What Is the Term Used to Describe the Science of Making and Breaking Secret Codes?
The Secret Linguistic communication
Ron Hipschman
When you were a child, did you have a "Captain Midnight" decoder ring? With information technology, you could transport messages to a friends that no 1 else could read. Or possibly you call up using special symbols to write notes to your "squeeze" in class. If the note was intercepted , your instructor, could larn aught most your romance.
In more than serious uses, codes and ciphers are used by our military and diplomatic forces to keep confidential information from unauthorized eyes. Businesses as well ship data that has been encoded to try and protect merchandise secrets and back-room deals. Later on all, you wouldn't want your competitor to know that you were about to acquire their visitor with a leveraged purchase-out.
The written report of enciphering and encoding (on the sending stop), and deciphering and decoding (on the receiving finish) is called cryptography from the Greek κρυπτός (kryptos), or hidden and γράφειν (graphia), or writing. If y'all don't know Greek (and not many of the states practise) the above letters could exist a grade of lawmaking themselves! Although the stardom is fuzzy, ciphers are dissimilar from codes. When y'all substitute one word for some other word or sentence, similar using a strange language dictionary, yous are using a code. When you mix up or substitute existing messages, you are using a zilch. (I told you the difference was fuzzy, and yous can combine codes and ciphers by substituting ane word for some other and so mixing upward the outcome.) We'll concentrate on ciphers.
For a goose egg to exist useful, several things must exist known at both the sending and receiving ends.
- The algorithm or method used to encipher the original message (known every bit the plaintext).
- The central used with the algorithm to allow the plaintext to be both enciphered and deciphered.
- The period or fourth dimension during which the primal is valid.
Past way of analogy, to get into your home y'all would put a key in a lock to open the door. This process (the use of a key and a lock) is the method or algorithm. At present this method just works if you accept the proper key to stick in the lock, and your central will be valid just as long as y'all are the resident of the particular abode. The next resident will have the locks changed to a different key to make sure that you cannot enter even though you lot may know the method.
The selection of the in a higher place three items - algorithm, key and catamenia - depend on your needs. If yous are in the battlefield and are receiving current tactical data, you desire an algorithm that makes information technology like shooting fish in a barrel to decipher the bulletin in the heat of battle. On the other manus, you lot must also assume that your opponent has intercepted your enciphered bulletin and is busy trying to break it. Therefore you must choose an algorithm (method) that is complicated enough so that by the time your opponent figures information technology out, the data will be worthless. The easier the algorithm you choose, the more than often y'all volition accept to change the central that unlocks the code - if you want to go along your enemy in the dark.
Ciphers are cleaved into two main categories; substitution ciphers and transposition ciphers. Substitution ciphers replace letters in the plaintext with other letters or symbols, keeping the club in which the symbols fall the aforementioned. Transposition ciphers proceed all of the original letters intact, but mix upward their society. The resulting text of either enciphering method is called the ciphertext. Of form, you tin can utilize both methods, one after the other, to further confuse an unintended receiver besides. To get a feel for these methods, allow'south take a look at some ciphers.
Exchange ciphers and decoder rings
We use substitution ciphers all the fourth dimension. (Actually, substitution ciphers could properly be chosen codes in most cases.) Morse code, shorthand, semaphore, and the ASCII code with which these characters are being stored in inside my Macintosh are all examples. (ASCII stands for American Standard Lawmaking for Information Interchange, just in case you're interested.) The only difference between these and the spy codes is that the above examples are standardized and so that everybody knows them.
The Captain Midnight decoder ring (which is an "encoder" ring as well) allows you to do a simple substitution cipher. It ordinarily has two concentric wheels of letters, A through Z. Yous rotate the outside ring and substitute the letters in your bulletin found on the exterior band with the letters directly below on the inside band (run across diagram). Here, the algorithm is to offset the alphabet and the key is the number of characters to offset information technology. Julius Caesar used this uncomplicated scheme, offsetting by 3 characters (He would take put the "A" on the outer band of letters over the "D" on the inner ring if he had endemic a Captain Midnight decoder band.) The give-and-take "EXPLORATORIUM" thus becomes "HASORUDWRULXP." Such a scheme was easily broken and showed a certain level of naivete on Caesar's part concerning the enemy's intelligence.
Substitution cipher wheels
Click hither to download a copy of the zip wheels (12k PDF). Copy and cutting out the two wheels. Place the smaller wheel on top of the larger cycle and rotate them then your "key alphabetic character" on the modest bike is beneath the "A" of the large cycle. Now you can encipher your plaintext and pass it to your friend who knows the proper key letter.
