Schemes of cryptography are endless in their variety. Bacon lays down the following as the "virtues" to be looked for in them:--"that they be not laborious to write and read; that they be impossible to decipher; and, in some cases, that they be without suspicion." These principles are more or less disregarded by all the modes that have been advanced, including that of Bacon himself, which has been unduly extolled by his admirers as "one of the most ingenious methods of writing in cypher, and the most difficult to be decyphered, of any yet contrived" (Thicknesse, p. 13).
The simplest and commonest of all the ciphers is that in which the writer selects in place of the proper letters certain other letters in regular advance. This method of transposition was used by Julius Caesar. He, "per quartam elementorum literam," wrote _d_ for _a_, _e_ for _b_, and so on. There are instances of this arrangement in the Jewish rabbis, and even in the sacred writers. An illustration of it occurs in Jeremiah (xxv. 26), where the prophet, to conceal the meaning of his prediction from all but the initiated, writes _Sheshak_ instead of Babel (Babylon), the place meant; i.e. in place of using the second and twelfth letters of the Hebrew alphabet (_b_, _b_, _l_) from the beginning, he wrote the second and twelfth (_sh_, _sh_, _k_) from the end. To this kind of cipher-writing Buxtorf gives the name Athbash (from _a_ the first letter of the Hebrew alphabet, and _th_ the last; _b_ the second from the beginning, and _h_ the second from the end). Another Jewish cabalism of like nature was called Albam; of which an example is in Isaiah vii. 6, where Tabeal is written for Remaliah. In its adaptation to English this method of transposition, of which there are many modifications, is comparatively easy to decipher. A rough key may be derived from an examination of the respective quantities of letters in a type-founder's bill, or a printer's "case." The decipherer's first business is to classify the letters of the secret message in the order of their frequency. The letter that occurs oftenest is _e_; and the next in order of frequency is _t_. The following groups come after these, separated from each other by degrees of decreasing recurrence:--_a_, _o_, _n_, _i_; _r_, _s_, _h_; _d_, _l_; _c_, _w_, _u_, _m_; _f_, _y_, _g_, _p_, _b_; _v_, _k_; _x_, _q_, _j_, _z_. All the single letters must be _a_, _I_ or _O_. Letters occurring together are _ee_, _oo_, _ff_, _ll_, _ss_, &c. The commonest words of two letters are (roughly arranged in the order of their frequency) _of_, _to_, _in_, _it_, _is_, _be_, _he_, _by_, _or_, _as_, _at_, _an_, _so_, &c. The commonest words of three letters are _the_ and _and_ (in great excess), _for_, _are_, _but_, _all_, _not_, &c.; and of four letters--_that_, _with_, _from_, _have_, _this_, _they_, &c. Familiarity with the composition of the language will suggest numerous other points that are of value to the decipherer. He may obtain other hints from Poe's tale called _The Gold Bug_. As to messages in the continental languages constructed upon this system of transposition, rules for deciphering may be derived from Breithaupt's _Ars decifratoria_ (1737), and other treatises.