Compiler error on line 12?

[Kernmaster34]

How can I fix my compiler error on line 12 of me code? It's not working for some reason. May I have feedback please?

This is the problem:

Your country is at war and your enemies are using a secret code to communicate
with each other. You have managed to intercept a message that
reads as follows:

:mmZ\dxZmx]Zpgy

Write a program that decrypts the intercepted message. You only know
that the key used is a number between 1 and 100. Your program should
try to decode the message using all possible keys between 1 and 100.
When you try the valid key, the message will make sense. For all other
keys, the message will appear as gibberish.

#include <iostream>
#include <string>
#include <vector>

using namespace std;

void input(char message[100], int& key);
vector<char> encrypt(char message[100], int key);

int main()
{
    input(message[100], key);
    vector<char> encrypt(char message[100], key);

    return 0;
}

void input(char message[100], int& key)

{
    cout << "Please enter an encrypted message: \n";
    cin.getline(message,100);
}

vector<char> encrypt(char message[100], int key)

{
    int counter;
    char output[100];

    for(key = 1; key <= 100; key++)
    {
        for(counter = 0; message[counter]! = '\0' ; counter++)
        {
            if(message[counter] - key < 32)
                output[counter] = message[counter] - key - 32 + 127;
            else
                output[counter] = message[counter] - key;
        }

        cout << "Decrypted message using Key" << key << " : ";
        cout << output << endl;
    }

}

[rssair]

You're getting a compiler error because you have not defined message or key.
In main, before you call the input function, you need to have those variables defined.