A Shamir's Secret Sharing App to Split a Secret Among People You Trust
Shamir's Secret Sharing is a well-studied way to split a secret into pieces so that any T of N pieces rebuild it and fewer reveal nothing. ShareMySecret turns it into an iPhone app you can use for seed phrases, master passwords and vital documents.
Shamir's Secret Sharing in one paragraph
Adi Shamir published the scheme in 1979. The secret becomes the constant term of a random polynomial of degree T − 1; each share is one point on that polynomial. Any T points determine the polynomial and therefore the secret. With fewer than T points, every possible secret is equally likely, so they reveal nothing. That is information-theoretic security, not a matter of computing power. For a longer explanation, read Understanding Shamir's Secret Sharing.
Why not just split the text?
Cutting a password or a seed phrase into halves or thirds feels similar, but it is not. Each piece leaks part of the secret, and losing any piece loses the whole thing. A threshold scheme fixes both problems: pieces below the threshold leak nothing, and you can lose up to N − T pieces safely.
How ShareMySecret applies it
- Encrypt on-device. Your secret (text and optional files) is encrypted on your iPhone with a fresh, random AES-256 key.
- Split the key. The AES key is split with Shamir's Secret Sharing over GF(256) into the number of shares and threshold you choose, for example 3 of 5.
- Share it. Each recipient receives one key share together with the encrypted secret, through the iOS share sheet or AirDrop, among other options in the app.
- Recover together. When needed, any threshold number of recipients combine their shares in the app to decrypt the secret.
There is no account and no server-side copy. Details are in the security whitepaper and threat model.
What people split with it
- Crypto seed phrases, so no fire or single person can cost you the wallet.
- Crypto inheritance plans that heirs can open together.
- Master passwords and recovery codes for the family.
- Break-glass company credentials that no single employee controls.
Choosing N and T
- Higher T means more people must cooperate: harder to abuse, harder to gather.
- Bigger gap between N and T means more shares can be lost without losing the secret.
- 3 of 5 is a balanced default for most personal secrets. 2 of 3 suits a couple plus one backup. Avoid T = N unless you accept that losing one share loses everything.
Limits
ShareMySecret runs on iPhone only, for both you and the people holding shares. It cannot protect a secret on a compromised device, and it cannot stop the threshold number of holders from deciding together to open it. Choose holders and thresholds accordingly.
Try it with your own secret
ShareMySecret is free to download for iPhone. No account, nothing stored on our servers.
Frequently asked questions
What is Shamir's Secret Sharing?
A cryptographic method that splits a secret into N shares so that any T of them reconstruct it, while fewer than T reveal nothing about it. It was published by Adi Shamir in 1979.
Is splitting a password into parts the same as secret sharing?
No. Cutting the text into pieces leaks part of the secret in each piece, and losing one piece loses everything. Shamir's scheme leaks nothing below the threshold and tolerates lost shares.
What does ShareMySecret split, the secret or a key?
It encrypts your secret on your device with AES-256 and splits the encryption key with Shamir's Secret Sharing. Each recipient gets one key share plus the encrypted secret.
Do recipients need to install anything?
Yes. Recipients need ShareMySecret on an iPhone to receive and store their share and to combine shares during recovery.