Современные информационные технологии / Вычислительная техника и программирование

V. Chychuzhko, M. Chychuzhko, I. Zubko

Cherkassy State Technological University, Ukraine

Optimization of the process of Caesar and Vigenere decoding methods in C # environment

Every year, importance of computer information in our lives is increasing, and the problems of its protection become more urgent. Protection from each type of danger implies not only universal approaches, but also their own solutions that can protect data against different threats. One of them is cryptography - applied science about methods and ways of information transformation, with the purpose of its protection from illegal users [1].

The purpose of work - is to implement an interactive environment for coding and decoding textual information using Caesar and Vigenere methods [2]. Were added a point to break code without knowing a key, using consequent search of right variant. Methods of encoding/decoding and user interface were implemented in the environment of Microsoft Visual Studio using language C #. Note: program is resistant to register, any text is converted into uppercase. All characters (including digits) will be kept unchanged, including spaces.

The user must enter the key and message/code and click on the appropriate button. TextBox element, which is empty by default, will display the code/message according to the selected function.

Regardless of Internet sources, were implemented my own methods of coding according to used programming language [3]. You can see code of these methods below:

private String Cesar(int key, String code,int e)

{String message = ""; code.ToUpper();

for (int i = 0; i < code.Length; i++)

{ if (!((int)code.ElementAt(i) <= 90 && (int)code.ElementAt(i)>=65))

{ message = message + code.ElementAt(i); }

else

{message=message+(char)(((int)code.ElementAt(i)-65+key*e+26)%26+65); }}

return message; }

private String Vigenere(String key, String code, int e)

{String message = ""; code.ToUpper();

for (int i = 0; i < code.Length; i++)

{if (!((int)code.ElementAt(i)<= 90 && (int)code.ElementAt(i) >= 65))

{message = message + code.ElementAt(i); }

else

{message=message+(char)(((int)code.ElementAt(i)-65+((int)key.ElementAt(i % key.Length) - -65) * e + 26) % 26 + 65); }}

return message; }

In both cases, the function has three input parameters and return processed text [4]. Methods are used for encoding and decoding and excludes any errors for given values. Whether encoding information or function decodes it affects input parameter e. By pressing the appropriate buttons to the appropriate method is transmitted e = 1 to encode or e = -1 for decoding. Here is code of encoding and decoding buttons calling Caesar method:

private void button1_Click(object sender, EventArgs e)

{textBox3.Text=""+Cesar(Int32.Parse(textBox1.Text.ToString()), textBox2.Text.ToString(), 1); }}

private void button2_Click(object sender, EventArgs e)

{textBox3.Text=""+Cesar(Int32.Parse(textBox1.Text.ToString()), textBox2.Text.ToString(), -1); }

Buttons of breaking have a slightly different structure, so as encryption key is unknown. By pressing a button, generates a loop that goes through all possible key, and sends them into the appropriate method. Caesar method has only 26 possible combinations (based on English alphabet, there are 26 letters), that’s why even to show all 26 variants on screen is not a problem.

private void button3_Click(object sender, EventArgs e)

{ textBox3.Text = "";

for (int i = 1; i < 26; i++)

{String c = Cesar(i, textBox2.Text.ToString(), -1);

if(isText(c)){textBox3.Text = textBox3.Text + i + " " + c + "\r\n";}}}

Vigenere method has more complicated aspects because key is presented not as number, but as a word or even as a phrase. For key, which length is n, we have 26ⁿ variants of possible keys [5]. Output of such amount of information on the screen is impractical. By the recommendation of teacher, were created methods to break 3 and 4-length keys. Thanks to feature of method, they will include keys with length 1 and 2. Were added additional TextBox, which is invisible by default. It’s used only for showing results of 4-length-key breaking. Code of Breaking button has to separated loops for counting 3 and 4 length keys. They go through all possible combinations of key. On every step this loop generates one-time-key, which is sent to coding method. Below you can see it’s code:

private void button6_Click(object sender, EventArgs e)

{ textBox3.Text = "3-key\r\n";

textBox4.Visible = true;

textBox4.Text = "4-key\r\n";

for (int i = 0; i < 26; i++)

{for (int j = 0; j < 26; j++)

{for (int k = 0; k < 26; k++)

{String key1 = ""+((char)(i + 65)) + ((char)(j + 65)) + ((char)(k + 65));

