Gossamer: A Decentralized Status-Sharing Network
Twitter's pretty great. The short format encourages brief, pithy remarks, and the default assumption of visibility makes it super easy to pitch in on a conversation, or to find new people to listen to. Unfortunately, Twitter is a centralized system: one Bay-area company in the United States controls and mediates all Twitter interactions.
From all appearances, Twitter, Inc. is relatively benign, as social media corporations go. There are few reports of censorship, and while their response to abuse of the Twitter network has not been consistently awesome, they can be made to listen. However, there exists the capacity for Twitter, Inc. to subvert the entire Twitter system, either voluntarily or at the behest of governments around the world.
(Just ask Turkish people. Or the participants in the Arab Spring.)
Gossamer is a Twitter-alike system, designed from the ground up to have no central authority. It resists censorship, enables individual participants to control their own data, and allows anyone at all to integrate new software into the Gossamer network.
Gossamer does not exist, but if it did, the following notes describe what it might look like, and the factors to consider when implementing Gossamer as software. I have made fatal mistakes while writing it; I have not rushed to build it specifically because Twitter, Gossamer's model, is so deeply woven into so many peoples' lives. A successor must make fewer mistakes, not merely different mistakes, and certainly not more mistakes.
The following is loosely inspired by Rumor Monger, at “whole world” scale.
Users must be in control of their own privacy and identity at all times. (This is a major failing with Diaspora, which limits access to personal ownership of data by being hard to run.)
Users must be able to communicate without the consent or support of an intermediate authority. Short of being completely offline, Gossamer should be resilient to infrastructural damage.
Any functional communication system will be used for illicit purposes. This is an unavoidable consequence of being usable for legitimate purposes without a central authority. Rather than revealing illicit conversations, Gossamer should do what it can to preserve the anonymity and privacy of legitimate ones.
All nodes are as equal as possible. The node I use is not more authoritative for messages from me than any other node. You can hear my words from anyone who has heard my words, and I can hear yours from anyone who has heard your words, so long as some variety of authenticity and privacy are maintained.
If an identity's secrets are removed, a node should contain no data that correlates the owner with his or her Gossamer identities. Relaying and authoring must be as indistinguishable as possible, to limit the utility of traffic analysis.
Public and Private Information
Every piece of data Gossamer uses, either internally or to communicate with other ndoes, is classified as either public or private. Public information can be communicated to other nodes, and is assumed to be safe if recovered out of band. Private information includes anything which may be used to associate a Gossamer identity with the person who controls it, except as noted below.
Gossamer must ensure users understand what information that they provide will be made public, and what will be kept private, so that they can better decide what, if anything, to share and so that they can better make decisions about their own safety and comfort against abusive parties.
Internally, Gossamer always stores private information encrypted, and never transmits it to another node. Gossamer must provide a tool to safely obliterate private data.
Details on the role of each piece of information are covered below.
Public status updates, obviously. Gossamer exists to permit users to easily share short messages with one another.
The opaque form of a user's incoming and outgoing private messages.
The users' identities' public keys. (But not their relationship to one another.)
Any information the user places in their profile. (This implies that profiles must not be auto-populated from, for example, the user's address book.)
The set of identities verified by the user's identity.
Any other information Gossamer retains must be private.
Gossamer is built on the assumption that every participant is willing to act as a relay for every other participant. This is a complicated assumption at the human layer.
Inevitably, someone will use the Gossamer network to communicate something morally repugnant or deeply illegal: the Silk Road guy, for example, got done for trying to contract someone to commit murder. Every Gossamer node is complicit in delivering those messages to the rest of the network, whether they're in the clear (status updates) or not (private messages). It's unclear how this interacts with the various legal frameworks, moral codes, and other social constructs throughout the world, and it's ethically troubling to put users in that position by default.
