Universally Composable Termination Analysis of Tendermint

Modern blockchain systems operating in adversarial environments require robust consensus protocols that guarantee both safety and termination under network delay attacks. Tendermint, a widely adopted consensus protocol in consortium blockchains, achieves high throughput and finality. However, previous analysis of the safety and termination has been done in a standalone fashion, with no consideration of the composition with other protocols interacting with it in a concurrent manner. Moreover, the termination properties under adaptive network delays caused by Byzantine adversaries have not been formally analyzed. This paper presents the first universally composable (UC) security analysis of Tendermint, demonstrating its resilience against strategic message-delay attacks. By constructing a UC ideal model of Tendermint, we formalize its core mechanisms: phase-base consensus procedure, dynamic timeouts, proposal locking, leader rotation, and others, under a network adversary that selectively delays protocol messages. Our main result proves that the Tendermint protocol UC-realizes the ideal Tendermint model, which ensures bounded termination latency, i.e., guaranteed termination, even when up to $f<n/3$ nodes are Byzantine (where $n$ is the number of nodes participating in the consensus), provided that network delays remain within a protocol-defined threshold under the partially synchronous net assumption. Specifically, through formal proofs within the UC framework, we show that Tendermint maintains safety and termination. By the composition theorem of UC, this guarantees that these properties are maintained when Tendermint is composed with various blockchain components.