String c = Vigenere(key1, textBox2.Text.ToString(), -1);

if (isText(c))

{textBox3.Text = textBox3.Text + key1 + " " + c + "\r\n";}}}}

for (int i = 0; i < 26; i++)

{ for (int j = 0; j < 26; j++)

{ for (int k = 0; k <26; k++)

{ for (int l = 0; l < 26; l++)

{Stringkey1=""+((char)(i+65))+((char)(j+65))+((char)(k+65))+((char)(l+65));

String c = Vigenere(key1, textBox2.Text.ToString(), -1);

if (isText(c))

{textBox4.Text = textBox4.Text + key1 + " " + c + "\r\n";}}}}}}

On the code above, you can see method. Its task is to check, is this string value a text or just set of letters. The main idea is to follow main rules of English language (in this case). After decoding, so-called “possible message” is divided into words by symbols. New array of word is checked according to the length of word. For example, there only two words in English, which length is 1. They are pronoun “I” and article “a”. All other found results are rejected and loop stops, moving to the next variant of decoding [6]. All of words, which are composed by two letters, have 1 vowel and 1 consonant, anyway. So, the main goal is to proof it or reject. There are special methods, which were implemented for this purpose. One of them tests if letter is a vowel, and other one is using previous to count number of vowels in word. Using them we can compare numbers and check 2-legth word. Methods are shown below:

private bool isVowel(char c)

{ bool b = false;

char[] vowels = { 'E', 'Y', 'U', 'I', 'O', 'A' };

for(int i=0; i<=5; i++)

{if (c.Equals(vowels[i]))

{ b = true; break; }}

return b; }

private int countVowels(String s)

{int b = 0; for (int i = 0; i < s.Length; i++)

{if (isVowel(s.ElementAt(i))) b++;}

return b; }

Words, which are composed of 3 letters, can have different number of vowels. But they still correspond to rules: there can’t be 3 or 0 consonants in the 3-length word. That is the main test for them, used in code. There is only one exception, it’s pronoun “YOU”, it has 3 vowels. Bigger words require special method to check them. The rule is that we cannot have more than 3 vowels or consonant in a row. So, method creates a loop, which goes through all word and count vowels and consonants, using method above. If their amount is more than 3 in the row, loop breaks and method returns false. Code of this method is below:

private bool isWord(String s)

{bool b = true;

int v = 0;

int c = 0;

for (int i = 0; i < s.Length; i++)

{if (isVowel(s.ElementAt(i)))

{ v++; c = 0; }

else

{c++;

v = 0; }

if (c > 3 || v > 3) b = false; }

return b; }

Here you can see main method, which unites all previous ones in on place and makes them work together, preventing unreal text output. When even one mistake is found on any step, loop stops, variant is got rejected and program starts testing the next variant of decoding. This method was also used in Caesar breaking, and depending on the text-length we can get even only one right variant. The main factor, which influences on the amount of results, is size of incoming text. The bigger text then less results we have, and easier for us to find right. While testing I’ve encode message and then I’ve tried to break this. Note: during test, I decreased amount of possible variants, to prevent problems with computer, counting 26³ or 26⁴ variants can be dangerous for system. I’ve reduce it to 3⁴ variants for 4-length word and 6³ for 3-length. I’ve coded message “I am from Ukraine” and as you can see on the screen, I have only 2 results instead of 216.

Considering the speed of development of computer technology, we can be assume that in the near future cryptography will become the "third literacy" along with mandatory possession of a computer and information technology.

LITERATURE

1. Martin, Keith M.  Everyday Cryptography. — Oxford University Press, 2012. — 142 p.

2. Knudsen, Lars R.  Block Ciphers—a survey. — London: Springer, 1997.

3. Henk C.A. van Tilborg.  Encyclopedia of Cryptography and Security. — Springer, 2005. — 115 p.

4. Wobst R. Cryptology Unlocked — Chichester: John Wiley & Sons Ltd, 2007. — 554 p. 

5. Martin, Keith M. Everyday Cryptography. — Oxford University Press, 2012. — 142 с.

6. Bruen, Aiden A. & Forcinito, Mario A. (2011). Cryptography, Information Theory, and Error-Correction: A Handbook for the 21st Century. John Wiley & Sons. p. 21.