CHAPTER I
General Information
The subject which we are about to study is the analysis and solution of cipher, though not including code, which is a very special form of cipher demanding something more than elementary knowledge; nor shall we enter at all into the subject of invisible inks, certainly a most important aspect of secret writing, but belonging to the province of chemistry rather than to that of cryptanalysis. Cipher machines, also, are not within our present scope.
The term cipher implies a method, or system, of secret writing which, generally speaking, is unlimited in scope; it should be possible, using any one given cipher, to transform any plaintext whatever, regardless of its length and the language in which it is written, into a cryptogram, or single enciphered message. The process of accomplishing this transformation is called encipherment; the opposite process, that of transforming the cryptogram into a plaintext, is called decipherment.
The word decrypt, with its various derivatives, is being used here to signify the process of solving and reading cryptograms without any previous knowledge as to their keys, or secret formulas; thus the word decipher has been left to convey only its one meaning, as mentioned above: the mechanical process of applying a known key. Our word decrypt, however, is an innovation borrowed from the modern French and Italian writers, and is somewhat frowned upon by leading cryptologists.
The word digram is being used to indicate a two-letter sequence; similarly, we have trigrams, tetragrams, pentagrams, etc., to indicate sequences of three, four, five, etc. letters.
Ciphers, in general, fall into three major classifications:
- Concealment Cipher
- Transposition Cipher
- Substitution Cipher
Minor types, such as “abbreviation,” are sometimes included, though, to the writer, these have never seemed to be truly of a cryptographic nature.
In concealment cipher, the true letters of the secret message are hidden, or disguised, by any device whatever; and this type of cipher, as a general rule, is intended to pass without being suspected as the conveyor of a secret communication.
In transposition cipher, the true letters of the secret message are taken out of their text-order, and are rearranged according to any pattern, or key, agreed upon by the correspondents.