BIP 340: Update reference code and test vectors as follows:

 - use evenness as tiebreaker
 - using different tags for nonce- and challenge hashing
 - add pubkey to nonce function.

3 files changed, 66 insertions, 41 deletions

diff --git a/bip-0340/reference.py b/bip-0340/reference.py
index f2a944f..9b5592e 100644
--- a/bip-0340/reference.py
+++ b/bip-0340/reference.py
@@ -51,7 +51,7 @@ def bytes_from_int(x):
 def bytes_from_point(P):
     return bytes_from_int(x(P))
 
-def point_from_bytes(b):
+def lift_x_square_y(b):
     x = int_from_bytes(b)
     if x >= p:
         return None
@@ -61,6 +61,13 @@ def point_from_bytes(b):
         return None
     return [x, y]
 
+def lift_x_even_y(b):
+    P = lift_x_square_y(b)
+    if P is None:
+        return None
+    else:
+        return [x(P), y(P) if y(P) % 2 == 0 else p - y(P)]
+
 def int_from_bytes(b):
     return int.from_bytes(b, byteorder="big")
 
@@ -73,6 +80,9 @@ def is_square(x):
 def has_square_y(P):
     return not is_infinity(P) and is_square(y(P))
 
+def has_even_y(P):
+    return y(P) % 2 == 0
+
 def pubkey_gen(seckey):
     x = int_from_bytes(seckey)
     if not (1 <= x <= n - 1):
@@ -87,13 +97,13 @@ 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 has_square_y(P) else n - seckey0
-    k0 = int_from_bytes(tagged_hash("BIPSchnorrDerive", bytes_from_int(seckey) + msg)) % n
+    seckey = seckey0 if has_even_y(P) else n - seckey0
+    k0 = int_from_bytes(tagged_hash("BIP340/nonce", bytes_from_int(seckey) + bytes_from_point(P) + msg)) % n
     if k0 == 0:
         raise RuntimeError('Failure. This happens only with negligible probability.')
     R = point_mul(G, 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
+    e = int_from_bytes(tagged_hash("BIP340/challenge", bytes_from_point(R) + bytes_from_point(P) + msg)) % n
     return bytes_from_point(R) + bytes_from_int((k + e * seckey) % n)
 
 def schnorr_verify(msg, pubkey, sig):
@@ -103,14 +113,14 @@ def schnorr_verify(msg, pubkey, sig):
         raise ValueError('The public key must be a 32-byte array.')
     if len(sig) != 64:
         raise ValueError('The signature must be a 64-byte array.')
-    P = point_from_bytes(pubkey)
+    P = lift_x_even_y(pubkey)
     if (P is None):
         return False
     r = int_from_bytes(sig[0:32])
     s = int_from_bytes(sig[32:64])
     if (r >= p or s >= n):
         return False
-    e = int_from_bytes(tagged_hash("BIPSchnorr", sig[0:32] + pubkey + msg)) % n
+    e = int_from_bytes(tagged_hash("BIP340/challenge", sig[0:32] + pubkey + msg)) % n
     R = point_add(point_mul(G, s), point_mul(P, n - e))
     if R is None or not has_square_y(R) or x(R) != r:
         return False
diff --git a/bip-0340/test-vectors.csv b/bip-0340/test-vectors.csv
index 3970803..096fe2e 100644
--- a/bip-0340/test-vectors.csv
+++ b/bip-0340/test-vectors.csv
@@ -1,16 +1,16 @@
 index,secret key,public key,message,signature,verification result,comment

-0,0000000000000000000000000000000000000000000000000000000000000001,79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798,0000000000000000000000000000000000000000000000000000000000000000,528F745793E8472C0329742A463F59E58F3A3F1A4AC09C28F6F8514D4D0322A258BD08398F82CF67B812AB2C7717CE566F877C2F8795C846146978E8F04782AE,TRUE,

-1,B7E151628AED2A6ABF7158809CF4F3C762E7160F38B4DA56A784D9045190CFEF,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,667C2F778E0616E611BD0C14B8A600C5884551701A949EF0EBFD72D452D64E844160BCFC3F466ECB8FACD19ADE57D8699D74E7207D78C6AEDC3799B52A8E0598,TRUE,

-2,C90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B14E5C9,DD308AFEC5777E13121FA72B9CC1B7CC0139715309B086C960E18FD969774EB8,5E2D58D8B3BCDF1ABADEC7829054F90DDA9805AAB56C77333024B9D0A508B75C,2D941B38E32624BF0AC7669C0971B990994AF6F9B18426BF4F4E7EC10E6CDF386CF646C6DDAFCFA7F1993EEB2E4D66416AEAD1DDAE2F22D63CAD901412D116C6,TRUE,

