I wonder. Assume you do not have arqbackup, and--for the sake of your short open format comment--arq_restore is outdated. You have downloaded all the data chunks (the many, many, many arq creates), how do you go by to decrypt it and get the original data in working state?
I'm not sure what you mean by outdated; it seems to work fine. If not, you read https://www.arqbackup.com/s3_data_format.txt and write your own. In contrast, what do you do with your Crashplan data if you don't have access to Crashplan? It's lost.