I have an information that I would like to encrypt and give it to someone else. I would like to encrypt it using N separate passwords (say 5) and give each password to some other persons. However the tricky part is that I would like to have any M of them (M<=N, say M=4) to be able to get together and decipher the message. Is that possible using some sort of conventional free software?

My use case is to write a secret message, give it to one of my friends and then distribute passwords to others. However if one were to lose their part, the entire message would be lost. That is why I would like to have a redundancy. If you have any other idea how to solve this, I am open to it as well.

Thank you.

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    $\begingroup$ This scheme is called, surprisingly enough, "secret sharing". Have you tried googling "secret sharing software"? I am getting at least 4 promising hits on the first page, have you explored these? Also, software requests are off-topic on this site. $\endgroup$ – Thomas Sep 25 '13 at 13:11
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    $\begingroup$ A common choice is Shamir's Secret Sharing $\endgroup$ – CodesInChaos Sep 25 '13 at 13:14
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    $\begingroup$ @Thomas: Actually no, I did not try to google for secret sharing. Had I known to google for it, I would not have asked here. $\endgroup$ – NeverStopLearning Sep 25 '13 at 13:29
  • $\begingroup$ @CodesInChaos Thank you. This is what I was looking for. $\endgroup$ – NeverStopLearning Sep 25 '13 at 13:30
  • $\begingroup$ @NeverStopLearning: If there's one thing I've learned over the years, it's that if you can imagine cryptography being used for some purpose, there is someone out there who has at least touched it... invariably. $\endgroup$ – Reid Sep 25 '13 at 14:52