-3,0B432B2677937381AEF05BB02A66ECD012773062CF3FA2549E44F58ED2401710,25D1DFF95105F5253C4022F628A996AD3A0D95FBF21D468A1B33F8C160D8F517,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF,8BD2C11604B0A87A443FCC2E5D90E5328F934161B18864FB48CE10CB59B45FB9B5B2A0F129BD88F5BDC05D5C21E5C57176B913002335784F9777A24BD317CD36,TRUE,test fails if msg is reduced modulo p or n

-4,,D69C3509BB99E412E68B0FE8544E72837DFA30746D8BE2AA65975F29D22DC7B9,4DF3C3F68FCC83B27E9D42C90431A72499F17875C81A599B566C9889B9696703,00000000000000000000003B78CE563F89A0ED9414F5AA28AD0D96D6795F9C63EE374AC7FAE927D334CCB190F6FB8FD27A2DDC639CCEE46D43F113A4035A2C7F,TRUE,

-5,,EEFDEA4CDB677750A420FEE807EACF21EB9898AE79B9768766E4FAA04A2D4A34,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,667C2F778E0616E611BD0C14B8A600C5884551701A949EF0EBFD72D452D64E844160BCFC3F466ECB8FACD19ADE57D8699D74E7207D78C6AEDC3799B52A8E0598,FALSE,public key not on the curve

-6,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,F9308A019258C31049344F85F89D5229B531C845836F99B08601F113BCE036F9935554D1AA5F0374E5CDAACB3925035C7C169B27C4426DF0A6B19AF3BAEAB138,FALSE,has_square_y(R) is false

-7,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,10AC49A6A2EBF604189C5F40FC75AF2D42D77DE9A2782709B1EB4EAF1CFE9108D7003B703A3499D5E29529D39BA040A44955127140F81A8A89A96F992AC0FE79,FALSE,negated message

-8,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,667C2F778E0616E611BD0C14B8A600C5884551701A949EF0EBFD72D452D64E84BE9F4303C0B9913470532E6521A827951D39F5C631CFD98CE39AC4D7A5A83BA9,FALSE,negated s value

-9,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,000000000000000000000000000000000000000000000000000000000000000099D2F0EBC2996808208633CD9926BF7EC3DAB73DAAD36E85B3040A698E6D1CE0,FALSE,sG - eP is infinite. Test fails in single verification if has_square_y(inf) is defined as true and x(inf) as 0

-10,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,000000000000000000000000000000000000000000000000000000000000000124E81D89F01304695CE943F7D5EBD00EF726A0864B4FF33895B4E86BEADC5456,FALSE,sG - eP is infinite. Test fails in single verification if has_square_y(inf) is defined as true and x(inf) as 1

-11,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,4A298DACAE57395A15D0795DDBFD1DCB564DA82B0F269BC70A74F8220429BA1D4160BCFC3F466ECB8FACD19ADE57D8699D74E7207D78C6AEDC3799B52A8E0598,FALSE,sig[0:32] is not an X coordinate on the curve

-12,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F4160BCFC3F466ECB8FACD19ADE57D8699D74E7207D78C6AEDC3799B52A8E0598,FALSE,sig[0:32] is equal to field size

-13,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,667C2F778E0616E611BD0C14B8A600C5884551701A949EF0EBFD72D452D64E84FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141,FALSE,sig[32:64] is equal to curve order

-14,,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC30,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,667C2F778E0616E611BD0C14B8A600C5884551701A949EF0EBFD72D452D64E844160BCFC3F466ECB8FACD19ADE57D8699D74E7207D78C6AEDC3799B52A8E0598,FALSE,public key is not a valid X coordinate because it exceeds the field size

+0,0000000000000000000000000000000000000000000000000000000000000003,F9308A019258C31049344F85F89D5229B531C845836F99B08601F113BCE036F9,0000000000000000000000000000000000000000000000000000000000000000,514F0E96BB9AD56A245A7F4ED1030D4DE3FB0F5DE285116514292B2F910C979201D5C686A9D968E169C3ED1C2249C81F2BD27D53C42D15FA275EA6445389410A,TRUE,

+1,B7E151628AED2A6ABF7158809CF4F3C762E7160F38B4DA56A784D9045190CFEF,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,74556372D3369E8C53E6B84B5D7EE9AE0220EB37A6EA5501EF828FBFBA90A864092EF727796DACA51118BE8FBD70B3EC50536E65DB6F3B3B3FE1049862018B02,TRUE,

+2,C90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B14E5C9,DD308AFEC5777E13121FA72B9CC1B7CC0139715309B086C960E18FD969774EB8,7E2D58D8B3BCDF1ABADEC7829054F90DDA9805AAB56C77333024B9D0A508B75C,FAD73AE779EDD67BA40772867FEF9F20F151EB4BFDDECC53B90DD3017FC5D6035670DB8C83BA96EAF51C069B2AA7CEEF556787AE897F84F8D822C4ED7115B851,TRUE,

