Fixes an ECDSA signature performed by OpenSSL.

fix-signature.py

#! /usr/bin/env python
import sys

if not sys.stdin.isatty():
  hex = sys.stdin.read()
elif len(sys.argv) < 2:
	print(f"usage: {sys.argv[0]} hex_signature", file=sys.stderr)
	exit(1)
else:
	hex = sys.argv[1]

b = bytes.fromhex(hex)
n = 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141; rSize = b[3]; s = int.from_bytes(b[rSize + 6:], 'big')
if s <= n / 2:
	print(b.hex())
else:
	s = n - s
	if s < 0: # pad
		s = f"00{-s:02x}"
	else:
		s = f"{s:02x}"

	size = rSize + len(s) // 2 + 4
	print(f"30{size:02x}{b[2:5 + rSize].hex()}{len(s) // 2:02x}{s}")