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<h2>Lagrange Polynomial Interpolation and Shamir secret sharing</h2>
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<p><em>2021-10-10</em></p>
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<blockquote>
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<p>If you read this post, be aware that I’m not a mathematician, I’m just an amateur on math studying in my free time, and this article is just an attempt to try to sort the notes that I took while learning about Lagrange polynomial interpolation and Shamir’s secret sharing.</p>
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</blockquote>
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<p>Imagine that you have a <em>secret</em> (for example a <em>private key</em> that can decrypt a file), and you want to backup that <em>secret</em>. You can split the <em>secret</em> and give each slice to a different person, so when you need to reconstruct the <em>secret</em> you just need to put together all the parts. But, what happens if one of the parts gets corrupted, or is lost? The secret would not be recoverable.
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A better solution can be done if we use <em>Shamir Secret Sharing</em>, which allows us to split the <em>secret</em> in <span class="math inline">\(k\)</span> different parts, and set a minimum threshold <span class="math inline">\(n\)</span>, which defines the number of required parts to recover the <em>secret</em>, so just by putting together any <span class="math inline">\(n\)</span> parts we will recover the original secret.</p>
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<p>This has interesting applications, such as social recovery of keys or distributing a secret and ensuring that cooperation is needed in order to recover it. In the following lines we will overview the concepts behind this scheme.</p>
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<h3>Lagrange polynomial interpolation</h3>
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<p>Lagrange interpolation is also used in many schemes that work with polynomials, for example in <a href="https://arnaucube.com/blog/kzg-batch-proof.html">KZG Commitments</a> (an actual implementation <a href="https://github.com/arnaucube/kzg-commitments-study/blob/master/arithmetic.go#L272">can be found here</a>).</p>
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<p>The main idea behind is the following: for any <span class="math inline">\(n\)</span> distinct points over <span class="math inline">\(\mathbb{R}^2\)</span>, there is a unique polynomial <span class="math inline">\(p(x) \in \mathbb{R[x]}\)</span> of degree <span class="math inline">\(n-1\)</span> which goes through all of them.
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From the ‘other side’ point of view, this means that if we have a polynomial of degree <span class="math inline">\(n-1\)</span>, we can take <span class="math inline">\(n\)</span> points (or more) from it, and we will be able to recover the original polynomial from those <span class="math inline">\(n\)</span> points.</p>
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<p>We can see this starting with a line. If we are given any two points <span class="math inline">\(P_0=(x_0, y_0)\)</span> and <span class="math inline">\(P_1=(x_1, y_1)\)</span> from that line, we are able to recover the original line.</p>
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<div style="text-align:center;">
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<img style="width:300px;margin-bottom:20px;" src="img/posts/shamir-secret-sharing/line.png" />
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</div>
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<p>We can map this into the previous idea, seeing that our line is a degree <span class="math inline">\(1\)</span> polynomial, so, if we pick <span class="math inline">\(2\)</span> points from it, we later can recover the original line.</p>
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<p>Same happens with polynomials of degree <span class="math inline">\(2\)</span>. Let <span class="math inline">\(p(x)\)</span> be a polynomial of degree <span class="math inline">\(2\)</span> defined by <span class="math inline">\(p(x)= x^2 - 5x - 6\)</span>. We can create infinity of polynomials of degree <span class="math inline">\(2\)</span> that go through <span class="math inline">\(2\)</span> points, but with 3 points there is a unique polynomial degree <span class="math inline">\(2\)</span></p>
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<p>As the degree is <span class="math inline">\(2\)</span>, if we pick <span class="math inline">\(3\)</span> points from the polynomial, we will be able to reconstruct it.
