In a Dual System Encryption system both ciphertexts and private keys can take on one of two indistinguishable forms. A private key or ciphertext will be normal if they are generated respectively from the system's key generation or encryption algorithm. These keys and ciphertexts will behave as one expects in an IBE system. In addition, we define semi-functional keys and ciphertexts. A semi-functional private key will be able to decrypt all normally generated ciphertexts; however, decryption will fail if one attempts to decrypt a semi-functional ciphertext with a semi-functional private key. Analogously, semi-functional ciphertexts will be decryptable only by normal private keys.
Dual System Encryption opens up a new way to prove security of IBE and related encryption systems. We define a sequence of games where we change first the challenge ciphertext and then the private keys one by one to be semi-functional. We finally end up in a game where the challenge ciphertext and all private keys are semi-functional at which point proving security is straightforward.Category / Keywords: public-key cryptography / Identity-Based Encryption Publication Info: Crypto 2009 Date: received 6 Aug 2009 Contact author: bwaters at cs utexas edu Available format(s): PDF | BibTeX Citation Note: Full version of the paper. Version: 20090810:215827 (All versions of this report) Short URL: ia.cr/2009/385 Discussion forum: Show discussion | Start new discussion