S4 is an Amazon S3-based application of Least-Authority File System, or LAFS. LAFS is a free, open source cloud storage system with verifiable end-to-end security. It distributes your data across multiple servers. Even if some of the servers fail or are taken over by an attacker, the entire filesystem continues to function correctly, preserving your privacy and security.
What is Least-Authority Backup good for? More Info »
It is good for securely backing up your data off-site. The "tahoe backup" command inspects your local filesystem for files that have changed since the last time you ran it. It uploads each file that has changed and it creates a directory in LAFS to hold the current "snapshot"—the current version of each of the files. You can browse or access old versions just by browsing the old snapshot directories.
Where is my data stored?
Your data, encrypted, is stored on Amazon's Simple Storage Service (S3), which is a convenient, reliable, and widely understood platform for storage.
Data is replicated according to the following policy decribed in Amazon's Overview of Security Processes:
Data stored in Amazon S3, Amazon SimpleDB, or Amazon Elastic Block Store (EBS) is redundantly stored in multiple physical locations as part of normal operation of those services and at no additional charge. Amazon S3 and Amazon SimpleDB provide object durability by storing objects multiple times across multiple Availability Zones on the initial write and then actively doing further replication in the event of device unavailability or detected bit-rot.
How much do the services cost?
S4 costs $25 per month for unlimited storage, for personal use.