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Vigenere Cipher - variation  - Part II

Posted on 2010-09-07
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http://www.experts-exchange.com/Programming/Languages/Pascal/Delphi/Q_26433255.html#a33622117

If you can show me how your function works wiith examples for each case......starting with

Standard Vigenere  (A...Z)

lowercase alphabet    'a' - 'z',
numeric                        0..9
special characters      ~`!@#\$%^&*()_-+=|\}]{[:;"'?/>.<,
Control characters

I call your function for the Standard Vigenere  (A...Z) using:

Encrypt
Memo1.Lines.Text:= VigenereExEncrypt(Trim(Memo1.Lines.Text), edtKey.Text);

Decrypt
Memo1.Lines.Text:= VigenereExEncrypt(Trim(Memo1.Lines.Text), edtKey.Text, True);

It looks like it may be encrypting it, but its not using just A..Z, it is showing other characters (special).  And the decrypt does not work.

thanks
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LVL 45

Expert Comment

ID: 33622353
What version of Delphi are you using?

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LVL 16

Expert Comment

ID: 33625448

As i said on the previous post there is a fault in this logic.

1) You need to know what method was used for encryption in order to decrypt it.
2) The whole Vigenere function should change to something completely different... It should NOT output any letters... Look at the example :

Lets say that you have to encrypt the word A717.
And you use some key that normally will encrypt A717 to 94FJ.

Now A717 word will be encrypted with the "leave A-Z untouched" mode,
OUTPUT :
A717 will be A4FJ.

If we know that the decryption we should use IS the "leave A-Z untouched",
then we should not mess with the first letter (since it wasn't encrypted) but we cannotnot decrypt also the 3rd and the 4th letter that were encrypted! (since  "leave A-Z untouched" mode is on)

So final output :

A7FJ   <-  the only character that was successfully decrypted was the second one!

Solution:

When "Leave A-Z untouched mode" is used we should also block the encrypted output of normal letters.
example :

if '%' -> C based on some key then we should output "-C" ... Some letter "signed" with a special character so we can tell the decryption function if you see this then decrypt it.
This would be at least idiotic.
Or a shift in the table should happen dynamically.
In another words dynamically changing the key word.
This will be equivalent with chaos.

Mixing encrypted text with plain-text don't play well with a decryption function !

Correct me if I am wrong.
Hope this helps.
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LVL 45

Expert Comment

ID: 33626424
@CodedK

I've been interpreting the requirement as always encrypt [A..Z] and, optionally, encrypt the other character sets.
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LVL 16

Accepted Solution

CodedK earned 500 total points
ID: 33629144
The problem still stands only reversed.

Plaintext : @Code
Encrypted with a key :  @\$#G4  [mode : encrypt only A-Z, character '@'  was not encrypted]

Decryption is impossible since  "\$" and "#" are also special characters. The function cannot decide which character is encrypted and which is plain text.
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Author Comment

ID: 33666344
ok, i dont undertand why it matters, but its both Delphi 7 & Delphi 2010
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Author Comment

ID: 33666356
I am reviewing and testing and will report back soon, thanks
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