Paper 2003/043

Parallel Signcryption with OAEP, PSS-R, and other Feistel Paddings

Yevgeniy Dodis, Michael J. Freedman, and Shabsi Walfish

Abstract

We present a new, elegant composition method for joint signature and encryption, also referred to as signcryption. The new method, which we call *Padding-based Parallel Signcryption* (PbPS), builds an efficient signcryption scheme from any family of trapdoor permutations, such as RSA. Each user U generates a single public/secret key pair f_U/f^{-1}_U used for both sending and receiving the data. To signcrypt a message m to a recipient with key f_{rcv}, a sender with key f_{snd} efficiently transforms m into a pair {w|s}, and simply sends { f_{rcv}(w) | f^{-1}_{snd}(s) }. PbPS enjoys many attractive properties: simplicity, efficiency, generality, parallelism of ``encrypting''/``signing'', optimal exact security, flexible and ad-hoc key management, key reuse for sending/receiving data, optimally-low message expansion, long message and associated data support, and, finally, complete compatibility with the PKCS#1 infrastructure. The pairs {w|s} sufficient for the security of PbPS are called *universal two-padding schemes*. Using one round of the Feistel transform, we give a very general construction of such schemes. Interestingly, we notice that all popular padding schemes with message recovery used for plain signature or encryption, such as OAEP, OAEP+, PSS-R, and ``scramble all, encrypt small'', naturally consist of two pieces {w|s}. Quite remarkably, we show that all such pairs become special cases of our construction. As a result, we find a natural generalization of all conventional padding schemes, and show that any such padding can be used for signcryption with PbPS. However, none of such paddings gives optimal message bandwidth. For that purpose and of independent interest, we define a new ``hybrid'' between PSS-R and OAEP, which we call *Probabilistic Signature-Encryption Padding* (PSEP). We recommend using PbPS with PSEP to achieve the most flexible and secure signcryption scheme up-to-date. To justify this point, we provide a detailed practical comparison of PbPS/PSEP with other previously-proposed signcryption candidates.

Note: This report is completely superceded by E-Print report 2004/020: http://eprint.iacr.org/2004/020