The OAEP construction is already 10 years old and well-established in many practical applications. But after some doubts about its actual security level, four years ago, the first efficient and provably IND-CCA1 secure encryption padding was formally and fully proven to achieve the expected IND-CCA2 security level, when used with any trapdoor permutation. Even if it requires the partial-domain one-wayness of the permutation, for the main application (with the RSA permutation family) this intractability assumption is equivalent to the classical (full-domain) one-wayness, but at the cost of an extra quadratic-time reduction. The security proof which was already not very tight to the RSA problem is thus much worse. However, the practical optimality of the OAEP construction is two-fold, hence its attractivity: from the efficiency point of view because of two extra hashings only, and from the length point of view since the ciphertext has a minimal bit-length (the encoding of an image by the permutation.) But the bandwidth (or the ratio ciphertext/plaintext) is not optimal because of the randomness (required by the semantic security) and the redundancy (required by the plaintext-awareness, the sole way known to provide efficient CCA2 schemes.) At last Asiacrypt '03, the latter intuition had been broken by exhibiting the first IND-CCA2 secure encryption schemes without redundancy, and namely without achieving plaintext-awareness, while in the random-oracle model: the OAEP 3-round construction. But this result achieved only similar practical properties as the original OAEP construction: the security relies on the partial-domain one-wayness, and needs a trapdoor permutation, which limits the application to RSA, with still a quite bad reduction. This paper improves this result: first we show the OAEP 3-round actually relies on the (full-domain) one-wayness of the permutation (which improves the reduction), then we extend the application to a larger class of encryption primitives (including ElGamal, Paillier, etc.) The extended security result is still in the random-oracle model, and in a relaxed CCA2 model (which lies between the original one and the replayable CCA scenario).

OAEP 3-round: A generic and secure asymmetric encryption padding

/pdf/text-encryption-by-indexing-ascii-of-characters-based-on-the-3dzy1o15.pdf

Text Encryption by Indexing ASCII of Characters Based on the Locations of Pixels of the Image

Public-key encryption is an efficient mechanism providing a panoply of fundamental security services as confidentiality, authentication, integrity and non-repudiation. Effective key management and digital signatures is one of the key factors for the successful widespread deployment of public-key encryption. The digital signature is considered as one of the important mechanisms providing, in most network-based applications, one of the well-required security services, namely the authentication. In this paper, we study the security of the identity-based multisignature scheme. The scheme is based on RSA and Shamir's identity-based encryption and the message signature is shared by a coalition of known signers. We demonstrate the vulnerability of this scheme to the forgery attack, which could be carried by compromised signers, and then we propose countermeasures through a more robust scheme.

Cryptanalysis and Improvement of Identity-Based Multisignature Scheme

Privacy-preserving Mechanisms for e-Commerce

We study how to make any deterministic encryption scheme probabilistic and secure against adaptively chosen ciphertext attacks. A new transpositional padding encryption scheme is proposed, with which we construct a universal scheme, namely, a transpositional padding encryption scheme, which has three novel attributes: 1) it can pad a given plaintext into several different values once the randomness is chosen while the previous padding schemes only pad a given plaintext into a fixed value; 2) it introduces the randomness into a ciphertext without employing hash function or random oracle, and; 3) it enables the encrypted message to contain more useful information that may exceed the upper limit of plaintext space. Then, we give this encryption scheme an instantiation of RSA, which is proven to be indistinguishable under adaptively chosen ciphertext attacks without random oracle that assumes a variant of the standard RSA problem. The variant problem is a novel arithmetic problem, and it is weaker than the standard RSA problem.

Using Transposition Padding to Get CCA2 Security From Any Deterministic Encryption Schemes

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Using Transposition Padding to Get CCA2 Security From Any Deterministic Encryption Schemes