I need a program that can read a four-digit integer 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, swap the second digit with
the fourth and print the encrypted integer.  Write a separate program
that inputs an encrypted four-digit integer and decrypts it to form
the original number.

Does this need to be in some particular language?

c++ code:

encrypt code:
#include <iostream>
#include <string>


int main()
{
	//get input
	string encrypt_me;
	cin >> encrypt_me;

	//add 7 mod 10
	int i,j;
	for (i=0,i<4,i++)
	{
		j = encrypt_me[i] - '0';
		j += 7;
		j %= 10;
		encrypt_me[i] = '0' + j;
	}

	//reorder number
	encrypt_me[0] &= encrypt_me[2];
	encrypt_me[1] &= encrypt_me[3];

	//print out result
	cout << encrypt_me;

	return 0;
}

decrypt code:
#include <iostream>
#include <string>


int main()
{
	//get input
	string decrypt_me;
	cin >> decrypt_me;

	//reorder number
	decrypt_me[0] &= decrypt_me[2];
	decrypt_me[1] &= decrypt_me[3];

	//undo 7 mod 10
	int i,j;
	for (i=0,i<4,i++)
	{
		j = decrypt_me[i] - '0';
		j += 3;
		j %= 10;
		encrypt_me[i] = '0' + j;
	}

	//print out result
	cout << decrypt_me;

	return 0;
}

--jeff