Core Concepts

“All non-trivial abstractions, to some degree, are leaky.”—Joel Spolsky

This chapter describes the intrinsic building blocks that make up neuropil. It is intended to help readers gain a deeper understanding of neuropil’s internals and its design, and ultimately make better use of the library.

The versatile concepts described in this chapter are primarily pervasive in the implementation of the neuropil cybersecurity mesh itself, but curious users of neuropil will most likely be exposed to them, eventually. After all, neuropil’s user facing abstractions, such as Realms, are constructed on top of these building blocks. By nature, primitive concepts escape the abstractions that contain them, and understanding them will demystify the real-world behavior of neuropil applications.


This introduction offers only an informal coverage of the foundations upon which neuropil is built. For an in-depth discussion we refer to the following papers:

The Overlay Network

neuropil decouples routing from the physical network by implementing a so-called overlay network. In this logical network, the physical network addresses of nodes are mapped to randomly chosen, virtual addresses in a 256‑bit address space. This virtual address space is large enough that most of it will never be used, and address collision is virtually impossible.

Overlay network

Any two addresses in the overlay network have a logical, relative distance, taken into account by nodes to coordinate routing. Specifically, a node will maintain connectivity to many nodes with virtual addresses close to itself, and to fewer nodes whose virtual addresses that are far from it. Because it is possible to tell which one of any two nodes is closer to a particular virtual address, scalable, distributed routing can be achieved by forwarding messages to the address closest to the destination. The node at that intermediary address will either pass on the message to its destination, or again forward it to the next closest node it knows, if any.


As an additional property, latency is taken into account for routing decisions. By accounting for latency, inefficient routing topologies that fail to reflect the physical host network can be avoided.

A different, useful routing variant involves intentionally sending messages to destination addresses that may not be occupied by any node at all. In this variant, messages are not routed to their final destination, but to the node that occupies the virtual address closest to the destination address. This works because the node that is closest to any given address is unambiguous across the overlay network .

Node discovery

neuropil uses this type of routing to allow nodes to automatically discover each others virtual and physical addresses. Using a cryptographic hash function, a subject string is mapped into the 256‑bit address space. The nodes then “meet” at whichever node is currently the closest to this address, and exchange their contact details by proxy. As a result, neuropil nodes need not know the addresses of the nodes with which they will communicate in advance, but can instead discover them dynamically on demand.


neuropil employs multiple layers of end-to-end encryption so that they can participate in cooperative routing without risk of confidentiality or integrity being compromised.


A token, sometimes specifically referred to as an aaatoken, is a record that represents the nodes of a directed acyclic graph. It consists of a number of predefined fields with conventional usage semantics, and can be extended with a set of arbitrary, user-defined fields. Refer to np_token for the aaatoken structure definition and predefined fields, and to neuropil_attributes.h for the API methods to interact with user defined attributes.

Token illustration

A token can be unambiguously referred to by its fingerprint, which is a 256‑bit hash value derived from all of its contents. Referencing a fingerprint in one of the fields of a token creates an unambiguous, directed edge from that token to another token. Conveniently, this fingerprint can also be used as a virtual address in our overlay network.


As a hint for readers who are familiar with JWT, neuropil’s token structure is loosely modeled after JSON Web Tokens.

neuropil defines three distinct types of tokens to implement its authentication, authorization, and accounting (AAA) capabilities on top of its overlay network.

Node tokens denote ephemeral key pairs used by nodes at the transport layer. Their fingerprint is the virtual address of a node in the overlay network. Whenever two nodes talk to each other, they use the public keys noted in each others node tokens to secure the communications end-to-end.

Identity tokens denote relatively long-lived key pairs used by the entities that sit on top of—or use—nodes. Identities are decoupled from nodes in a way that allows them to communicate using multiple nodes, even at the same time. Identities are used to restrict who accesses a given application, and are possibly exchanged out of band. Messages from one identity to another are secured end-to-end, on top of the secure node-to-node channels, using the keys specified in their identity tokens.

Relationships between the various kinds of tokens

Message intent tokens link nodes and identities together. They are signed by an identity so that receivers of a message intent can authenticate it after matching it to an identity they trust—i.e., the issuer of the message intent. They also include a routing hint that contains the fingerprint of one of the nodes in use by the identity. This fingerprint is also the virtual address of the node to and from which messages will be sent. Finally, the subject field in a message intent is used to discover nodes to exchange messages with via the fuzzy routing technique described earlier. It is mapped into the virtual address space of our overlay network, and signifies a channel over which identities will communicate.

To summarize, users of neuropil must validate identity and message intent tokens in their respective AAA callbacks according to the policies they desire to implement. They do this by matching identity and message intent tokens with identities they know and trust. Subsequently, they must take care of identity management as well. Beyond that, neuropil ensures that data produced and consumed by message handler callbacks remains authentic and confidential, and transparently abstracts message routing.