Error passing values into array/values in array missing - c++

Question

I have a text file and I passed it to a char array. It was fine when I display in a getline() loop but after going through encryption, only the last line is encrypted and displayed. I have tried to display values in array before entering shift key and what is inside the array is the last line of the text only. What has went wrong? Please help!!
char msg[5000];
string plaintext;
int choice, shift;
char ch;
// Read file into char array
ifstream infile("plaintext.txt");
while (getline(infile, plaintext))
{
strcpy_s(msg, plaintext.c_str());
for (int i = 0; i < plaintext.length(); i++)
{
cout << msg[i];
}
}
cout << endl;
cout << "Enter shift key: ";
cin >> shift; //take the shift as input
cout << "Enter your choice" << endl;
cout << "1.Encryption" << endl;
cout << "2.Decryption" << endl;
cin >> choice;
// Encryption
if (choice == 1)
{
for (int i = 0; msg[i] != '\0'; i++) {
ch = msg[i];
//encrypt for lowercase letter
if(ch >= 'a' && ch <= 'z') {
ch = ch + shift;
if (ch > 'z') {
ch = ch - 'z' + 'a' - 1;
}
msg[i] = ch;
}
//encrypt for uppercase letter
else if (ch >= 'A' && ch <= 'Z') {
ch = ch + shift;
if (ch > 'Z') {
ch = ch - 'Z' + 'A' - 1;
}
msg[i] = ch;
}
}
printf("Encrypted message: %s", msg);
}
result

Answer 1

This is wrong because you need to use strcat too
// Read file into char array
ifstream infile("plaintext.txt");
while (getline(infile, plaintext))
{
strcpy_s(msg, plaintext.c_str());
for (int i = 0; i < plaintext.length(); i++)
{
cout << msg[i];
}
}
Instead:
// Read file into char array
ifstream infile("plaintext.txt");
getline(infile, plaintext)
if (!plaintext.empty())
{
strcpy_s(msg, plaintext.c_str());
plaintext.clear();
while (getline(infile, plaintext))
{
strcat_s(msg, sizeof msg, plaintext.c_str());
for (int i = 0; i < plaintext.length(); i++)
{
cout << msg[i];
}
plaintext.clear();
}
}