def retrieve_cors_header(response, key):
    """
    Method to retrieve a CORS header value from a HTTP response.

    :param response: A HTTPResponse object.
    :param key: A key representing the desired header value to retrieve.
    :return: The header value or None if the header do not exists.
    """
    headers = response.get_headers()

    for header_name in headers:
        if header_name.upper().strip() == key.upper():
            return headers[header_name].strip()

    return None