Node Keys in Tahoe-LAFS

“Node Keys” are cryptographic signing/verifying keypairs used to identify Tahoe-LAFS nodes (client-only and client+server). The private signing key is stored in NODEDIR/private/node.privkey , and is used to sign the announcements that are distributed to all nodes by the Introducer. The public verifying key is used to identify the sending node from those other systems: it is displayed as a “Node ID” that looks like “v0-abc234xyz567..”, which ends with a long base32-encoded string.

These node keys were introduced in the 1.10 release (April 2013), as part of ticket #466. In previous releases, announcements were unsigned, and nodes were identified by their Foolscap “Tub ID” (a somewhat shorter base32 string, with no “v0-” prefix).

Why Announcements Are Signed

All nodes (both client-only and client+server) publish announcements to the Introducer, which then relays them to all other nodes. These announcements contain information about the publishing node’s nickname, how to reach the node, what services it offers, and what version of code it is running.

The new private node key is used to sign these announcements, preventing the Introducer from modifying their contents en-route. This will enable future versions of Tahoe-LAFS to use other forms of introduction (gossip, multiple introducers) without weakening the security model.

The Node ID is useful as a handle with which to talk about a node. For example, when clients eventually gain the ability to control which storage servers they are willing to use (#467), the configuration file might simply include a list of Node IDs for the approved servers.

TubIDs are currently also suitable for this job, but they depend upon having a Foolscap connection to the server. Since our goal is to move away from Foolscap towards a simpler (faster and more portable) protocol, we want to reduce our dependence upon TubIDs. Node IDs and Ed25519 signatures can be used for non-Foolscap non-SSL based protocols.

How The Node ID Is Computed

The long-form Node ID is the Ed25519 public verifying key, 256 bits (32 bytes) long, base32-encoded, with a “v0-” prefix appended, and the trailing “=” padding removed, like so:


The Node ID is displayed in this long form on the node’s front Welcome page, and on the Introducer’s status page. In most other places (share-placement lists, file health displays), the “short form” is used instead. This is simply the first 8 characters of the base32 portion, frequently enclosed in square brackets, like this:


In contrast, old-style TubIDs are usually displayed with just 6 base32 characters.

Version Compatibility, Fallbacks For Old Versions

Since Tahoe-LAFS 1.9 does not know about signed announcements, 1.10 includes backwards-compatibility code to allow old and new versions to interoperate. There are three relevant participants: the node publishing an announcement, the Introducer which relays them, and the node receiving the (possibly signed) announcement.

When a 1.10 node connects to an old Introducer (version 1.9 or earlier), it sends downgraded non-signed announcements. It likewise accepts non-signed announcements from the Introducer. The non-signed announcements use TubIDs to identify the sending node. The new 1.10 Introducer, when it connects to an old node, downgrades any signed announcements to non-signed ones before delivery.

As a result, the only way to receive signed announcements is for all three systems to be running the new 1.10 code. In a grid with a mixture of old and new nodes, if the Introducer is old, then all nodes will see unsigned TubIDs. If the Introducer is new, then nodes will see signed Node IDs whenever possible.

Share Placement

Tahoe-LAFS uses a “permuted ring” algorithm to decide where to place shares for any given file. For each potential server, it uses that server’s “permutation seed” to compute a pseudo-random but deterministic location on a ring, then walks the ring in clockwise order, asking each server in turn to hold a share until all are placed. When downloading a file, the servers are accessed in the same order. This minimizes the number of queries that must be done to download a file, and tolerates “churn” (nodes being added and removed from the grid) fairly well.

This property depends upon server nodes having a stable permutation seed. If a server’s permutation seed were to change, it would effectively wind up at a randomly selected place on the permuted ring. Downloads would still complete, but clients would spend more time asking other servers before querying the correct one.

In the old 1.9 code, the permutation-seed was always equal to the TubID. In 1.10, servers include their permutation-seed as part of their announcement. To improve stability for existing grids, if an old server (one with existing shares) is upgraded to run the 1.10 codebase, it will use its old TubID as its permutation-seed. When a new empty server runs the 1.10 code, it will use its Node ID instead. In both cases, once the node has picked a permutation-seed, it will continue using that value forever.

To be specific, when a node wakes up running the 1.10 code, it will look for a recorded NODEDIR/permutation-seed file, and use its contents if present. If that file does not exist, it creates it (with the TubID if it has any shares, otherwise with the Node ID), and uses the contents as the permutation-seed.

There is one unfortunate consequence of this pattern. If new 1.10 server is created in a grid that has an old client, or has a new client but an old Introducer, then that client will see downgraded non-signed announcements, and thus will first upload shares with the TubID-based permutation-seed. Later, when the client and/or Introducer is upgraded, the client will start seeing signed announcements with the NodeID-based permutation-seed, and will then look for shares in the wrong place. This will hurt performance in a large grid, but should not affect reliability. This effect shouldn’t even be noticeable in grids for which the number of servers is close to the “N” number (e.g. where num-servers < 3*N). And the as-yet-unimplemented “share rebalancing” feature should repair the misplacement.

If you wish to avoid this effect, try to upgrade both Introducers and clients at about the same time. (Upgrading servers does not matter: they will continue to use the old permutation-seed).