When was this file encrypted?

I’ve run into a few situations where I wanted to know more about a miniLock file. I can get the original file name, file size and sender identity from the current file format which is great. But I’d like to also know when the file was encrypted.

For example, I keep `My miniLock IDs.txt.minilock` on my thumb drive so that I can do crypto operations on the computers at the library. Sometimes I end up with a few copies of this file after I’ve updated the list a few times and I’m not certain which minilock file is the most up-to-date. It would be really nice in those situations to know when a file was encrypted so I can quickly sort the keepers from the goners.

This proposal adds an optional `encryptedAt` member to the encrypted `fileInfo` properties in the header of the file. I suggest miniLock programs should always add the `encryptedAt` member by default because it is helpful. I made it strictly optional so that programs are backwards compatible with the original format and to let people override the default when they want to exclude the encryption time for some reason.

What do ya’ll think of this idea?