Secure and practical identity-based encryption

In this paper, we present a variant of Waters’ Identity-Based Encryption scheme with a much smaller public-key size (only a few kilobytes). We show that this variant is semantically secure against passive adversaries in the standard model. In essence, the new scheme divides Waters’ public key size by a factor ` at the cost of (negligibly) reducing security by ` bits. Therefore, our construction settles an open question asked by Waters and constitutes the first fully secure practical Identity-Based Encryption scheme.