You can say that you signed an NDA or your employment agreement with the company forbids you from sharing those details. It might also be the case the interview

You can say that you signed an NDA or your employment agreement with the company forbids you from sharing those details. It might also be the case the interviewers are testing you and assessing how easy it is go get proprietary information out of you. If the HR insists, be polite and repeat your message. Explicitly mention that you are not comfortable to disclose information related to your previous company and happy to share more details about your expertise.