It is one of those cases where the art of the reasoner should be used rather for the sifting of details than for the acquiring of fresh evidence." -- Holmes (SILV)

No one can accurately trace when the art of cryptography began, but it dates back a long, long time - as long as humans have tried to keep their written intentions and thoughts safe from the prying eyes of others. Cryptography seeks to change the form of a message either through letters or numbers so that only the intended recipient can read the message (known as a cipher). The best codes also mask even the fact that it is a code such as a letter - which can be read as a simple letter - with a coded message hidden inside it. Therefore, if you are not looking for the code, you would not see it nor even suspect that it was there (also known as steganography).

This sort of code was found in GLOR. The recipient of a common letter suffered a stroke and died at it's receipt. This is what he read:

The supply of game for London is going steadily up. Head-keeper Hudson, we believe, has been now told to receive all orders for fly-paper and for preservation of your hen-pheasant's life.

But Holmes, after reviewing the document, realized that a secret message was hidden - conveyed as every third word.

The supply of game for London is going steadily up. Head-keeper Hudson, we believe, has been now told to receive all orders for flypaper and for preservation of your hen-pheasant's life.

So the coded message, within the plain text message, read The game is up. Hudson has told all. Fly for your life.

Transposition Ciphers

The earliest ciphers were simple transposition ciphers where the encoder simply scrambled the letters in each word, i.e. "veens rentcep lunstioo" for "seven percent solution."

The earliest known use of a transposition cipher was by Lysander of Sparta (Greece) in 404 B.C. where a series of "random" letters were drawn on a belt. But when wound around the right diameter stick (called a "scytale"), the encoded message would appear. Because the letters are jumbled, this is considered a transposition cipher.

Substitution Ciphers

Substitution ciphers date back almost as far as transposition ciphers and are equally easy to solve. The simplest type of substitution cipher replaced each letter in the message to be encoded with the letter following it in the alphabet. Therefore, if you received a message such as "j uijol, uifsfgpsf j bn" and wished to read it, you would reverse the process (replacing every letter with the letter before it in the alphabet), making it "I think, therefore I am."

A slightly more difficult version of the Substitution cipher shifts each letter a fixed number of positions down the alphabet. The example above uses a simple shift of one - but you could shift each letter seven places in the alphabet - or any number you wish up to 25. Such ciphers are called Caesar ciphers, for Julius Caesar used this type of cipher (with a transposition shift of 3) to communicate secretly with his generals.

Progressive Keys

The next level of difficulty in a substitution cipher is when you start by shifting the first letter N times, the next letter N+1, the next N+2, and so on. So, let's say you begin with a message and you decide to shift the first letter in the alphabet 3 places (just like a Caesar cipher), but the next letter you shift 4 places, and the next you shift 5 places, and so on.

One Time Pads

Now, imagine if you could randomize the amount of shifts for each letter in the cipher, such as shifting the first letter 9 places in the alphabet, the second letter 18 places, the third letter 2 places, the fourth letter 25 places, the fifth letter 16 places, and so on. But how would the recipient know how to solve this, you ask? Up to now, as long as the encoder and the reader had previously agreed upon a system, it would be easy to break any subsequent cipher they sent each other. But with this new cipher's apparent randomness, it would become much more difficult to crack. Ciphers using this method require a pad of paper of which both the sender and the recipient have identical copies. The pad has random sequences of numbers from 1 - 15. The encoder then uses the topmost sheet (which matches the recipient's topmost sheet) to encode his message. When the recipient receives the message, he uses his topmost sheet to decode the message. They use this type of cipher (usually once for each sequence or sheet on the pad - hence the name) and then throw it away. Because the same sequence is not used more than once, the probability of breaking the cipher becomes decidedly smaller.

Complications

Thomas Jefferson created a more complex cipher machine using 25 rows of alphabets, each alphabet jumbled in a different order. To solve a cipher from this machine, one would have to have an identical machine and the proper order of the wheels.

In World War II, the German Enigma machine was used to encrypt their messages. It used electrical connections to encrypt messages.

Today, codes are encrypted using complex mathematical equations that, for all intents and purposes, are unsolvable. The computer equations are so long that without the key, it would take millions of supercomputers millions of years to come up with the correct answer.

Solving Ciphers

Up to now, we have delved into ciphers which exchange one letter, number, or symbol for one letter. But if one is familiar with letter and word patterns, you can begin to solve these 1:1 ciphers through logic and intuition. For instance, if a single letter stands alone in a cipher (such as the x below) . . .

silekh jskklo x lsi jsxegi
(FYI this is just an example, not a real cipher...)

. . . then we know that X must stand for either "A" or "I" as those are the only two one-letter words in the English language. And if any other X's appeared in the message, the must also be "a's" or "i's." Given this new information, we may find patterns in words which allow us to solve them (think of guessing the puzzle on "Wheel of Fortune"). Knowing which letters occur most frequently in a given language, which letters appear least frequently, which letters almost always appear in combination with specific other letters -- all of this can help a person on his way to "deciphering" the message.

In DANC Holmes solves a simple sequence of cryptograms (a message or writing in code or cipher) using this method.

Here each symbol stood for a letter. Upon a single glance, these figures resemble flag symbols used by various Navies. In Naval codes, each flag symbol also represented one letter - and that may have given Holmes an important first clue that these symbols each stood for a single alphabet letter. When Holmes had received enough of the messages (five to be exact), he was able to use letter-frequency analysis to help decrypt all of the codes. By knowing which letters occur most often in the English language, he could hypothesize what letters to substitute for what symbols. Of course, the greater the pool of data, the more accurately one can predict which letter stands for which symbol (hence the reason he needed to wait for more messages before he could break the cryptogram).

Ciphers to Codes

Forget 1:1 letters or numbers. What if you substitute 1 number for 2 letters, or 2 letters for 1 letter or a much more complicated version of that, or what if you used symbols? Enter the code. Codes replace "syntactic entities" in a message with other entities. Whereas ciphers are blind to "syntactic entities" (or the spacing between entities), codes use them to encrypt the data. Thus, each separate word of the message could be translated into a number or a symbol which would have to be looked up individually.

Enter the message Holmes received in (VALL).

534 C2 13 127 36 31 4 17 21 41 DOUGLAS 109 293 5 37 BIRLSTONE 26 BIRLSTONE 9 127 171

This is a perfect example of such a code - it uses references within a book's passage to specific words within the passage. When one knows the proper book and substitutes the numbers for words in the passages to which the code points, it translates to

There is danger may come very soon one Douglas rich country no at Birlstone House Birlstone confidence is pressing.

Unless you were the recipient and already knew the book to which the code references (or can brilliantly deduce which, of the millions of books in the world, this refers to -- such as Holmes did), you would have no way of solving this code. And because each number stands for a word, not a letter, a person trying to break the code could not use any of knowledge we explored to break ciphers (in the section above).

Information for this article was adapted from Wikipedia.org and here. For more on Codes and Holmes, see here.