For emergencies: share part of pass with 4 people, decryptable with 2 persons

Question

Let's say I have a password:

AAABBBCCCDDD I could easily give person A the first part (AAA), person B the second part and so on.

But is there an option any two of the four people can decrypt/form the password from a part of text I give them? Obviously, from only the parts AAA and DDD the password can not be formed.

How? :)

Answer 1

This is known as "Secret sharing", "Key splitting" or "Key distribution", and can be done with some public key cryptography algorithms.

Here are a few links on the subject:

• Wikipedia article: Key distribution
• Wikipedia article: Secret sharing

An easy way to do 2 out of 4 would be to split the key into 4 pieces, and distribute 3 pieces to everyone. The distributed parts would know which parts they have.

Here's a sample distribution:

1. 1+2+3
2. 1+2+4
3. 1+3+4
4. 2+3+4

As you can see, no matter which two people you pick, they will always have all four pieces. There are more mathematical methods the more people and combinations you get to, but it is certainly doable.

Answer 2

Is giving up 3/4 of the password to each person acceptable?

Person A: AAABBBCCC
Person B: AAABBBDDD
Person C: AAACCCDDD
Person D: BBBCCCDDD

Any two people would have the whole thing. They'd have to do a little trial and error with with splitting and merging their components, but it's probably doable.

Answer 3

Say your password is "password", give each person the following:

A: XXssword

B: paXXword

C: passXXrd

D: passwoXX