Syncordian: A New CRDT That Locks Out Intruders and Fixes Text Interleaving

Collaborative editing has become so seamless in everyday tools like Google Docs that most users never stop to consider the machinery underneath. When two people type into the same paragraph at the same moment, the document somehow resolves their conflicting keystrokes into a single coherent text. In the dominant commercial model, that magic depends on a costly central server that arbitrates every change. A new data structure called Syncordian, presented by Mateo Sanabria and Nicolás Cardozo of Universidad de los Andes in Bogotá in the journal Cluster Computing, takes a different path: it lets a closed group of peers keep a shared document consistent without any central authority, while simultaneously defending the document against outsiders who try to inject malicious operations.

The foundation of the work is the conflict-free replicated data type, or CRDT, a family of decentralized data structures introduced in the early 2010s. CRDTs allow every replica of a document to accept edits independently and then converge to the same state, provided that all operations eventually reach all peers. The mathematical property that makes this possible is commutativity: operations are designed so that applying them in any order yields the same result. This eliminates the need for a coordinator, which is why CRDTs have attracted intense interest for peer-to-peer collaboration, offline-first applications, and distributed systems where central services are expensive or unavailable.

Yet the authors identified a gap that no existing CRDT proposal had closed. Real collaborative documents are often written by a closed group of admitted peers, and two problems arise in that setting. First, an adversary outside the group can forge operations and flood the network, corrupting every replica that accepts them. Second, even among honest peers, concurrent edits to the same region of text can produce interleaving anomalies, in which characters from different users’ insertions become tangled together, scrambling words in ways the authors never intended. To date, the researchers note, no CRDT had simultaneously supported a closed peer group, prevented interleavings, and rejected operations from distrusted nodes outside that group.

Syncordian addresses the interleaving problem by detecting overlapping edit blocks and re-issuing the lower-priority one. When two peers concurrently edit text that overlaps, the system recognizes that their edit blocks collide and re-executes the block with lower priority in a way that preserves the author’s intention without tangling the characters. This mechanism builds on earlier research by Kleppmann and colleagues, who catalogued interleaving anomalies in collaborative text editors, and on work by Weidner and Kleppmann on minimizing interleaving in collaborative text editing. The result is that concurrent insertions remain visually and semantically separate, so a reader sees the text as the authors meant it rather than a character-by-character mosaic of competing inputs.

The security mechanism is deliberately lightweight. Every operation in Syncordian is authenticated with a keyed message authentication code, or MAC, rooted in a secret shared by the peers admitted when the document is created. Because the secret is established at document creation time and shared only among admitted members, the system needs no public key infrastructure, no certificate authorities, and no heavyweight cryptographic ceremony. Any operation arriving without a valid authentication tag is rejected, which means nodes outside the admitted group cannot inject content into the document. The approach draws on established cryptographic primitives, including HMAC as specified in RFC 2104, the HKDF key derivation function of RFC 5869, and the KMAC construction from NIST Special Publication 800-185.

The authors are careful to state precisely what these guarantees cover. Syncordian authenticates group membership rather than individual peer identity. Its protections are therefore directed against nodes outside the admitted group, and a compromised member who legitimately holds the shared secret falls outside the scope of the system’s guarantees. This is an honest and important boundary: the design trades fine-grained per-user accountability for simplicity and decentralization, a trade-off that suits scenarios where the threat model is hostile outsiders rather than traitors within. It places Syncordian in a broader research conversation about Byzantine fault tolerance for collaborative editing, where earlier work by Zhao and colleagues and more recent proposals such as Kleppmann’s Byzantine fault tolerant CRDTs, secure CRDTs by Barbosa and colleagues, and the blocklace of Almeida and Shapiro have explored related territory.

Evaluating a collaborative editing system requires realistic workloads, and the team went beyond synthetic benchmarks. They replayed two collaborative documents taken from GitHub, together containing more than 1,000 edit operations contributed by more than 100 network peers. Replayed histories, however, cannot recreate the genuine concurrency of live co-editing, since a recorded history is already a linearized sequence of events. To fill that gap, the researchers supplemented the replays with a suite of controlled workloads specifically designed to build the concurrent authoring situations that a replayed history cannot produce, forcing overlapping edits and adversarial traffic under laboratory conditions.

The results validate the system’s two central claims. Across all workloads, Syncordian kept every peer’s document consistent, satisfying strong eventual convergence, and it preserved the intention of each author’s edits, even in the presence of distrusted nodes attempting to participate. Just as importantly for practitioners, the paper reports what these guarantees cost. The authors quantify the overhead in memory, in the number of messages exchanged, and in bytes transferred, and they measure how each cost grows as the number of peers increases. This kind of accounting matters because CRDTs are often criticized for metadata bloat, and a security layer that silently multiplied communication costs would undermine the decentralized promise of the design.

The implementation itself is written in Elixir, a language built on the BEAM virtual machine that is well suited to the message-passing style of distributed systems, and the full source code along with all experiment data has been released publicly on GitHub. In their evaluation the authors compared Syncordian against existing approaches, noting that no prior CRDT implementation offers protection against adversarial outsiders, and that the closest comparable systems are general collaborative editing frameworks, which they included as reference points in their benchmarks.

The significance of the work extends beyond text editors. CRDTs underpin a growing ecosystem of decentralized applications, from shared whiteboards and databases to blockchain-adjacent infrastructure, and the questions Syncordian tackles, namely who is allowed to write and how concurrent edits should be ordered, are fundamental to any system that replicates state across untrusted networks. By combining membership authentication with a principled solution to interleaving anomalies, and by publishing both the costs and the code, Sanabria and Cardozo have given the distributed systems community a concrete, measurable starting point for building collaborative tools that are decentralized by design and resistant to outsiders by default. As collaboration moves away from centralized services toward peer-to-peer architectures, designs like Syncordian suggest that the security and consistency problems once solved by a server in the middle can instead be solved by the data structure itself.

Subject of Research: A decentralized CRDT for secure collaborative editing that resists adversarial outsiders and interleaving anomalies

Article Title: Syncordian: managing adversarial behavior and interleaving anomalies in CRDTs

Article References: Sanabria, M., & Cardozo, N. (2026). Syncordian: managing adversarial behavior and interleaving anomalies in CRDTs. Cluster Computing, 29(12), Article 721. https://doi.org/10.1007/s10586-026-06430-0

Image Credits: AI Generated

DOI: 10.1007/s10586-026-06430-0

Keywords: CRDTs, collaborative editing, distributed systems, strong eventual consistency, message authentication codes, interleaving anomalies, outsider resistance, membership authentication, peer-to-peer, Elixir, optimistic replication, Cluster Computing