The strong alternative, that each node only relay content with the controlling user's explicit and ongoing consent, is also troubling: it limits the Gossamer network's ability to deliver messages at all, and exposes information about which identities each node's owner considers interesting and publishable.
I don't have an obvious resolution to this. Gossamer's underlying protocol relies on randomly-selected nodes being more likely to propagate a message than to ignore it, because this helps make Gossamer resilient to hostile users, nosy intelligence agencies, and others who believe communication must be restrictable. On the other hand, I'd like not to put a user in Taiwan at risk of legal or social reprisals because a total stranger in Canada decided to post something vile.
(This is one of the reasons I haven't built the damn thing yet. Besides being A Lot Of Code, there's no way to shut off Gossamer once more than one node exists, and I want to be sure I've thought through what I'm doing before creating a prototype.)
Identity in the Gossamer Network
Every Gossamer message carries with it an identity. Gossamer identities are backed by public-key cryptography. However, unlike traditional public key systems such as GPG, Gossamer identities provide continuity, rather than authenticity: two Gossamer messages signed by the same key are from the same identity, but there is no inherent guarantee that that identity is legitimate.
Gossamer maintains relationships between identities to allow users to verify the identities of one another, and to publish attestations of that to other Gossamer nodes. From this, Gossamer can recover much of GPG's “web of trust.”
TODO: revocation of identities, revocation of verifications. Both are important; novice users are likely to verify people poorly, and there should be a recovery path less drastic than GPG's “you swore it, you're stuck with it” model.
Gossamer encourages users to create additional identities as needed to, for example, support the separation of work and home conversations, or to provide anonymity when discussing reputationally-hazardous topics. Identities are not correlated by the Gossamer codebase.
Each identity can optionally include a profile: a block of data describing the person behind the identity. The contents of a profile are chosen by the person holding the private key for an identity, and the profile is attached to every new message created with the corresponding identity. A user can update their profile at will; potentially, every message can be sent with a distinct profile. Gossamer software treats the profile it's seen with the highest timestamp as authoritative, retroactively applying it to old messages.
Multiple Devices and Key Security
A Gossamer identity is entirely contained in its private key. An identity's key must be stored safely, either using the host operating system's key management facilities or using a carefully-designed key store. Keys must not hit long-term storage unprotected; this may involve careful integration with the underlying OS's memory management facilities to avoid, eg., placing identities in swap. This is necessary to protect users from having their identities recovered against their will via, for example, hard drive forensics.
Gossamer allows keys to be exported into password-encrypted archive files, which can be loaded into other Gossamer applications to allow them to share the same identity.
GOSSAMER MUST TREAT THESE FILES WITH EXTREME CARE, BECAUSE USERS PROBABLY WON'T. Identity keys protect the user's Gossamer identity, but they also protect the user's private messages (see below) and other potentially identifying data. The export format must be designed to be as resilient as possible, and Gossamer's software must take care to ensure that “used” identity files are automatically destroyed safely wherever possible and to discourage users from following practices that weaken their own safety unknowingly.
Exported identity files are intrinsically vulnerable to offline brute-force attacks; once obtained, an attacker can try any of the worryingly common passwords at will, and can easily validate a password by using the recovered keys to regenerate some known fact about the original, such as a verification or a message signature. This implies that exported identities must use a key derivation system which has a high computational cost and which is believed to be resilient to, for example, GPU-accelerated cracking.
Secure deletion is a Hard Problem; where possible, Gossamer must use operating system-provided facilities for securely destroying files.
Status messages are messages visible to any interested Gossamer users. These are the primary purpose of Gossamer. Each contains up to 140 Unicode characters, a markup section allowing Gossamer to attach URLs and metadata (including Gossamer locators) to the text, and an attachments section carrying arbitrary MIME blobs of limited total size.
All three sections are canonicalized (TODO: how?) and signed by the publishing identity's private key. The public key, the identity's most recent profile, and the signed status message are combined into a single Gossamer message and injected into the user's Gossamer node exactly as if it had arrived from another node.
