Javakurs/Übungsaufgaben/Cäsar-Chiffre/Musterlösung - Versionsgeschichte

98.210.208.240 am 27. Mai 2011 um 09:13 Uhr

2011-05-27T09:13:16Z

Jörg F: hat „Javakurs2007/Cäsar-Chiffre/Musterlösung“ nach „Javakurs/Übungsaufgaben/Cäsar-Chiffre/Musterlösung“ verschoben: wikistruktur

2010-08-01T11:50:59Z

hat „Javakurs2007/Cäsar-Chiffre/Musterlösung“ nach „Javakurs/Übungsaufgaben/Cäsar-Chiffre/Musterlösung“ verschoben: wikistruktur

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  /* Decodes texts having been encoded with caesar */     import java.io.*;   public class Caesar {        public static void main(String arg[])     {       ...“

2010-03-22T11:25:38Z

Die Seite wurde neu angelegt: „<pre> ‎Decodes texts having been encoded with caesar: import java.io.*; public class Caesar { public static void main(String arg[]) { ...“

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<pre>

/* Decodes texts having been encoded with caesar */

import java.io.*;

public class Caesar {

public static void main(String arg[])

{

int menuCase;

String text= new String();
String textState= "nothing loaded";
String output= new String(); /*will always be shown under menu*/
int key;

do

{

System.out.println("[1] load text");
System.out.println("[2] show text");
System.out.println("[3] save text");
System.out.println("[4] encrypt text");
System.out.println("[5] decrypt text without key");
System.out.println("[6] decrypt text with key");
System.out.println("[7] quit");
System.out.println( );
System.out.println("text state: ["+ textState +"]");

System.out.println( );
System.out.println( output );

System.out.print("input: ");

String s= readln();

menuCase= Integer.valueOf(s).intValue();

switch (menuCase) {

case 1:

{

System.out.print("filename: ");

String fileIn= readln();

text= readFile(fileIn);

output = "";
textState= "loaded";

break;

}

case 2:
{
output= text;
break;
}
case 3:
{
System.out.print("filename: ");

String fileName= readln();

String[] fileToSave= {text};
writeArrayToFile( fileName, fileToSave );

output = "";
break;
}
case 4:
{
System.out.print("key: ");
key= Integer.valueOf(readln()).intValue();

text= encryptCaesar(text, key);

output = "";
textState= "encrypted";
break;
}
case 5:

{
if( determineKoinzidenzindex(text) < 0.06 ) {
output= "Text might not be Caesar encrypted! ";
}

key= findKeyCaesar(text);

text= decryptCaesar(text, key);

output= new String("key is: "+ key);
textState= "decrypted";

break;

}
case 6:

{
System.out.print("key: ");
key= Integer.valueOf(readln()).intValue();

text= decryptCaesar(text, key);

output = "";
textState= "decrypted";

break;

}

}//switch

}while(menuCase!=7);

} //main

/* reads in a text from console */

public static String readln() {

BufferedReader consoleLine =

new BufferedReader(new InputStreamReader(System.in));

String input = null;

try {

input = consoleLine.readLine();

} catch (IOException e) {

e.printStackTrace();

}

return input;

}

/**

* Schreibt einen String-Array in eine Datei.

* @param filename : Pfad zu der Datei in Form eines Strings,
also z.B. "hallo.txt" oder "/etc/fstab", ...
* @param data String-Array: welches in die angegebene Datei
geschrieben werden soll.
* @return
*/

public static boolean writeArrayToFile(String filename,
String[] data ) {

BufferedWriter bufferedWriter;

try {

// Datei zum schreiben oeffnen

bufferedWriter= new BufferedWriter(

new OutputStreamWriter( new FileOutputStream(filename)) );

// Array Elementenweise in Datei schreiben
for (int i = 0; i < data.length; i++) {

bufferedWriter.write(data[i]);
bufferedWriter.newLine();

}

bufferedWriter.flush();

} catch (Exception e) {
// Fehlerbehandlung

e.printStackTrace();
return false;

}

return true;
}

/* reads in a text */

public static String readFile( String fileName ) {

String text= new String();

try {

FileReader file = new FileReader (fileName);