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<div style="text-align:center;">
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<img style="width:300px;margin-bottom:20px;" src="img/posts/shamir-secret-sharing/degree2.png" />
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</div></p>
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<p>This is generalized by using <em>Lagrange polynomial interpolation</em>, which defines:</p>
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<p>For a set of points <span class="math inline">\((x_0, y_0), (x_1, y_1), ..., (x_n, x_n)\)</span>,</p>
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<p><span class="math display">\[
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I(x) = \sum_{i=0}^n y_i l_i(x)\newline
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where \space\space\space l_i(x) = \prod\_{0\leq j \leq n, j\neq i} \frac{x-x_j}{x_i - x_j}
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\]</span></p>
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<h3>Shamir’s secret sharing</h3>
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<p>As we’ve seen, for a degree <span class="math inline">\(n-1\)</span> polynomial we can pick <span class="math inline">\(n\)</span> or more points and we will be able to reconstruct the original polynomial from it. This is the main idea used in <em>Shamir’s secret sharing</em>.</p>
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<p>Let <span class="math inline">\(s\)</span> be our secret. We want to generate <span class="math inline">\(k\)</span> pieces and set a threshold <span class="math inline">\(n\)</span> which is the minimum number of pieces that are needed to reconstruct the secret <span class="math inline">\(s\)</span>. We can define a polynomial of degree <span class="math inline">\(n-1\)</span>, and pick <span class="math inline">\(k\)</span> points from that polynomial, so in this way with just putting together <span class="math inline">\(n\)</span> points of <span class="math inline">\(k\)</span> we will be able to reconstruct the original polynomial. And, we can place our secret <span class="math inline">\(s\)</span> in the <em>constant term</em> of the polynomial (the one that has <span class="math inline">\(x^0\)</span>), in this way, when we reconstruct the polynomial using <span class="math inline">\(n\)</span> out of <span class="math inline">\(k\)</span> points, we will be able to recover the secret <span class="math inline">\(s\)</span>.</p>
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<p>We can see this with an example with actual numbers (we will use small numbers):
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Imagine that we want to generate <span class="math inline">\(5\)</span> pieces from our secret, and define that just by putting together <span class="math inline">\(3\)</span> of the pieces we can recover the secret, this means setting <span class="math inline">\(n=3\)</span> and <span class="math inline">\(k=5\)</span>. Then we will generate a polynomial of degree <span class="math inline">\(n-1=2\)</span>, by <span class="math inline">\(p(x) = \alpha_0 + \alpha_1 x + \alpha_2 x^2\)</span>, where <span class="math inline">\(\alpha_0 = s\)</span> (the secret).</p>
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<p>We will work over a finite field of size <span class="math inline">\(p\)</span>, where <span class="math inline">\(p\)</span> is a prime number. For our example we will work over <span class="math inline">\(\mathbb{F}_{19}\)</span>, in real world we would work with much more bigger field. You can find an <a href="https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing#Example">example without finite fields in Wikipedia</a>.</p>
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<p>Let our secret be <span class="math inline">\(s=14\)</span>. We now generate our polynomial of degree <span class="math inline">\(n-1=2\)</span>, where <span class="math inline">\(s\)</span> will be the constant coefficient: <span class="math inline">\(p(x)= s + \alpha_1 x^1 + \alpha_2 x^2\)</span>. We can set <span class="math inline">\(\alpha_1\)</span> and <span class="math inline">\(\alpha_2\)</span> into any random value, as example <span class="math inline">\(\alpha_1=4\)</span> and <span class="math inline">\(\alpha_2=6\)</span>. So we have our polynomial: <span class="math inline">\(p(x) = 14 + 4 x + 6 x^2\)</span>.</p>
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<p>Now that we have the polynomial, we can pick <span class="math inline">\(k\)</span> points from it, using incremental indexes for the <span class="math inline">\(x\)</span> coordinate: <span class="math inline">\(P_1=(1, p(1)), P_2=(2, p(2)), \space\ldots\space, P_k=(k, p(k))\)</span>. With the numbers of our example this is (remember, we work over <span class="math inline">\(\mathbb{F}\_{19}\)</span>):</p>
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<p><span class="math display">\[
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p(x) = 14 + 4 x + 6 x^2,\newline
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p(1)=14 + 4 \cdot 1 + 6 \cdot 1^2 = 24 \space (mod \space 19) = 5\newline
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p(2)=14 + 4 \cdot 2 + 6 \cdot 2^2 = 46 \space (mod \space 19) = 8\newline
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p(3)=14 + 4 \cdot 3 + 6 \cdot 3^2 = 80 \space (mod \space 19) = 4\newline
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p(4)=14 + 4 \cdot 4 + 6 \cdot 4^2 = 126 \space (mod \space 19) = 12\newline
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p(5)=14 + 4 \cdot 5 + 6 \cdot 5^2 = 184 \space (mod \space 19) = 13
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\]</span></p><p>So our <span class="math inline">\(k\)</span> points are: <span class="math inline">\((1,5), (2,8), (3,4), (4,12), (5,13)\)</span>. We can distribute these points as our ‘secret parts’.
