AI Agent Policy Signature Verification: Ed25519 Key Rotation Playbook (2026)

The production problem

Policy files are high-impact config. They decide deny, approval, throttle, and allow behavior for autonomous jobs. Treating them like unsigned text files is a control gap.

If an attacker or misconfigured pipeline changes policy bytes, your enforcement logic changes instantly. No code deploy is required. That makes policy integrity a first-class security boundary.

Signature verification closes this gap only when enforced, monitored, and rotated correctly.

What top results miss

Source	Strong coverage	Missing piece
RFC 8032 (EdDSA: Ed25519 and Ed448)	Algorithm and verification behavior for Ed25519 digital signatures.	No operational playbook for policy reload paths in autonomous AI control planes.
Sigstore verify docs	Signature verification workflows, identity constraints, and offline bundle verification.	No guidance for runtime policy file verification before in-process policy evaluation.
TUF specification	Signed metadata model, role separation, and threshold-based trust management.	No direct mapping to single-key, env-driven policy verification in hot-reloaded control planes.

The missing part is policy-engine runtime behavior: exactly when verification runs and what fails closed during reload.

They also skip real rotation pressure: what happens when key and signature rollout are out of sync across replicas.

Signing workflow model

Stage	Objective	Failure mode
Build policy artifact	Generate deterministic policy file bytes	Non-deterministic formatting causes signature mismatch
Sign policy bytes	Apply Ed25519 private key signature to exact file content	Signing transformed bytes (newline/encoding drift) fails verification
Distribute public key	Expose trusted verifier key to runtime via env/config	Missing or malformed key blocks policy load in required mode
Verify before parse	Reject unsigned or invalid policies before evaluation pipeline	Late verification can allow temporary unsafe policy use
Rotate keypair safely	Roll new key and signatures atomically with rollback plan	Key/signature skew causes startup or reload failures

Cordum runtime behavior

Boundary	Current behavior	Operational impact
Signature requirement	Verification required in production or when `SAFETY_POLICY_SIGNATURE_REQUIRED=true`.	Prevents unsigned policy from entering evaluation path in hardened environments.
Public key source	`SAFETY_POLICY_PUBLIC_KEY` accepts base64/hex raw Ed25519 public key.	Operator controls trust root at deployment level.
Signature source order	Kernel checks `SAFETY_POLICY_SIGNATURE`, then `SAFETY_POLICY_SIGNATURE_PATH`, then `<policy-file>.sig` fallback.	Supports inline, detached-file, and local file-based workflows.
Remote source signature behavior	For `http/https` policy sources, missing explicit signature input returns `no signature provided`.	Remote policy fetch cannot silently rely on local `.sig` conventions.
Verification checks	Code validates key/signature length and runs `ed25519.Verify` over exact policy bytes.	Rejects malformed key material and tampered policy artifacts early.
Load pipeline order	`loadPolicyBundle` reads source, verifies signature, then parses policy and computes snapshot hash.	No snapshot or policy object is produced for unsigned/invalid payloads.
Reload failure semantics	On verification failure during watch reload, kernel logs `policy reload failed` and keeps current snapshot.	Fail-stale behavior protects from unsigned updates but can hide drift unless monitored.
Trust threshold	Verifier uses one Ed25519 public key from env; threshold multi-key verification is not implemented.	Rotation needs tight key/signature rollout sequencing to avoid transient reload failures.

Implementation examples

Minimal signing tool (Go)

sign_policy.go

// sign_policy.go
package main

import (
  "crypto/ed25519"
  "encoding/base64"
  "fmt"
  "os"
)

func main() {
  policy, _ := os.ReadFile(os.Args[1])
  priv, _ := os.ReadFile(os.Args[2]) // raw 64-byte Ed25519 private key
  sig := ed25519.Sign(ed25519.PrivateKey(priv), policy)
  fmt.Println(base64.StdEncoding.EncodeToString(sig))
}

Verification path (Go)

verify_policy_signature.go

func verifyPolicySignature(data []byte, source string) error {
  pubRaw := strings.TrimSpace(os.Getenv("SAFETY_POLICY_PUBLIC_KEY"))
  requireSignature := env.IsProduction() || env.Bool("SAFETY_POLICY_SIGNATURE_REQUIRED")

  if pubRaw == "" {
    if requireSignature {
      return errors.New("policy signature required but SAFETY_POLICY_PUBLIC_KEY not configured")
    }
    return nil
  }

  pubKey, _ := decodeKey(pubRaw)
  sig, _ := readSignature(source)
  if !ed25519.Verify(ed25519.PublicKey(pubKey), data, sig) {
    return errors.New("policy signature verification failed")
  }
  return nil
}

Rotation runbook

policy_signature_rotation.sh

Bash

# 1) Configure verifier key and strict mode
export SAFETY_POLICY_SIGNATURE_REQUIRED=true
export SAFETY_POLICY_PUBLIC_KEY="<base64-or-hex-ed25519-pubkey>"

# 2) Provide detached signature
export SAFETY_POLICY_SIGNATURE_PATH=/etc/cordum/policy.sig

# 3) Restart safety kernel and verify no signature errors
kubectl logs deploy/cordum-safety-kernel -n cordum | grep -E "signature|policy updated"

# 4) Rotation drill
# - sign with new private key
# - roll new public key + new signature together
# - validate reload
# - keep old key only for rollback window

Reload drift guard

policy_signature_reload_guard.sh

Bash

# Catch fail-stale quickly after key/signature rollout
kubectl logs deploy/cordum-safety-kernel -n cordum --since=10m   | rg "policy reload failed|policy snapshot updated"

# Expected after healthy rollout:
# - at least one "policy snapshot updated"
# - zero "policy reload failed"

Limitations and tradeoffs

- Signature enforcement improves integrity but adds key management overhead.
- Failed verification can block policy reload and temporarily preserve older policy snapshot.
- Single-key verification has no threshold overlap, so rotation windows are operationally brittle.
- Inline env signatures are convenient but increase secret-handling risk in deployment pipelines.
- Fast key rotation requires disciplined artifact handling and rollback rehearsal.

If signature checks are optional in production, attackers only need one path to unsigned policy injection.

Next step

Run this hardening sequence this week:

1. Enable required signature mode in production environments.
2. Add CI step that signs policy bundle and publishes detached signature artifact.
3. Alert on `policy reload failed` to catch fail-stale conditions immediately.
4. Rehearse key/signature rollout order on a canary replica before fleet-wide rotation.
5. Execute a full key rotation drill with rollback timer and success criteria.

Continue with Policy Decision Cache Invalidation and LLM Safety Kernel.