BIP324 updates

Includes:
* Simpler (but equivalent) ElligatorSwift encoding function & spec
* Improved test vectors
* Test vector generation code
* Code for converting test vectors for libsecp256k1 code.
* Code for running test vectors against SwiftEC paper authors' code.
* Miscellaneous reference code improvements (style, comments).

bip-0324/test_sage_decoding.py

@@ -0,0 +1,78 @@
+"""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']