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Merged
fmontesi merged 1 commit into from
Oct 6, 2026
Merged

fix(DiffieHellman): reject mismatched moduli #1042

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7 changes: 4 additions & 3 deletions Cslib/Algorithms/StatefulProcesses/DiffieHellman/Basic.lean
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Original file line number Diff line number Diff line change
Expand Up @@ -139,12 +139,13 @@ def bob : Process Pid Var params.Val FunId params.SelLabel params.ProcName :=

/-! ## Semantics -/

/-- Implementation of local function call evaluation. -/
/-- Implementation of local function call evaluation.
Calls with a mismatched modulus are rejected. -/
def funEval : FunCallEval FunId params.Val
| .computePublicMessage, [.nat p, .zMod g, .nat privateExp], v =>
(h : p = params.p) → v = (.zMod <| h ▸ computePublicMessage p (h ▸ g) privateExp)
p = params.p ∧ v = .zMod (computePublicMessage params.p g privateExp)
| .computeSharedSecret, [.nat p, .zMod msg, .nat privateExp], v =>
(h : p = params.p) → v = (.zMod <| h ▸ computeSharedSecret p (h ▸ msg) privateExp)
p = params.p ∧ v = .zMod (computeSharedSecret params.p msg privateExp)
| _, _, _ => False

/-- DH network. -/
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20 changes: 10 additions & 10 deletions CslibTests/DiffieHellman.lean
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Expand Up @@ -16,30 +16,30 @@ variable {Pid Var : Type*} (params : Params Pid Var)
example (σ : LocalStore Var params.Val) :
(funEval params).EvalExpr σ (aliceComputeMesg params) (.zMod (params.g ^ params.a)) := by
apply FunCallEval.EvalExpr.call (.cons .val (.cons .val (.cons .val .nil)))
intro hmod
rfl
exact ⟨rfl, rfl⟩

example (σ : LocalStore Var params.Val) :
(funEval params).EvalExpr σ (bobComputeMesg params) (.zMod (params.g ^ params.b)) := by
apply FunCallEval.EvalExpr.call (.cons .val (.cons .val (.cons .val .nil)))
intro hmod
rfl
exact ⟨rfl, rfl⟩

-- Each role must also use its own exponent on the received message.
example (σ : LocalStore Var params.Val) (message : ZMod params.p)
(h : σ params.y = .zMod message) :
(funEval params).EvalExpr σ (aliceComputeSharedSecret params) (.zMod (message ^ params.a)) := by
apply FunCallEval.EvalExpr.call (.cons .val (.cons .var (.cons .val .nil)))
simp only [h, funEval]
intro hmod
rfl
simp [h, funEval, computeSharedSecret]

example (σ : LocalStore Var params.Val) (message : ZMod params.p)
(h : σ params.x = .zMod message) :
(funEval params).EvalExpr σ (bobComputeSharedSecret params) (.zMod (message ^ params.b)) := by
apply FunCallEval.EvalExpr.call (.cons .val (.cons .var (.cons .val .nil)))
simp only [h, funEval]
intro hmod
rfl
simp [h, funEval, computeSharedSecret]

-- A mismatched modulus must reject every result for either function.
example (f : FunId) (p : ℕ) (hp : p ≠ params.p) (message : ZMod params.p)
(privateExp : ℕ) (v : params.Val) :
¬ funEval params f [.nat p, .zMod message, .nat privateExp] v := by
cases f <;> exact fun h => hp h.1

end CslibTests.DiffieHellman
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