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+"""Compare ellswift decoding in the BIP-324 test vectors against the SwiftEC reference code.
+
+Instructions:
+
+* Clone the SwiftEC repository, and enter the directory:
+
+ git clone https://github.com/Jchavezsaab/SwiftEC
+ git checkout 5320a25035d91addde29d14164cce684b56a12ed
+ cd SwiftEC
+
+* Generate parameters for the secp256k1 curve:
+
+ sage --python generate_parameters.py -p secp256k1
+
+* Copy over this file and the CSV test vectors:
+
+ cp PATH_TO_BIPS_REPO/bips/bip-0324/{*.csv,test_sage_decoding.py} .
+
+* Run the tests:
+
+ sage --python test_sage_decoding.py -p secp256k1
+
+No output = good.
+"""
+
+import sys
+import csv
+from config import F
+from Xencoding_0 import Xdecode
+
+
+FILENAME_PACKET_TEST = 'packet_encoding_test_vectors.csv'
+FILENAME_XSWIFTEC_INV_TEST = 'xswiftec_inv_test_vectors.csv'
+FILENAME_ELLSWIFT_DECODE_TEST = 'ellswift_decode_test_vectors.csv'
+
+def ellswift_decode_sage(ellswift):
+ """Given a 64-byte ellswift encoded public key, get the 32-byte X coordinate."""
+
+ u = F(int.from_bytes(ellswift[:32], 'big'))
+ t = F(int.from_bytes(ellswift[32:], 'big'))
+
+ # Reimplement the input correction step.
+ if u == F(0):
+ u = F(1)
+ if t == F(0):
+ t = F(1)
+ if u**3 + t**2 + 7 == F(0):
+ t = F(2) * t
+
+ # Invoke reference code
+ x, z = Xdecode(u, t)
+
+ # Convert to bytes.
+ return int(x / z).to_bytes(32, 'big')
+
+with open(FILENAME_PACKET_TEST, newline='', encoding='utf-8') as csvfile:
+ reader = csv.DictReader(csvfile)
+ for row in reader:
+ bytes_ellswift_ours = bytes.fromhex(row['in_ellswift_ours'])
+ bytes_ellswift_theirs = bytes.fromhex(row['in_ellswift_theirs'])
+ assert row['mid_x_ours'] == ellswift_decode_sage(bytes_ellswift_ours).hex()
+ assert row['mid_x_theirs'] == ellswift_decode_sage(bytes_ellswift_theirs).hex()
+
+with open(FILENAME_XSWIFTEC_INV_TEST, newline='', encoding='utf-8') as csvfile:
+ reader = csv.DictReader(csvfile)
+ for row in reader:
+ udat = bytes.fromhex(row['u'])
+ xdat = bytes.fromhex(row['x'])
+ for case in range(8):
+ tdat = bytes.fromhex(row[f"case{case}_t"])
+ if tdat:
+ assert ellswift_decode_sage(udat + tdat) == xdat
+
+with open(FILENAME_ELLSWIFT_DECODE_TEST, newline='', encoding='utf-8') as csvfile:
+ reader = csv.DictReader(csvfile)
+ for row in reader:
+ ellswift = bytes.fromhex(row['ellswift'])
+ assert ellswift_decode_sage(ellswift).hex() == row['x']