What is a red block in Kaspa, and how does it affect security?

A red block is a block that arrives outside Kaspa's expected propagation window, causing a small, bounded reduction in the security guarantee for that round. Kaspa's protocol uses a parameter called k to define how many parallel blocks are expected per round. When a block falls outside this window, the security threshold for that round is reduced by 1/(k+2). At the current setting of k=124 — which targets 10 blocks per second — a single red block reduces the security threshold by 1/126, less than 0.8%. This bounded, predictable impact matters for beginners because it shows that Kaspa's security scales with its throughput: the faster the network runs, the smaller the effect of any one late block.