BufferedReader buff = new BufferedReader(file);

boolean eof = false;

while ( !eof ) {

String line = buff.readLine();

if ( null == line ) {

eof = true;

} else {

text+= line;

}

}

buff.close();

} catch (IOException e) {

System.out.println( e.toString() );

}

return text;

}

/* takes out punctuation and sets all letters to lower case */

public static String takeOutPunctuation( String origin ) {

String originLowCase= origin.toLowerCase();
String text= new String();

for ( int i= 0; i<originLowCase.length(); i++ ) {
/* filtering signs 'a' to 'z' using ASCII numbers */

if ( ((int) originLowCase.charAt(i)>= 97)
&& ((int) originLowCase.charAt(i)<= 122) )

text+= originLowCase.charAt(i);
}

return text;

}

public static int[] frequencyAnalysis( String text ) {

/* holds frequency of chars a-z, last digit (index 26) is total
* number of chars in text*/

int chr[]= new int[27];

for ( int j= 0; j< text.length(); j++ ) {

chr[ ((int) text.charAt(j)) - 97 ]++;

chr[26]++;

}

return chr;

}

/*see also: http://de.wikipedia.org/wiki/Koinzidenzindex */

public static double determineKoinzidenzindex( String text ) {

text= takeOutPunctuation( text );

int cnt= 0;
double kindex= 0;

/* holds frequency of chars a-z, last digit (index 26) is total
* number of chars in text*/

int chr[]= frequencyAnalysis(text);

/* chr.length-1 remember: last digit holds number of chars */

for ( int k= 0; k< chr.length-1; k++ ) {

kindex+= ( ((double) chr[k]/chr[26])*((double) chr[k]/chr[26]));

}

return kindex;

}

/* count the number of nigramms and trigramms in a german text*/

public static int germanText( String text ) {

int cntbitri= 0; //counter for bi- and trigramms

/* count bigramms */

for ( int i= 0; i< text.length()-1; i++ ) {

if (

(text.substring(i, i+2).equals("er"))

||(text.substring(i, i+2).equals("en"))

||(text.substring(i, i+2).equals("ch"))

||(text.substring(i, i+2).equals("de"))

||(text.substring(i, i+2).equals("ei"))

||(text.substring(i, i+2).equals("nd"))

||(text.substring(i, i+2).equals("te"))

||(text.substring(i, i+2).equals("in"))

||(text.substring(i, i+2).equals("ie"))

||(text.substring(i, i+2).equals("ge"))

)

cntbitri++;

}

/* count trigramms */

for ( int i= 0; i< text.length()-2; i++ ) {

if (

(text.substring(i, i+3).equals("ein"))

||(text.substring(i, i+3).equals("ich"))

||(text.substring(i, i+3).equals("nde"))

||(text.substring(i, i+3).equals("die"))

||(text.substring(i, i+3).equals("und"))

||(text.substring(i, i+3).equals("der"))

||(text.substring(i, i+3).equals("che"))

||(text.substring(i, i+3).equals("end"))

||(text.substring(i, i+3).equals("gen"))

||(text.substring(i, i+3).equals("sch"))

) cntbitri++;

}

return cntbitri;

}

/* count the number of nigramms and trigramms in an english text*/

public static int englishText( String text ) {

int cntbitri= 0; /*counter for bi- and trigramms */

/* count bigramms */

for ( int i= 0; i< text.length()-1; i++ ) {

if (

(text.substring(i, i+2).equals("th"))

||(text.substring(i, i+2).equals("he"))

||(text.substring(i, i+2).equals("an"))

||(text.substring(i, i+2).equals("in"))

||(text.substring(i, i+2).equals("er"))

||(text.substring(i, i+2).equals("re"))

||(text.substring(i, i+2).equals("on"))

||(text.substring(i, i+2).equals("es"))

||(text.substring(i, i+2).equals("ti"))

||(text.substring(i, i+2).equals("at"))

)

cntbitri++;

}

/* count trigramms */

for (int i= 0; i< text.length()-2; i++) {

if (

(text.substring(i, i+3).equals("the"))

||(text.substring(i, i+3).equals("ing"))

||(text.substring(i, i+3).equals("and"))

||(text.substring(i, i+3).equals("ion"))

||(text.substring(i, i+3).equals("tio"))

||(text.substring(i, i+3).equals("ent"))

||(text.substring(i, i+3).equals("ere"))

||(text.substring(i, i+3).equals("her"))

||(text.substring(i, i+3).equals("ate"))

||(text.substring(i, i+3).equals("ver"))

) cntbitri++;

}

return cntbitri;

}

public static int findKeyCaesar(String origin) {
/* proccedding:

* search for most frequent char in encoded text and assume it is
* one of the three most frequent letters in german or english
* language.

