Step 1: An Introduction to Ciphers
Each letter has a number. A is 1, B is 2, etc. If I wanted to say hello, it would turn into 8(h)-5(e)-9(l)-9(l)-15(o). If you want to decipher 1-16-16-12-5, you would match the numbers to the letters and see that it means apple. This cipher is so simple that I don't recommend using it for deceiving anyone but small children, and even they might figure it out. This one is just to introduce you the idea.
When learning these ciphers, get a some pen and paper and try using each cipher. Doing them will make them stick in your memory and make sure that you fully understand each cipher.
Step 2: Caesar Cipher
1. Cut out a circle of construction paper or something thicker if you want.
2. Write the alphabet along the outside of the circle so that it takes up the entire outside edge.
3. Cut out another circle that is small enough to fit inside the first circle and not cover up any of the alphabet.
4. Write the alphabet along the outside of the second circle so that it takes up the entire outside edge, and so that the letters on the second circle match up with the letters on the first.
5. Attach the second circle to the middle of the first circle with one of those pins that you push through paper and then flatten the two ends so that it stays in place.
So know you have your coding wheel you need to know how to use it. First spin the middle wheel so that A on the outside matches with B on the inside, B with C, C with D, and so on. Let's say the message you want to code is "Go to the store in five minutes." On the outside wheel, find the letter in the message, then write down the letter that matches them in the second wheel. "Go to the store in five minutes" becomes "fn sn sgd rsnqd hm ehud lhmtdr."
To decode the message, you do the complete opposite. Find the coded letters in the second wheel and match them with the first. You can see why they call it a shift cipher. You're basically shifting the alphabet over. Because shifting the alphabet only one space down is a semi-obvious thing to do, you can shift the alphabet as much as you want. You could make B U, C V, D W, like in the picture.
Before you send a message like this, make sure that you and whoever you are contacting have agreed on how many times you are shifting the alphabet. It won't work if the coder shifts it twice and the decoder shifts it thrice.
Also, if you don't want to go through the hassle of making a coder, you can wright out the alphabet and write the shifted alphabet underneath it.
Security Level- Low. This cipher is good for messages that aren't that important, things you just want to make a bit more complicated to figure out. It will deter the casual observer and beginner snoop, but there are only 26 ways a message can be coded. If someone cares enough, all they have to do is try each combonation until they decode your message.
Step 3: Masonic Cipher
To code a message, you find the letter in your message and draw the symbol that goes with it. A would be a corner open on the top left side, E would be a square, S is a v shape facing upwards, and so on. To decode a message you use your key, the first image, to match the symbols with the letter.
Security level- Low. This cipher can be cracked by anyone who knows about this style of cipher.
Here are some links to different variations of the masonic cipher.
Step 4: Playfair Cipher
The first step in using the Playfair cipher is preparing your message. Let's say it's, "I will be at the clock in five minutes."
-Get rid of all punctuation and spacing. Put the letters in pairs of two. You should have, "iw il lb ea tt he cl oc ki nf iv em in ut es"
-If you have any double letters, like tt, you need to put an X between them. It should look like this, "iw il lb ea tx th ec lo ck in fi ve mi nu te s"
-If you have a letter at the end that is by itself, add an X to make it a pair. Your code now looks like, "iw il lb ea tx th ec lo ck in fi ve mi nu te sx"
-If you have any J's in your message, replace them with an I. There are no J's in this message, so it doesn't matter.
Now you nee to make the coding square.
-Choose a word or phrase that you want to be you keyword. The longer it is the more secure your code will be. Let's say your keyword is "instructables".
-Write your keyword into a 5x5 grid without repeating any letters. Then fill in the rest of the alphabet, excluding the letter J. It should look like this.
It's finally time to code your message! You want take each pair of letters individualy, find them on the letter block, and use these three rules to code your message.
1.If the two letters on different rows and columns (diagonal from each other) imagine that the two letters form the opposite corner of a box. Write down the letters that are in the opposite corner. It is important the order you write them in. Whatever letter is first in the original pair of numbers, the new coded letter that is in the same row as it( in the corner of the imaginary box) will go first. If that didn't make sense, the first picture may help.
2.If the letters are on the same row, write down the letter that is to the immediate right of your original letter. If your letter is at the edge of the alphabet square, you need to wrap around the box and write the first letter in that row. Again, the order of the coded letters should be in the same order as the original letters. If that didn't make sense, the second picture may help.
