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The definition of IND-CCA security model for public key encryption allows an adversary to obtain (adaptively) decryption of ciphertexts of its choice. That is, the adversary is given oracle access to the decryption function corresponding to the decryption key in use. The adversary may make queries that do not correspond to a valid ciphertext, and the answer will be accordingly (i.e., a special "failure" symbol). In this article, we investigate the case where we restrict the oracle to only determine if the query made is a valid ciphertext or not. That is, the oracle will output 1 if the query string is a valid ciphertext (do not output the corresponding plaintext) and output 0 otherwise. We call this oracle as "ciphertext verification oracle" and the corresponding security model as Indistinguishability against chosen ciphertext verification attack (IND-CCVA). We point out that this seemingly weaker security model is meaningful, clear and useful to the extent where we motivate that certain cryptographic functionalities can be achieved by ensuring the IND-CCVA security where as IND-CPA is not sufficient and IND-CCA provides more than necessary. We support our claim by providing nontrivial construction (existing/new) of: · public key encryption schemes that are IND-CCVA secure but not IND-CCA secure, · public key encryption schemes that are IND-CPA secure but not IND-CCVA secure. · public key encryption schemes that are IND-CCA1 secure but not IND-CCVA secure. Our discoveries are another manifestation of the subtleties that make the study of security notions for public key encryption schemes so attractive and are important towards achieving the definitional clarity of the target security.

In this paper, we show how to construct an Identity Based Signcryption Scheme (IBSC) using an Identity Based Encryption (IBE) and an Identity Based Signature (IBS) schemes. We show that the security of the IBSC scheme-indistinguishability as well as unforgeablity-is derived from the security of the underlying IBE and IBS schemes. Moreover, we show that under mild (reasonable) assumptions, the scheme is both space and time efficient compared to the Sign-then-Encrypt approach.

In a recent paper [10], Mishra and Dimitrov have proposed a window-based Elliptic Curve (EC) scalar multiplication using double-base number representation. Their methods were rather heuristic. In this paper, given the window lengths w2 and w3 for the bases 2 and 3, we first show how to fix the number of windows, ρ, and then obtain a Double Base Number System (DBNS) representation of the scalar n suitable for window-based EC scalar multiplication. Using the DBNS representation, we obtain our first algorithm that uses a small table of precomputed EC points. We then modify this algorithm to obtain a faster algorithm by reducing the number of EC additions at the cost of storing a larger number of precomputed points in a table. Explicit constructions of the tables are also given.