How it works

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.

Here's how it works:

A "storage grid" is made up of one or more storage servers. In the case of S4, the grid has one server which is run by Least Authority, and configured to store its data in Amazon Simple Storage Service (S3). A "gateway" uses the storage servers and provides access to the filesystem over HTTP(S), SFTP, or FTP.

Users do not rely on storage servers to provide confidentiality nor integrity for their data -- instead all of the data is encrypted and integrity-checked by the gateway, so that the servers can neither read nor modify the contents of the files. Users do rely on storage servers for availability.

In the typical deployment mode each user runs her own gateway on her own machine. This way she relies on her own machine for the confidentiality and integrity of the data.

An alternate deployment mode is that the gateway runs on a remote machine and the user connects to it over HTTPS or SFTP. This means that the operator of the gateway can view and modify the user's data (the user relies on the gateway for confidentiality and integrity), but the advantage is that the user can access the filesystem with a client that doesn't have the gateway software installed, such as a cell phone.

LAFS also supports access control. In LAFS, there are two kinds of files: immutable and mutable. When you upload a file to the storage grid you can choose which kind of file it will be in the grid. Immutable files can't be modified once they have been uploaded. A mutable file can be modified by someone with read-write access to it. A user can have read-write access to a mutable file or read-only access to it, or no access to it at all.

A user who has read-write access to a mutable file or directory can give another user read-write access to that file or directory, or they can give read-only access to that file or directory. A user who has read-only access to a file or directory can give another user read-only access to it.

When linking a file or directory into a parent directory, you can use a read-write link or a read-only link. If you use a read-write link, then anyone who has read-write access to the parent directory can gain read-write access to the child, and anyone who has read-only access to the parent directory can gain read-only access to the child. If you use a read-only link, then anyone who has either read-write or read-only access to the parent directory can gain read-only access to the child.

For more technical detail, please see the LAFS project's docs page.

What is verifiable end-to-end security?

Every seller of cloud storage services will tell you that their service is "secure". But what they mean by that is something fundamentally different from what we mean. What they mean by "secure" is that after you've given them the power to read and modify your data, they try really hard not to let this power be abused. This turns out to be difficult! Bugs, misconfigurations, or operator error can accidentally expose your data to another customer or to the public, or can corrupt your data. Criminals routinely gain illicit access to corporate servers. Even more insidious is the fact that the employees themselves sometimes violate customer privacy out of carelessness, avarice, or mere curiousity. The most conscientious of these service providers spend considerable effort and expense trying to mitigate these risks. However, some privacy breach is inevitable--if by nothing else, then by government order via programs like PRISM.

What we mean by "security" is something different. The service provider never has the ability to read or modify your data in the first place: never. If you use S4, then all of the threats described above are non-issues to you. Not only is it easy and inexpensive for the service provider to maintain the security of your data, but in fact they couldn't violate its security if they tried. This is what we call provider-independent security.

This guarantee is integrated naturally into S4 (and LAFS, on which S4 is based) and and doesn't require you to perform a manual pre-encryption step or cumbersome key management. After all, having to do cumbersome manual operations when storing or accessing your data would nullify one of the primary benefits of using cloud storage in the first place: convenience.

What is Least-Authority Backup good for?

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.

How secure is my credit or debit card information? does not hold any credit or debit card information on our servers. When you sign up, a connection is made to our payment processor, Stripe, which holds your card information on's behalf. Stripe is certified to PCI Service Provider Level 1, the most stringent certification level. The connections to both and the Stripe API are made over TLS (with forward secrecy in modern browsers), so the security of your card information is at least as good as for other e-commerce sites.

Are there limits on how much data I can store?

There are no limits on the total data you can store, but there is currently a limit on the size of an individual file. Files larger than about 3 GB might fail to upload. All upload failures, including failures caused by this, are shown in your LAFS client software, so you'll know if this happens to you.

We are working on lifting this limitation and will keep you informed about our progress.

Can I delete old files to save space?

For now, it is not possible to remove data from the LAE service. If you unlink a file, you lose your ability to access it, but it does not free up the space in your S3 bucket, so you still get charged for it.

We are working on fixing this issue and will keep you informed about our progress.

What is the relationship between and the LAFS open-source project?

Zooko Wilcox-O'Hearn, founder and CEO of Least Authority Enterprises, was one of the inventors of LAFS. Least Authority Enterprises contributes all of the code we write to the free and open-source software project. We believe that's the best thing for our customers.

The Python source code for this website, our automation and monitoring scripts, and our fork of LAFS to support Amazon S3 are all open-sourced under the same licenses as LAFS itself. The S3 backend depends on a fork of the txaws library that is open-sourced under the MIT / X / Expat License.

To obtain this source code, you need to install git. Then use:

  git clone -b 2237-cloud-backend-s4
  git clone
  git clone

How can I communicate securely with

If you are able to use PGP, here is our public key: Its fingerprint is 9007 F6BF 5E17 416A 3B7E 57E7 2374 FCC9 1B2B 5E6B.

How do I start?

Go to our S4 subscription page to provide your payment details.