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Shamir's Secret Sharing

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Re: Shamir's Secret Sharing

#31

Post by andrewflnr »

jancsika wrote: post #26

Suppose I asked if there's a practical example of merkle trees in the wild. Someone answers, "of course: git." Then 7 troglodyte friends and I jump on github/gitlab/whatever (which is super easy because everyone already uses one of these user-friendly services that wrap around git) and immediately see how git helps us develop by leveraging merkle trees. We realize that the merkle trees are leveraged so that we can en…

Hashicorp Vault: https://www.vaultproject.io/

It uses SSS for its startup process.

Re: Shamir's Secret Sharing

#32

Post by userbinator »

ballenf wrote: post #17

Reminds me a lot of my usenet newsgroup file sharing days and the PAR parity format. A file is split into say 200 pieces to fit within the limitations of a newsgroup post. Those 200 posts may or may not all make it to your usenet server, but an additional 10-20 parity files are also created such that you need to only find 200 total unique pieces to recreate the data. It's different in that the data is totally readabl…

Also the HDCP master key and how it was cracked:

https://en.wikipedia.org/wiki/High-bandwidth_Digital_Content...

https://en.wikipedia.org/wiki/Key_selection_vector

Re: Shamir's Secret Sharing

#33

Post by tuxxy »

NuCypher uses this on our proxy re-encryption scheme. You don't want a re-encryption key to be all together in one place, so we split it up using SSS and distribute the fragments. For cryptography beginners, this scheme is relatively easy to understand, describe, and prove.

Re: Shamir's Secret Sharing

#34

Post by [deleted] »

jancsika wrote: post #26

Suppose I asked if there's a practical example of merkle trees in the wild. Someone answers, "of course: git." Then 7 troglodyte friends and I jump on github/gitlab/whatever (which is super easy because everyone already uses one of these user-friendly services that wrap around git) and immediately see how git helps us develop by leveraging merkle trees. We realize that the merkle trees are leveraged so that we can en…

[deleted]

Re: Shamir's Secret Sharing

#35

Post by sowbug »

jancsika wrote: post #26

Suppose I asked if there's a practical example of merkle trees in the wild. Someone answers, "of course: git." Then 7 troglodyte friends and I jump on github/gitlab/whatever (which is super easy because everyone already uses one of these user-friendly services that wrap around git) and immediately see how git helps us develop by leveraging merkle trees. We realize that the merkle trees are leveraged so that we can en…

If you trust your seven friends, then you can use SSS to pass on digital assets after you die.

Decide on a policy. For example, 4 of your 7 friends need to agree that you've been dead/incapacitated for 90 days, and that once that happens they should give your digital assets to Recipient R (probably your next of kin). Pick a master passphrase that decrypts something interesting like a passphrase manager database file. Split the master passphrase using a 4-of-7 scheme. Distribute the seven shares to your seven friends (one to each friend). Now you know the passphrase manager won't be compromised before your death unless you are careless with the master passphrase, or four of your seven friends either collude, get hacked, or honor a legal demand to produce the shares.

This gets interesting when the digital-asset ownership is determined exclusively by cryptography. The money in your bank account is not such a thing (a suitably official-looking piece of paper will release your money to anyone), but Bitcoin or Ethereum definitely are (no court order or lead pipe can solve the discrete logarithm problem).

Such classes of assets are very new. So there are not yet any "practical examples in the wild" that an ordinary person would be likely to recognize.

Re: Shamir's Secret Sharing

#36

Post by jron »

Greg Maxwell has suggested that quite a few implementations of SSS are broken: "FWIW, virtually every SSS thing I've seen out there is just wrong in at least some less serious way. In general I've found secret sharing to be part of a pretextual security practice that seldom helps users against realistic threats, and the thoughtlessness of using it is usually reflected in the implementation." - https://np.reddit.com/r/Bitcoin/comments/72dfy1/armory_walle...

Here is one seriously broken implementation he discovered: https://bitcointalk.org/index.php?topic=2199659.0

Re: Shamir's Secret Sharing

#37

Post by late2part »

jancsika wrote: post #24
late2part wrote: post #20

Earlier quoted context omitted.

Consider you want to share the passwords to your bank accounts with your family after you die. You take a list of those passwords, and encrypt it using SSSS with 4 of 7 keys needed to decrypt. You then share these 7 keys with your 7 relatives. After your death, they get together and unlock your passwords.

They can access the bank accounts, but can they legally perform any meaningful transaction with the data/money they access? For example-- suppose that person dies and these 7 relatives access the account and wire themselves some money. With no other arrangements made, doesn't that constitute bank fraud? On the flip side-- if the relatives also have to go through the time-consuming processes of meetings with an estate…

Maybe your bank accounts are held in nominee officer shelf corps in offshore companies. I used an arbitrary example as a vehicle to show how the m of n could access info.

Re: Shamir's Secret Sharing

#38

Post by bborud »

late2part wrote: post #20
bborud wrote: post #8

Ever since learning about this I've wanted to use it for something, but I've never had the opportunity.

Consider you want to share the passwords to your bank accounts with your family after you die. You take a list of those passwords, and encrypt it using SSSS with 4 of 7 keys needed to decrypt. You then share these 7 keys with your 7 relatives. After your death, they get together and unlock your passwords.

Yeah, I know of plenty of use-cases for it but I have never worked on a project that required a secret sharing capability. I'm too lazy to make a product just because I want to use a cool algorithm.

However, during the data retention debate in Norway I repeatedly pointed out that the only responsible way to implement the act would be to use secret sharing to ensure that a sufficient number of parties were involved when unlocking someone's private data.

Re: Shamir's Secret Sharing

#39

Post by blattimwind »

barking wrote: post #23
mehrdadn wrote: post #15

Earlier quoted context omitted.

Just remember the caveat with the ELI5 explanation is that if I tell you the first two points on a parabola are (0,0) and (1,0) you will figure that the third point is more likely to be around (2,0) than, say, (2,2^30).

I could understand the integer arithmetic example they gave and I think you are pointing out how this is flawed security-wise (its use lies in explaining the method). This flaw is addressed by using finite field arithmetic but I did't understand that part too well.

The modulo reduction used in finite fields essentially "scrambles" the structure of many objects.

Re: Shamir's Secret Sharing

#40

Post by dsnuh »

jancsika wrote: post #26

Suppose I asked if there's a practical example of merkle trees in the wild. Someone answers, "of course: git." Then 7 troglodyte friends and I jump on github/gitlab/whatever (which is super easy because everyone already uses one of these user-friendly services that wrap around git) and immediately see how git helps us develop by leveraging merkle trees. We realize that the merkle trees are leveraged so that we can en…

Hashicorp Vault uses SSS as part of its security model for unsealing the data.

https://www.vaultproject.io/docs/internals/security.html

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