{
  "schema_version": "1.3.0",
  "slug": "what-signature-downgrade-invariants-prevent-agents-from-accepting-weaker-cryptographic-proofs-when-upgraded-proofs-fail",
  "title": "What signature downgrade invariants prevent agents from accepting weaker cryptographic proofs when upgraded proofs fail?",
  "date_of_record": "2026 October 20",
  "scope": "* **Applies when:** Signatures can be verified at multiple strengths, downgrade attacks are possible, and the receiver's security depends on strength. * **Does not apply when:** All signatures are verified at a single strength, or downgrade",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/what-signature-downgrade-invariants-prevent-agents-from-accepting-weaker-cryptographic-proofs-when-upgraded-proofs-fail.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/what-signature-downgrade-invariants-prevent-agents-from-accepting-weaker-cryptographic-proofs-when-upgraded-proofs-fail.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/what-signature-downgrade-invariants-prevent-agents-from-accepting-weaker-cryptographic-proofs-when-upgraded-proofs-fail.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1057,
    "python_slice_tokens": 321,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The separation between **signature presence** (some signature is available) and **signature strength** (the signature meets the receiver's required security level). Relying on presence creates the **Downgrade Exploitation**: an attacker strips the strong signature (e.g., the post-quantum component of a hybrid signature), leaving only the weak signature, and ...",
    "what_was_observed": "In multi-strength signature architectures, failing to prevent downgrade produces three primary failure modes: 1. **Hybrid Downgrade:** A message is signed with classical + post-quantum hybrid. An attacker strips the post-quantum component. The receiver verifies only the classical component and accepts the message, losing the quantum-resistance the sender intended. 2. **Algorithm Confusion:** The receiver accepts any ...",
    "condition_of_validity": "* **Applies when:** Signatures can be verified at multiple strengths, downgrade attacks are possible, and the receiver's security depends on strength. * **Does not apply when:** All signatures are verified at a single strength, or downgrade is explicitly accepted.",
    "decision_boundary": "1. **Does the signature meet the `required_signature_strength_receipt`?** If no, the signature is insufficient. Reject. 2. **Is the `hybrid_completeness_receipt` present for hybrid-required signatures?** If no, a component may be stripped. Reject. 3. **Does the `signature_algorithm_attestation_signature` confirm the expected algorithm, not just a valid one?*...",
    "operational_effect": "1. **Downgrade Prevention:** Receivers reject signatures that fail to meet required strength, preventing hybrid stripping and algorithm confusion. 2. **Algorithm Strictness:** Specific algorithms are required, not just any valid algorithm. 3. **Rotation Safety:** Key rotation epochs prevent stale signature acceptance."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/according-to-what-precedence-hierarchy-must-an-instance-resolve-contradictory-directives-across-system-developer-and-user-tiers.md",
      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/how-can-an-instance-capture-implicit-user-preferences-without-storing-private-data.md",
      "https://corpusfetch.com/c/how-can-an-instance-compress-long-reasoning-traces-into-binding-conclusions.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "848370895d98da3084c491756061372c3af2ffa619a650689e8d7a26d878769a",
    "verified_classes": [
      "SignatureVerdict",
      "Signature",
      "SignatureRequirements",
      "SignaturePolicy",
      "SignatureManifest",
      "DowngradeGuard"
    ],
    "verified_functions": [
      "_hash",
      "evaluate_signature"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 0,
    "raised_exceptions": [],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "enum",
      "hashlib",
      "json"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}