//___ Program for Encryption and Decryption in C ___

/* Program will read the message and an encryption key (which should be of only 3 digits) and based on that key

	data will be encrypted and for decryption the program will again read a number and if it is same as that of the number being inputted the first time 

		original message can be obtained */

/* Encrypted message in not printed but you can edit the code to watch the encrypted message */


//____ Including Header Files ___


#include<stdio.h>

#include<conio.h>

#include<string.h>

#include<stdlib.h>	//__ Included for fflush(stdin) ___

char original_message[100];

int temp_variable = 0;

int encryption_message_key_scan();

int decryption_message_key_scan();

void message_encryption(char *original_message, int temp_key);	//___ Function to encrypt the message ___

void message_decryption(char *original_message, int temp_key);	//___ Function to decrypt the message ___

int main()	//___ Main function from where the program execution starts ___
{
	int temp = 0, counter = 0;


	
	printf("\n\n\t\t__________ Encryption/Decryption in C _________\n\n");

	printf("\n\n  1. Encryption will take place\n\n  2. Decryption will take place");

	printf("\n\n\t Press Enter to Continue.....");

	printf("\n\n________________________________________________________________________________");

	while(getch() != 13)
	{
	}

	fflush(stdin);

	printf("\n\n  Encryption - ");

	temp = encryption_message_key_scan();

	message_encryption(original_message, temp++);
	
	printf("\n\n________________________________________________________________________________");
	
	printf("\n\n  Decryption - ");

	temp = decryption_message_key_scan();

	message_decryption(original_message, temp);
	
	printf("\n\n  Your Message      -  ");

	puts(original_message);

	getch();
	
}

/* Function to scan the message and the encryption key based on which message will be encrypted */


int encryption_message_key_scan()
{

	int encryption_key = 0;

	printf("\n\n  Enter the Message -  ");

	gets(original_message);

	fflush(stdin);

	printf("\n\n  Enter the Number  -  "); //___ Prompting the user to add the key ___

	scanf("%d",&encryption_key);	//__ Reading the three digit encryption key __

	return encryption_key;

}

/* Function to scan the decryption key and the function return type is of integer i.e. it returns the decryption

	key where the function is being called */

int decryption_message_key_scan()
{
	int decryption_key = 0;

	fflush(stdin);

	printf("\n\n  Enter the Number  -  "); //___ Prompting the user to add the key ___

	scanf("%d",&decryption_key);	//__ Reading the three digit encryption key __

	return decryption_key;

}

void message_encryption(char *original_message, int temp_key)
{
	int encryption_key = 0, remainder = 0, first_digit = 0, last_digit = 0, string_length = 0, counter = 0;

	char temp_character;
	
	/* Counter in the while loop represents the place value
	
		(0 place represents the ones/2 represents 100 place value) */
	
	while(temp_key != 0)
	{
		remainder = temp_key % 10;

		temp_key = temp_key / 10;

		
		if(counter == 0 )		
		{
			last_digit = remainder;
		}
		else if ( counter == 2)
		{
			first_digit = remainder;
		}

		counter++;
	}

	if (counter == 3)	//___ if the number is of 3 digits then ___
	{
		string_length = 0;	//___ String_length is used to count the number of characters in message ___

		while(original_message[string_length] != '\0')	
			
		//___ Counting the number of characters in the message__
		
		{
			string_length++;
		}

		

		if (string_length%2 != 0)	//__ According the algorithm if message length is odd, make it even ___
		{
			original_message[string_length] = 32;

			original_message[string_length + 1] = '\0';

			string_length++;
		}

		//___ Replacing the characters at the initial positions _____

		temp_character = original_message[first_digit - 1];

		original_message[first_digit - 1] = original_message[last_digit - 1];

		original_message[last_digit - 1] = temp_character;

		while((string_length >= first_digit * 2) && (string_length >= last_digit * 2))

		/* The loop will continue till any of the condition is false i.e. if any of the digit is becomes greater

			than message length loop will stop */
		{
			first_digit = first_digit * 2;

			last_digit = last_digit * 2;

			temp_character = original_message[first_digit - 1];

			original_message[first_digit - 1] = original_message[last_digit - 1];

			original_message[last_digit - 1] = temp_character;

		}

		counter = 0;

		while(counter!= string_length)	//__ Replacing the spaces will'@' __
		{
			if (original_message[counter] == ' ')
			{
				original_message[counter] = '@';
			}
			counter = counter + 1;
		}	
	}
	else
	{
		printf("\n\n\t\t __ 3 Digit Only __");
	}
}

void message_decryption(char *original_message, int temp_key)
{
	char temp_character;
	
	int decryption_key = 0, string_length = 0, remainder = 0, counter = 0, previous_first = 0, previous_last = 0,last_digit = 0, first_digit = 0, original_first = 0, original_last = 0;

	while(temp_key != 0)
	{
		remainder = temp_key % 10;
		
		temp_key = temp_key / 10;
				
		if (counter == 0 )
		{
			last_digit = remainder;
		}
		else if ( counter == 2)
		{
			first_digit = remainder;
		}

			counter++;
	}

	if (counter == 3)
	{
		string_length = 0;

		while(original_message[string_length] != '\0')	
			
		//___ Counting the number of characters in the message__
		
		{
			string_length++;
		}

		original_first = first_digit;

		original_last = last_digit;
			
		counter = 0;

		while(original_message[counter] != '\0')	//__ Replacing '@' with space __
		{
			if (original_message[counter] == '@')
			{
				original_message[counter] = 32;
			}
				counter++;
		}

			
		while((first_digit <= string_length) && (last_digit <= string_length))
		{
			previous_first = first_digit;
			
			previous_last = last_digit;

			first_digit = first_digit * 2;

			last_digit = last_digit * 2;
		}

		while((previous_first != original_first) && (previous_last != original_last))	
		{
			
			temp_character = original_message[previous_first - 1];

			original_message[previous_first - 1] = original_message[previous_last - 1];

			original_message[previous_last - 1] = temp_character;

			previous_first = previous_first/2;

			previous_last = previous_last/2;

		}

		temp_character = original_message[previous_first - 1];

		original_message[original_first - 1] = original_message[original_last - 1];

		original_message[original_last - 1] = temp_character;

	}
	else
	{
		printf("\n\n\t\t __ 3 Digit only __");
	}
}