3. If the letters are in the same column, write down the letters immediately below your original letter. if your letter is at the bottom of the alphabet square, you will need to write the letter that is at the top of that column. Again, keep the coded letters in the same order as the original letters. If that didn't make sense, the third picture may help.
IW becomes NV. IL becomes RU. LB becomes UL. VE becomes IK, and so on and so forth. The entire message when coded should be, "nvrufusyrgduaqumnsesikknciigas"
To decode the message go through the steps of coding, only this time, when letters are on the same row write the letter to the left, and when letters are on the same column write the letter above it. Basically do the inverse operation.
Remember, for your friend to understand your message he needs to know the key word. You can have an agreed upon keyword before hand, or if you want something a little more complicated, keep reading.
For this to work you need to write the date along with the coded message. First, choose twelve books that you and you friend both have access to. Assign each book a number. If the date is 7/17/12, then you go the the seventh book, the seventeenth page, and the twelfth word. That will be the keyword to solve the cipher.
Security Level- High. Unless your messages are being intercepted by someone with cipher cracking computer programs, your message will be safe.
Step 5: Columnar Transposition
This cipher works by writing your message into columns and then rearranging them.
First you need you choose a keyword. When you choose a keyword I would suggest using the method I described at the end of the last page. Let's use the word, "fancy" as the keyword. The message that you will code is, "Meet me at next midnight."
The number of columns you have is decided by the length of the code word. Fancy has five letters, so you have five columns. Now you write your message across the columns, starting another row when you reach the end of your columns just like in the picture. If the last line of your message didn't fill the last row, you would fill it in with random letters.
Next you need to assign each column a number. Again, this is decided by the keyword. Number the letters by ow close they are to the beginning of the alphabet. A is 1, C is 2, just like in the picture. Align the keyword and its numbers above the columns if you haven't already.
The number above each column determines which order it is written in. This turns you message into, "eatitnihmexnetmgmedt"
To decode the message, first dived the message into columns. You know how many columns there are because you have the keyword. You that the first group of five letters is column 1, the next one 2, and so on. Then you just need to reorder the columns according to the keyword.
Security Level-High. This code can be cracked with the right knowledge and a lot of time, but as long as you guard your keyword, you should be safe.
Step 6: Invisible Ink
First you want to pour some lemon juice into a small bowl and add a few drops of water. Take a cotton swab and dip it in the mixture. Use your swab to write out your message on the paper. When ever the swab starts to get dry dip it in the lemon juice. if you want to be really sneaky, use a cipher to code the message.
When the lemon juice dries, the message will be invisible. To make it reappear, hold it to a heat source, like a light bulb.
A blank piece of paper is a little suspicious, so you should write something on top of it. Because you have an invisible message and a obvious message, you have several ways to be devious.
First, you can write in plain English some misleading information to fool anyone who intercepts your message. You could write a series of random letters, making someone think there is some kind of code when there really isn't. You your enemies are smart, you write some misleading information coded with a cipher. The difficulty of the cipher you choose depends on how smart your enemies are.
Step 7: Dead Drops
Dead drops were used a lot during the Cold War when spies were sharing information. Basically, one spy would hide some documents, usually disguised, somewhere in a city. Another spy, who had been given directions to said dead drop, would come by after the other spy had left and pick up the documents. The documents could be disguised as trash and put in plane sight, or it could be in a hidden spot in an envelope.
An easy way to hide a small message is with a pen. As you can see in the second picture you just take apart the pen, roll the message around the inner tube, and put the pen back together. Preferably you want to use a pen that isn't see through for obvious reasons.
Hide your message in a carved out book, an empty pop can, an old box, a hollowed out phone. Hide it in a dumpster, in a tree, under a bench, by a gutter; the possibilities are literally endless! Whatever way you hide your dead drop, make sure you take the necessary precaution so that your documents aren't ruined by weather.
Step 8: Electronic Dead Drop
First, make an email account. Don't use your real name or information to create it. Now, give the password and username to the person you want to talk with.
To send him a message, write an email, but then save it as a draft instead of sending it. When your friend logs on, he'll be able to see the message and send one to you.
If you friend is compromised, all you have to do is log on to the email and delete everything.
Step 9: Conclusion
-Always use a cipher to code your messages
-Never assume that your phone calls, emails, or daily conversations are secure
-The best way to keep information from your enemies is to not let them know your sending a message.
This is my first instructables. i hope you learned something and please vote for me in the Spy Challenge.