Proof-Carrying Code Generator

Generate executable code bundled with formal proofs that certify key safety and correctness properties.

Overview

Proof-carrying code (PCC) is executable code accompanied by formal proofs that certify specific properties. This skill helps generate:

1. Verified implementations with correctness proofs
2. Safety certificates for memory safety, bounds checking, null safety
3. Functional specifications with pre/postconditions
4. Invariant proofs for data structure integrity
5. Extracted code from verified specifications

The generated code can be extracted to mainstream languages (OCaml, Haskell, SML) while maintaining proof certificates.

PCC Generation Workflow

Requirements
    ↓
Formal Specification
    ↓
Verified Implementation
    ↓
Proof Obligations
    ↓
Certified Code + Proofs
    ↓
Code Extraction

Core Approaches

Approach 1: Specification-First

Start with formal specification, implement, then prove.

Steps:

1. Write formal specification
2. Implement function/algorithm
3. State correctness theorem
4. Prove theorem
5. Extract code

Example (Isabelle):

(* Specification *)
definition sorted_spec :: "nat list ⇒ nat list ⇒ bool" where
  "sorted_spec input output ≡ sorted output ∧ mset output = mset input"

(* Implementation *)
fun insertion_sort :: "nat list ⇒ nat list" where
  "insertion_sort [] = []" |
  "insertion_sort (x # xs) = insert x (insertion_sort xs)"

(* Correctness theorem *)
theorem insertion_sort_correct:
  "sorted_spec input (insertion_sort input)"
proof (induction input)
  case Nil
  then show ?case by (simp add: sorted_spec_def)
next
  case (Cons x xs)
  then show ?case
    using insert_sorted mset_insert
    by (auto simp: sorted_spec_def)
qed

(* Extract code *)
export_code insertion_sort in SML file "sort.sml"

Approach 2: Refinement-Based

Start abstract, refine to concrete with proofs at each step.

Steps:

1. Abstract specification
2. Refine to intermediate level
3. Prove refinement correct
4. Refine to executable code
5. Prove final refinement
6. Extract code

Example (Coq):

(* Abstract specification *)
Definition find_spec {A} (P : A → bool) (l : list A) : option A :=
  (* Nondeterministic: any element satisfying P *)
  ...

(* Concrete implementation *)
Fixpoint find {A} (P : A → bool) (l : list A) : option A :=
  match l with
  | [] => None
  | x :: xs => if P x then Some x else find P xs
  end.

(* Refinement proof *)
Theorem find_refines : forall A P l,
  find P l = find_spec P l.
Proof.
  (* Proof that concrete refines abstract *)
Qed.

(* Extract *)
Extraction "find.ml" find.

Approach 3: Program-First

Write code with annotations, generate proof obligations, prove them.

Steps:

1. Write function with contracts
2. Generate verification conditions
3. Prove verification conditions
4. Certify code

Example (Coq with Program):

Require Import Program.

Program Definition safe_div (a b : nat) (H : b ≠ 0) : nat :=
  a / b.

(* Proof obligation automatically generated *)
Next Obligation.
  (* Prove division is safe *)
Qed.

Safety Properties

Memory Safety

Property: No buffer overflows, out-of-bounds access.

Pattern:

lemma array_access_safe:
  "⟦ i < length arr ⟧ ⟹ ∃v. arr ! i = v"
  by auto

fun safe_get :: "'a list ⇒ nat ⇒ 'a option" where
  "safe_get [] _ = None" |
  "safe_get (x # xs) 0 = Some x" |
  "safe_get (x # xs) (Suc n) = safe_get xs n"

lemma safe_get_bounds:
  "safe_get xs i = Some v ⟹ i < length xs"
  by (induction xs i rule: safe_get.induct) auto

Null Safety

Property: No null pointer dereferences.

Pattern:

Definition safe_head {A} (l : list A) (H : l ≠ []) : A :=
  match l with
  | [] => match H eq_refl with end
  | x :: _ => x
  end.

Lemma safe_head_correct : forall A (l : list A) H,
  l ≠ [] → safe_head l H = hd_error l.
Proof.
  intros. destruct l; auto. contradiction.
Qed.

Bounds Checking

Property: All array accesses within bounds.

Pattern:

definition bounded_access :: "'a array ⇒ nat ⇒ 'a option" where
  "bounded_access arr i = (if i < array_length arr
                           then Some (array_get arr i)
                           else None)"

lemma bounded_access_safe:
  "bounded_access arr i = Some v ⟹ i < array_length arr"
  by (simp add: bounded_access_def split: if_splits)

Correctness Properties

Functional Correctness

Property: Function computes correct result.

Pattern:

fun factorial :: "nat ⇒ nat" where
  "factorial 0 = 1" |
  "factorial (Suc n) = Suc n * factorial n"

function fact_spec :: "nat ⇒ nat" where
  "fact_spec 0 = 1" |
  "fact_spec n = n * fact_spec (n - 1)"
  by auto

theorem factorial_correct:
  "factorial n = fact_spec n"
  by (induction n) auto

Invariant Preservation

Property: Data structure invariants maintained.

Pattern:

Inductive BST : tree → Prop :=
  | BST_leaf : BST Leaf
  | BST_node : forall l x r,
      BST l → BST r →
      (forall y, In y l → y < x) →
      (forall y, In y r → x < y) →
      BST (Node l x r).

Fixpoint insert (x : nat) (t : tree) : tree :=
  match t with
  | Leaf => Node Leaf x Leaf
  | Node l y r =>
      if x <? y then Node (insert x l) y r
      else if y <? x then Node l y (insert x r)
      else t
  end.

