Problem with decryption (. turn to A's)

I've created this program to decrypt sentences with a key that shifts the alphabet. The problem with it is that when i have any non-Alphabetic characters, the punctuation or numbers will turns it into a Alphabetic character for some reason. I cannot find the problem with the code. I know the error occurs when it's checking for the uppercase letters because uppercase letters are always displayed. If you guys could shed any light onto this that would be great. I also commented out the quick fix I made...but it does not allow some characters as well.

Code:

#include <iostream>
#include <fstream> //read from file
using namespace std;

int main()
{
	int offset;
	const int ENCRYPTED = 250;
	char message[ENCRYPTED];
	char AlphabetLower[]="abcdefghijklmnopqrstuvwxyz";
	char AlphabetHigher[]="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	char AlphabetLowerKey[26];
	char AlphabetHigherKey[26];
//	char OtherChars[]="1234567890- ;:,.?!$&*()=+-";
	int length;
	int k = 0;
	int l = 0;
	int m = 0;
	int n = 0;
	int loop = 1;
	int num = 0;
	int arraynum = 0;

	cout << "Please enter the encrypted message: ";
	cin.getline (message, ENCRYPTED, '\n');
	cin.ignore(100, '\n');
	cout << "Please enter the decryption key: ";
	cin >> offset;

	int a = offset; //Have to be initialized after offset is inputted
	int b = offset;

	length = strlen(message);

	for (a; a < 26; a++)
	{
		AlphabetLowerKey[k]=AlphabetLower[a];
		k++;
		
	}
	for (b; b < 26; b++)
	{
		AlphabetHigherKey[l]=AlphabetHigher[b];
		l++;
	}

	for (m; m < offset; m++)
	{
		AlphabetLowerKey[k]=AlphabetLower[m];
		k++;
	}
	for (n; n < offset; n++)
	{
		AlphabetHigherKey[l]=AlphabetHigher[n];
		l++;
	}
	while (arraynum < length)
	{
		while (loop == 1)
		{			
			if (message[arraynum] == AlphabetLower[num])
			{
				message[arraynum] = AlphabetLowerKey[num];
				loop = 0;
			}
			else
				if (message[arraynum] == AlphabetHigher[num])
				{
					message[arraynum] = AlphabetHigherKey[num];
					loop = 0;
				}
				else
//					if (message[arraynum] == OtherChars[num])
//					{
//						loop = 0;
//					}
//					else
						num++;
		}
		num = 0;
		loop = 1;
		arraynum++;
	}
	cout << message << endl;
	return 0;
}

You can check whether or not a single character is alphabetical, numerical, punctuation, etc. Have a look at the <cctype> library - in particular, isalpha()

Also, i'd reccomend researching C++ strings from the <string> header, instead of C-style null-terminated character arrays... you should find they make your life alot easier. (C++ strings automatically determine their own size, and have a wide variety of functions attached which make general manipulation much simpler).