Encrypt / Decrypt String do Menu

using namespace std;

char* decrypt(const char* plaintext)
{
int len = strlen(plaintext);
char* cyphertext = new char[len+1];

for(int i=0 ; i<len ; ++i)
{
cyphertext[i] = plaintext[i] - 4;
}
cyphertext[len] = 0; 
return cyphertext;
}