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.

Whitfield Diffie on Cryptography

There's a good video from a talk that Whitfield Diffie gave at the Computer History Museum. The subject is Information Security before and after Public Key Cryptography. Whitfield tells about the development of cryptography, the problems with key distribution, the solutions to that by public key cryptography and the future of cryptography. Most interesting lecture for the novice, without any technical details or math stuff. A good introduction in cryptography.

Whitfield Diffie knows what he's talking about. He's one of the inventors of the Diffie-Hellman (what's in a name) public key protocol and is now Security Officer at Sun Microsystems. If you want to read up on Whitfield, there's an interview with Diffie on Wired.com. You can view the talk by Whitfield (1h16 ) below. Enjoy!

Information Security—Before & After Public-Key Cryptography

One-time Pad Tool

I just uploaded the new software tool CT-46 OTP to perform one-time pad encryption. The training tool takes a plaintext and one-time pad key, converts the plaintext into digits and calculates the ciphertext. The CT-46 checkerboard is used to convert the text into digits. The program contains instructions on how to use the pencil-and-paper version and the conversion table.

CT-46 OTP Training Tool (click to visit download page)

The software is meant for training purposes only, as we all know there's no such thing as a secure computer. The only secure computer is a stand-alone laptop, stored in a 100% burglar proof vault. It's useless to run a perfect crypto system, or any crypto software for that matter, on a normal computer, given the many viruses, trojans, spyware and countless unidentified processes. The software however is ideal to train your encryption skills. The only truly secure way to apply one-time pad encryption still is simply with pencil and paper.

More about one-time pads at my one-time pad page, including an extensive Guide To Secure Communications with One-time Pad (pdf). The freeware CT-46 Tool is available at my download page.