oh, that is easy, also. Oldest known security trick will work for this.
Take USB stick. Mold a clay figure or tablet around it. Sunbake it. -it does not have to be a pretty figurine - in fact, it kind of helps if it is not, harder to copy.  Mail it. Email a photo. Have the recipient compare before smashing. But yhea, you will need a pad for everyone you want secure communications with.