Secret Sharing Schemes

```Secret Sharing Schemes
Russ Martin
May 14, 2012
Table of Contents
 What is Secret Sharing?
 Traditional Schemes
 Shamir’s
 Simplified
 Blakley’s
 Theory of More Efficient Schemes
 Short Share Secret Sharing
 Robust Secret Sharing
What is Secret Sharing
 A method of distributing data between a group of persons so
that any subset of a specified size can access the data, and a
subset of size smaller can not.
 A (t,w) Threshold Scheme is a method of sharing a key K
among w participants in such a way that any t participants
can compute the value of K, but no group of t-1 participants
can
Definitions
 Perfect Secret Sharing Scheme (PSS) – A scheme in which t-1
shares provide absolutely no information on the hidden data
 Information Rate – Ratio of # of bits in the secret being
hidden to the # of bits in the size of each share
 1 is ideal, as the size of the shares are the size of the secret
 Must be less than or equal to 1 for any perfect secret sharing
scheme
Traditional Schemes – Shamir’s
 Based on polynomial interpolation – given t points on the
plane, only one polynomial q(x) degree of t-1 exists that
satisfies q(x) = y for all xi (the key given to each participant).
 K = the data being hidden by the scheme, in numeric form
 q(x) = a0 + a1x + … + ak-1xk-1, where K = a0
Shamir’s Scheme – Key Distribution
 To Distribute data: Choose w unique elements in Zp, where p>w.
These are the x values.
 For i in 1 to w: Give xi to each of the participants. These x values
are public
 Choose t-1 values in Zp randomly. These values are secret to the
person distributing the shares. These are the a values.
 Privately give each member y = q(x) corresponding to their x
value, where
t 1
q( x)  K 
a
j 1
j
j
x mod p
Shamir’s Scheme – Key
Reconstruction
 Goal is to solve for the a values used during distribution, notably
a0 = K
 With t participants, one can form t linear equations in the form:
t 1
q ( xand
)  taunknowns,
 a1 x  ...
 aist 1axunique solution.
0
 With t equations
there
Shamir’s Scheme - Example
 p = 19, t = 3, w =4, xi = i
 K = a0 = 12
 Randomly Choose a1 = 14 , a2 = 3
q ( x )  12  14 x  3 x mod 19
2
 q(1) = 10, q(2) = 14 , q(3) = 5 , q(4) = 2
Shamir’s Scheme – Example (Solving)
 (1,2,3)
• (1,2,4)
10  a 0  a1  a 2
10  a 0  a1  a 2
14  a 0  2 a1  4 a 2
14  a 0  2 a1  4 a 2
5  a 0  3 a1  9 a 2
2  a 0  4 a1  16 a 2
 (1,3,4)
10  a 0  a1  a 2
5  a 0  3 a1  9 a 2
2  a 0  4 a1  16 a 2
• (2,3,4)
14  a 0  2 a1  4 a 2
5  a 0  3 a1  9 a 2
2  a 0  4 a1  16 a 2
• In all cases, Equations solve for 12, 14,
and 3, the values chosen
Shamir’s Scheme - Alternate
Reconstruction
 Each participant computes a value of b for each possible subset of
participants they could reconstruct the secret with.
 This can be done prior to reconstruction, as all x values are public
xk
b jare
 computed,
mod
p
 Once b values
for reconstruction
as
 xcanbexused
1 k  t , k  j
k
j
such:
t
K 
b
j 1
j
y j mod p
Shamir’s Scheme
 Size of all shares are the size of the hidden key (Information
Rate = 1)
 For t-1 people, forms a line of possible answers – providing
no information, making this a PSS
 If a person is “more important”, increase their ability by
giving them multiple shares
 Recommended # of shares: w = 2t – 1
 Allows recovery with loss/destruction of t-1 shares, but no
reconstruction with same number
Simplified Shamir’s Scheme
 Works only with a (t,t) threshold scheme
 Over any finite integer field Zm
 Randomly choose t-1 integers from i = 1 to t-1, denoted y1
… yt-1
t 1
yt  K 

y i mod m
i 1
 yi = Shares given to participants
Simplified Shamir’s Scheme
 Reconstruction:
t
K 

y i mod m
i 1
 With t-1 particpants, only can compute K-yi
 Still a PSS
Traditional Schemes – Blakley’s
 t different (t-1)-dimensional hyperplanes will always
intersect at exactly one point.
