Surely all you are asked to sign is that the photo is an accurate likeness of the person in front of you! It doesn't matter whether you know them or not, does it? I haven't done it for many years, so I may be wrong.
I used to be eligible to sign when I was a teacher and later when I was a tax inspector. I am still the same person, so why can't I sign now?
Many years ago, I asked somebody to witness my signature on a will and she got very uncomfortable, as if it might cost her some money. All she had to testify was that she had seen me sign my name. People are weird.