* decode text end test if it is decipheable
*/
origin= takeOutPunctuation( origin );

String text= new String();
/* number of occurence of most frequent char: */

int mostFrequentChrValue= 0;

int mostFrequentChr= 0; //most frequent char in text

char en[]= {'e','t','a',}; //most frequent letters in english

char ge[]= {'e','n','i',}; //most frequent letters in german

int biTriMax= 0; //maximal value of bi- and trigramms

int biTriTemp= 0;

int displacement= 0;
int key= 0;

double kindex= determineKoinzidenzindex(origin);

/*

System.out.print("Koinzidenzindex: "+ kindex );

if ( kindex < 0.07 ) {

System.out.println( " english text" );

} else {

System.out.println(" german text" );
}

System.out.println( );

*/

/* define most frequent char */

int chr[]= frequencyAnalysis(origin);

for ( int i= 0; i< chr.length-1; i++) {

if ( mostFrequentChrValue< chr[i] ) {

mostFrequentChrValue= chr[i];

mostFrequentChr= i;

}

}//for

/*assume most frequent char as (e,t,a)(engl.) or (e,n,i)(germ.) */

for ( int k= 0; k< en.length; k++ ) {
/* decide if text german or english */

if ( kindex < 0.07 ) {
/*assumed char causes certain displacement */

displacement= (mostFrequentChr+97) - (int) en[k] ;

} else {

displacement= (mostFrequentChr+97) - (int) ge[k] ;

}

if ( kindex < 0.07) {

text= decryptCaesar(origin, displacement);

biTriTemp= englishText(text);

} else {

text= decryptCaesar(origin, displacement);

biTriTemp= germanText(text);

}

/* displacement causing highest biTri value is key */

if ( biTriMax< biTriTemp )

{

biTriMax= biTriTemp;

key= displacement;

}

}

System.out.println("Schluessel lautet: "+ key );

return key;

}

public static String encryptCaesar( String text, int key ) {
return ( modifyText(text, key) );
}

public static String decryptCaesar( String text, int key ) {
key= key*(-1);
return ( modifyText(text, key) );
}

public static String modifyText( String text, int key ) {
String encrypted= new String("");

key= key%26;

int code= 0;
int currentChar= 0;

for ( int i= 0; i< text.length(); i++ ) {

currentChar= (int) text.charAt(i);
code= currentChar;

/*lower case letters*/
code= filterSigns(currentChar, code, key, 97, 122);

/* upper case letters */
code= filterSigns(currentChar, code, key, 65, 90);

/* numbers */
code= filterSigns(currentChar, code, key, 48, 57);

encrypted+= (char) code;
}

return encrypted;
}

/*replaces char only if it is situated in given boundarys*/
public static int filterSigns(int currentChar, int code, int key,
int lowerBoundary, int upperBoundary ) {

int range= upperBoundary - lowerBoundary + 1;

if ( (currentChar >= lowerBoundary) &
(currentChar <= upperBoundary) ) {

code= currentChar + key;

while ( code > upperBoundary ) {

code-= range;

}

while( code < lowerBoundary ) {

code+= range;

