Polyalphabetic Case: Probable Words

Abstract

Pattern finding, using the positive coincidence of two text patterns, is necessarily restricted to monoalphabetic encryptions. But for a wide class of polyalphabetic encryptions, namely for those with fixpoint-free encryption steps, whose alphabets fulfill the condition “no letter may represent itself,” there is no coincidence between plaintext and cryptotext. This allows us to exclude certain positions of a probable word and thus establishes the remaining ones as possible positions. It is a probable word attack by exhausting positions. Exhaustion runs only over the length of the text and is feasible.