Some guy (a former co-worker) decided to virtual stalk my friend starting several years ago. He's hacked and gotten her contact list, so that even if she gets a new communication device - which she has, multiple times, he just monitors her contacts until she communicates with them, and presto - he's got her new device.

What he does generally is either message her directly, or send messages pretending to be her to her friends, or saying bad things about her to her friends, or pretending to be her friends to her.

I have made two suggestions: One is to tell her contacts to leave voice messages instead of texts, so they can immediately be identified as being who they say they are. She can do the same to them, so that she can't be impersonated online either.

The second idea was to embed something identifiable within messages that would make it clear that the message was from one of her actual contacts (or actually her to her contacts).

There would be two ways to do this: One way is to give all of her contacts a list of words on paper, and each time they send a message, they use one of the words, crossing it off as used, once they do. She would do the same.

A simpler way (where I would love suggestions to pass on), is to in some way modify the message where the stalker would not notice, but she (or her contacts) who know what to look for, would be able to tell. It would have to be a modification that would be almost completely unnoticeable. If something could be thought of, it would mean that the more complicated paper word-list method would not have to be used.

So do you guys have ideas for a modification that is so small in a text message that a sharp-eyed stalker would not notice it?

Personally, I think that the method of just using voice messages is the best, but I'd like to give her some other fallback options.

This guy has not been threatening, but he's definitely harassing. She's tried to make police reports in her country, but since there have been no overt threats, the police can't and won't follow up.

So - any ideas for a tiny, almost invisible modification that her friends and contacts (and she as well) can put in all of their messages to identify the sender as genuine? It would have to be something recognizable in all messages, so something like 'don't use the word 'and' or the word 'the', would not work, because not all messages would contain that anyway...

Note that there might not be anything that would work for this, in which case the voice-messages-only or (as a fallback), the random list of words would be the only options.