signing document app

I am looking for a developer to create an app that allows users to securely sign legal documents. The app must have the capability for document encryption, timestamping, and should be compatible with Android operating systems. Multi-signatory must also be supported, allowing for multiple parties to sign a single document. The resulting document must be organized, easy to use and understand, and legally valid. The developer should have proven experience in developing data security solutions and creating apps for mobile platforms.