Secret Splitting

An interesting way to use one-time pad encryption is Secret Splitting. We all know what sharing a secret is (people tend to have problems not to share secrets). The opposite of sharing secrets is to split them.

Suppose Charlie has a secret password or a safe combination he wants to share with someone in case of emergency. Wouldn't it be great if he could split his secret and give one share to Alice and one share to Bob? The secret could only be retrieved when both Alice and Bob agree on sharing their secret. Of course, cutting it simply in two and give each person half the secret would reveal already that part, so that would not be safe.

Well, there's a simple solution and it's called one-time pad. Secret Splitting is a special way of using one-time pad, since you don't send a secret message, and the key isn't destroyed. Here's how it goes: Charlie subtracts a truly random key, digit by digit, modulo 10 (without borrowing), from his safe combination. For instance, 4 - 6 = 8 because [1]4 - 6 = 8. He gives one share (they key) to Alice and one share (the result) to Bob.

Charlie's Combination      21 46 03 88
Random key (first share) - 25 01 77 61
                           -----------
Results (second share)     06 45 36 27

Alice's share = 2501 7761
Bob's share   = 0645 3627

To retrieve the original combination, Alice and Bob just add their keys together, again modulo 10 (without carry). For instance 7 + 6 = 3 because 7 + 6 = [1]3. Of course, one could also split text by first converting the letters into digits (f.i. A=01, B=02 and so on through Z=26). You can download a practical Secure Code Splitter (pdf) template that supports up to 4 shares of 10 digits.

It's mathematically impossible to retrieve the secret information without having all shares, under the condition that the random share contains truly random digits, and all shares are physically and securely separated.

There are some interesting applications for this system. It's a secure method to give a code or password in the custody of multiple persons when you're abroad. Each person receives a share. All persons involved have to agree to combine their shares to retrieve the code or password. But if you want to have the final decision to disclose your secret code, there's another solutions.

You can create two shares, take one with you on a business trip and give the second share to a trustee from your company. In case of emergency, you can sent your share by open e-mail or telephone to the trustee. Combined with his own share, he can retrieve the secret code and open your safe or whatever. Because the share and result share are trule random, sending one of the shares by insecure channel will never disclose the secret code, as long as both share are never disclosed simultaneously.

 

It's also a clever way to share things with people who don't trust each other. Suppose grandpa, old and sick, splits the secret combination of a safe that contains his savings and gives each of his children one share of that secret. They can only get their hands on his money if they all agree (not that this will make grandpa live longer).

Of course, after using the shares, you should always set a new secret code or password to your safe and create a completely new set of shares to avoid unauthorised use of the old combination.

More about Secret Splitting and one-time pad on my Secret Splitting page.

Update: a new version of the Secure Code Splitter is available. More information at this blog post.