A company wants to transmit data over the telephone

A company wants to transmit data over the telephone, but they are concerned that their phones may be tapped. All their data are transmitted as four-digit integers. They have asked you to write a program that will encrypt their data so that it may be transmitted more securely. Your application should read a four-digit integer entered by the user in an input dialog and encrypt it as follows: Replace each digit by (the sum of that digit plus 7) modulus 10. Then swap the first digit with the third, and swap the second digit with the fourth. Print the encrypted integer. Write a separate application that inputs an encrypted four-digit integer and decrypts it to form the original number.

Here's some code to deal with the encryption part:

using System;

class Encrypt

{

static void Main()

{

string input;

while(true)

{

Console.Write("Enter a 4 digit number : ");

input = Console.ReadLine();

// check it's a four digit number - allow leading zeros

int number;

bool isValid = int.TryParse(input, out number);

if (!isValid || input.Length != 4)

{

Console.WriteLine("Not a 4 digit number, please try again");

}

else

{

break;

}

// now encrypt it

int[] digits = new int[4];

for(int i = 0; i < 4; i++)

{

digits[i] = (int)input[i] - 48; // the digits '0' to '9' are ASCII 48 to 57

digits[i] = (digits[i] + 7) % 10;

}

int temp = digits[2];

digits[2] = digits[0];

digits[0] = temp;

temp = digits[3];

digits[3] = digits[1];

digits[1] = temp;

Console.Write("The encrypted number is : ");

foreach(int digit in digits) Console.Write(digit);

Console.WriteLine();

Console.ReadKey();

}

}

See now if you can figure out how to write the decryption application - very few changes are needed to the code.

1 Reply

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Vulpes14y ago
Accepted answer
Here's some code to deal with the encryption part:

using System;

class Encrypt
{
static void Main()
{
string input;

while(true)
{
Console.Write("Enter a 4 digit number : ");
input = Console.ReadLine();
// check it's a four digit number - allow leading zeros
int number;
bool isValid = int.TryParse(input, out number);
if (!isValid || input.Length != 4)
{
Console.WriteLine("Not a 4 digit number, please try again");
}
else
{
break;
}
}

// now encrypt it
int[] digits = new int[4];

for(int i = 0; i < 4; i++)
{
digits[i] = (int)input[i] - 48; // the digits '0' to '9' are ASCII 48 to 57
digits[i] = (digits[i] + 7) % 10;
}

int temp = digits[2];
digits[2] = digits[0];
digits[0] = temp;

temp = digits[3];
digits[3] = digits[1];
digits[1] = temp;

Console.Write("The encrypted number is : ");
foreach(int digit in digits) Console.Write(digit);
Console.WriteLine();
Console.ReadKey();
}
}

See now if you can figure out how to write the decryption application - very few changes are needed to the code.
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