https://reinproject.org/static/bitcoin-signature-tool/index.html

This is a public domain open source java script implement about BTC signing and verifying. See here for a print screen of signing and here for a print screen of verifying.

Being java script, it means you can operate it directly in the browser. Further, if you insist a high security for your private keys, you can download the whole source code from github mentioned in its page and inspect the source code line by line and signing on a standalone offline browser with no any private key info leaking possibility.