Layer 2 (L2) protocols, payment channels, sidechains, and rollups, are central to blockchain scalability, enabling off-chain execution while preserving on-chain security. Despite growing deployment, existing security models remain protocol-specific and monolithic, hindering compositional reasoning and principled comparison of assumptions and requirements. We present a general security framework for L2 protocols in the IITM-style Universal Composability (iUC) model. At its core is a modular ideal functionality F_layer2 that abstracts mechanism-specific details while capturing the essential structure of L2 systems through composable subroutines for joining, submission, updating, reading, and settlement under adversarial conditions. This yields uniform definitions of safety, liveness, and data availability across a broad class of L2 protocols. We demonstrate generality by instantiating the framework for three representative constructions: the Brick payment channel, the Liquid sidechain, and the Arbitrum Nitro rollup. Each case study yields a protocol-specific ideal functionality derived from F_layer2 and tailored to its assumptions. Our analysis (i) establishes security via simulation-based proofs, (ii) exposes inherent trade-offs among safety, liveness, and data availability, and (iii) derives lower bounds characterizing fundamental limitations of each design class. Finally, we illustrate the framework as a design tool by presenting FRoll, the first optimistic rollup protocol with fast-finality guarantees, together with a security analysis in our model, showing how the framework supports requirement-driven design of L2 protocols.

翻译：第二层（L2）协议（包括支付通道、侧链和卷叠）是实现区块链可扩展性的核心，它们能在链下执行操作的同时维护链上安全。尽管这些协议部署日益广泛，现有的安全模型仍局限于特定协议且结构单一，这阻碍了对不同协议假设与需求的组合推理及规范化比较。本文提出了一种基于IITM风格通用可组合性（iUC）模型、面向L2协议的通用安全框架。该框架的核心是一个模块化理想功能函数F_layer2，它通过抽象具体机制细节，并借助自组织子程序（用于在对抗条件下实现加入、提交、更新、读取和结算操作），捕捉了L2系统的本质结构。这为广泛类别的L2协议提供了关于安全性、活跃性及数据可用性的统一定义。我们通过对三种代表性协议（Brick支付通道、Liquid侧链和Arbitrum Nitro卷叠）的框架实例化，验证了其通用性。每个案例研究均从F_layer2中推导出协议特定的理想功能函数，并针对其假设进行定制。我们的分析实现了以下目标：（i）通过基于模拟的证明确立安全性；（ii）揭示安全性、活跃性与数据可用性之间的固有权衡；（iii）推导出表征每类设计固有局限性的下界。最后，我们通过提出FRoll协议（首个具有快速最终性保证的乐观卷叠协议）及其在本模型中的安全性分析，展示了该框架作为设计工具的效能，从而阐明框架如何支持L2协议的需求驱动设计。