You could make your ciphertext a little tougher to decode if you threw 26 pieces of paper into a chapeau, each with a letter of the alphabet written on it, drew them out i at a fourth dimension, and put them side-by-side under a normal alphabet. The result might look similar this (I just used the order of the keys on my keyboard, so you might call this a "Qwerty" lawmaking):
Plaintext letter A B C D E F G H I J K Fifty M Due north O P Q R Due south T U V W X Y Z
Ciphertext letter Q W E R T Y U I O P A S D F 1000 H J Thou L Z X C V B N M
Y'all can construct a clandestine bulletin from the to a higher place tabular array. Every time you see an "I" you lot would substitute the "O" beneath and so on for the other characters. The message "See me after school behind the gym," would read
"DTTZ DT QYZTK LEIGGS WTIOFR ZIT UND."
Give-and-take lengths - especially the short words - give great clues as to the nature of the lawmaking (see frequency charts). To help conceal your bulletin, ignore the spaces and break the message up into equal-sized chunks. Five letters are customary in the spy biz, so your message comes out like this (Note that an extra "dummy" graphic symbol "1000" is added at the stop to brand information technology come out with a five-letter group. Your recipient should accept no trouble with the actress character.):
DTTZD TQYZT KLEIG GSWTI OFRZI TUNDM
Another pop system called a diagrammatic cipher, used by many children in school, substitutes symbols for letters instead of other messages. This system is, in essence, the same as the alphabetic character commutation system, but information technology's easier to recall than 26 randomly picked letters. Information technology uses the tic-tac-toe boards and two 10's as shown beneath.
The same secret message as above, using the line-shapes that environment each letter of the alphabet (and including a dot where needed) becomes:
Fifty-fifty though it looks like undecipherable outer-space alien text, this would take an arm-chair cryptologist only almost 10 minutes or less to effigy out. Why? Given enough ciphertext, certain patterns go obvious. Notice how often the empty four-sided box appears: half-dozen times out of a total of 29 characters or near 20% of the fourth dimension. This would immediately point that the empty box was well-nigh certainly the symbol for "E," the most frequently used letter in English language. Other messages tin also be adamant by their frequency and by their association with other nearby characters (see "Frequencies"). Nigh all substitution ciphers are open to this kind of analysis.
Francis Bacon created one of the more interesting commutation ciphers. He used two different type faces slightly differing in weight (boldness). He broke up his ciphertext into v character groups, each of which would stand for one character in his plaintext. Depending on which characters of the group were bold, one could determine the plaintext character using the following table (* stands for a plain character and B for a bold character)
A=***** G=**BB* Thousand=*BB** S=B**B* Y=BB*** B=****B H=**BBB N=*BB*B T=B**BB Z=BB**B C=***B* I=*B*** O=*BBB* U=B*B** D=***BB J=*B**B P=*BBBB V=B*B*B E=**B** M=*B*B* Q=B**** W=B*BB* F=**B*B L=*B*BB R=B***B Ten=B*BBB
Our same secret message equally above would appear thusly (Bacon's bold and manifestly characters were less obvious than those beneath):
To be or not to be that is thdue east question. Whether 'tis nobler in the mind to suf fer the southwardlings and arrowests of outrageous fortunortheastward or to take arms aga inst a body of water of troubles and past opposing end them?
To decipher, we just pause the characters into groups of 5 and use the central above to detect the plaintext bulletin.
M E E T M E B E Tobeo rnott obeth atist heque stion Wheth ertis H I North D T H E Thou noble rinth emind tosuf ferth eslin gsand arrodue west Y M A F T E R S thenfou trage ousfo rtunorthwarde ortot akdue eastar msaga insta C H O O L seaof troub lesan dbyop posouthwardin gendt hem?
Transposition ciphers
Going back to your school days, oo-solar day oo-yay emember-ray ig-pay atin-lay? Pig-latin is a grade of transposition zippo where the original letters are kept intact (albeit with the addition of the suffix "ay"), but rearranged in some way.
Going back way before your school days, to the 5th century B.C., the Spartans used an interesting transposition zippo called a scytale. The scytale utilized a cylinder with a ribbon wrapped helically around information technology from ane stop to the other. The message was written across the ribbons, and so unwrapped from the cylinder. Only someone with an identical bore cylinder could re-wrap and read the message.
The scytale depended on a slice of hardware, the cylinder, which if captured past the enemy, compromised the whole organization. As well, the receiver could lose or break the cylinder and therefore lose the ability to decipher any message. It would be better if the method were completely "intellectual" and could be remembered and used without resorting to a physical device.
Since both the sender and receiver of a transposed ciphertext must agree on and recall this algorithm or method for enciphering and deciphering, something piece of cake would be nice. Since geometrical figures are like shooting fish in a barrel to remember, they serve as the basis for a whole course of transposition ciphers. Let's put our message into the shape of a box. Since there are 29 characters, we'll add a dummy ("O") to brand 30 and write the message in a half dozen by v box.
