C 中的子字符串误差

我正在编写一个程序，该程序将根据两个公式计算密码强度。它要求用户输入2个密码，一个是八个或以下的密码，另一个是20个字符或更多。第一部分执行问题。但是，当我去执行第二部分时，它有效，但是提取接下来十二个字符的子字符串函数不断提取18个字符。我已经几次查看了我的代码，不明白为什么这样做。任何帮助将不胜感激！

#include <iostream>
#include <cmath>
#include <string>
#include <iomanip>
using namespace std;
int main()
{
//All variables and constants are declared
string eight_password, first_char, next_seven, twenty_password, first_char_twenty, next_seven_twenty, next_twelve, remaining;
int ep_length, character_set, first_char_length, next_seven_length, character_set_20, twenty_length, first_char_twenty_length, next_seven_twenty_length, next_twelve_length, remaining_length;
double eight_ent_strength, eight_nist_strength, twenty_ent_strength, twenty_nist_strength;
const int NIST_FIRST = 4, NIST_SEVEN = 2, NIST_REM = 1, NIST_CHARACTER=94, NIST_BONUS=6;
const double NIST_TWELVE = 1.5; 
//Console prompts for user to input password and character set
cout << "Hello! Please enter a password of 8 characters or less (including spaces!):" << endl;
getline(cin, eight_password);
cout << "What character set is being used?";
cin >> character_set;
cout << endl;
//Password length and information entropy strength are calculated and saved to the appropriate variables
ep_length = eight_password.length();
eight_ent_strength = (ep_length*((log(character_set))/(log(2))));
//First character and next seven characters are extracted and saved to the appropriate variables
first_char = eight_password.substr(0, 1);
next_seven = eight_password.substr(1, 7);
//First character and next seven characters lengths are calculated and saved to the appropriate variables
first_char_length = first_char.length();
next_seven_length = next_seven.length();
//NIST strength is calculated and saved to the appropriate variable
eight_nist_strength = (first_char_length*NIST_FIRST) + (next_seven_length*NIST_SEVEN)+((character_set/NIST_CHARACTER)*NIST_BONUS);
//The information that was calculated is now printed back out on the console to be viewed by the user
cout << "Your password " << eight_password << " is " << ep_length << " characters long. According to the information " << endl;
cout<<"entropy formula, it has a strength of " << eight_ent_strength << "." << endl;
cout << "The first character is "" << first_char << "" and the next seven characters are "" << next_seven << "". " << endl;
cout << "According to the NIST formula, it has a strength of " << eight_nist_strength << "." << endl << endl;
cin.ignore();
cout << "Now, please enter a password of 20 characters (including spaces!):" << endl;
getline(cin, twenty_password);
cout << "What character set is being used?";
cin >> character_set_20;
cout << endl;
twenty_length = twenty_password.length();
twenty_ent_strength = (twenty_length*((log(character_set_20)) / (log(2))));
first_char_twenty = twenty_password.substr(0, 1);
next_seven_twenty = twenty_password.substr(1, 7);
next_twelve = twenty_password.substr(8, 19);
remaining = twenty_password.substr(19);
first_char_twenty_length = first_char_twenty.length();
next_seven_twenty_length = next_seven_twenty.length();
next_twelve_length = next_twelve.length();
remaining_length = remaining.length();
twenty_nist_strength = (first_char_twenty_length*NIST_FIRST) + (next_seven_twenty_length*NIST_SEVEN) + (next_twelve_length*NIST_TWELVE) + (remaining_length*NIST_REM) + ((character_set_20 / NIST_CHARACTER)*NIST_BONUS);
cout << "Your password " << twenty_password << " has a length of " << twenty_length << "." << endl;
cout << "According to the information entropy formula, it has a strength of " << twenty_ent_strength << endl;
cout << "The first character is "" << first_char_twenty << ""." << endl;
cout<< "The next seven characters are "" << next_seven_twenty <<""."<< endl;
cout << "The next twelve characters are "" << next_twelve << ""." << endl;
cout<< "The remaining characters are "" << remaining << ""." << endl;
cout << "According to the NIST formula, it has a strength of " << twenty_nist_strength <<"." << endl;
return 0;
}