 t = 3, 2-dimensional planes in the form a1x1 + a2x2+ … atxt
=b
 K = x1
Blakley’s Scheme - Distribution
 Choose a prime p and F = finite, t-1 dimensional field
 Select a secret, random point x, where x1=K, rest of values
are random.
 All a values are also random and public
 Privately give each person yi = ai1x1 + ai2x2 + … aitxt
 Forms a w x t matrix, with Ax = y
Blakley’s Scheme - Reconstruction
 Solve system of equations Ax = y, only with the t users that
are combining shares.
 K = xi
Blakley’s Scheme
 Not fully secure – all participants know the point exists on
their plane
 Public share is much larger than K – t times in magnitude.
n*t a values are needed.
 a values are not sensitive, may be public
 Information Rate is 1
More Efficient Schemes
 Note that for large secrets or number of participants, there is
a large amount of data needed to be transferred
 Ideally, size of each share would be equal to size of the secret
divided by the threshold
 Since Information Rate is now greater than 1, it can no
longer be guaranteed to be a perfect secret sharing scheme
 Security can not be proved for any scheme with shares shorter
than secret, as there will be some information revealed.
Computationally Secure Secret Sharing
Scheme
 Proposed by Hugo Krawczyk
 Computationally Secure – No Information can be efficiently
computed from a single share
 Polynomial Indistinguishability – Two Probability
Distributions that cannot be told apart through any
polynomial-time algorithm
 Can be applied to encryptions – An encryption function is
computationally secure if for any pair of messages M’ and
M’’, their encryptions under all possible keys are
polynomially indistinguishable
Computationally Secure Secret Sharing
Scheme
 Applied to a Secret Sharing Scheme
 Computationally Secure if for any pair of secrets of same length
S’ and S’’, the distribution of their shares are polynomially
indistinguishable
 Information Dispersal Algorithm (IDA)
 A split of a file F into n partitions, where m are needed to
reconstruct the original file.
 Each partition size F/m, with a little redundancy attached
Short Share Secret Sharing
 Distribution
 Encrypt the secret S using a random key in a polynomially
indistinguishable algorithm
 Split the encrypted file into w fragments using IDA
 Encode the key using a PSS to create w shares of the key
 Give each participant one part of the key and one part of the
encrypted file
 Reconstruction:
 Use IDA to reconstruct the file
 Use PSS to recover the key
 Decrypt the file using the key to uncover the secret
 Share Size ≈ Size(File) / t + Size(Key)
Robust Secret Sharing
 A scheme that can recover the secret with up to m
corrupted/malicious shares
 m < t and t ≤ w-m
 Same Distribution and Reconstruction of Short Share, but signed
shares
 Sign file after encrypting, but before IDA
 Sign each of the shares
 Additional size of shares is not dependent on secret, only the signing
system
 Downsides
 Requires a public key signature verification system
 Much more computationally complex
 Entity distributing the secret needs to be known
Works Cited
[1]Stinson, Douglas R. Cryptography:Theory and Practice. CRC
Press 2006.
[2]Shamir, Adi. How to Share a Secret. November 1979.
[3]Krawczyk, Hugo. Secret Sharing Made Short. 1993.
[4]RSA Laboratories. What are some secret sharing schemes?
[5]http://www.cs.bilkent.edu.tr/~selcuk/publications/BSS_I
SC08.pdf
Questions?
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