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The purpose of the Nymble project is to allow for responsible, anonymous access online. It provides a mechanism for server administrators to block misbehaving users while allowing for honest users to stay anonymous; in fact even the blocked users remain anonymous.

The name "Nymble" comes from a play on the word "pseudonym" and "nimble". Instead of giving users a simple pseudonym, the Nymble system assigns users "nymbles"; that is, a pseudonym with better anonymity properties.

The Problem: Abuse of Anonymizing Networks

Tor is an anonymizing network—it hides a client's identity (actually, your computer's IP address) from the servers that it accesses. Tor keeps a client's IP-address anonymous by bouncing its data packets through a random path of relays. Each relay knows only of the relay that sent it data and the next relay in the random path. As long as the entry and exit nodes do not collude, the client's connections remain anonymous.

Tor provides anonymity, but some people abuse this anonymity. Since website administrators depend on blocking the IP addresses of misbehaving users, they are unable to block misbehaving users who connect through Tor—their IP address is hidden after all. Frustrated by repeated offenses through the Tor network, the usual response for websites such as Slashdot and Wikipedia is to block the entire Tor network. This is hardly an optimal solution, as honest users are denied anonymous access to these websites through Tor (or any anonymizing network for that matter). For an extensive list of the many legitimate uses of Tor, see Who uses Tor?

The Solution: Using Nymble for Blacklisting Anonymous Users

By providing a mechanism for server administrators to block anonymous misbehaving users, we hope to make the use of anonymizing networks such as Tor more acceptable for server administrators everywhere. All users remain anonymous— misbehaving users can be blocked without deanonymization, and their activity prior to being blocked remain unlinkable (anonymous).

How Nymble Works
Nymble is based on two administratively-separate "manager" servers, the Pseudonym Manager (PM) and the Nymble Manager (NM). The PM is responsible for pairing a user's IP address with a pseudonym deterministically generated based on the user's IP address. The NM pairs a user's pseudonym with the target server. As long as the two managers are not colluding, the user's connections remain anonymous to the PM, pseudonymous to the NM (note that the user does not communicate directly with the NM, and connects to the NM through Tor), and anonymous to servers that the user connects to.

Pseudonym Manager

The user (in this case, Alice) must first demonstrate control over a resource, that is the Alice's IP-address. To do this Alice must first connect directly with the PM before receiving a pseudonym. The PM has knowledge of existing Tor routers, and thus can ensure that Alice is communicating with it directly. Note that the PM has no knowledge of the user's destination, similar to the entry node in Tor. The PM's sole responsibility it to map IP addresses to pseudonyms. The reason for this is explained next.

Nymble Manager

Alice then connects to the NM through Tor presenting her pseudonym and her target server. The NM does not know the IP address of the user, but the pseudonym provided by the PM guarantees that some unique IP address maps to the pseudonym. She receives a set of nymble tickets as her credential for the target server. These nymble tickets are unlinkable, and therefore Alice can present these nymble tickets (once each) to gain anonymous access at the target server.