+3,0B432B2677937381AEF05BB02A66ECD012773062CF3FA2549E44F58ED2401710,25D1DFF95105F5253C4022F628A996AD3A0D95FBF21D468A1B33F8C160D8F517,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF,60DC1A7E50D2269424060FF66361601075EB4B516DE89BF1D91B1D6AD78900DACDA5AC4B697491430CAA7604C8D819B2150DEC26E8D01E2981DDA071D7556CD3,TRUE,test fails if msg is reduced modulo p or n

+4,,D69C3509BB99E412E68B0FE8544E72837DFA30746D8BE2AA65975F29D22DC7B9,4DF3C3F68FCC83B27E9D42C90431A72499F17875C81A599B566C9889B9696703,00000000000000000000003B78CE563F89A0ED9414F5AA28AD0D96D6795F9C630EC50E5363E227ACAC6F542CE1C0B186657E0E0D1A6FFE283A33438DE4738419,TRUE,

+5,,EEFDEA4CDB677750A420FEE807EACF21EB9898AE79B9768766E4FAA04A2D4A34,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,74556372D3369E8C53E6B84B5D7EE9AE0220EB37A6EA5501EF828FBFBA90A864092EF727796DACA51118BE8FBD70B3EC50536E65DB6F3B3B3FE1049862018B02,FALSE,public key not on the curve

+6,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,F9308A019258C31049344F85F89D5229B531C845836F99B08601F113BCE036F995A579DA959FA739FCE39E8BD16FECB5CDCF97060B2C73CDE60E87ABCA1AA5D9,FALSE,has_square_y(R) is false

+7,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,0B3E00AF0641F28B4B52F7E7AD3DDEB9BD313F9E382563BA9C9A8274F45D3D72D8F733F2901432C8DD99C739B0C1EE4030E79A94318278EC4E7160A65CDE8015,FALSE,negated message

+8,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,74556372D3369E8C53E6B84B5D7EE9AE0220EB37A6EA5501EF828FBFBA90A864F6D108D88692535AEEE74170428F4C126A5B6E80D3D965007FF159F46E34B63F,FALSE,negated s value

+9,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,00000000000000000000000000000000000000000000000000000000000000009915EE59F07F9DBBAEDC31BFCC9B34AD49DE669CD24773BCED77DDA36D073EC8,FALSE,sG - eP is infinite. Test fails in single verification if has_square_y(inf) is defined as true and x(inf) as 0

+10,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,0000000000000000000000000000000000000000000000000000000000000001C7EC918B2B9CF34071BB54BED7EB4BB6BAB148E9A7E36E6B228F95DFA08B43EC,FALSE,sG - eP is infinite. Test fails in single verification if has_square_y(inf) is defined as true and x(inf) as 1

+11,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,4A298DACAE57395A15D0795DDBFD1DCB564DA82B0F269BC70A74F8220429BA1D092EF727796DACA51118BE8FBD70B3EC50536E65DB6F3B3B3FE1049862018B02,FALSE,sig[0:32] is not an X coordinate on the curve

+12,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F092EF727796DACA51118BE8FBD70B3EC50536E65DB6F3B3B3FE1049862018B02,FALSE,sig[0:32] is equal to field size

+13,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,74556372D3369E8C53E6B84B5D7EE9AE0220EB37A6EA5501EF828FBFBA90A864FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141,FALSE,sig[32:64] is equal to curve order

+14,,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC30,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,74556372D3369E8C53E6B84B5D7EE9AE0220EB37A6EA5501EF828FBFBA90A864092EF727796DACA51118BE8FBD70B3EC50536E65DB6F3B3B3FE1049862018B02,FALSE,public key is not a valid X coordinate because it exceeds the field size

diff --git a/bip-0340/test-vectors.py b/bip-0340/test-vectors.py
index 195b61b..d4674e9 100644
--- a/bip-0340/test-vectors.py
+++ b/bip-0340/test-vectors.py
@@ -2,14 +2,24 @@ import sys
 from reference import *
 
 def vector0():
-    seckey = bytes_from_int(1)
+    seckey = bytes_from_int(3)
     msg = bytes_from_int(0)
     sig = schnorr_sign(msg, seckey)
     pubkey = pubkey_gen(seckey)
 
-    # The point reconstructed from the public key has an even Y coordinate.
-    pubkey_point = point_from_bytes(pubkey)
-    assert(pubkey_point[1] & 1 == 0)
+    # We should have at least one test vector where the seckey needs to be
+    # negated and one where it doesn't. In this one the seckey doesn't need to
+    # be negated.
+    x = int_from_bytes(seckey)
+    P = point_mul(G, x)
+    assert(y(P) % 2 == 0)
+
+    # For historic reasons (pubkey tiebreaker was squareness and not evenness)
+    # we should have at least one test vector where the the point reconstructed
+    # from the public key has a square and one where it has a non-square Y
+    # coordinate. In this one Y is non-square.
+    pubkey_point = lift_x_even_y(pubkey)
+    assert(not has_square_y(pubkey_point))
 
     return (seckey, pubkey, msg, sig, "TRUE", None)
 
