ltl-accumulator-verified/verification/Proofs/Theorem3.lean

/- S5.4 — **Theorem 3 (Consistency soundness)**, assembled: the explicit
   extractor 𝓔′ for history rewrites. Joins consRecBinding (steps 1-2,
   S5.3) to extractMTH (step 3, S4).

   Statement design per the corpus discipline: a NAMED function whose
   correctness is about ITS OUTPUT (pigeonhole/choice cannot discharge
   it), guarded by a permanent non-vacuity witness. -/
import Proofs.Binding3

namespace LTLAcc

/-- The consistency extractor 𝓔′ (paper Theorem 3). On a claimed rewrite
    — an accepted consistency proof `C` between the pinned root of `D₀`
    and the head of `D₁`, where `D₀` is NOT the real prefix — return the
    node collision found while walking the fold, or descend into the two
    same-root prefix trees. -/
noncomputable def extractCons (n₀ : Nat) (C : List Hash)
    (D₀ D₁ : List Bytes) : List UInt8 × List UInt8 :=
  match extractConsNode n₀ D₁.length C true (MTH D₀) D₁ with
  | some c => c
  | none => extractMTH D₀ (D₁.take n₀)

/-- **Theorem 3 (Consistency soundness), explicit form**: if the
    consumer's verifier accepts `C` between the pinned head `MTH D₀`
    (size `n₀`) and the offered head `MTH D₁`, but `D₀` is not the real
    prefix of `D₁`, then `extractCons` outputs a genuine SHA-256
    collision. -/
theorem extractCons_correct (n₀ : Nat) (C : List Hash) (D₀ D₁ : List Bytes)
    (hlen0 : D₀.length = n₀) (hn0 : 0 < n₀) (hle : n₀ ≤ D₁.length)
    (hne : D₀ ≠ D₁.take n₀)
    (hacc : ConsRec n₀ D₁.length C true (MTH D₀) = some (MTH D₀, MTH D₁)) :
    IsCollision (extractCons n₀ C D₀ D₁).1 (extractCons n₀ C D₀ D₁).2 := by
  have hbind := consRecBinding (MTH D₀) n₀ D₁.length C true D₁
    (MTH D₀) (MTH D₁) rfl hn0 hle hacc rfl
  rw [extractCons]
  cases hrec : extractConsNode n₀ D₁.length C true (MTH D₀) D₁ with
  | some c =>
      rw [hrec] at hbind
      exact hbind
  | none =>
      rw [hrec] at hbind
      -- hbind : MTH D₀ = MTH (D₁.take n₀); descend
      have htklen : (D₁.take n₀).length = n₀ := by
        rw [List.length_take]; omega
      exact extractMTH_correct D₀ (D₁.take n₀) (by omega) hne hbind

/-- Permanent non-vacuity witness: on a NON-rewrite input (the pinned
    list IS the real prefix), the extractor's output is provably NOT a
    collision — so the correctness conclusion is false for some inputs
    and cannot be discharged by pigeonhole or choice. Uses the honest
    n₀ = n base: D₀ = D₁ = [[7]], C = [], where ConsRec accepts and
    extractConsNode returns none, so extractCons = extractMTH D₀ D₀ =
    the equal leaf pair. -/
theorem extractCons_nonvacuous :
    ¬ IsCollision (extractCons 1 [] [([7] : List UInt8)] [([7] : List UInt8)]).1
                  (extractCons 1 [] [([7] : List UInt8)] [([7] : List UInt8)]).2 := by
  rw [extractCons]
  have hrec : extractConsNode 1 ([([7] : List UInt8)]).length [] true
      (MTH [([7] : List UInt8)]) [([7] : List UInt8)] = none := by
    rw [extractConsNode]; simp
  rw [hrec]
  rw [extractMTH]
  simp only [List.length_singleton, if_pos (by omega : (1:Nat) ≤ 1)]
  intro hcol
  exact hcol.1 rfl

end LTLAcc