schemes significantly improve the security guarantee of forward-secure schemes [A97,BM99], in which exposure of the secret key at even a single time period (necessarily) compromises the security of the system for all future time periods. This improvement is achieved with minimal cost: infrequent key updates with a (possibly untrusted) secure device. We focus primarily on key-insulated public-key encryption. We construct a (t,N)-key-insulated encryption scheme based on any (standard) public-key encryption scheme, and give a more efficient construction based on the DDH assumption. The latter construction is then extended to achieve chosen-ciphertext security.