import re


def find_checksum(s):
    """Accepts a manifest-style filename and returns the embedded
    checksum string, or None if no such string could be found."""
    m = re.search(r"\b([a-z0-9]{32})\b", s, flags=re.IGNORECASE)
    md5 = m.group(1).lower() if m else None
    return md5