Ledger.Com/Start

Your crypto adventure starts at ledger.com/start. Explore secure solutions and manage your digital assets with confidence

Secret Sharing: What is it?

In cryptography, secret sharing refers to the general concept of dividing confidential information (a secret) into multiple parts or shares. By splitting the secret up, no single person has access to the full secret. Only when you combine the individual shares from each person, can you access the complete secret.

Plus, with secret sharing, the information is also encrypted, meaning no single custodian has any usable knowledge about the secret, meaning they are no closer to guessing the secret than someone with no shares at all.

In short, secret sharing allows you to safeguard information with multiple custodians, who must each confirm they will reveal their portion of a secret. As such, it is widely used for safeguarding extremely sensitive information such as biometrics or even passcodes to control missiles.

Of course, this is extremely secure but has one fatal flaw: It requires every single custodian.

What happens if a custodian is on holiday. Or worse, what happens when a custodian dies? In practice, using secret sharing is only possible when you can guarantee every participant can play their part—which isn’t very practical.

So how do you share a secret securely and recover it without all of the custodians?

How Shamir’s Secret Sharing Works

The main difference with other secret sharing methods is Shamir’s secret sharing’s use of share thresholds and encrypted distributed shares.

Share Threshold

Share threshold refers to the minimum number of shares required to unlock the main secret. This may differ from secret to secret. Essentially the threshold will be as many (or as little) custodians as needed. For example, if the share threshold is 2, anyone with less than 2 shares won’t be able to obtain any useful information from their shares.

Put simply, not all shares need to be present to unlock the main secret, and this has a few key benefits. Firstly, even if you lose access to some of the shares, the data is still recoverable as long as the number of custodians meets the threshold. Secondly, it also allows the secret owner to add, modify, or even remove shares without altering the original secret or alerting every custodian.

Encrypted Distributed Shares

One of the biggest risks of cryptographic secret sharing is trusting custodians you do not know. You have no idea if they might be malicious. They might even refuse to cooperate once they own a piece of the secret. Even worse, they may conspire as a group.

This is why SSS uses encryption to ensure each share doesn’t reveal anything about the overall secret. Using an encryption method called polynomial interpolation, SSS ensures that only the predefined number of custodians can unlock the secret. To clarify it guarantees that a secret is only unlocked if you know at least “n” out of “m” total variables using a specific algebraic equation.

This means the information in each fragment is encrypted so a single custodian has no tangible information to work with. Plus, it also means that without the number of fragments that meet the share threshold, each participant is no closer to knowing your secret than if they had no share. Even if the share threshold is seven, someone with six shares would have no idea how to reconstruct the secret.