Showing papers on "Ciphertext published in 1986"

Cryptanalysis of DES with a reduced number of rounds

Abstract: A blockcipher is said to have a linear factor if, for all plaintexts and keys, there is a fixed non-empty set of key bits whose simultaneous complementation leaves the exclusive-or sum of a fixed non-empty set of ciphertext bits unchanged.

Is the data encryption standard a group

Abstract: The Data Encryption Standard (DES) defines an indexed set of permutations acting on the message space M = {0, l}64. If this set of permutations were closed under functional composition, then DES would be vulnerable to a known-plaintext attack that runs in 228 steps, on the average. It is unknown in the open literature whether or not DES has this weakness.We describe two statistical tests for determining if an indexed set of permutations acting on a finite message space forms a group under functional composition. The first test is a "meet-in-the-middle" algorithm which uses O[?K) time and space, where K is the size of the key space. The second test, a novel cycling algorithm, uses the same amount of time but only a small constant amount of space. Each test yields a known-plaintext attack against any finite, deterministic cryptosystem that generates a small group.The cycling test takes a pseudo-random walk in the message space until a cycle is detected. For each step of the pseudo-random walk, the previous ciphertext is encrypted under a key chosen by a pseudo-random function of the previous ciphertext. Results of the test are asymmetrical: long cycles are overwhelming evidence that the set of permutations is not a group; short cycles are strong evidence that the set of permutations has a structure different from that expected from a set of randomly chosen permutations.Using a combination of software and special-purpose hardware, we applied the cycling test to DES. Our experiments show, with a high degree of confidence, that DES is not a group.

A method of authenticating stream ciphers

Abstract: A problem associated with any encrypted message occurs when an interceptor attempts to not necessarily cryptanalyse that message, but instead to intercept the ciphertext and to alter it in such a way as to convey an alternative meaning This may take the form of altering one or more selected bits of the ciphertext stream to invert the meaning of the decrypted plaintext Accordingly an aim of the present invention is to provide a stream cipher having an authentication code added, but which has a further property of rendering the "known plaintext" cryptanalytic attack useless A cryptosystem is used to encrypt an authentication message which is added to the plaintext message to provide a pseudo-random output ciphertext message

On DDos Attack against Proxy in Re-encryption and Re-signature

Abstract: In 1998, Blaze, Bleumer, and Strauss proposed new kind of cryptographic primitives called proxy re-encryption and proxy resignature[BBS98]. In proxy re-encryption, a proxy can transform a ciphertext computated under Alice’s public key into one that can be opened under Bob’s decryption key. In proxy re-signature, a proxy can transform a signature computated under Alice’s secret key into one that can be verified by Bob’s public key. In 2005, Ateniese et al proposed a few new re-encryption schemes and discussed its several potential applications especially in the secure distributed storage[AFGH05]. In 2006, they proposed another few re-signature schemes and also discussed its several potential applications[AH06]. They predicated that re-encryption and resignature will play an important role in our life. Since then, researchers are sparked to give new lights to this area. Many excellent schemes have been proposed. In this paper, we introduce a new attackDDos attack against proxy in the proxy re-cryptography. Although this attack can also be implemented against other cryptographic primitives, the danger caused by it in proxy re-cryptography seems more serious. We revisit the current literature, paying attention on their resisting DDos attack ability. We suggest a solution to decline the impact of DDos attacking. Also we give a new efficient re-encryption scheme which can achieve CCA2 secure based on Cramer-Shoup encryption scheme and prove its security. We point out this is the most efficient proxy re-encryption schemes for the proxy which can achieve CCA2 secure in the literature. At last we give our conclusions with hoping researchers give more attention on this