# Tag Info

20

In Shamir's scheme is a secret sharing scheme, that is, someone that has fewer shares than is required get no information about the secret. For example, if we have a system where we require 3 shares to reveal the shared secret, then someone with 2 shares cannot be able to reconstruct it. This is true if we make the shared secret the zero-th coefficient; ...

15

Here's an simpler but analogous problem that may illustrate what's going on: Given that $X=Y+Z$ and $Y=5$, compute $X$. The problem isn't that the answer is difficult to compute, the problem is that there isn't enough information there to give you any idea (any information about) what the answer is. That's what's meant by information-theoretic security. (...

13

It's simply not secure. Sure, it "works", in the sense that you can generate shares and reconstruct the secret from a sufficient number of them, but the essential security property of Shamir's secret sharing — namely, that knowing less than the required threshold number of shares reveals no information about the secret — does not hold. Since it'...

13

This is about the Theory of Computability not the Theory of Complexity. The halting problem is a decision problem in CS. From Wikipedia's introduction; In computability theory, the halting problem is the problem of determining, from a description of an arbitrary computer program and an input, whether the program will finish running, or continue to run ...

12

Here is an active attack on the privacy of out-of-the-box SSS. For this attack, we'll assume that the attacker (without a valid share) is allowed to participate (with $T-1$ friends with honest key shares), jointly use the protocol to recover a 'shared secret' (which might not be the real shared secret); we'll assume that this shared secret recovery process ...

11

The process is pretty simple. As you say, each party multiplies their two shares. They then use Shamir secret sharing to share the resulting value with the other parties. Once they have received a "subshare" from each other party, each party simply runs Lagrangian interpolation on the subshares they received (plus their own subshare). The result is a share ...

11

In RSA, assuming knowledge of the public key but not the private key, analyzing any number of triplets of matching message, encrypted message, and signature $(m,M,sg)$, does not help (as far as we know) towards recovering the private key $s$ (nor an equivalent). That's regardless of the sensible padding or RSA variant used (as long as neither the padding nor ...

11

let's say have used a 64-bit secret key to encrypt a file and then we split the key into 2 pieces of 32-bit That right there is your misunderstanding. Shamir secret sharing does not split a secret into pieces that are smaller than the original secret. Instead, it splits the secret into pieces that are the same size as the original secret. A simple ...

10

No, the Runge phenomenon is known not to affect Shamir's scheme. Remember, the point of Shamir's scheme is not actually to form an approximation over an interval; instead, it's to encode a secret in a randomly chosen polynomial, and then divide up clues to that polynomial so that, with enough clues (shares), someone can reconstruct the entire polynomial (...

10

I worked on a secure document management project 20 years ago which used secret sharing. It is widely used in financial networks. Actual use cases with public details that are easily accessible include: Threshold key sharing is used in Hardware Security Modules, for example to unlock the administrator account. In the Amazon CloudHSM documentation, this is ...

9

The point is that the dealer generating the update needn't know what the shared secret is. If we had a dealer that remembered what the shared secret was (or we asked enough people to contribute their shares so that the dealer could reconstruct it), then yes, the dealer could generate new shares. However, this would require is a dealer that did know the ...

9

Nowadays, the most standard method is to use oblivious transfers. Oblivious transfer involve a sender with two messages $(m_0,m_1)$ and a receiver with a selection bit $b$. At the end of the protocol, the receiver learns $m_b$ (and gets no information about $m_{1-b}$) while the sender gets no information about $b$. Suppose Alice and Bob want to generate a ...

9

As others have noted, information-theoretic security really has no connection to computational complexity. Yes, with sufficient computing power, you could enumerate all the solutions (including the right one). But without enough information (e.g. the key in OTP, or enough shares in SSS) you would still have no way of telling which of the solutions is the ...

8

Actually, you can do Shamir Secret Sharing over any finite field $GF(p^k)$, for any prime $p$ and any integer $k$. If $k=1$, you have the $GF(p)$ field you mentioned; however it works on extension fields as well. We often pick $p=2$ and $k$ a multiple of 8; this makes everything nice even number of bytes (at the cost of doing our calculations in $GF(2^k)$). ...

8

Is the running times of corresponding steps true? No. Step 3 of the dealer has to be executed $n$ times (once for each party) with each execution taking $O(t)$ time. So it must be $O(t\cdot n)$. Step 4 of the dealer needs $O(n)$ to distribute each share to every party. I count $O(t\cdot n)$ as the overall time complexity for the dealer. Of course, you ...

