An Efficient Technique for Secure Message Broadcasting

In contemporary cryptographic discourse, traditional broadcast encryption (BE) schemes have emerged as a robust method for secure broadcasting to specific subsets of members within a group. However, these schemes typically necessitate the involvement of a trusted entity for the distribution of decryption keys, which introduces vulnerabilities. Conversely, group key agreement (GKA) protocols offer a means of negotiating a shared encryption key among a group of members across open networks, thereby ensuring that only authorized group members possess the capability to decrypt ciphertexts encrypted under this key. Nonetheless, GKA protocols lack the flexibility to allow senders to selectively exclude specific members from decrypting ciphertexts. In response to this dichotomy, we present a pioneering hybrid cryptographic primitive termed Contributory Broadcast Encryption (ConBE), which ingeniously amalgamates the strengths of both BE and GKA paradigms. In this innovative approach, a cohesive group of members collaboratively negotiates a shared public encryption key, with each member possessing an associated decryption key. This setup empowers senders, upon accessing the public group encryption key, to exercise granular control over decryption, selectively restricting access to a subset of designated recipients. Expounding upon this foundational framework, we introduce a ConBE scheme distinguished by succinct ciphertexts, thus optimizing computational efficiency without compromising on security guarantees. Through rigorous analysis, our scheme is proven to be fully collusion-resistant under the decision n- Bilinear Diffie-Hellman Exponentiation (BDHE) assumption, thereby ensuring robust security within the standard cryptographic model. Furthermore, in a complementary contribution, we unveil a novel BE scheme endowed with aggregatable properties, which hold paramount significance in the construction of advanced cryptographic protocols. These properties facilitate the aggregation of decryption rights across multiple ciphertexts, thereby enhancing scalability and versatility in cryptographic applications. Our comprehensive exploration and formalization of ConBE, alongside the development of efficient cryptographic schemes, represent a significant stride forward in the realm of secure data transmission and access control mechanisms. This endeavor underscores our commitment to advancing cryptographic methodologies that reconcile security, efficiency, and flexibility in contemporary distributed computing environments