Thread by @zndtoshi

Shamir's Secret Sharing is an algorithm in cryptography created by Adi Shamir. It is a form of secret sharing, where a secret is divided into parts. I have used this scheme to backup my passphrase for my Bitcoin wallet. Here is what I did.
Thread:

First I used an old Android phone and downloaded "Shamir Secret Sharing Scheme Encoder & Decoder" app from Playstore and "Backup and Restore" app to save the apk of the SSS app in case it would disappear from the store.

After installing the apps I closed all internet / bluetooth connections and will never enable them again.
Next in the app you can write the secret (passphrases) and select how many shards to have in total and how many needed for reconstruction.

After getting a QR code for each shard I took polaroid photos with my wife's Zoemini S. This camera takes insta photos and has no memory for storing. So this is ideal for getting a backup of each shard. You can find many cheap cameras like this on the market.

The zink photo papers that you get are also stickers so you can hide them in different spots. Remember that it's best to put them in dry places because the humidity can make the color go away.

I used 3 of 5 shards for the SSS and tried them all to verify if the app can read them correctly. After this I just shut down the old android phone and left it in a drawer. If you ever do this.. consider verifying the integrity of the photos from time to time. Enjoy!

Here is the link to the Shamir Secret Sharing app in Android Playstore:
play.google.com/store/apps/det…

Of course this option can be used to backup your seed too if you want more redundancy.

I didn't mention in the thread but before shutting down the network connection I emailed myself the apk backup of the app in order to have it even if I lose the phone.