Partially identified Babassu cipher and attack on it

Abstract

In the present article, we consider a new class of one-pad ciphers. We define its encryption algorithm as a partial function with two arguments. The first argument is a key, which is a random and equiprobable sequence of natural language letters. The length of the key is N. The second argument is a coherent (readable) text of length L, where L < N. The partiality feature of the encryption function does not allow us to apply C. Shannon's mathematical model. For this class of ciphers, we present two operational modes. We chose positions (numbers) of letters in a given key with the encrypted coherent text in the first mode. The positions (numbers) of letters represent the encrypted text. If there is not enough key information to encrypt the whole text, we may repeat the encryption using the next chosen key. To illustrate cryptographic capabilities of this mode, we widely use a widely known perfect cipher secrecy concept of C. Shannon as a prototype to the concept of perfect partial secrecy. Newly presented concepts allow us to build a hypothesis stating that it is impossible to decrypt the presented cipher without finding the information about its key. The second operational mode of the cipher consists of multiple usage of the key to save the key information. We also calculate the complexity of finding the key.