c++ simple Caesar cipher algorithm

Question

I am trying to make a very simple Caesar cipher algorithm to encrypt and decrypt the player's data in my game , but i am getting some weird results.The task of the algorithm is simple, just push foward or backwards the characters in the ascii table.

std::string Encrypt(std::string in,int key)
{
    const char* chars=in.data();
    char* newchar=(char*)malloc(sizeof(char)*in.length());
    for(int c=0;c<in.length();c++)
    {
        newchar[c]=char(((int)chars[c])+key);//I suspect somewhere here is the problem
    }

    std::string out(newchar);
    return out;
}

LOGI("encrypt:%s",Encrypt("hello",10).data());
LOGI("decrypt:%s",Encrypt(Encrypt("hello",10),-10).data());

Output:

encrypt:rovvyu@ 
decrypt:hellok

I dont know much about encryption and i know less about how ascii and the whole character thing works in c

Answer 1

std::string Encrypt(const std::string & in, int key)
{
    std::string out(in);
    for(int i=0; i < in.length(); ++i)
    {
        out[i] += key;
    }
    return out;
}

Answer 2

The problem is that adding "key" to a value in a..z may not land back in that range. Something like

if (chars[c] >= 'a' && chars[c] <='z') newchar[c] = 'a' + ((chars[c]-'a'+key)%26);
else if (chars[c] >= 'A' && chars[c] <='Z') newchar[c] = 'A' + ((chars[c]-'A'+key)%26);
else newchar[c] = chars[c];

Hope you have a good reason for using something so weak. BTW, use 26-key to invert.

Answer 3

If you want to just know how to make a Caesar cipher, cool. If you actually want to protect some data, don't do this! It is no more a cryptographic method than pig latin.

Instead, use a pre-written encryption library. Don't write your own because it takes a long time to get it right. AES is fast and good enough for most cases, and has libraries for just about every programming language.