Title: Securely share a secret using Shamir's secret sharing

	Title: Securely share a secret using Shamir's secret sharing
	Author: Solène
	Date: 21 March 2021
	Tags: openbsd security
	Description:

	# Introduction

	I will present you the program ssss (for Shamir's Secret Sharing
	Scheme), a cryptography program to split a secret into n parts,
	requiring at least t parts to be recovered (with t <= n).

	Shamir Secret Sharing (method is mathematically proven to be secure)

	# Use case

	The project website list a few use cases for real life and I like them,
	but I will share another use case.

	ssss project website

	I used to run a community but there was no person in charge apart me,
	which made me a single point of failure. I decided to make the
	encrypted backup available to a few kind of trustable community
	members, and I gave each a secret. There were four members and I made
	the backup password available only if the four members agreed to share
	their secrets to get the password. For privacy reasons, I didn't want
	any of these people to be able to lurk into the backup, at least, if
	someone had happened to me, they could agree to recover the database
	only if the four persons agreed on it.

	# How to use

	ssss-split is easy to use, you can only share text with it. So you can
	use a very long passphrase to encrypt files and share this passphrase
	into many secrets that you share.

	You can install it on OpenBSD using pkg_add ssss.

	In the following examples, I will create a simple passphrase and then
	use the generated secrets to get the original passphrase back.

	```ssss-split example
	$ ssss-split -t 3 -n 3
	Generating shares using a (3,3) scheme with dynamic security level.
	Enter the secret, at most 128 ASCII characters: [Note=>hidden input where I typ…
	1-cfef7c2fcd283133612834324db968ef47e52997d23f9d6eae0ecd8f8d0e898b65
	2-e414b5b4de34c0ee2fbb14621201bf16e4a2df70a4b5a16a823888040d332d47a8
	3-0d4d2cebcc67851ed93da3c80c58fce745c34d1fb2d1341da29b39a94b98e0f353
	```

	When you want to recover a secret, you will have to run ssss-combine
	and tell it how many secrets you have, they can be provided in any
	order.

	```ssss-combine example
	$ ssss-combine -t 3
	Enter 3 shares separated by newlines:
	Share [1/3]: 2-e414b5b4de34c0ee2fbb14621201bf16e4a2df70a4b5a16a823888040d332d47…
	Share [2/3]: 3-0d4d2cebcc67851ed93da3c80c58fce745c34d1fb2d1341da29b39a94b98e0f3…
	Share [3/3]: 1-cfef7c2fcd283133612834324db968ef47e52997d23f9d6eae0ecd8f8d0e898b…
	Resulting secret: this is a very very long password
	```

	# Tips

	If you want to easily store a secret or share it to a non-IT person (or
	in a vault), you can create a QR code and then print the picture. QR
	code has redundancy so if the paper is damaged you can still recover
	it, it's quite big on a paper so if it fades of you may not lose data
	and it also checks integrity.

	# Conclusion

	ssss is a wonderful program to share a secret among a few people or put
	a few secrets here and there for a recovery situation. The program can
	receive the passphrase on its standard input allowing it to be
	scripted.

	Interesting fact, if you run ssss-combine multiple times on the same
	text, you always get different secrets, so if you give a secret, no
	brute force can be used to find which input produced the secret.