Each Gossamer node maintains a follow list of identities whose messages the user is interested in seeing. When Gossamer receives a novel status message during a gossip exchange, it displays it to the user if and only if its identity is on the node's follow list. Otherwise, the message is not displayed, but will be shared onwards with other nodes. In this way, every Gossamer node acts as a relay for every other Gossamer node.
If Gossamer receives a message signed by an identity it has seen attestations for, it attaches those attestations to the message before delivering them onwards. In this way, users' verifications of one another's identity spread through the network organically.
Gossamer can optionally encrypt messages, allowing users to send one another private messages. These messages are carried over the Gossamer network as normal, but only nodes holding the appropriate identity key can decrypt them and display them to the user. (At any given time, most Gossamer nodes hold many private messages they cannot decrypt.)
Private messages do not carry the author's identity or full profile in the clear. The author's bare identity is included in the encrypted part of the message, to allow the intended recipient to identify the sender.
TODO: sign-then-encrypt, or encrypt-then-sign? If sign-then-encrypt, are private messages exempted from the “drop broken messages” rule above?
Each Gossamer node maintains a database of followed identities. (This may or may not include the owner's own identity.) Any message stored in the node published by an identity in this database will be shown to the user in a timeline-esque view.
Gossamer's follow list is purely local, and is not shared between nodes even if they have identities in common. The follow list is additionally stored encrypted using the node's identities (any one identity is sufficient to recover the list), to ensure that the follow list is not easily available to others without the node owner's permission.
Exercises such as Finding Paul Revere have shown that the collection of graph edges showing who communicates with whom can often be sufficient to map identities into people. Gossamer attempts to restrict access to this data, believing it is not the network's place to know who follows who.
Gossamer allows identities to sign one anothers' public keys. These signatures form verifications. Gossamer considers an identity verified if any of the following hold:
Gossamer has access to the identity key for the identity itself.
Gossamer has access to the identity key for at least one of the identity's verifications.
The identity is signed by at least three (todo: or however many, I didn't do the arithmetic yet) verified identities.
Verified identities are marked in the user interface to make it obvious to the user whether a message is from a known friend or from an unknown identity.
Gossamer allows users to sign new verifications for any identity they have seen. These verifications are initially stored locally, but will be published as messages transit the node as described below. Verification is a public fact: everyone can see which identities have verified which other identities. This is a potentially very powerful tool for reassociating identities with real-world people; Gossamer must make this clear to users.
(I'm pretty sure you could find me, personally, just by watching whose identities I verify.)
Each Gossamer node maintains a database of every verification it has ever seen or generated. If the node receives a message from an identity that appears in the verification database, and if the message is under some total size, Gossamer appends verifications from its database to the message before reinjecting it into the network. This allows verifications to propagate through
Any social network will attract hostile users who wish to disrupt the network or abuse its participants. Users must be able to filter out these users, and must not provide too much feedback to blocked users that could otherwise be used to circumvent blocks.
Each Gossamer node maintains a database of blocked identities. Any message from an identity in this database, or from an identity that is verified by three or more identities in this database, will automatically be filtered out from display. (Additionally, transitively-blocked users will automatically be added to the block database. Blocking is contagious.) (TODO: should Gossamer drop blocked messages? How does that interact with the inevitable “shared blocklist” systems that arise in any social network?)
As with the follow list, the block database is encrypted using the node's identities.
Gossamer encourages users to create new identities as often as they see fit and attempts to separate identities from one another as much as possible. This is fundamentally incompatible with strong blocking. It will always be possible for a newly-created identity to deliver at least one message before being blocked. This is a major design problem; advice encouraged.
Gossamer Network Primitives
The Gossamer network is built around a gossip protocol, wherein nodes connect to one another periodically to exchange messages with one another. Connections occur over the existing IP internet infrastructure, traversing NAT networks where possible to ensure that users on residential and corporate networks can still participate.
