import json


def extract_entanglement_data(block_data):
    """
    Extract and list the entangling information from the blocks header
    Args:
        block_data(str): A data block with an entanglement header
    Returns:
        list((str, int)): A list of the blocks used for entanglement
    """
    header_delimiter = chr(29)
    pos = block_data.find(header_delimiter)
    if pos <= 0:
        return ""
    raw_header = block_data[:pos]
    formatted_header = json.loads(raw_header)
    return formatted_header