8

Yes, preprocessing Beaver triples in an offline phase leads to a faster online phase. The online phase of an AND gate requires just two openings plus local computations. But there are other advantages as well. Define a "linear representation" $[x]$ to be any way of representing/distributing a value $x$ among parties such that the following properties hold: ...

8

This cannot be achieved information-theoretically. This is typically the task that requires multiparty computation protocol to be achieved. In particular, the common method for what you want is called "secure multiplication protocol", and is in general constructed from an additively homomorphic encryption scheme (over the ring $R$), or from oblivious ...

8

With Shamir's Secret Sharing you can add as many shares as you want, as long as the threshold and the secret is unchanged. You can see that neither the generation of the polynomial that produces the shares nor the reconstruction of the secret by polynomial interpolation require the parameter $N$ (number of shares), but only the threshold $k$. Of course ...

8

Yes. It is possible to construct secret sharing schemes for general (monotone) access structures. You can read about the first construction in the paper called Secret Sharing Scheme Realizing General Access Structure. I also suggest reading this survey by Amos Beimel.

8

The proposed sharing scheme does allow reconstruction of key $K$ from any two of the three shares $K_A$, $K_B$, $K_C=(X_A,X_B)$, because: $K=K_A+K_B$ from $K_A$ and $K_B$ $K=K_A\oplus X_A$ from $K_A$ and $K_C$ $K=K_B\oplus X_B$ from $K_B$ and $K_C$ Problem is, the scheme leaks information about key $K$: always less than 1 bit worth to participants $A$ and $... 8 The maximum number of shares in Shamir's secret sharing is limited by the size of the underlying finite field. In particular, the maximum is one less than the number of elements in the field, since each share must be associated with a distinct element of the field, and one of the elements (usually the zero element) must be reserved for the secret being ... 8 Let's recall Shamir's Secret Sharing. We work in a finite field$\mathbb{F}_q$of cardinal$q$. The secret to share is$s$; we want$n$shares with a threshold$t$. We suppose that$n < q$(otherwise, the scheme does not work). We conventionally name$n$non-zero values of$\mathbb{F}_q$:$x_1$,$x_2$...$x_n$. Exactly how we choose them is unimportant, ... 8 If you reuse the same multiplication triples, then you leak information about the secret shared values that you multiply. Let's recall how the multiplication works in Beaver's protocol for secure evaluation of arithmetic circuits. Protocols like the ones from the SPDZ family follow Beaver's blueprint and "just" add some additional magic on top for getting ... 7 Shamir's secret sharing works in any finite field. A field is a mathematical structure that follows the usual laws of addition and multiplication. A finite field is a field with a finite number of elements, unlike for example the real numbers, which have an infinite number of elements. Fields exist for all prime powers pk where p is a prime and k a positive ... 7 Assuming that$p$is prime, then you are in a cyclic group. Consequently, this is identical to considering the shares$s_i$as "exponents" of a generator$g$of$Z_p^*$. Now we can write:$s_1 = g^{s'_1}, \ldots,s_{k}=g^{s'_{k}}$and$s=\prod_{i=1}^{k} s_i$Or we can view this as:$s = g^{\sum_{i=1}^{k} s'_i}$. Consequently it looks like a perfect (= ... 7 If you perform the distribution digitally (using networks) then you have a problem. Unless you use another one time pad you lose the perfect confidentiality as the distribution itself won't deliver perfect security. But using another one time pad is pointless: you would lose exactly as many key bits as you are distributing, while you are only protecting the ... 7 Full disclosure: In 2007 I founded an association aiming at voting transparency. I'm proud that my efforts may have had some role, however small, in the fact that the number of French cities using electronic voting machines for political elections, then growing, has been declining since then. The book defining the protocol of the question is made freely ... 7 Let us first consider the problem without involving Shamir secret-sharing at all. Suppose that$n = 140$and that the secret$\sigma$is a 140-byte Twitter message. The space is thus restricted considerably, from all possible$256$byte values to the printable characters permitted to be used in Twitter messages, and the distribution in this restricted space ... 7 Your understanding is correct. The SPDZ protocol can be used for any number of two or more parties. In fact, this is one of the strengths of the SPDZ protocol. Namely, many recent secure computation protocols such as the various versions of the Yao protocol or the TinyOT protocol are limited to two parties. So it may sometimes be overemphasized that SPDZ ... 7 Here's one more way in which a dishonest participant can mess with Shamir's secret sharing: Let's briefly review how secret reconstruction in Shamir's$(k,n)$secret sharing works. Given the$x$-coordinates of$k$participants$(x_1, x_2, \dots, x_k)$, one way to reconstruct the secret is to compute the Lagrange basis polynomials:$\$\ell_j(x) = \prod_{1 \...

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