Key Escrow with Tree-Based Access Structure

In this paper we propose a system in which a set of people is able to confidentially communicate using a common session key. Due to required governmental surveillance properties, this key will be escrowed using a multi-party version of the ElGamal cryptosystem. The resulting shares of the ciphertext are stored over a set of trusted servers to provide availability and to hamper ciphertext-based attacks. Using a particular tree-based multi-party decryption, the session key can be reconstructed by a tree-structured set of escrow agencies without reconstructing the private ElGamal key and the ciphertext.