OneTaskBy Tim King · Updated
Secret Splitter
Shamir Secret Sharing for a Password, Recovery Phrase or Key, and Combining Shares
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How the Secret Splitter works
This secret splitter uses Shamir's secret sharing to break a password, recovery phrase or key into several shares. You choose how many shares to make and how many are needed to rebuild the secret. The defaults are 5 shares with any 3 required. To recover it, switch to Combine shares and paste enough shares, one per line.
How the secret splitter works
Each byte of the secret becomes the constant term of a random polynomial over the finite field GF(256). Every share is that polynomial evaluated at a different point. With as many points as the threshold, the polynomial can be rebuilt by Lagrange interpolation, which gives back the constant term: the secret. With one point fewer, every possible secret is equally likely, so a partial set of shares leaks nothing.
Share format
Each share looks like 3-2-56705570…. The first number is the threshold, the second is the share's
position, and the rest is hex. Shares are the same length as the secret in bytes. Keep the whole line,
because the position number matters when combining.
Good uses
Split a password manager's master password between family members for an emergency. Store a crypto wallet's recovery phrase in three places so that one lost or stolen copy does no harm. Give two of three directors the ability to unlock a company account between them.
FAQ
What happens if I lose some shares?
Nothing, as long as you still have at least the threshold number. With a 3-of-5 split any three shares work, so two can be lost. Below the threshold the secret cannot be recovered by anyone, including you, so plan where each share will live.
Are the random numbers secure?
Yes. The polynomial coefficients come from the browser's cryptographic random generator, the same source used for encryption keys. Splitting the same secret twice gives completely different shares, and shares from different splits cannot be mixed.
Does the share reveal the secret's length?
Yes. Each share has one byte per byte of the secret plus four for the checksum, so the length is visible. If that matters, pad the secret with extra words or characters that you will recognise and remove after combining.
See also: all Security & Identifiers tools, Dev Utilities, Formatters & Validators, Data & Encoding, SEO, Accessibility, AI.
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