diff --git a/week3.py b/week3.py
new file mode 100644
index 0000000..e5410c3
--- /dev/null
+++ b/week3.py
@@ -0,0 +1,201 @@
+from __future__ import annotations
+
+
+import random
+
+from crypto.week1 import BloodType, blood_cell_compatibility_lookup
+
+
+class Protocol:
+    def __init__(self, alice, bob):
+        self.alice: Alice = alice
+        self.bob: Bob = bob
+        self.dealer = Dealer()
+
+        self.alice.share_inputs(bob)
+        self.bob.share_inputs(alice)
+
+    def xor(self, x_name, y_name, z_name):
+        self.alice.xor(x_name, y_name, z_name)
+        self.bob.xor(x_name, y_name, z_name)
+
+    def xor_with_constant(self, x_name, c, z_name):
+        self.alice.xor_with_constant(x_name, c, z_name)
+        self.bob.xor_with_constant(x_name, c, z_name)
+
+    def and_(self, x_name, y_name, z_name):
+        self.dealer.gen_randoms()
+        self.alice.set_randoms(self.dealer.randoms_alice)
+        self.bob.set_randoms(self.dealer.randoms_bob)
+
+        self.xor(x_name, "u", "d")
+        self.xor(y_name, "v", "e")
+
+        self.open("d")
+        self.open("e")
+
+        # z = w ^ (e & x) ^ (d & y) ^ (e & d)
+        # z = w ^    a    ^    b    ^    c
+        # z =   z1        ^         z2
+        self.and_with_public(x_name, "e", "a")
+        self.and_with_public(y_name, "d", "b")
+        self.and_publics("e", "d", "c")
+
+        self.xor("w", "a", "z1")
+        self.xor_with_public("b", "c", "z2")
+
+        self.xor("z1", "z2", z_name)
+
+    def and_with_public(self, x_name, y_name, z_name):
+        self.alice.and_with_public(x_name, y_name, z_name)
+        self.bob.and_with_public(x_name, y_name, z_name)
+
+    def and_publics(self, x_name, y_name, z_name):
+        self.alice.and_publics(x_name, y_name, z_name)
+        self.bob.and_publics(x_name, y_name, z_name)
+
+    def xor_with_public(self, x_name, c_name, z_name):
+        self.alice.xor_with_public(x_name, c_name, z_name)
+        self.bob.xor_with_public(x_name, c_name, z_name)
+
+    def open(self, x_name):
+        self.alice.open(x_name, self.bob.send(x_name))
+        self.bob.open(x_name, self.alice.send(x_name))
+
+    def output(self, z_name):
+        return self.alice.open(z_name, self.bob.send(z_name))
+
+
+class Party:
+    def __init__(self, **inputs) -> None:
+        self.inputs = inputs
+        self.secrets = {}
+        self.publics = {}
+
+    def share(self, x_name):
+        x_other = random.randint(0, 1)
+        self.secrets[x_name] = self.inputs[x_name] ^ x_other
+        return x_other
+
+    def share_inputs(self, other_party: Party):
+        for name, value in self.inputs.items():
+            other_party.receive(name, self.share(name))
+
+    def set_randoms(self, randoms):
+        self.secrets["u"], self.secrets["v"], self.secrets["w"] = randoms
+
+    def xor(self, x_name, y_name, z_name):
+        x = self.secrets[x_name]
+        y = self.secrets[y_name]
+        self.secrets[z_name] = x ^ y
+
+    def xor_with_constant(self, x_name, c, z_name):
+        x = self.secrets[x_name]
+        self.secrets[z_name] = c ^ x
+
+    def and_with_public(self, x_name, y_name, z_name):
+        x = self.secrets[x_name]
+        y = self.publics[y_name]
+        self.secrets[z_name] = x & y
+
+    def and_publics(self, x_name, y_name, z_name):
+        x = self.publics[x_name]
+        y = self.publics[y_name]
+        self.publics[z_name] = x & y
+
+    def xor_with_public(self, x_name, c_name, z_name):
+        x = self.secrets[x_name]
+        c = self.publics[c_name]
+        self.secrets[z_name] = c ^ x
+
+    def open(self, x_name, other_share):
+        x = self.secrets[x_name]
+        z = x ^ other_share
+        self.publics[x_name] = z
+        return z
+
+    def send(self, x_name):
+        return self.secrets[x_name]
+
+    def receive(self, x_name, x):
+        self.secrets[x_name] = x
+
+
+class Alice(Party):
+    pass
+
+
+class Bob(Party):
+    def xor_with_constant(self, x_name, c, z_name):
+        """
+        xor with constant is asymmetric, so bob simply saves x to z.
+        """
+        x = self.secrets[x_name]
+        self.secrets[z_name] = x
+
+    def xor_with_public(self, x_name, c_name, z_name):
+        """
+        xor with constant is asymmetric, so bob simply saves x to z.
+        """
+        x = self.secrets[x_name]
+        self.secrets[z_name] = x
+
+
+class Dealer:
+    def gen_randoms(self):
+        u = random.randint(0,1)
+        v = random.randint(0,1)
+        w = u & v
+
+        u_b = random.randint(0,1)
+        u_a = u_b ^ u
+
+        v_b = random.randint(0,1)
+        v_a = v_b ^ v
+
+        w_b = random.randint(0,1)
+        w_a = w_b ^ w
+
+        assert((u_a ^ u_b) & (v_a ^ v_b) == w)
+        assert(w_a ^ w_b == w)
+
+        self.randoms_alice = (u_a, v_a, w_a)
+        self.randoms_bob = (u_b, v_b, w_b)
+
+
+def run(da, db, ds, ra, rb, rs):
+    proto = Protocol(Alice(da=da, db=db, ds=ds), Bob(ra=ra, rb=rb, rs=rs))
+
+    proto.xor_with_constant("ra", 1, "k1")
+    proto.xor_with_constant("rb", 1, "k2")
+    proto.xor_with_constant("rs", 1, "k3")
+
+    proto.and_("da", "k1", "l1")
+    proto.and_("db", "k2", "l2")
+    proto.and_("ds", "k3", "l3")
+
+    proto.xor_with_constant("l1", 1, "m1")
+    proto.xor_with_constant("l2", 1, "m2")
+    proto.xor_with_constant("l3", 1, "m3")
+
+    proto.and_("m1", "m2", "n")
+    proto.and_("n", "m3", "z")
+
+    z = proto.output("z")
+    return z
+
+
+def main():
+    green = 0
+    red = 0
+    for i, recipient in enumerate(BloodType):
+        for j, donor in enumerate(BloodType):
+            z = run(*donor.value, *recipient.value)
+            lookup = blood_cell_compatibility_lookup(recipient, donor)
+            if lookup == z:
+                green += 1
+            else:
+                print(f"'{BloodType(donor).name} -> {BloodType(recipient).name}' should be {lookup}.")
+                red += 1
+    print("Green:", green)
+    print("Red  :", red)