Theorem insert_preserves_BST : forall x t,
  BST t → BST (insert x t).
Proof.
  induction t; intros; simpl.
  - constructor; constructor.
  - inversion H; subst.
    destruct (x <? n) eqn:E1.
    + (* Insert left *)
    + destruct (n <? x) eqn:E2.
      * (* Insert right *)
      * (* Equal, no change *)
Qed.

Termination

Property: Function always terminates.

Pattern:

function gcd :: "nat ⇒ nat ⇒ nat" where
  "gcd a 0 = a" |
  "gcd a b = gcd b (a mod b)"
  by auto
termination
  by (relation "measure (λ(a, b). b)") auto

lemma gcd_terminates:
  "∃result. gcd a b = result"
  by auto

Framework-Specific Guidance

Isabelle/HOL PCC

For Isabelle-specific code generation, extraction, and certification patterns, see references/isabelle_pcc.md.

Key features:

• Code generation to SML, OCaml, Haskell, Scala
• Refinement framework for verified implementations
• Sepref for imperative code with proofs
• Export with proof certificates

Coq PCC

For Coq-specific extraction, Program framework, and certification, see references/coq_pcc.md.

Key features:

• Extraction to OCaml, Haskell, Scheme
• Program for proof obligations
• Equations for well-founded recursion
• Certified compilation

Common Safety Properties

For detailed safety and correctness property patterns, see references/safety_properties.md.

Categories include:

• Memory safety (bounds, null, use-after-free)
• Type safety (no type confusion)
• Arithmetic safety (no overflow, division by zero)
• Concurrency safety (no data races, deadlocks)
• Information flow security (no leaks)

Complete Example: Verified Binary Search

Specification:

definition binary_search_spec :: "nat list ⇒ nat ⇒ nat option" where
  "binary_search_spec xs target = (
    if sorted xs ∧ target ∈ set xs
    then Some (THE i. i < length xs ∧ xs ! i = target)
    else None)"

Implementation:

fun binary_search :: "nat list ⇒ nat ⇒ nat option" where
  "binary_search xs target = bs_aux xs target 0 (length xs)"

fun bs_aux :: "nat list ⇒ nat ⇒ nat ⇒ nat ⇒ nat option" where
  "bs_aux xs target low high = (
    if low ≥ high then None
    else let mid = (low + high) div 2 in
         if xs ! mid = target then Some mid
         else if xs ! mid < target then bs_aux xs target (mid + 1) high
         else bs_aux xs target low mid)"

Safety proofs:

lemma bs_aux_bounds:
  "⟦ bs_aux xs target low high = Some i; low ≤ high; high ≤ length xs ⟧
   ⟹ low ≤ i ∧ i < high"
proof (induction xs target low high rule: bs_aux.induct)
  case (1 xs target low high)
  then show ?case
    by (auto simp: Let_def split: if_splits)
qed

lemma bs_aux_no_overflow:
  "⟦ low ≤ high; high ≤ length xs ⟧
   ⟹ (low + high) div 2 < length xs"
  by auto

Correctness proof:

theorem binary_search_correct:
  "sorted xs ⟹ binary_search xs target = binary_search_spec xs target"
proof -
  assume "sorted xs"
  show ?thesis
  proof (cases "target ∈ set xs")
    case True
    then show ?thesis
      using binary_search_finds_target[OF `sorted xs` True]
      by (simp add: binary_search_spec_def)
  next
    case False
    then show ?thesis
      using binary_search_not_found[OF `sorted xs` False]
      by (simp add: binary_search_spec_def)
  qed
qed

Code extraction:

export_code binary_search in SML file "binary_search.sml"
export_code binary_search in OCaml file "binary_search.ml"

Best Practices

1. Start with Specifications: Clear specs before implementation
2. Incremental Verification: Verify small pieces, compose
3. Reuse Lemmas: Build library of certified components
4. Automate Proofs: Use tactics and automation where possible
5. Test Extracted Code: Verify extraction produces correct code
6. Document Properties: Clearly state what's certified
7. Modular Design: Separate concerns, verify independently

Verification Checklist

Before finalizing proof-carrying code:

• All safety properties proven
• Functional correctness established
• Termination proven (if applicable)
• Invariants maintained
• Bounds checking verified
• Code extracts successfully
• Extracted code tested
• Proof certificates included
• Documentation complete

Integration Patterns

Integrating with Existing Code

Pattern: Verify critical components, interface with unverified code.

Example:

(* Verified core *)
definition verified_sort :: "nat list ⇒ nat list" where
  "verified_sort = insertion_sort"

theorem verified_sort_correct:
  "sorted (verified_sort xs) ∧ mset (verified_sort xs) = mset xs"
  by (simp add: verified_sort_def insertion_sort_correct)

(* Export for use in larger system *)
export_code verified_sort in OCaml file "verified_sort.ml"

Certified Libraries

Pattern: Build library of verified functions.

Example:

Module VerifiedList.
  (* Verified operations *)
  Definition safe_nth := ...
  Definition safe_update := ...
  Definition verified_map := ...

  (* Correctness theorems *)
  Theorem safe_nth_correct : ...
  Theorem safe_update_correct : ...
  Theorem verified_map_correct : ...
End VerifiedList.

(* Extract entire module *)
Extraction "verified_list.ml" VerifiedList.

Additional Resources

For detailed guidance on specific aspects:

• Isabelle PCC - Isabelle/HOL code generation and certification
• Coq PCC - Coq extraction and Program framework
• Safety Properties - Common safety and correctness properties