From baa40a2b4a1f92101c700935152ca1c012fc22dd Mon Sep 17 00:00:00 2001
From: arnaucube <root@arnaucube.com>
Date: Mon, 2 Jan 2023 10:48:28 +0100
Subject: [PATCH] Add KZG commitments Sage impl

---
 kzg.sage | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 kzg.sage

diff --git a/kzg.sage b/kzg.sage
new file mode 100644
index 0000000..4496b12
--- /dev/null
+++ b/kzg.sage
@@ -0,0 +1,82 @@
+# toy implementation of BLS signatures in Sage
+#
+# Scheme overview: https://arnaucube.com/blog/kzg-commitments.html
+# Go implementation: https://github.com/arnaucube/kzg-commitments-study
+
+load("bls12-381.sage")
+
+e = Pairing()
+
+def new_ts(l):
+    Fr = GF(e.r)
+    s = Fr.random_element()
+    print("s", s)
+    tauG1 = [None] * l
+    tauG2 = [None] * l
+    for i in range(0, l): # TODO probably duplicate G1 & G2 instead of first powering s^i and then * G_j
+        sPow = Integer(s)^i
+        tauG1[i] = sPow * e.G1
+        tauG2[i] = sPow * e.G2
+
+    return (tauG1, tauG2)
+
+def commit(taus, p):
+    return evaluate_at_tau(p, taus)
+
+# evaluates p at tau
+def evaluate_at_tau(p, taus):
+    e = 0
+    for i in range(0, len(p.list())):
+        e = e + p[i] * taus[i]
+    return e
+
+def evaluation_proof(tau, p, z, y):
+    # (p - y)
+    n = p - y
+    # (t - z)
+    d = (t-z)
+    # q, rem = n / d
+    q = n / d
+    print("q", q)
+    q = q.numerator()
+    den = q.denominator()
+    print("q", q)
+    print("den", den)
+    # check that den = 1
+    assert(den==1) # rem=0
+    # proof: e = [q(t)]₁
+    return evaluate_at_tau(q, tau)
+
+def verify(tau, c, proof, z, y):
+    # [t]₂ - [z]₂
+    sz = tau[1] - z*e.G2
+
+    # c - [y]₁
+    cy = c - y*e.G1
+
+    print("proof", proof)
+    print("sz", sz)
+    print("cy", cy)
+    lhs = e.pair(proof, sz)
+    rhs = e.pair(cy, e.G2)
+    print("lhs", lhs)
+    print("rhs", rhs)
+    return lhs == rhs
+
+
+(tauG1, tauG2) = new_ts(5)
+
+R.<t> = PolynomialRing(e.F1)
+p = t^3 + t + 5
+
+c = commit(tauG1, p)
+
+z = 3
+y = p(z) # = 35
+
+proof = evaluation_proof(tauG1, p, z, y)
+print("proof", proof)
+
+v = verify(tauG2, c, proof, z, y)
+print(v)
+assert(v)