How to do vigenere cipher??

[nelsonkkyy]

What is it you are trying to achieve?

How to do vigenere cipher??

Can someone post a solution to help me plz?
Thank you

[DtY]

Are you looking for a solution, or help?
If you just want help, you'll want to start with a simple caeser cipher. Once you get that, it should be relatively simple to apply a different one on each letter for the vigenere cipher.

[nelsonkkyy]

How to do a simple caeser cipher?!
i have no idea.

[Tony]

Wikipedia FTW -- http://en.wikipedia.org/wiki/Caeser_cipher

[nelsonkkyy]

can some one make an example?

[Dan]

The wikipedia artical tony linked expilains it rather well but the pesdo code would be somthing like this:

code:

CipherText = "" For letter x in PlainText     y = (x + Key) mod 26     append y to ChiperText End

This assumes the letter x is repsented by a system such as A=0, B=1, C=3, .... Z=25. You can do this easly if you give some thought as to how chars are repesented in ASCII.

Note that the value of 'A' in ASCII is 65 and 'B' in ASCII is 66...... 'Z' is 90.

[nelsonkkyy]

it does not work/.\

[Tony]

What do you mean it does not work?

[nelsonkkyy]

the mod dosen't work

[nelsonkkyy]

can someone make a vigenere cipher example for me plz??

[nelsonkkyy]

Dan @ Tue Nov 10, 2009 6:02 pm wrote:

The wikipedia artical tony linked expilains it rather well but the pesdo code would be somthing like this:

code:

CipherText = "" For letter x in PlainText     y = (x + Key) mod 26     append y to ChiperText End

This assumes the letter x is repsented by a system such as A=0, B=1, C=3, .... Z=25. You can do this easly if you give some thought as to how chars are repesented in ASCII.

Note that the value of 'A' in ASCII is 65 and 'B' in ASCII is 66...... 'Z' is 90.

SO how do i write like"a=0" for each letters

[Dan]

It's pseudocode and is not turing: http://en.wikipedia.org/wiki/Pseudocode

mod in this case means the modulo operation: http://en.wikipedia.org/wiki/Modulo_operation


If you look at an ASCII table ( http://www.asciitable.com/asciifull.gif )the capital letters values should look like this:

code:

A    65 B    66 C    67 D    68 E    69 F    70 G    71 .      . .      . .      . Z    90

So if you where to say the value of a letter is X then X - 65 would get you:

code:

letter   value (x)      x - 65 A        65                0 B        66                1 C        67                2 D        68                3 E         69               4 F         70                5 G         71               6 .          .                  . .          .                  . .          .                  . Z        90                 25

In turing you can covert a character to it's ASCII value with the ord command: http://compsci.ca/holtsoft/doc/ord.html

And you can convert an integer to it's conrpsonding character value with chr: http://compsci.ca/holtsoft/doc/chr.html



So the code for converting a character to a number such that A=0, B=1, C=2, .... Z=25 would look somthing like:

Turing:

x := ord(myChar) -65

and to convert it back:

Turing:

myChar := chr(x + 65)

Note that this only works for capital letters as lower case letter have diffrent values. If you want your chiper to handel both you will either have to convert them to upper case before encrypting or change how the chiper works to keep the case intack.

[nelsonkkyy]

Turing:

var plaintext,key,ciphertext:string put "Enter a plaintext to encrypt" get plaintext put "Enter a key word" get key for i:1..length(plaintext) ciphertext := ord(plaintext) -65 put ciphertext %like that???%

[Dan]

Well thats a start, however there are some problems.


First "x := ord(myChar) -65" witch i posted is how to go from ascii code values of capaital letters to A=0, B=1, C=2, ... Z=25 type values. So you still need to use the key on the chipertext.

Also the ord command is used on characters and not strings.

Here is some code to work with:

Turing:

for i:1..length(plaintext)    x := ord(plaintext(i)) -65    % use key on x here    % convert x back to a char    % append x to chipertext string end for

plaintext(i) Should return the character at index i in the string plaintext. So if "Hello World" was input and i was 2, it would be equal to 'e' (if i rember how string indexs in turing work correctly).

ord(plaintext(i)) converts the char from plaintext(i) to an integer (from the ASCII table).

Subtracting it by 65 bring it from A=65, B=66, ... Z=90 to A=0, B=1, ... Z=25.

The result is stroed in x (witch you need to declair some where above the for loop).


Give some thought to it and try to replace the comments with the correct code.


Also i should note that you are inputing key as a string. For this chiper the key is a integer, so you should either input an integer or convert the string to an integer.

[nelsonkkyy]

can you send an example for me plz. I still can't do it. my email:nelsonmilk@hotmail.com