In ProvSec 2014, Wang and Tanaka proposed a transformation which converts weakly existentially unforgeable (wEUF) signature schemes into strongly existentially unforgeable (sEUF) ones in the bounded leakage model. To obtain the construction, they combined leakage resilient (LR) chameleon hash functions with the Generalised Boneh-Shen-Waters (GBSW) transformation proposed by Steinfeld, Pieprzyk, and Wang. However, their transformation cannot be used in a more realistic model called continual leakage model since secret keys of LR chameleon hash functions cannot be updated. In this paper, we propose a transformation which can convert wEUF signature schemes into sEUF ones in the continual leakage model. To achieve our goal, we give a new definition of continuous leakage resilient (CLR) chameleon hash function and construct it based on the CLR signature scheme proposed by Malkin, Teranishi, Vahlis, and Yung. Although our CLR chameleon hash functions satisfy the property of strong collision-resistance, due to the existence of the updating algorithm, an adversary may find the kind of collisions such that messages are the same but randomizers are different. Hence, we cannot combine our chameleon hash functions with the GBSW transformation directly, or the sEUF security of the transformed signature schemes cannot be achieved. To solve this problem, we improve the original GBSW transformation by making use of the Groth-Sahai proof system and then combine it with CLR chameleon hash functions.

Boneh and Franklin considered to add the revocation functionality to identity-based encryption (IBE). Though this methodology is applicable to any IBE and hierarchical IBE (HIBE), the resulting scheme is non-scalable. Therefore, a generic transformation of scalable revocable (H)IBE (R(H)IBE) from non-scalable R(H)IBE is really desirable. Towards this final goal, in this paper we introduce prototype RHIBE which does not require to be scalable (but requires some conditions), and propose a generic transformation of scalable RHIBE from prototype RHIBE. Moreover, we construct a prototype RHIBE scheme based on the decisional bilinear Diffie-Hellman (DBDH) assumption. Since our prototype RHIBE provides history-free update, insider security, and decryption key exposure resistance, our construction yields the first RHIBE scheme based on the static assumption with these desirable properties.