What is recognized as legal in the digital age is not always clear. If there is something recording your agreement is likely valid or at least evidence. Verbal agreements are binding, the problem with them is proving or providing evidence. Disputes fall to arguments among the parties and in the absence of evidence there is little t0 be argued.
Small business is familiar with the handshake agreement. Some people are capitalizing by writing a smartphone app. This provides more formalized evidence in a manner that is a lot closer to a handshake. It provides evidence that backs up your agreement if you need to go back to the well and get people to do what they said they would do.
I would often ask my lawyer what is the formal steps and the answer would return that whatever you can prove was intended. In the past beyond a handshake there was only a document and a signature. If some sends an email agreeing to something as long as the email is determined to not be a fraud it is considered evidence and provides leverage to get both parties to do what was agreed to.
How legal something is depends upon how recognized it is as being representative of the people who used it. So whether you sign a document, send an email, digitally sign a pdf or use an app, it’s only as good as the technology can be considered as secure and proving it came from the parties involved.
As always consult your lawyer for specifics of any situation. Legal matters get tricky but going from handshake to other tech may be able to help you. It will be fun to see how these tech services develop.