From bde2442514d08f1aa4299f52886eb0ad3cfd7f9f Mon Sep 17 00:00:00 2001
From: David Florness <david@florness.com>
Date: Sun, 7 Jun 2020 14:40:55 -0600
Subject: [PATCH] Polynomials

---
 poly.go | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 poly.go

diff --git a/poly.go b/poly.go
new file mode 100644
index 0000000..8ebf89c
--- /dev/null
+++ b/poly.go
@@ -0,0 +1,69 @@
+package main
+
+import (
+	"crypto/rand"
+	"math"
+	"math/big"
+)
+
+type Poly struct {
+	constant *big.Int
+	coefs    []*big.Int
+}
+
+func RandomBigInt(numBytes uint, allowAllZeros bool) (*big.Int, error) {
+	randBytes := make([]byte, numBytes)
+	_, err := rand.Read(randBytes)
+	if err != nil {
+		return nil, err
+	}
+	bi := big.Int{}
+	bi.SetBytes(randBytes)
+	if bi.Cmp(big.NewInt(0)) == 0 {
+		return RandomBigInt(numBytes, allowAllZeros)
+	}
+	return bi.SetBytes(randBytes), nil
+}
+
+// creates a random polynomial of the given degree, uses at least `entropy' bits
+// of entropy for the random coefficients, and assigns the ballot to the
+// constant term
+func NewRandomPoly(degree uint, entropy uint, ballot []byte) *Poly {
+	constant := &big.Int{}
+	constant.SetBytes(ballot)
+	p := &Poly{constant, make([]*big.Int, degree)}
+
+	// number of bits per coefficient
+	numBits := uint(math.Ceil(float64(entropy)/float64(degree)))
+
+	// number of bytes per coefficient
+	numBytes := numBits / 8
+	if numBits % 8 > 0 {
+		numBytes += 1
+	}
+
+	var err error
+	for i := range p.coefs {
+		p.coefs[i], err = RandomBigInt(numBytes, false)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	return p
+}
+
+func (p *Poly) Eval(input *big.Int) *big.Int {
+	res := &big.Int{}
+	res.Set(p.constant)
+
+	for i, coef := range p.coefs {
+		degree := big.NewInt(int64(i+1))
+		term := &big.Int{}
+		term.Exp(input, degree, nil)
+		term.Mul(term, coef)
+		res.Add(res, term)
+	}
+
+	return res
+}
-- 
2.38.4