Settle on notation: is_square(y), has_square_y(P)

2 files changed, 19 insertions, 17 deletions

diff --git a/bip-schnorr/reference.py b/bip-schnorr/reference.py
index e5d9557..7de37c2 100644
--- a/bip-schnorr/reference.py
+++ b/bip-schnorr/reference.py
@@ -11,6 +11,9 @@ def tagged_hash(tag, msg):
     tag_hash = hashlib.sha256(tag.encode()).digest()
     return hashlib.sha256(tag_hash + tag_hash + msg).digest()
 
+def is_infinity(P):
+    return P is None
+
 def x(P):
     return P[0]
 
@@ -59,11 +62,11 @@ def int_from_bytes(b):
 def hash_sha256(b):
     return hashlib.sha256(b).digest()
 
-def jacobi(x):
-    return pow(x, (p - 1) // 2, p)
+def is_square(x):
+    return pow(x, (p - 1) // 2, p) == 1
 
-def is_quad(x):
-    return jacobi(x) == 1
+def has_square_y(P):
+    return not is_infinity(P) and is_square(y(P))
 
 def pubkey_gen(seckey):
     x = int_from_bytes(seckey)
@@ -79,12 +82,12 @@ def schnorr_sign(msg, seckey0):
     if not (1 <= seckey0 <= n - 1):
         raise ValueError('The secret key must be an integer in the range 1..n-1.')
     P = point_mul(G, seckey0)
-    seckey = seckey0 if is_quad(y(P)) else n - seckey0
+    seckey = seckey0 if has_square_y(P) else n - seckey0
     k0 = int_from_bytes(tagged_hash("BIPSchnorrDerive", bytes_from_int(seckey) + msg)) % n
     if k0 == 0:
         raise RuntimeError('Failure. This happens only with negligible probability.')
     R = point_mul(G, k0)
-    k = n - k0 if not is_quad(y(R)) else k0
+    k = n - k0 if not has_square_y(R) else k0
     e = int_from_bytes(tagged_hash("BIPSchnorr", bytes_from_point(R) + bytes_from_point(P) + msg)) % n
     return bytes_from_point(R) + bytes_from_int((k + e * seckey) % n)
 
@@ -104,7 +107,7 @@ def schnorr_verify(msg, pubkey, sig):
         return False
     e = int_from_bytes(tagged_hash("BIPSchnorr", sig[0:32] + pubkey + msg)) % n
     R = point_add(point_mul(G, s), point_mul(P, n - e))
-    if R is None or not is_quad(y(R)) or x(R) != r:
+    if R is None or not has_square_y(R) or x(R) != r:
         return False
     return True
 
diff --git a/bip-schnorr/test-vectors.py b/bip-schnorr/test-vectors.py
index 52e3b05..768d9f1 100644
--- a/bip-schnorr/test-vectors.py
+++ b/bip-schnorr/test-vectors.py
@@ -30,10 +30,10 @@ def vector2():
     msg = bytes_from_int(0x5E2D58D8B3BCDF1ABADEC7829054F90DDA9805AAB56C77333024B9D0A508B75C)
     sig = schnorr_sign(msg, seckey)
 
-    # This singature vector would not verify if the implementer checked the
-    # jacobi symbol of the X coordinate of R instead of the Y coordinate.
+    # This signature vector would not verify if the implementer checked the
+    # squareness of the X coordinate of R instead of the Y coordinate.
     R = point_from_bytes(sig[0:32])
-    assert(jacobi(R[0]) != 1)
+    assert(not is_square(R[0]))
 
     return (bytes_from_int(seckey), pubkey_gen(seckey), msg, sig, "TRUE", None)
 
@@ -43,15 +43,14 @@ def vector3():
     sig = schnorr_sign(msg, seckey)
     return (bytes_from_int(seckey), pubkey_gen(seckey), msg, sig, "TRUE", "test fails if msg is reduced modulo p or n")
 
-# Signs with a given nonce. Results in an invalid signature if y(kG) is not a
-# quadratic residue.
+# Signs with a given nonce. Results in an invalid signature if y(kG) is not a square
 def schnorr_sign_fixed_nonce(msg, seckey0, k):
     if len(msg) != 32:
         raise ValueError('The message must be a 32-byte array.')
     if not (1 <= seckey0 <= n - 1):
         raise ValueError('The secret key must be an integer in the range 1..n-1.')
     P = point_mul(G, seckey0)
-    seckey = seckey0 if (jacobi(P[1]) == 1) else n - seckey0
+    seckey = seckey0 if has_square_y(P) else n - seckey0
     R = point_mul(G, k)
     e = int_from_bytes(tagged_hash("BIPSchnorr", bytes_from_point(R) + bytes_from_point(P) + msg)) % n
     return bytes_from_point(R) + bytes_from_int((k + e * seckey) % n)
@@ -84,9 +83,9 @@ def vector6():
     k = 3
     sig = schnorr_sign_fixed_nonce(msg, seckey, k)
 
-    # Y coordinate of R is not a quadratic residue
+    # Y coordinate of R is not a square
     R = point_mul(G, k)
-    assert(jacobi(R[1]) != 1)
+    assert(not has_square_y(R))
 
     return (None, pubkey_gen(seckey), msg, sig, "FALSE", "incorrect R residuosity")
 
@@ -121,7 +120,7 @@ def vector9():
     sig = schnorr_sign_fixed_nonce(msg, seckey, k)
     bytes_from_point.__code__ = bytes_from_point_tmp
 
-    return (None, pubkey_gen(seckey), msg, sig, "FALSE", "sG - eP is infinite. Test fails in single verification if jacobi(y(inf)) is defined as 1 and x(inf) as 0")
+    return (None, pubkey_gen(seckey), msg, sig, "FALSE", "sG - eP is infinite. Test fails in single verification if has_square_y(inf) is defined as true and x(inf) as 0")
 
 def bytes_from_point_inf1(P):
     if P == None:
@@ -140,7 +139,7 @@ def vector10():
     sig = schnorr_sign_fixed_nonce(msg, seckey, k)
     bytes_from_point.__code__ = bytes_from_point_tmp
 
-    return (None, pubkey_gen(seckey), msg, sig, "FALSE", "sG - eP is infinite. Test fails in single verification if jacobi(y(inf)) is defined as 1 and x(inf) as 1")
+    return (None, pubkey_gen(seckey), msg, sig, "FALSE", "sG - eP is infinite. Test fails in single verification if has_square_y(inf) is defined as true and x(inf) as 1")
 
 # It's cryptographically impossible to create a test vector that fails if run
 # in an implementation which merely misses the check that sig[0:32] is an X