Array Application Programming. COMP102 Prog. Fundamentals I: Arrays Application / Slide 2 Character...

Array Application

Programming

COMP102 Prog. Fundamentals I: Arrays Application / Slide 2

Character Array: Ex. 1

Note: a palindrome is a word (or words) which read the same forward and backward kayak solos radar racecar deified evil olive Malayalam lonely Tylenol Re-divider Madam I'm Adam step on no pets won ton? not now

Run Example 1 to find out which words the computer

considers a palindrome


An Encoded Message What does the following message

represent? YMJ%HTRU657%HQFXX%NX%YTT%JFX^& Answer: THE COMP102 CLASS IS EASY! Problem statement: Write a function that

encodes a message (in English) using some encryption method, and another function that decodes the encoded message.


Offset Cipher

We use the simplest method, the “offset cipher”. Define offset value, which will be used in both the

encoding and decoding processes. The encoding function shifts each character in the

message higher by the offset amount.Example: if offset is 1, then a becomes b.

The decoding function shifts each character in the encoded message lower by the same offset amount.Example: if offset is 1, then b becomes a.


Ex. 2: Main Functionint main()

{

char wait; //Temporary input control character

// Print out original plain text

dump("input.txt");

cout << endl << endl;

cout << "Please press a key to encode this file:";

cin >> wait;

// Call encode function

encode1("input.txt", "cipherfile.txt");


Ex. 2: Main Function// Print out encoded (cipher) textdump("cipherfile.txt");cout << endl << endl;

cout << "Please press a key to decode this file:"; cin >> wait;

// Call decode functiondecode1("cipherfile.txt", "output.txt");// Print out decoded textdump("output.txt");

return 0;}


Ex. 2: encode1 Function

void encode1(const char plaintext[], const char ciphertext[])

{// plaintext is filename where the original // message (in English) is stored// ciphertext is filename in which to store// the encoded message

const int offset = 5; // secret offset value char tempchar; // declare input and output file streams ifstream ins; ofstream outs;

// open input and output data files ins.open(plaintext);

outs.open(ciphertext);


Ex. 2: encode1 Function // read original text message character-by-char ins.get(tempchar); while (!ins.eof()) {

// encode the character tempchar += offset; // write encoded character to

// the output file outs.put(tempchar);

// read next character ins.get(tempchar);

} // close files ins.close(); outs.close(); }


Ex. 2: decode1 Function

void decode1(const char ciphertext[], const char plaintext[])

{ // ciphertext is filename where the // encoded message is stored // plaintext is filename in which to store // the decoded message

const int offset = 5; // same offset value char tempchar;

// declare input and output file streams ifstream ins;

ofstream outs; // open input and output files

ins.open(ciphertext); outs.open(plaintext);


Ex. 2: decode1 Function// read encoded message character-by-

character ins.get(tempchar); while (!ins.eof())

{ // decode the character tempchar -= offset;

// write decoded character

//to the output file outs.put(tempchar);

// read next encoded character ins.get(tempchar);

} // close files

ins.close(); outs.close(); }


Ex. 2: Dump File to Screenvoid dump(const char filename[]){ // this function outputs the contents of a file // whose name is given in the parameter filename

char tempchar; ifstream ins; // declare input file stream ins.open(filename); // open input file

// output the file content character by character ins.get(tempchar);

while (!ins.eof()) { cout << tempchar; ins.get(tempchar);

} // close input file ins.close();

}


Cipher Improvement The simple offset cipher is very easy to break (for

example, by using frequency analysis to determine the constant offset value).

Better security is provided by a cipher where a key sequence is used for offset values. The first character in the plain text is offset by the

first key value in the key sequence. The second is offset by the second key value, and so on.

The longer the key sequence, the harder it is to break the encryption.

Both the sender and receiver must share the same secret key sequence.


A New Message

What does the following message represent?

Z¡¬ L¥]X6¼}un¢┤^╨ÑIXƒ[osåæ

Answer:

THE COMP102 CLASS IS FUN!


Ex. 3: Main Functionint main()

{

char wait; //Temporary input control character

int key[KEY_SIZE], index; // declare key array

// declare the input and output file names

char input_file_name[16], output_file_name[16];

// user supplies the input and output file names

cout << "Enter input file name (max 15 chars):\n";

cin >> input_file_name;

cout << "Enter output file name (max 15 char):\n";

cin >> output_file_name;


Ex. 3: Main Function // Print out original plain text dump(input_file_name); cout << endl << endl;

//create the seed for the random number generator srand(time(NULL));

//Create decode key array;for (index = 0; index < KEY_SIZE; index++) {

//For plain ASCII text, //the offset should be less than 128key[index] = rand()% MAX_OFFSET;//Print out the random number//cout << key[index] << " ";

}


Ex. 3: Main Function

cout << "Please press a key to encode this file: ";

cin >> wait;

// Call encode function

encode(input_file_name, "cipherfile.txt", key);

// Print out encoded (cipher) text

dump("cipherfile.txt");

cout << endl << endl;


Ex. 3: Main Function

cout << "Please press a key to decode this file: "; cin >> wait;

// Call decode functiondecode("cipherfile.txt", output_file_name, key);

// Print out decoded textdump(output_file_name);cout << endl << endl;

return 0;}


Ex. 3: encode Functionvoid encode(const char plaintext[],

const char ciphertext[],

const int keylist[]){

// plaintext is filename where the input// message (in English) is stored

// ciphertext is filename in which to // store the encoded message

// keylist is the array with the coding key

char tempchar;

int index = 0;


Ex. 3: encode Function

// declare input and output file streams ifstream ins; ofstream outs;

// open input and output data files ins.open(plaintext); outs.open(ciphertext);


Ex. 3: encode Function // read original text character-by-character ins.get(tempchar); while (!ins.eof()) {

// encode the character tempchar += keylist[index];

//Use the key sequence in a cycle index = (index+1) % KEY_SIZE;

// write encoded character to the output file outs.put(tempchar); // read next character ins.get(tempchar);

} // close files ins.close(); outs.close(); }


Ex. 3: decode Function

void decode(const char ciphertext[],

const char plaintext[],

const int keylist[]){

// ciphertext is filename where the encoded// message is stored

// plaintext is filename in which to store // the decoded message

// keylist is the array with the coding key

char tempchar;

int index = 0;


Ex. 3: decode Function

// declare input and output file streams ifstream ins; ofstream outs;

// open input and output data files ins.open(ciphertext); outs.open(plaintext);


Ex. 3: decode Function// read encoded message character-by-

character ins.get(tempchar); while (!ins.eof())

{ tempchar -= keylist[index]; // decode char //Use the key sequence in a cycle

index = (index+1) % KEY_SIZE;

// write decoded character to output file outs.put(tempchar);

// read next encoded character ins.get(tempchar);

} // close files

ins.close(); outs.close();

}


Ex. 3: Dump File to Screenvoid dump(const char filename[]){ // this function outputs the contents of a file // whose name is given in the parameter filename

char tempchar; ifstream ins; // declare input file stream ins.open(filename); // open input file

// output the file content character by character ins.get(tempchar);

while (!ins.eof()) { cout << tempchar; ins.get(tempchar);

} // close input file ins.close();

}


Cipher Improvement

Much stronger encryption systems exist (such as PGP, DES, etc.) today. ( For more information on data encryption in general, refer to the book by David Kahn, “The Code-Breakers” The Mac Millan New York 1976.)

Array Application Programming. COMP102 Prog. Fundamentals I: Arrays Application / Slide 2 Character...

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