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In order to recover the secret, we need at least <span class="math inline">\(n=3\)</span> points, for example <span class="math inline">\(P_1\)</span>, <span class="math inline">\(P_3\)</span>, <span class="math inline">\(P_5\)</span>, and we compute the <em>Lagrange polynomial interpolation</em> to recover the original polynomial (remember, we work over <span class="math inline">\(\mathbb{F}\_{19}\)</span>):</p>
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<p><span class="math display">\[
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I(x) = \sum_{i=0}^n y_i l_i(x) \space\space
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where \space\space\space l_i(x) = \prod\_{0 \leq j \leq n \\ j\neq i} \frac{x-x_j}{x_i - x_j}
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\]</span></p><p><span class="math display">\[
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l_1(x) = \frac{x-3}{1-3} \cdot \frac{x-5}{1-5} = \frac{x-3}{17} \cdot \frac{x-5}{15}=\frac{x^2+11x+15}{8}\newline
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l_3(x) = \frac{x-1}{3-1} \cdot \frac{x-5}{3-5} = \frac{x-1}{2} \cdot \frac{x-5}{17} =\frac{x^2+13x+5}{15}\newline
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l_5(x) = \frac{x-1}{5-1} \cdot \frac{x-3}{5-3} = \frac{x-1}{4} \cdot \frac{x-3}{2} = \frac{x^2 + 15x + 3}{8}\newline
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\]</span></p><p><span class="math display">\[
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I(x) = y_2 \cdot l_2(x) + y_4 \cdot l_4(x) + y_5 \cdot l_5(x)\newline
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= 5 \cdot (\frac{x^2+11x+15}{8}) + 4 \cdot (\frac{x^2+13x+5}{15}) + 13 \cdot (\frac{x^2 +15x + 3}{8})\newline
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= \frac{5x^2+17x+18}{8} + \frac{4x^2+14x+1}{15} + \frac{13x^2+5x+1}{8}\newline
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= 3x^2+14x+7 + 18x^2+6x+14 + 4x^2+3x+12\newline
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= 6x^2 + 4x + 14
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\]</span></p><p>We can now take the <em>constant coefficient</em>, or just evaluate the obtained polynomial at 0, <span class="math inline">\(p(0) = 6 \cdot 0^2 + 4 \cdot 0 + 14 = 14\)</span>, and we obtain our original secret <span class="math inline">\(s=14\)</span>.</p>
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<h3>Conclusions</h3>
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<p>As an example of an use case of <em>Shamir Secret Sharing</em> we can think of social recovery of keys, there is an useful implementation of this scheme is used in the <a href="https://bs.parity.io/">banana split by Parity</a>. Also, here it is an implementation of the scheme in <code>Go</code>&<code>Rust</code> done a couple of years ago: <a href="https://github.com/arnaucube/shamirsecretsharing">https://github.com/arnaucube/shamirsecretsharing</a>.</p>
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<p><em>Lagrange Interpolation</em> in its own way, is a very useful tool in many schemes, it is also used in KZG Commitments, in zkSNARKs, zkSTARKs, PLONK, etc. In most of the schemes where polynomials are involved it becomes a very useful tool.</p>
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