Ways to keep information and letters a secret and verifiable to others as a preppers with historical ways during war, SHTF and bug in scenario's.
https://www.bol.com/be/nl/p/bloederige-kruisbeelden/9300000111446643/?bltgh=svJZp-4dhEIGsnd40cVKMw.2_10.11.ProductTitle
https://www.survival-training-belgie.be/