BIP 66 — Strict DER signatures

BIP 66 (Strict DER signatures) is a consensus soft fork requiring signatures to follow strict DER encoding, eliminating a class of malleability and dependence on OpenSSL's permissive parsing. It activated in mid-2015; its rollout is associated with a brief July 2015 chain split caused by miners building on blocks they had not fully validated. Repository status: Deployed.

Debate relevance: Comes up in validation debates ('SPV mining'), in malleability history, and as evidence that even soft forks require attentive node validation. Common misstatements: Blaming the 2015 fork on a flaw in BIP 66 itself — the rule worked; the incident stemmed from miners signaling readiness while skipping full validation.