@@ -17,28 +27,33 @@ def vector1():
     seckey = bytes_from_int(0xB7E151628AED2A6ABF7158809CF4F3C762E7160F38B4DA56A784D9045190CFEF)
     msg = bytes_from_int(0x243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89)
     sig = schnorr_sign(msg, seckey)
-    pubkey = pubkey_gen(seckey)
-
-    # The point reconstructed from the public key has an odd Y coordinate.
-    pubkey_point = point_from_bytes(pubkey)
-    assert(pubkey_point[1] & 1 == 1)
-
-    return (seckey, pubkey, msg, sig, "TRUE", None)
+    return (seckey, pubkey_gen(seckey), msg, sig, "TRUE", None)
 
 def vector2():
     seckey = bytes_from_int(0xC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B14E5C9)
-    msg = bytes_from_int(0x5E2D58D8B3BCDF1ABADEC7829054F90DDA9805AAB56C77333024B9D0A508B75C)
+    msg = bytes_from_int(0x7E2D58D8B3BCDF1ABADEC7829054F90DDA9805AAB56C77333024B9D0A508B75C)
     sig = schnorr_sign(msg, seckey)
 
+    # The point reconstructed from the public key has a square Y coordinate.
+    pubkey = pubkey_gen(seckey)
+    pubkey_point = lift_x_even_y(pubkey)
+    assert(has_square_y(pubkey_point))
+
     # 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])
+    R = lift_x_square_y(sig[0:32])
     assert(not is_square(R[0]))
 
-    return (seckey, pubkey_gen(seckey), msg, sig, "TRUE", None)
+    return (seckey, pubkey, msg, sig, "TRUE", None)
 
 def vector3():
     seckey = bytes_from_int(0x0B432B2677937381AEF05BB02A66ECD012773062CF3FA2549E44F58ED2401710)
+
+    # Need to negate this seckey before signing
+    x = int_from_bytes(seckey)
+    P = point_mul(G, x)
+    assert(y(P) % 2 != 0)
+
     msg = bytes_from_int(0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
     sig = schnorr_sign(msg, seckey)
     return (seckey, pubkey_gen(seckey), msg, sig, "TRUE", "test fails if msg is reduced modulo p or n")
@@ -53,9 +68,9 @@ def insecure_schnorr_sign_fixed_nonce(msg, seckey0, k):
     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 has_square_y(P) else n - seckey0
+    seckey = seckey0 if has_even_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
+    e = int_from_bytes(tagged_hash("BIP340/challenge", bytes_from_point(R) + bytes_from_point(P) + msg)) % n
     return bytes_from_point(R) + bytes_from_int((k + e * seckey) % n)
 
 # Creates a singature with a small x(R) by using k = 1/2
@@ -78,7 +93,7 @@ def vector5():
     sig = schnorr_sign(msg, seckey)
 
     pubkey = bytes_from_int(0xEEFDEA4CDB677750A420FEE807EACF21EB9898AE79B9768766E4FAA04A2D4A34)
-    assert(point_from_bytes(pubkey) is None)
+    assert(lift_x_even_y(pubkey) is None)
 
     return (None, pubkey, msg, sig, "FALSE", "public key not on the curve")
 
@@ -156,7 +171,7 @@ def vector11():
 
     # Replace R's X coordinate with an X coordinate that's not on the curve
     x_not_on_curve = bytes_from_int(0x4A298DACAE57395A15D0795DDBFD1DCB564DA82B0F269BC70A74F8220429BA1D)
-    assert(point_from_bytes(x_not_on_curve) is None)
+    assert(lift_x_square_y(x_not_on_curve) is None)
     sig = x_not_on_curve + sig[32:64]
 
     return (None, pubkey_gen(seckey), msg, sig, "FALSE", "sig[0:32] is not an X coordinate on the curve")
@@ -201,10 +216,10 @@ def vector14():
 
     pubkey_int = p + 1
     pubkey = bytes_from_int(pubkey_int)
-    assert(point_from_bytes(pubkey) is None)
+    assert(lift_x_even_y(pubkey) is None)
     # If an implementation would reduce a given public key modulo p then the
     # pubkey would be valid
-    assert(point_from_bytes(bytes_from_int(pubkey_int % p)) is not None)
+    assert(lift_x_even_y(bytes_from_int(pubkey_int % p)) is not None)
 
     return (None, pubkey, msg, sig, "FALSE", "public key is not a valid X coordinate because it exceeds the field size")