Having problems with C++ related issues - c++

Question

I wrote a C++ program that does a Vigenere cipher, but I have bumped into couple c++ issues. One is that the program encrypts, but it doesn't decrypt its encryption.Another issue is how the last for loop is, it doesn't seem to work correctly.The third issue is that c++ is not adding space on where I type space. and also it only prints out one letter. I don't really get c++ because I'm new to it.
#include <iostream>
using namespace std;
int main()
{
string Message; //What The User Inputs
string Key; // What Key To Go By
string Encryption; // The Secret
cout << "\n\nEnter Your Message: ";
getline(cin, Message);
cout << "\nEnter The Main Key: ";
getline(cin, Key);
cout << "\n\n"<<endl;
for (int i=0; i<=Message.size(); i++) //letter i is less than the length of the message
{
int k=0;
Encryption[i] = (( (Message[i]-97) + (Key[k]-97)) %26) + 97; //The Algorithm
k++;
if ( k==Key.size() )
{
k=0;
}
}
for (int i=0; i<=Message.size(); i++)
{
string Result;
Result = Encryption[i];
if ( i == Message.size() )
{
cout <<"Encryption: "<< Result <<endl;
cout << "\n\n"<<endl;
}
}
return 0;
}
/*
INPUT:
Enter Your Message: Hello There
Enter The Main Key: Secret
OUTPUT:
Encryption: Z
*/

Answer 1

Point 1: Program does not decrypt the encrypted message
Of course it doesn't. The program does not contain any code that would decrypt the encrypted message. I can't help on point 1.
Point 2: The last for loop does not work.
You do not need a loop to print out the encrypted message.
cout << "Encryption: " << Encryption<< endl;
cout << "\n\n" << endl;
Point 3: "c++ is not adding space on where I type space"
I do not understand what you mean here. Please explain.
Point 4: Only one character is printed out
As per Point 2, this loop is not needed, but to explain what went wrong:
for (int i=0; i<=Message.size(); i++)
{
string Result;
Create an empty temporary string named Result. A new Result will be created every time the loop goes around and the previous one will be destroyed.
Result = Encryption[i];
Set Result to the ith character in string Encryption. Result now contains exactly one character.
if ( i == Message.size() )
{
If i has reached the length of the message
cout <<"Encryption: "<< Result <<endl;
print out the one character in Result.
cout << "\n\n"<<endl;
}
}
In addition:
No space was allocated inside string Encryption;. By default a string is created empty. It has no string length, so attempting to index the string, as in Encryption[i], is meaningless. There is no Encryption[i] to be accessed, and attempting to do so has no defined result. It may crash your program. It may look like it is running, and crash your program later. It may do anything including look like it is working.
To fix this, one needs to allocate space with string::resize. After the message to be encoded has been read in,
cout << "\n\nEnter Your Message: ";
getline(cin, Message);
add
Encryption.resize(Message.size());
to allocate the storage you need.