Gossamer bootstraps its network using a number of paths:
Gossamer nodes in the same broadcast domain discover one another using UDP broadcasts as well as Bonjour/mDNS.
Gossamer can generate locator strings, which can be shared “out of band” via email, SMS messages, Twitter, graffiti, etc.
Gossamer nodes share knowledge of nodes whenever they exchange messages, to allow the Gossamer network to recover from lost nodes and to permit nodes to remain on the network as “known” nodes are lost to outages and entropy.
A Gossamer locator is a URL in the
g scheme, carrying an encoding of one
or more network addresses as well as an encoding of one or more identities
(see below). Gossamer's software attempts to determine an appropriate
identifier for any identities it holds based on the host computer's network
configuration, taking into account issues like NAT traversal wherever
TODO: Gossamer and uPNP, what do locators look like?
When presented with an identifier, Gossamer offers to follow the identities it contains, and uses the nodes whose addresses it contains to connect to the Gossamer network. This allows new clients to bootstrap into Gossamer, and provides an easy way for users to exchange Gossamer identities to connect to one another later.
(Clever readers will note that the address list is actually independent of the identity list.)
Each Gossamer node maintains a pair of “freshness” databases, associating some information with a freshness score (expressed as an integer). One freshness database holds the addresses of known Gossamer nodes, and another holds Gossamer messages.
Whenever two Gossamer nodes interact, each sends the other a Gossamer node from its current node database, and a message from its message database. When selecting an item to send for either category, Gossamer uses a random selection that weights towards items with a higher “freshness” score. (TODO: how?)
When sending a fact, if the receiving node already knows the fact, both nodes decrement that fact's freshness by one. If the receiving node does not already know the fact, the sending node leaves its freshness unaltered, and the receiving node sets its freshness to the freshest possible value. This system encourages nodes to exchange “fresh” facts, then cease exchanging them as the network becomes aware of them.
During each exchange, Gossamer nodes send each other one Gossamer node address, and one Gossamer message. Both nodes adjust their freshness databases, as above.
If fact exchange fails while communicating with a Gossamer node, both nodes decrement their peer's freshness. Unreliable nodes can continue to initiate connections to other nodes, but will rarely be contacted by other Gossamer nodes.
TODO: How do we avoid DDOSing brand-new gossamer nodes with the full might of Gossamer's network?
TODO: Can we reuse Bittorrent's DHT system (BEP-5) to avoid having every node know the full network topology?
TODO: Are node-to-node exchanges encrypted? If so, why and how?
Gossamer node addresses are not authenticated. Gossamer relies on freshness to avoid delivering excess traffic to systems not participating in the Gossamer network. (TODO: this is a shit system for avoiding DDOS, though.)
Gossamer messages are partially authenticated: each carries with it a public key, and a signature. If the signature cannot be verified with the included public key, it must be discarded immediately and it must not be propagated to other nodes. The node delivering the message may also be penalized by having its freshness reduced in the receiving node's database.
Gossamer triggers a new Gossip exchange under the following circumstances:
15 seconds, plus a random jitter between zero and 15 more seconds, elapse since the last exchange attempt.
Gossamer completes an exchange wherein it learned a new fact from another node.
A user injects a fact into Gossamer directly.
Gossamer exchanges that fail, or that deliver only already-known facts, do not trigger further exchanges immediately.
TODO: how do we prevent Gossamer from attempting to start an unbounded number of exchanges at the same time?
Gossamer must not exhaust the user's disk. Gossamer discards extremely un-fresh messages, attempting to keep the on-disk size of the message database to under 10% of the total local storage, or under a user-configurable threshold.
Gossamer rejects over-large messages. Public messages carry with them the author's profile and a potentially large collection of verifications. Messages over some size (TODO what size?) are discarded on receipt without being stored, and the message exchange is considered to have failed.