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This is a C++ Program to implement monoalphaetic cipher. In cryptography, a substitution cipher is a method of encoding by which units of plaintext are replaced with ciphertext, according to a regular system; the “units” may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.

Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered.

There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.

Here is source code of the C++ Program to Implement the Monoalphabetic Cypher. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include <iostream>
```
```
#include <istream>
```
```
#include <ostream>
```
```
#include <fstream>
```
```
#include <cstdlib>
```
```
#include <string>
```
```
 
```
```
// the rot13 function
```
```
std::string rot13(std::string s)
```
```
{
```

    static std::string const lcalph = "abcdefghijklmnopqrstuvwxyz", ucalph =

            "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

```
 
```
```
    std::string result;
```
```
    std::string::size_type pos;
```
```
 
```
```
    result.reserve(s.length());
```
```
 
```

    for (std::string::iterator it = s.begin(); it != s.end(); ++it)

```
    {
```

        if ((pos = lcalph.find(*it)) != std::string::npos)

            result.push_back(lcalph[(pos + 13) % 26]);

        else if ((pos = ucalph.find(*it)) != std::string::npos)

            result.push_back(ucalph[(pos + 13) % 26]);

```
        else
```
```
            result.push_back(*it);
```
```
    }
```
```
 
```
```
    return result;
```
```
}
```
```
 
```

// function to output the rot13 of a file on std::cout

// returns false if an error occurred processing the file, true otherwise

// on entry, the argument is must be open for reading

```
int rot13_stream(std::istream& is)
```
```
{
```
```
    std::string line;
```
```
    while (std::getline(is, line))
```
```
    {
```

        if (!(std::cout << rot13(line) << "\n"))

```
            return false;
```
```
    }
```
```
    return is.eof();
```
```
}
```
```
 
```
```
// the main program
```
```
int main(int argc, char* argv[])
```
```
{
```

    if (argc == 1) // no arguments given

        return rot13_stream(std::cin) ? EXIT_SUCCESS : EXIT_FAILURE;

```
 
```
```
    std::ifstream file;
```
```
    for (int i = 1; i < argc; ++i)
```
```
    {
```

        file.open(argv[i], std::ios::in);

```
        if (!file)
```
```
        {
```

            std::cerr << argv[0] << ": could not open for reading: " << argv[i]

```
                    << "\n";
```
```
            return EXIT_FAILURE;
```
```
        }
```
```
        if (!rot13_stream(file))
```
```
        {
```
```
            if (file.eof())
```

                // no error occurred for file, so the error must have been in output

                std::cerr << argv[0] << ": error writing to stdout\n";

```
            else
```

                std::cerr << argv[0] << ": error reading from " << argv[i]

```
                        << "\n";
```
```
            return EXIT_FAILURE;
```
```
        }
```
```
        file.clear();
```
```
        file.close();
```
```
        if (!file)
```

            std::cerr << argv[0] << ": warning: closing failed for " << argv[i]

```
                    << "\n";
```
```
    }
```
```
    return EXIT_SUCCESS;
```
```
}
```

$ g++ MonoalphabeticCipher.cpp
$ a.out
 
Dharmendra
Qunezraqen
Hingu
Uvath
Sanfoundry
Fnasbhaqel
------------------
(program exited with code: 0)
Press return to continue

C++ Program to Implement the Monoalphabetic Cipher