Substitution Cipher, Strings and Functions - c++

Question

Where I am stuck at is the EncryptString function. The EncryptFile function will take a .txt file and convert the .txt to a C string equivalent, then pass that into the EncrpytString function. When I pass it in, the Substitution function will do its work with the cipher string and encrypt the individual characters, then spit that back out into a new string called encrypted_string. However, I cannot get this function to accept the argument passed into it. Any guidance?
This is where the program currently is:
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
using namespace std;
char substitution_cipher(string cipher_key, char char_to_encrypt);
char reverse_substitution_cipher(string cipher_key, char char_to_encrypt);
string EncryptString(string &cipher_key, string string_to_be_encrypted);
string DecryptString(string &cipher_key, string string_to_be_decrypted);
void RotateCipherKey(string &cipher_key);
void DisplayFile(string filename);
void EncryptFile(string cipher_key, string filename_from, string filename_to);
void DecryptFile(string cipher_key, string filename_from, string filename_to);
int main()
{
string cipher_key = "qwertyuiopasdfghjklzxcvbnm";
EncryptFile(cipher_key, "test.txt", "test-encrypted.txt");
DecryptFile(cipher_key, "test-encrypted.txt", "test-ed.txt");
DisplayFile("test.txt");
DisplayFile("test-encrypted.txt");
DisplayFile("test-ed.txt");
system("PAUSE");
return 0;
}
// Rotate the cipher key. Example: abcdef becames bcdefa
void RotateCipherKey(string &cipher_key)
{
rotate(cipher_key.begin(), cipher_key.begin() + 1, cipher_key.end());
}
// Perform a substitution cipher on a single character
// using the specified cipher key
char SubstitutionCipher(string cipher_key, char char_to_encrypt)
{
for (int iii = 0; iii < cipher_key.length(); iii++)
{
RotateCipherKey(cipher_key);
char_to_encrypt = cipher_key[iii];
}
return char_to_encrypt;
}
// Perform a "reverse" substitution cipher on a single character
// using the specified cipher key
char ReverseSubstitutionCipher(string cipher_key, char char_to_decrypt)
{
for (int iii = 0; iii < cipher_key.length(); iii++)
{
RotateCipherKey(cipher_key);
char_to_decrypt = cipher_key[iii];
}
return char_to_decrypt;
}
// Encrypt String and return it
// You will use the SubstitutionCipher() function to encrypt the
// individual characters
//
// Note: We will call RotateCipherKey() after each time we encrypt
// a character.
string EncryptString(string &cipher_key, string string_to_be_encrypted)
{
char *y = string_to_be_encrypted.c_str();
{
//SubstitutionCipher(cipher_key, string_to_be_encrypted);
}
cout << " " << string_to_be_encrypted;
string encrypted_string = string_to_be_encrypted;
return encrypted_string;
}
// Decrypt String and return it
// You will use the ReverseSubstitutionCipher() function to decrypt the
// individual characters
//
// Note: We will call RotateCipherKey() after each time we encrypt
// a character.
string DecryptString(string &cipher_key, string string_to_be_decrypted)
{
string decrypted_string = string_to_be_decrypted;
return decrypted_string;
}
// Display file specified by the filname parameter
void DisplayFile(string filename)
{
string str;
ifstream infile;
infile.open(filename);
infile >> str;
while (infile)
{
cout << " " << str;
infile >> str;
}
cout << endl;
}
// Encrypt the specified file using the specified cipher key and
// write the output to a different file
// This function is complete
void EncryptFile(string cipher_key, string filename_from, string filename_to)
{
string input;
ifstream infile;
ofstream outfile;
infile.open(filename_from.c_str());
outfile.open(filename_to.c_str());
if (!infile)
{
cout << "Can not open input file " + filename_from << endl;
exit(0);
}
if (!outfile)
{
cout << "Can not open Output file " + filename_to << endl;
exit(0);
}
while (getline(infile, input))
{
outfile << EncryptString(cipher_key, input) << endl;
}
infile.close();
outfile.close();
}
// Decrypt the specified file using the specified cipher key and
// write the output to a different file
// This function is complete
void DecryptFile(string cipher_key, string filename_from, string filename_to)
{
string input;
ifstream infile;
ofstream outfile;
infile.open(filename_from.c_str());
outfile.open(filename_to.c_str());
if (!infile)
{
cout << "Can not open input file " + filename_from << endl;
exit(0);
}
if (!outfile)
{
cout << "Can not open Output file " + filename_to << endl;
exit(0);
}
while (getline(infile, input))
{
outfile << DecryptString(cipher_key, input) << endl;
}
infile.close();
outfile.close();
}
EDIT//
string EncryptString(string &cipher_key, string string_to_be_encrypted)
{
char new_char;
for (int iii = 0; iii < string_to_be_encrypted.length(); iii++)
{
new_char = SubstitutionCipher(cipher_key, string_to_be_encrypted[iii]);
RotateCipherKey(cipher_key);
}
string encrypted_string = string_to_be_encrypted;
cout << " " << encrypted_string;
return encrypted_string;
}
Ok here is new code now with some modifications.

Answer 1

The problem you have is that c_str() returns a pointer to const, const char*, which you can not assign to a char* in the line
char* y=string_to_be_encrypted.c_str(); // cannot assign a `const char*` to `char*`
Use directly the string to perform your work. There is no other way of getting a pointer to non-const char to the data of a std::string.