Task: Substitution cipher
C#: using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SubstitutionCipherProject
{
    class SubstitutionCipher
    {
        static void Main(string[] args)
        {
            doEncDec("e:\\source.txt", "enc.txt", true);
            doEncDec("enc.txt", "dec.txt", false);
            Console.WriteLine("Done");
            Console.ReadKey();
        }
        static void doEncDec(String source, String target, bool IsEncrypt)
        {
            ITransform trans;

            if (IsEncrypt)
                trans = new Encrypt();
            else
                trans = new Decrypt();

            FileInfo sfi = new FileInfo(source);
            FileStream sstream = sfi.OpenRead();
            StreamReader sr = new StreamReader(sstream);

            FileInfo tfi = new FileInfo(target);
            FileStream tstream = tfi.OpenWrite();
            TransformWriter tw = new TransformWriter(tstream, trans);
            StreamWriter sw = new StreamWriter(tw);

            String line;
            while ((line = sr.ReadLine()) != null)
                sw.WriteLine(line);
            sw.Close();
        }
    }
    public interface ITransform
    {
        byte transform(byte ch);
    }
    public class Encrypt : ITransform
    {
        const String str = "xyfagchbimpourvnqsdewtkjzl";
        byte ITransform.transform(byte ch)
        {
            if (char.IsLower((char)ch))
                ch = (byte)str[ch - (byte)'a'];
            return ch;
        }
    }
    class Decrypt : ITransform
    {
        const String str = "xyfagchbimpourvnqsdewtkjzl";
        byte ITransform.transform(byte ch)
        {
            if (char.IsLower((char)ch))
                ch = (byte)(str.IndexOf((char)ch) + 'a');
            return ch;
        }
    }
    class TransformWriter : Stream, IDisposable
    {
        private Stream outs;
        private ITransform trans;

        public TransformWriter(Stream s, ITransform t)
        {
            this.outs = s;
            this.trans = t;
        }

        public override bool CanRead
        {
            get { return false; }
        }

        public override bool CanSeek
        {
            get { return false; }
        }

        public override bool CanWrite
        {
            get { return true; }
        }
        public override void Flush()
        {
            outs.Flush();
        }

        public override long Length
        {
            get { return outs.Length; }
        }
        public override long Position
        {
            get
            {
                return outs.Position;
            }
            set
            {
                outs.Position = value;
            }
        }
        public override long Seek(long offset, SeekOrigin origin)
        {
            return outs.Seek(offset, origin);
        }

        public override void SetLength(long value)
        {
            outs.SetLength(value);
        }

        public override void Write(byte[] buf, int off, int len)
        {
            for (int i = off; i < off + len; i++)
                buf[i] = trans.transform(buf[i]);
            outs.Write(buf, off, len);
        }

        void IDisposable.Dispose()
        {
            outs.Dispose();
        }

        public override void Close()
        {
            outs.Close();
        }

        public override int Read(byte[] cbuf, int off, int count)
        {
            return outs.Read(cbuf, off, count);
        }
    }
}

C++: #include <iostream>
#include <string>
#include <fstream>

class cipher {
public:
    bool work( std::string e, std::string f, std::string k ) {
        if( e.length() < 1 ) return false;
        fileBuffer = readFile( f );
        if( "" == fileBuffer ) return false;
        keyBuffer = readFile( k );
        if( "" == keyBuffer ) return false;

        outName = f;
        outName.insert( outName.find_first_of( "." ), "_out" );

        switch( e[0] ) {
            case 'e': return encode();
            case 'd': return decode();
        }
        return false;
    }
private:
    bool encode() {
        size_t idx, len = keyBuffer.length() >> 1;
        for( std::string::iterator i = fileBuffer.begin(); i != fileBuffer.end(); i++ ) {
            idx = keyBuffer.find_first_of( *i );
            if( idx < len ) outBuffer.append( 1, keyBuffer.at( idx + len ) );
            else outBuffer.append( 1, *i );
        }
        return saveOutput();
    }
    bool decode() {
        size_t idx, l = keyBuffer.length(), len = l >> 1;
        for( std::string::iterator i = fileBuffer.begin(); i != fileBuffer.end(); i++ ) {
            idx = keyBuffer.find_last_of( *i );
            if( idx >= len && idx < l ) outBuffer.append( 1, keyBuffer.at( idx - len ) );
            else outBuffer.append( 1, *i );
        }
        return saveOutput();
    }
    bool saveOutput() {
        std::ofstream o( outName.c_str() );
        o.write( outBuffer.c_str(), outBuffer.size() );
        o.close();
        return true;
    }
    std::string readFile( std::string fl ) {
        std::string buffer = "";
        std::ifstream f( fl.c_str(), std::ios_base::in );
        if( f.good() ) {
            buffer = std::string( ( std::istreambuf_iterator<char>( f ) ), std::istreambuf_iterator<char>() );
            f.close();
        }
        return buffer;
    }
    std::string fileBuffer, keyBuffer, outBuffer, outName;
};

int main( int argc, char* argv[] ) {
    if( argc < 4 ) {
        std::cout << "<d or e>\tDecrypt or Encrypt\n<filename>\tInput file, the output file will have"
        "'_out' added to it.\n<key>\t\tfile with the key to encode/decode\n\n";
    } else {
        cipher c;
        if( c.work( argv[1], argv[2], argv[3] ) ) std::cout << "\nFile successfully saved!\n\n";
        else std::cout << "Something went wrong!\n\n";
    }
    return 0;
}