Is there any open source, simple project which just for implement the signing function. It must be open source, no one will trust this kind of software, even Electrum .if I want to sign a message with more than 100 bitcoins, I really can not trust any software even with big companies. Do you think so?
I have difficulty to understand what you said. But Electrum is open source where the code is available on
https://github.com/spesmilo/electrum. And i doubt you can find simple project since signing involve so many things including ECDSA implementation and checking whether you use valid private key.