Function gcs_request_signature 

pub fn gcs_request_signature(
    secret: &str,
    date: &str,
    region: &str,
    message: &[u8],
) -> [u8; 32]

Google V4 HMAC signature using the account’s interoperability secret. Date and region are the exact credential-scope components in the request. Intermediate signing keys and the prefixed secret are erased on drop.