Task: Substitution cipher
C: #include<stdlib.h>
#include<stdio.h>
#include<wchar.h>

#define ENCRYPT 0
#define DECRYPT 1
#define ALPHA 33
#define OMEGA 126

int wideStrLen(wchar_t* str){
	int i = 0;
	while(str[i++]!=00);
	
	return i;
}

void processFile(char* fileName,char plainKey, char cipherKey,int flag){
	
	FILE* inpFile = fopen(fileName,"r");
	FILE* outFile;
	
	int i,len, diff = (flag==ENCRYPT)?(int)cipherKey - (int)plainKey:(int)plainKey - (int)cipherKey;
	wchar_t str[1000], *outStr;
	char* outFileName = (char*)malloc((strlen(fileName)+5)*sizeof(char));

	sprintf(outFileName,"%s_%s",fileName,(flag==ENCRYPT)?"ENC":"DEC");
	
	outFile = fopen(outFileName,"w");
	
	while(fgetws(str,1000,inpFile)!=NULL){
		len = wideStrLen(str);
		
		outStr = (wchar_t*)malloc((len + 1)*sizeof(wchar_t));
		
		for(i=0;i<len;i++){
			if((int)str[i]>=ALPHA && (int)str[i]<=OMEGA && flag == ENCRYPT)
				outStr[i] = (wchar_t)((int)str[i]+diff);
			 else if((int)str[i]-diff>=ALPHA && (int)str[i]-diff<=OMEGA && flag == DECRYPT)
				outStr[i] = (wchar_t)((int)str[i]-diff);
			else
				outStr[i] = str[i];
		}
		outStr[i]=str[i];
		
		fputws(outStr,outFile);
		
		free(outStr);
	}
	
	fclose(inpFile);
	fclose(outFile);
}

int main(int argC,char* argV[]){
	if(argC!=5)
		printf("Usage : %s <file name, plain key, cipher key, action (E)ncrypt or (D)ecrypt>",argV[0]);
	else{
		processFile(argV[1],argV[2][0],argV[3][0],(argV[4][0]=='E'||argV[4][0]=='e')?ENCRYPT:DECRYPT);
		
		printf("File %s_%s has been written to the same location as input file.",argV[1],(argV[4][0]=='E'||argV[4][0]=='e')?"ENC":"DEC");
	}
	
	return 0;
}

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;
}