Up to now, we have analyzed ciphertext assuming the method of encipherment was known. Diagnosis is a process used to discover the nature of the encipherment system. The toy diagnostic procedure to be described next assumes that each of the six ciphertext files that follow cipherEx1.A, cipherEx1.B,…, cipherEx1.F has been produced using one of the following encipherment systems:
1-gram monoalphabetic substitution
Some other cryptosystem.
The process of identifying which cryptographic system has produced given ciphertext is referred to as diagnosis. It will be carried out by making a sequence of tests whose objective is to accept or reject one of the hypotheses , , , or .
Test #i Compute …