Question

I am writing an application using the crypto++ library. For those not familiar with it, the ECB_Mode template classes inherit from CipherModeBase. The program compiles and runs, but the output I am getting is incorrect. When I call the encryption method from the cipher_object, it doesn’t work the same way as if I use an ECB_Mode object directly. I have verified that the options object instance variables are getting assigned correctly. I would like to create the instances within an if_then_else structure or switch_case so I can keep the code nice and DRY. What am I doing wrong?

Here is what I am trying but doesn’t work:

    CipherModeBase *cipher_object;
    cipher_object == NULL;

    if(options->cipher == BS_AES)
    {
        ECB_Mode<AES >::Encryption ecbEncryption(options->key, options->keylen);
        cipher_object = &ecbEncryption;
    }
    else if(options->cipher == BS_TWOFISH)
    {
        ECB_Mode<Twofish >::Encryption ecbEncryption(options->key, options->keylen);
        cipher_object = &ecbEncryption;     
    }
cipher_object->processData(args);

Here is what does work:

ECB_Mode<AES >::Encryption ecbEncryption(options->key, options->keylen);
ecbEncryption.processData(args);

PS. I know not to use ECB mode. I just don’t want to mess with IVs until I can get everything working. I am also relatively inexperienced with C++.

Answer 1

Here:

cipher_object == NULL;

if(options->cipher == BS_AES)
{
    // v
    ECB_Mode<AES >::Encryption ecbEncryption(options->key, options->keylen);
    cipher_object = &ecbEncryption;
}

ecbEncryption is local to that scope. You are storing the address of a local, and using it after it goes out of scope. That’s undefined behaviour. You should allocate it on the heap using the new keyword:

if(options->cipher == BS_AES)
{
    cipher_object = new ECB_Mode<AES >::Encryption(options->key, options->keylen);
}

You should do the same for the other if statement.

Also note that this:

cipher_object == NULL;

Should be changed to this:

cipher_object = NULL;

The problem should be solved using the above code, though.