Is it permissible to design computer programs that deal with different types of data, especially financial data such as stock markets and banks, provided that the work does not involve programming usurious loans or prohibited banking activities?

The basic ruling is that it is permissible to design software that is used for permissible and other purposes. However, if the designer knows that it will be used for forbidden purposes, then it is not permissible to design it, because that would be assisting in sin. It is also not permissible to design software for interest-based banks for any of their activities, nor to process their data, even if it is not directly related to interest-based loans.