Paper 2005/377

Exclusion-Intersection Encryption

Sherman S. M. Chow and Siu-Ming Yiu

Abstract

Identity-based encryption (IBE) has shown to be a useful cryptographic scheme enabling secure yet flexible role-based access control. We propose a new variant of IBE named as exclusion-intersection encryption: during encryption, the sender can specify the targeted groups that are legitimate and interested in reading the documents; there exists a trusted key generation centre generating the intersection private decryption keys on request. This special private key can only be used to decrypt the ciphertext which is of all the specified groups' interests, its holders are excluded from decrypting when the documents are not targeted to all these groups (e.g., the ciphertext of only a single group's interest). While recent advances in cryptographic techniques (e.g., attribute-based encryption or wicked IBE) can support a more general access control policy, the private key size may be as long as the number of attributes or identifiers that can be specified in a ciphertext, which is undesirable, especially when each user may receive a number of such keys for different decryption power. One of the applications of our notion is to support an ad-hoc joint project of two or more groups which needs extra helpers that are not from any particular group. We also present an online/offline variant such that encryption can be computed quickly after offline pre-computation.