Jik Hyun Chang

Parallel Parsing on a One-Way Array of Finite-State Machines

We show that a one-way two-dimensional iterative array of finite-state machines (2-DIA) can recognize and parse strings of any context-free language in linear time. What makes this result interesting and rather surprising is the fact that each processor of the array holds only a fixed amount of information (independent of the size of the input) and communicates with its neighbors in only one direction. This makes for a simple VLSI implementation. Although it is known that recognition can be done on a 2-DIA, previous parsing algorithms require the processors to have unbounded memory, even when the communication is two-way. We also consider the problem of finding approximate patterns in strings, the string-to-string correction problem, and the longest common subsequence problem, and show that they can be solved in linear time on a 2-DIA.

On the power of one-way communication

We look at a very simple model of parallel computation and study the question of how restricting the flow of data to be one-way compares with two-way flow. A one-way linear iterative array (1LIA) is a finite one-dimensional array of identical finite-state machines (cells) in which information is allowed to move only in one direction- from left to right. For inputs of length n, the array uses n cells which are initially set to the quiescent state. The serial input, which is applied to the leftmost cell, is accepted if the rightmost cell ever enters an accepting state. We give results which show that 1LIAs are surprisingly very powerful in that they can accept languages which seemingly require two-way communication.

Security flaw of authentication scheme with anonymity for wireless communications

Recently, Wu et al. discussed some security flaws of enhanced authentication scheme with anonymity for wireless environments proposed by Lee et al. and showed how to overcome the problems regarding anonymity and backward secrecy. However, in the paper, we will show that Wu et al.s improved scheme still did not provide user anonymity as they claimed.

Comment on Saeednia et al.'s strong designated verifier signature scheme

In 1996, Jakobsson et al. proposed a designated verifier signature scheme in which only one specified person, called a designated verifier, can be convinced of the validity of the signature and the identity of the signer. This is possible by giving the designated verifier the ability to simulate a signature him/herself in an indistinguishable way. Therefore, the other third party cannot determine whether the signature is from the signer or the designated verifier. However, in some circumstances, the third party may be convinced that a signature intended for the designated verifier is actually generated by the signer. In 2003, Saeednia et al. proposed a strong designated verifier signature scheme to overcome this problem. However, we found that Saeednia et al.s scheme would reveal the identity of the signer if the secret key of this signer is compromised. In this paper, we provide a new strong designated verifier signature scheme that provides signer ambiguity, even if the secret key of the signer is compromised. We also analyze the proposed scheme.

Some observations concerning alternating Turing machines using small space

Abstract We make some observations concerning alternating Turing machines operating in small space. For example, we show that alternating Turing machines using o(log n) space are more powerful than nondeterministic Turing machines using the same space-bound. In fact, we show that there is a language over a unary alphabet that can be accepted by an on-line alternating Turing machine in log n space, but not by any off-line nondeterministic Turing machine in o(log n) space. We also investigate the weak vs. strong space bounds and on-line vs. off-line machines at these low tape bounds.

Two-dimensional convolution on a pyramid computer

An algorithm for convolving a k*k window of weighting coefficients with an n*n image matrix on a pyramid computer of O(n/sup 2/) processors in time O(logn+k/sup 2/), excluding the time to load the image matrix, is presented. If k= Omega ( square root log n), which is typical in practice, the algorithm has a processor-time product O(n/sup 2/ k/sup 2/) which is optimal with respect to the usual sequential algorithm. A feature of the algorithm is that the mechanism for controlling the transmission and distribution of data in each processor is finite state, independent of the values of n and k. Thus, for convolving two (0, 1)-valued matrices using Boolean operations rather than the typical sum and product operations, the processors of the pyramid computer are finite-state. >

Forgery attacks on Kang et al.'s identity-based strong designated verifier signature scheme and its improvement with security proof

Recently, Kang et al. proposed a new identity-based strong designated verifier signature scheme (ID-SDVS) and identity-based designated verifier proxy signature scheme (ID-DVPS). They claimed that their schemes are unforgeable. However, we found out that their schemes are universally forgeable in the sense that anyone can forge valid ID-SDVS and ID-DVPS on an arbitrary message without the knowledge of the secret key of either the signer or the designated verifier. Finally, we propose an improved ID-SDVS which is unforgeable. We give formal security proof of universal unforgeability of our scheme. We also give an improved ID-DVPS.

Systolic tree implementation of data structures

Systolic tree architectures are presented for data structures such as stacks, queues, dequeues, priority queues, and dictionary machines. The stack, queue, and dequeue have a unit response time and a unit pipeline interval. The priority queue also has a unit response time, but the pipeline interval is 2. The response time and pipeline interval for the dictionary machine are O(log n) and O(1), respectively, where n is the number of data elements currently residing in the tree. In each node of the tree, the mechanism for controlling the transmission and distribution of data is finite state. This feature makes the designs presented here suitable for VLSI. If there are n data elements in the data structure, the depth of the tree is O(log n). >

On pebble automata

Abstract We look at two simple variations of space-bounded Turing machines (TMs): An off-line S(n) -space bounded TM where S(n) is below log n , which can use a pebble on the input tape, and a TM with two (or three) pebbles and no workspace. We show that in the former case, a pebble increases the recognition power of the device. The latter model(s) can accept large families of languages. For example, 2-pebble automata can accept languages accepted by deterministic checking stack automata, and 3-pebble automata can accept languages accepted by ‘stack-resetting’ two-way deterministic pushdown automata. Moreover, the pebble automata are halting (i.e., halt on all inputs).

Strong Designated Verifier Signature Scheme with Message Recovery

In this paper, we propose a strong designated verifier signature scheme with message recovery mechanism. In designated verifier signature scheme, the designated verifier can simulate a signature which is indistinguishable from the signature by the real signer. Our scheme guarantees that only the designated verifier can recover the message, verify the signature, and be convinced of the real signer. If the verifier reveals his secret key, then anyone can recover the message and verify the signature. However, still no one can be convinced that who the real signer is. The security assumption of our scheme only depends on the discrete logarithm problem.