Function to do decryption - c++

Question

I need help with function string decryptedpassword() which is a part of a program to decrypt a password. I have to utilize const VOWELS to identify presence of vowels and insert 0 before and e after each vowel dynamically.
The main goal of program is to make an authentication system that begin with some initial arguments containing his or her credentials to be validated by the program.
These arguments are stated in the command line and should consist of two values, one for username and the other for password. Fortunately, the client has already specified that Kalle will use the following credentials:
Username: Kalle
Password: bAnanASplit
These arguments are passed to main(..) which in turn hands them over to mainArgumentsParser(..), a function devoted to the process of parsing out values. You are free to study how this is done but may not modify any of the code you find in these functions. The parser will return a string value containing user
credentials in the form username,password, or simply ”fail” to inform main(..) that too few arguments were provided.
The string of credentials are then passed to authenticateUser(..) which will need to separate the username from password and store them in separate strings. As can be seen in the function definition, there already exists two string constants.
const string USERNAME = "Kalle";
const string PASSWORD = "i0J0u0j0u0J0Zys0r0{";
#include "Prototypes.h"
using std::string;
using std::stringstream;
using std::cout;
using std::endl;
// If argument less than 3 so print fail.
string mainArgumentParser(int argc, char* argv[]) {
if (argc < 3)
return "fail";
std::stringstream ss;
ss << argv[1] << "," << argv[2];
return ss.str();
}
//bool function to separate username and password and validate them.
bool authenticateUser(string value)
{
const string USERNAME = "Kalle";
const string PASSWORD = "i0J0u0j0u0J0Zys0r0{";
bool authPassed = false;
string username = value.substr(0, 5);
size_t pos = value.find(",");
string pass = value.substr(pos + 1);
if (username == USERNAME && decryptPassword(pass) == PASSWORD )
{
authPassed = true;
}
return authPassed;
}
// bool to detremine if the number is even or not.
bool even(int x)
{
return x % 2 == 0;
}
//function to decrypt the paasword and return the result to validate the authentication.
string decryptPassword(string password)
{
const int ROT7 = 7, ROT9 = 9;
const string VOWELS = "AEIOUYaeiouy";
string decryptedpass;
for (size_t i = 0; i < password.length(); i++) {
if (even(i)) {
decryptedpass += password[i] + ROT7;
}
else {
decryptedpass += password[i] + ROT9;
}
decryptedpass += '0';
}
return decryptedpass;
}