In the Shamir secret sharing scheme, we distribute a secret among a different users as follows. If our secret is a message (m₁, ..., mk) from V (k, q) then, we encode it as a codeword of the Reed-Solomon RSk(q) and give one coordinate to each user. In this problem, we will use q = 7, k = 4 and the parity check matrix H₁ below for RS(7). НА = 1 1 1 1 1 1 1 0 1 2 3 4 5 6 01 4 2 2 4 1 A new secret is selected and user #1 receives share value 0, user #2 receives share value 6 and user #3 receives share value 1 and are collaborating to discover the new secret. They can't recover the secret with only this information Now, suppose users #1, #2 and #3 discover, in addition to the values of their own shares, that users #4 and #5 have identical shares (but they don't necessarily know what the common value is). Using this information, first explain how they can collaborate to recover the secret and then find the secret.

Linear Algebra: A Modern Introduction
4th Edition
ISBN:9781285463247
Author:David Poole
Publisher:David Poole
Chapter2: Systems Of Linear Equations
Section2.2: Direct Methods For Solving Linear Systems
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In the Shamir secret sharing scheme, we distribute a secret among q different users as
follows. If our secret is a message (m₁,..., mk) from V(k, q) then, we encode it as a
codeword of the Reed-Solomon RS(q) and give one coordinate to each user. In this
problem, we will use q = 7, k = 4 and the parity check matrix H4 below for RS4(7).
HA
=
1 1 1 1 1 1 1
1 2 3 4 5 6
0 1 4 2 2 4 1
A new secret is selected and user #1 receives share value 0, user #2 receives share value 6
and user #3 receives share value 1 and are collaborating to discover the new secret. They
can't recover the secret with only this information
Now, suppose users #1, #2 and #3 discover, in addition to the values of their own shares, that
users #4 and #5 have identical shares (but they don't necessarily know what the common
value is). Using this information, first explain how they can collaborate to recover the secret
and then find the secret.
Transcribed Image Text:In the Shamir secret sharing scheme, we distribute a secret among q different users as follows. If our secret is a message (m₁,..., mk) from V(k, q) then, we encode it as a codeword of the Reed-Solomon RS(q) and give one coordinate to each user. In this problem, we will use q = 7, k = 4 and the parity check matrix H4 below for RS4(7). HA = 1 1 1 1 1 1 1 1 2 3 4 5 6 0 1 4 2 2 4 1 A new secret is selected and user #1 receives share value 0, user #2 receives share value 6 and user #3 receives share value 1 and are collaborating to discover the new secret. They can't recover the secret with only this information Now, suppose users #1, #2 and #3 discover, in addition to the values of their own shares, that users #4 and #5 have identical shares (but they don't necessarily know what the common value is). Using this information, first explain how they can collaborate to recover the secret and then find the secret.
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In Step 2 part,  could you please explain why you said:

H4 * [s1, s2, s3, s4, s5] = [0, 6, 1] 

As to me, H4 is a 3x7 matrix, while [s1, s2, s3, s4, s5] is a 1x5 matrix. We cannot perform that multiplication

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