Moving cfrag proof and verification logic to _pre

umbral/_pre.py

@@ -3,6 +3,105 @@ from umbral.config import default_params
 from umbral.params import UmbralParameters
 
 
+def prove_cfrag_correctness(cfrag: "CapsuleFrag",
+                            kfrag: "KFrag",
+                            capsule: "Capsule",
+                            metadata: bytes=None,
+                            params: UmbralParameters=None
+                      ) -> "CorrectnessProof":
+    params = params if params is not None else default_params()
+
+    rk = kfrag._bn_key
+    t = CurveBN.gen_rand(params.curve)
+    ####
+    ## Here are the formulaic constituents shared with `verify_correctness`.
+    ####
+    e = capsule._point_e
+    v = capsule._point_v
+
+    e1 = cfrag._point_e1
+    v1 = cfrag._point_v1
+
+    u = params.u
+    u1 = kfrag._point_commitment
+
+    e2 = t * e
+    v2 = t * v
+    u2 = t * u
+
+    hash_input = [e, e1, e2, v, v1, v2, u, u1, u2]
+    if metadata is not None:
+        hash_input.append(metadata)
+    h = CurveBN.hash(*hash_input, params=params)
+
+    z1 = kfrag._bn_sig1
+    z2 = kfrag._bn_sig2
+    z3 = t + h * rk
+    ########
+
+    cfrag.attach_proof(e2, v2, u1, u2, z1, z2, z3, metadata)
+
+    # Check correctness of original ciphertext (check nº 2) at the end
+    # to avoid timing oracles
+    if not capsule.verify(params):
+        raise capsule.NotValid("Capsule verification failed.")
+
+
+def assess_cfrag_correctness(cfrag,
+                             capsule: "Capsule",
+                             pubkey_a_point,
+                             pubkey_b_point,
+                             params: UmbralParameters = None):
+    params = params if params is not None else default_params()
+
+    ####
+    ## Here are the formulaic constituents shared with `prove_cfrag_correctness`.
+    ####
+    e = capsule._point_e
+    v = capsule._point_v
+
+    e1 = cfrag._point_e1
+    v1 = cfrag._point_v1
+
+    u = params.u
+    u1 = cfrag.proof._point_kfrag_commitment
+
+    e2 = cfrag.proof._point_e2
+    v2 = cfrag.proof._point_v2
+    u2 = cfrag.proof._point_kfrag_pok
+
+    hash_input = [e, e1, e2, v, v1, v2, u, u1, u2]
+    if cfrag.proof.metadata is not None:
+        hash_input.append(cfrag.proof.metadata)
+    h = CurveBN.hash(*hash_input, params=params)
+
+    z1 = cfrag.proof._bn_kfrag_sig1
+    z2 = cfrag.proof._bn_kfrag_sig2
+    z3 = cfrag.proof._bn_sig
+    ########
+
+    xcomp = cfrag._point_noninteractive
+    kfrag_id = cfrag._bn_kfrag_id
+
+    g = params.g
+
+    g_y = (z2 * g) + (z1 * pubkey_a_point)
+    signature_input = [g_y, kfrag_id, pubkey_a_point, pubkey_b_point, u1, xcomp]
+    kfrag_signature1 = CurveBN.hash(*signature_input, params=params)
+    valid_kfrag_signature = z1 == kfrag_signature1
+
+    correct_reencryption_of_e = z3 * e == e2 + (h * e1)
+
+    correct_reencryption_of_v = z3 * v == v2 + (h * v1)
+
+    correct_rk_commitment = z3 * u == u2 + (h * u1)
+
+    return valid_kfrag_signature \
+           & correct_reencryption_of_e \
+           & correct_reencryption_of_v \
+           & correct_rk_commitment
+
+
 def verify_kfrag(kfrag,
                 pubkey_a_point,
                 pubkey_b_point,