## Cryptology ePrint Archive: Report 2007/127

CCA2-Secure Threshold Broadcast Encryption with Shorter Ciphertexts

Vanesa Daza and Javier Herranz and Paz Morillo and Carla Ràfols

Abstract: In a threshold broadcast encryption scheme, a sender chooses (ad-hoc) a set of $n$ receivers and a threshold $t$, and then encrypts a message by using the public keys of all the receivers, in such a way that the original plaintext can be recovered only if at least $t$ receivers cooperate. Previously proposed threshold broadcast encryption schemes have ciphertexts whose length is $\O(n)$.

In this paper, we propose new schemes, for both PKI and identity-based scenarios, where the ciphertexts' length is $\O(n-t)$. The construction uses secret sharing techniques and the Canetti-Halevi-Katz transformation to achieve chosen-ciphertext security. The security of our schemes is formally proved under the Decisional Bilinear Diffie-Hellman (DBDH) Assumption.

Category / Keywords: cryptographic protocols / threshold broadcast encryption, ad-hoc groups, secret sharing