could delegate to Charlie by combining his secret key with Charlie's public key. Transitivity – Transitive proxy re-encryption schemes allow for a ciphertext to be re-encrypted an unlimited number of times. For example, a ciphertext might be re-encrypted from Bob to Charlie, and then again from Charlie to David and so on. Non-transitive schemes allow for only one (or a limited number) of re-encryptions on a given ciphertext. Most known schemes are bi-directional and transitive. Currently, the only known uni-directional, transitive proxy re-encryption is done through the use of Homomorphic Encryption. Cloud Computing – Proxy re-encryption has potential applications for secure sharing in a cloud computing environment. In the cloud scenario the re-encryption key is provided to the cloud operator/admin. Looking at the Bob, Charlie, David example, the cloud would take the place of Charlie. Bob generates a re-encryption key to supply to the cloud. The cloud operator/admin completes a re-encrypt of Bob’s encrypted files into David’s files whenever David downloads Bob’s files. Challenges exist with the cloud solution. A user could conspire with a cloud operator to gain access to all a user’s, such as Bob, files. A second potential challenge is segmentation via access control. A cloud user can restrict access to files via the assignment of conditional values. However, the number of re-encryption keys grows proportionately with the number of conditional values. This situation is not optimal for resource constrained devices. Proxy re-encryption should not be confused with proxy signatures, which is a separate construction with a different purpose. See also Identity-based conditional proxy re-encryption References M. Blaze, G. Bleumer, M. Strauss. Divertible Protocols and Atomic Proxy Cryptography. Bertino, E., Sandhu, R. "Database security - concepts, approaches, and challenges." IEEE Transactions on Dependable and Secure Computing 2 (2005): 2-19 G. Ateniese, K. Fu, M. Green, S. Hohenberger. Improved Proxy Re-encryption Schemes with Applications to Secure Distributed Storage. Proceedings of the 12th Annual Network and Distributed Systems Security Symposium (NDSS 2005), San Diego, California, 2005. M. Green, G. Ateniese. Identity-Based Proxy Re-encryption. Applied Cryptography and Network Security Conference, June 2007. S. Hohenberger, G. Rothblum, a. shelat, and V. Vaikuntanathan. Securely Obfuscating Re-encryption. Proceedings of the Theory of Cryptography Conference (TCC), 2007. The JHU-MIT Proxy Re-cryptography Library Bibliography on Proxy Re-Cryptography Public-key cryptography