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Imagine something like TrueCrypt where user A can decrypt his files, or any 3 of the 10 directors in his organization can decrypt user A's files. As I understand it this is similar to the way the DNSSEC Root Keys are secured. Obligatory wiki entries. Does anyone know of any commercial or open source implementation of file encryption utilizing secret-sharing?

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Thanks to all who responded! Now to get my head around the math involved! –  scuzzy-delta Jul 20 '11 at 22:17

6 Answers 6

up vote 3 down vote accepted

I want to say that the commercial PGP product has had this feature for at least 10 years. It currently has this feature now:

http://www.pgpi.org/doc/pgpintro/#p24 and http://www.symantec.com/business/support/index?page=content&id=HOWTO41916

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Here's a Shamir's Secret Sharing library I put together in Python: https://github.com/rxl/secret-sharing.

It's really simple to split secrets:

>>> from secretsharing import SecretSharer
>>> shares = SecretSharer.split_secret("c4bbcb1fbec99d65bf59d85c8cb62ee2db963f0fe106f483d9afa73bd4e39a8a", 2, 3)
['1-58cbd30524507e7a198bdfeb69c8d87fd7d2c10e8d5408851404f7d258cbcea7', '2-ecdbdaea89d75f8e73bde77a46db821cd40f430d39a11c864e5a4868dcb403ed', '3-80ebe2cfef5e40a2cdefef0923ee2bb9d04bc50be5ee308788af98ff609c380a']

...and recover secrets:

>>> SecretSharer.recover_secret(shares[0:3])
'c4bbcb1fbec99d65bf59d85c8cb62ee2db963f0fe106f483d9afa73bd4e39a8a'

And it supports a whole bunch of secret and share formats, like Bitcoin private keys and base32:

>>> from secretsharing import BitcoinToZB32SecretSharer
>>> shares = BitcoinToZB32SecretSharer.split_secret("5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS", 2, 3)
['b-aweuzkm9jmfgd7x4k595bzcm3er3epf4dprfwzpprqa3exbuocs9byn4owfuqbo', 'n-btetgqqu8doacarsbyfdzpyycyj6gfdeaaxrpfx33pdjk4ou1d5owjdmdi1iegm9', 'd-njh33f14q7smucmh8iq8uaewc8mzub3mzptrwsegfiz3hc1fozkkjtguc4trh6sq']
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The key sharing is also used in Hardware Security Modules, for example to unlock the administrator account. In the Amazon CloudHSM documentation, the key sharing is explained pretty well: Amazon CloudHSM: About M of N

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There are at least two free implementations that are part of Ubuntu linux and implement Shamir's secret splitting and combining:

  • gfshare: Ubuntu Manpage: gfshare - explanation of Shamir Secret Sharing in gf(2**8) provides both tools for secret sharing (gfsplit and gfcombine) which can split an arbitrary file into shares, as well as a library for use by developers.

  • The "ssss" package provides the ssss-split program which prompts you for a pass phrase which can't be longer than 128 characters, to split up into a set of phrases to share. Then the ssss-combine program prompts for enough of the shared phrases and prints out the secret.

gfshare directly works with files, while ssss would be used to split up a pass phrase which can then be used with gpg or openssl or another encryption utility. So gfshare would seem simpler for your use case.

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props for the manpage link, interesting and easy read. +1 –  jcinacio Feb 11 '12 at 20:44

The physical/IT crossover comes with Break Glass scenarios for emergencies - holders of part of the secret retrieve them from their safes and input a passphrase together in order to approve short-term emergency access permissions. This is pretty common in large organisations - especially in financial services.

I haven't seen a real world implementation of the purely technical solution that I think you mean - where the encryption algorithm copes with x out of y keys to unlock.

I wonder what the scenario would be that would require it as a solution over something simple like the break glass scenario I mentioned.

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The company I currecntly work at has this in place using PGP. It is used for key escrow procedures. The master-key which can decrypt everything is stored in components in sealbags. Only when 3 persons come together, they can decrypt another persons secrets.

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what are sealbags? Are there only three components and are all three components needed? –  this.josh Jul 18 '11 at 17:22
1  
Sealbags are special bags which can only be opened by destroying it. So if someone opens it, it is obvious. This ensures that nobody can peek into his component secretly. There are three components and two of the three are needed for the escrow procedure. –  Henri Jul 19 '11 at 14:50

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