K Eastward Eastward T M Due east A F T E R S C H O O L B E H I N D T H Due east Thousand Y 1000 O
We tin at present transcribe the message past moving down the columns instead of across the rows. Over again nosotros'll interruption the characters into groups of five to give no clues about word sizes. The result looks similar this :
MACEH EFHHE ETOIG TEONY MRLDM ESBTO
The existent variety begins when you realize that you don't take to write your plaintext into the box row by row. Instead, you can follow a blueprint that zig-zags horizontally, vertically or diagonally, or one that spirals in or spirals out (clockwise or counterclockwise), or many other variations (see diagram below).
Once you've put the text in the chosen form using ane route, you can then encipher it by choosing a dissimilar road through the text. You and your partner just have to agree on the reading route, the transcription (enciphering) road, and the starting point to have yourselves a organization. These systems are chosen route transcriptions.
Here's our message once more. The reading route spirals counterclockwise inward, starting at the lower right corner (left diagram). The transcription route (right diagram) is zig-zag diagonal starting at the lower left corner. The ciphertext becomes:
EAMTN FTDIE EHOTE RHMEM BYESC GLOHO
To decipher, you make full the in box following the zig-zag route and read the message using the spiral route.
Some other blazon of transposition nada uses a cardinal word or phrase to mix upwardly the columns. This is called columnar transposition. It works similar this: Start, call back of a hole-and-corner key give-and-take. Ours will be the word Secret. Side by side, write it above the columns of letters in the square, and number the letters of the primal word as they would fall if we placed them in alphabetical lodge. (If in that location are indistinguishable messages, like the "East", they are numbered from left to right.)
5 two 1 four three 6 S Eastward C R E T M East Eastward T M East A F T East R Due south C H O O 50 B E H I Due north D T H E G Y M O
Now write the columns down in the order indicated by the numbers. The resulting ciphertext looking similar this:
ETOIG EFHHE MRLDM TEONY MACEH ESBTO
As y'all tin can see, this is only a unlike arrangement of the previous ciphertext, but at least it isn't in some regular pattern. We could have hands made information technology a little more difficult by filling the foursquare following a more complicated path. We could also use a geometric shape other than a rectangle and combine commutation and transposition. The only problem that might occur is that the deciphering may become so complicated that information technology will remain a secret at the receiving end forever! Come to call up of it, she never did run across me behind the gym...
Frequencies
Club of frequency of unmarried letters:
Eastward T O A North I R Southward H D L C W U K F YG P B Five K Ten Q J Z
Social club of frequency of digraphs (two letter of the alphabet combinations):
th er on an re he in ed nd ha at en es of or nt ea ti to it st io le is ou ar as de rt ve
Lodge of frequency of trigraphs:
the and tha ent ion tio for nde has nce edt tis oft sth men
Order of frequency of most mutual doubles:
ss ee tt ff 11 mm oo
Order of frequency of initial letters:
T O A W B C D S F M R H I Y E 1000 L N P U J 1000
Society of frequency of final letters:
E S T D Due north R Y F L O Chiliad H A R M P U W
One-letter of the alphabet words:
a, I, 0.
Most frequent two-letter words:
of, to, in, it, is, be, every bit, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am...
Most frequent iii-letter words:
the, and, for, are, just, not, you, all, any, can, had, her, was, one, our, out, day, get, has, him, his, how, human being, new, now, old, see, ii, way, who, boy, did, its, let, put, say, she, too, utilise...
Most frequent 4-letter words:
that, with, have, this, will, your, from, they, know, desire, been, good, much, some, time, very, when, come, here, only, like, long, brand, many, more, only, over, such, take, than, them, well, were...
Bibliography:
Gardner, Martin. Codes, Ciphers and Secret Writing.
New York, NY: Dover Publications Inc., 1972.
A wonderful, fun, and like shooting fish in a barrel to read introduction to codes and ciphers.
Smith, Laurence Dwight. Cryptography, the Science of Underground Writing.
New York, NY: Dover Publications Inc., 1943.
A good account of codes and ciphers with many historical examples.
Konheim, Alan G. Cryptography: A Primer.
New York, NY: John Wiley & Sons, 1981.
A highly technical (and mathematical) volume on more than modern methods of code making and breaking.
Gaines, Helen Fouché. Cryptanalysis: A Study of Ciphers and their Solution.
New York, NY: Dover Publications Inc., 1956.
The title says it all.
Net Resources
- Cryptography Tutorial
- Learn Cryptography
- Cryptology for Beginners (PDF)
- Basic cryptology concepts
- International Spy Museum in Washington D.C.
- Frode Weierud's CryptoCellar: Cryptology and Its History
- Cryptology paper by Oliver Pell
- Navajo Code Talkers: World War Ii Fact Sheet
Source: https://www.exploratorium.edu/ronh/secret/secret.html
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