How would one write a coded message to which you need a single keyword to decode?

Question

Suppose I want to write a coded message to my friend. I want to give my friend a piece of paper with letters on it that appear random... out of place, what have you. I want to leave my friend a code word like, "raven" somewhere in his house. Once he has this codeword, he can decode the message.

How would I go about producing a coded message to which you need a codeword to decode?

Why do I always end up finding it myself before someone answers my question?

http://www.wikihow.com/Encode-and-Decode-Using-the-Vigenere-Cipher

Answer 1

A very simple and hard to crack (though possible) cipher is the Vigenere Cipher. You use the tabula recta (a table with letters) to encode and decode you messages. There's an image of it at http://en.wikipedia.org/wiki/Image:Vigenere-square.png (if you're reading this, open this table now because it will greatly help you understand). You encode the first letter on your plaintext with the first letter of your keyword, the second ith the second, etc. Once you run out of letters in the keyword, you repeat the keyword. The intersections on the tabula recta between the two letters in a pair is your coded letter (the pair of letters would be the plaintext letter and the keyword letter). So, for example, if the word OCTOPUS were coded using the keyword TIME, then O would encoded with T, C with I, T with M, O with E, P with T, U with I, and S with E.
Encoded, it would be HKFSICW.
If this all just seemed confusing or it's just beginning to settle in your brain, see http://en.wikipedia.org/wiki/Vigenere_cipher

Answer 2

So you want to encrypt and decrypt a message. Well there an unlimited encryption systems, from the very simple to the unbreakable.

Here is a simple one. Write out the message, but instead of using letters use a number in the alphabet for the letter, like a is 1, b is 2, ... Now take those number and to the first letter add the number in the alphabet for "r", if that is larger then 26, subtract 26. Now for the second letter (number) in the message add the value for a (1), and again if that is greater then 26, subtract 26. Now repeat that over and over reusing the letters (numbers) in raven for the whole message.

The message can be reproduced by using a raven again, but this time subtracting, and if less then 1 then adding 27 to it.

P.S. Why do you always find the answer before someone answers it? Most likely because you choose to post it here, and then do some quick research, I find the truth that most questions posted here can be answered with just a little research.

Answer 3

I know you found the answer but maybe this will help you.
I write to my sister in code. I put for example:

1 uf rvjfsp

The one means go one letter over, so instead of A being A it's B, I always change the first number I can change it for a 3 or a 4 and since we are bilingual, this is in Spanish.

So this means in Spanish I love you=te quiero, for example. We almost always use 1 or 2 it's easier.

Greetings from Baja!

Answer 4

the answer to your question is xor. it is a boolean function that can be used with the binary of the ascii characters to be encoded against the "key". since this doesnt tell someone who has no experience with the topic anything here is the idea...

1) Create a text file
2) Name it something.VBS (the VBS is important the something isnt)
3) Copy this code to the file:

'CODE
data=InputBox("Data to cipher.","")
key=InputBox("Key","")
Msgbox Cipher(data,key)
Function Cipher(key,data)
tmp=vbNullString
For i=1 to len(data)
j=j+1: if j>len(key) then j=1
if mid(key,j,1)=mid(data,i,1) then Cipher="Bad key": Exit Function
tmp=tmp & chr(asc(mid(key,j,1)) xor asc(mid(data,i,1)))
Next
Cipher=tmp
End Function

4) Open the file... jerk.