Summary: We investigate public key encryption that allows the originator of a ciphertext to retrieve a “forgotten” plaintext from the ciphertext. This type of public key encryption with “backward recovery” contrasts more widely analyzed public key encryption with “forward secrecy”. We advocate that together they form the two sides of a whole coin, whereby offering complementary roles in data security, especially in cloud computing, 3G/4G communications and other emerging computing and communication platforms. We formalize the notion of public key encryption with backward recovery, and present two construction methods together with formal analyses of their security. The first method embodies a generic public key encryption scheme with backward recovery using the “encrypt then sign” paradigm, whereas the second method provides a more efficient scheme that is built on D. Hofheinz and D. Kiltz’s public key encryption [Advances in cryptology – EUROCRYPT 2009, Lect. Notes Comput. Sci. 5479, 313–332 (2009; Zbl 1239.94052)] in conjunction with target collision resistant hashing. Security of the first method is proved in a two-user setting, whereas the second is in a more general multi-user setting.
For the entire collection see [Zbl 1233.94004].