}
}

return code;
}

}

</pre>

@@ Zeile 20: / Zeile 20: @@
      int menuCase;
-     String text= new String();
+     String text= "";
      String textState= "nothing loaded";
-     String output= new String(); /*will always be shown under menu*/
+     String output= ""; /*will always be shown under menu*/
      int key;
@@ Zeile 47: / Zeile 47: @@
       String s= readln();
-      menuCase= Integer.valueOf(s).intValue();
+      menuCase= Integer.parseInt(s);
@@ Zeile 91: / Zeile 91: @@
+                {
                   System.out.print("key: ");
-                  key= Integer.valueOf(readln()).intValue();
+                  key= Integer.parseInt(readln());
                   text= encryptCaesar(text, key);
@@ Zeile 111: / Zeile 111: @@
                   text= decryptCaesar(text, key);
-                  output= new String("key is: "+ key);
+                  output= "key is: "+ key;
                   textState= "decrypted";
@@ Zeile 122: / Zeile 122: @@
+                {
                   System.out.print("key: ");
-                  key= Integer.valueOf(readln()).intValue();
+                  key= Integer.parseInt(readln());
                   text= decryptCaesar(text, key);
@@ Zeile 223: / Zeile 223: @@
-     String text= new String();
+     String text= "";
@@ Zeile 232: / Zeile 232: @@
        BufferedReader buff = new BufferedReader(file);
+       while ( true ) {
          String line = buff.readLine();
@@ Zeile 241: / Zeile 239: @@
          if ( null == line ) {
-              eof = true;
+              break;
          } else {
@@ Zeile 272: / Zeile 270: @@
     String originLowCase= origin.toLowerCase();
-    String text= new String();
+    String text= "";
@@ Zeile 278: / Zeile 276: @@
       /* filtering signs 'a' to 'z' using ASCII numbers */
-      if (   ((int) originLowCase.charAt(i)>= 97)
+      if (   (originLowCase.charAt(i)>= 'a')
-          && ((int) originLowCase.charAt(i)<= 122) )
+          && (originLowCase.charAt(i)<= 'z') )
       text+= originLowCase.charAt(i);
@@ Zeile 300: / Zeile 298: @@
       * number of chars in text*/
-     int chr[]= new int[27];
+     int[] chr= new int[27];
@@ Zeile 306: / Zeile 304: @@
      for ( int j= 0; j< text.length(); j++ ) {
-       chr[ ((int) text.charAt(j)) - 97 ]++;
+       chr[ text.charAt(j) - 'a' ]++;
        chr[26]++;
@@ Zeile 331: / Zeile 329: @@
       * number of chars in text*/
-     int chr[]= frequencyAnalysis(text);
+     int[] chr= frequencyAnalysis(text);
@@ Zeile 514: / Zeile 512: @@
-    String text= new String();
+    String text= "";
     /* number of occurence of most frequent char: */
@@ Zeile 522: / Zeile 520: @@
-    char en[]= {'e','t','a',};  //most frequent letters in english
+    char[] en= {'e','t','a',};  //most frequent letters in english
-    char ge[]= {'e','n','i',};  //most frequent letters in german
+    char[] ge= {'e','n','i',};  //most frequent letters in german
@@ Zeile 558: / Zeile 556: @@
     /* define most frequent char */
-    int chr[]= frequencyAnalysis(origin);
+    int[] chr= frequencyAnalysis(origin);
@@ Zeile 583: / Zeile 581: @@
         /*assumed char causes certain displacement */
-        displacement= (mostFrequentChr+97) -  (int) en[k]  ;
+        displacement= (mostFrequentChr+'a') -  (int) en[k]  ;
       } else {
-        displacement= (mostFrequentChr+97) -  (int) ge[k]  ;
+        displacement= (mostFrequentChr+'a') -  (int) ge[k]  ;
+      }
@@ Zeile 630: / Zeile 628: @@
    public static String encryptCaesar( String text, int key ) {
-     return ( modifyText(text, key) );
+     return modifyText(text, key);
+   }
    public static String decryptCaesar( String text, int key ) {
-     key= key*(-1);
+     key*= -1;
-     return ( modifyText(text, key) );
+     return modifyText(text, key);
+   }
    public static String modifyText( String text, int key ) {
-     String encrypted= new String("");
+     String encrypted= "";
-     key= key%26;
+     key%= 26;
      for ( int i= 0; i< text.length(); i++ ) {
-       currentChar= (int) text.charAt(i);
+       int currentChar= (int) text.charAt(i);
-       code= currentChar;
+       int code= currentChar;
        /*lower case letters*/

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