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/secp256k1_test_vectors.py

@@ -0,0 +1,52 @@
+"""Convert the BIP-324 test vectors to secp256k1 code."""
+
+import csv
+import reference
+import os
+import sys
+
+FILENAME_XSWIFTEC_INV_TEST = os.path.join(sys.path[0], 'xswiftec_inv_test_vectors.csv')
+FILENAME_ELLSWIFT_DECODE_TEST = os.path.join(sys.path[0], 'ellswift_decode_test_vectors.csv')
+
+def format_int(v):
+    """Format 0 as "0", but other integers as 0x%08x."""
+    if v == 0:
+        return "0"
+    return f"0x{v:08x}"
+
+def format_fe(fe):
+    """Format a field element constant as SECP256K1_FE_CONST code."""
+    vals = [(int(fe) >> (32 * (7 - i))) & 0xffffffff for i in range(8)]
+    strs = ", ".join(format_int(v) for v in vals)
+    return f"SECP256K1_FE_CONST({strs})"
+
+def output_xswiftec_inv_cases():
+    """Generate lines corresponding to the xswiftec_inv test cases."""
+    with open(FILENAME_XSWIFTEC_INV_TEST, newline='', encoding='utf-8') as csvfile:
+        reader = csv.DictReader(csvfile)
+        print("xswiftec_inv cases:")
+        for row in reader:
+            u = int.from_bytes(bytes.fromhex(row['u']), 'big')
+            x = int.from_bytes(bytes.fromhex(row['x']), 'big')
+            pat = sum(1<<c for c in range(8) if row[f"case{c}_t"])
+            tstrs = []
+            for c in range(8):
+                tstrs.append(format_fe(int.from_bytes(bytes.fromhex(row[f"case{c}_t"]), 'big')))
+            print(f"    {{0x{pat:02x}, {format_fe(u)}, {format_fe(x)}, {{{', '.join(tstrs)}}}}},")
+    print()
+
+def output_ellswift_decode_cases():
+    """Generate lines corresponding to the ellswift_decode test cases."""
+    with open(FILENAME_ELLSWIFT_DECODE_TEST, newline='', encoding='utf-8') as csvfile:
+        reader = csv.DictReader(csvfile)
+        print("ellswift_decode cases:")
+        for row in reader:
+            enc = bytes.fromhex(row['ellswift'])
+            tval = int.from_bytes(enc[32:], 'big') % reference.FE.SIZE
+            x = int.from_bytes(bytes.fromhex(row['x']), 'big')
+            encstr = ", ".join(f"0x{b:02x}" for b in enc)
+            print(f"    {{{{{encstr}}}, {format_fe(x)}, {tval & 1}}},")
+    print()
+
+output_xswiftec_inv_cases()
+output_ellswift_decode_cases()