Poker Player, if you want to add a timestamp to your message, I suggest that you incorporate the block number and the nonce from the last mined block into the message. When you publish the signature/hash of the message, you also publish the block number and nonce that will be in the message. That way, you will have another provable way of confirming when the message was created.
I understand that would confirm that the message was signed at least when that block was mined or after, and not before, but it could have been created after.
That is true. That's why I said that you should make the block nonce public along with the digital signature of the message. There is no way to know the nonce of future blocks.
But I agree that it's not really necessary, unless you want to add a timestamp to the message.
I was away yesterday but I was reading your response and Googled some articles about email security for different email services. I had a wrong idea that Google do PGP encryption but knowing they don't, just broke my trust. You are right most regular email system don't. Protonmail has it as an additional feature, which needs additional setting to enable it in your end. And if the receiver do not have the same setting then they will not be able to read your email. It become complicated when the email is sent to another service instead of proton to proton.
That's right. Most regular email providers use only transport-level encryption, or TLS (SSL) layer over the plaintext communication, which means that the message is protected from eavesdropping only during transit between individual SMTP relays, but not all the way between the sender and the recipient.