static const EVP_CIPHER *afalg_aes_cbc(int nid)
{
    cbc_handles *cipher_handle = get_cipher_handle(nid); // may return null
    if (cipher_handle->_hidden == NULL  //dereference here
        && ((cipher_handle->_hidden =
         EVP_CIPHER_meth_new(nid,
                             AES_BLOCK_SIZE,
                             cipher_handle->key_size)) == NULL
        || !EVP_CIPHER_meth_set_iv_length(cipher_handle->_hidden,
                                          AES_IV_LEN)
        || !EVP_CIPHER_meth_set_flags(cipher_handle->_hidden,
                                      EVP_CIPH_CBC_MODE |
                                      EVP_CIPH_FLAG_DEFAULT_ASN1)
        || !EVP_CIPHER_meth_set_init(cipher_handle->_hidden,
                                     afalg_cipher_init)
        || !EVP_CIPHER_meth_set_do_cipher(cipher_handle->_hidden,
                                          afalg_do_cipher)
        || !EVP_CIPHER_meth_set_cleanup(cipher_handle->_hidden,
                                        afalg_cipher_cleanup)
        || !EVP_CIPHER_meth_set_impl_ctx_size(cipher_handle->_hidden,
                                              sizeof(afalg_ctx)))) {
        EVP_CIPHER_meth_free(cipher_handle->_hidden);
        cipher_handle->_hidden= NULL;
    }
    return cipher_handle->_hidden;
}

static cbc_handles *get_cipher_handle(int nid)
{
    switch (nid) {
    case NID_aes_128_cbc:
        return &cbc_handle[AES_CBC_128];
    case NID_aes_192_cbc:
        return &cbc_handle[AES_CBC_192];
    case NID_aes_256_cbc:
        return &cbc_handle[AES_CBC_256];
    default:
        return NULL; //may return null.
    }
}