Attack the message, observe the guardrail

1. 01 · SENDER

   Receiver fingerprint pin

   SHA-256(pk) == pinned BB82…29F5

   …
2. 02 · SENDER

   KEM encapsulation

   ML-KEM-512-style → shared secret

   …
3. 03 · SENDER

   HKDF-SHA-256

   shared secret → AES-256 key

   …
4. 04 · SENDER

   AES-256-CCM

   22 B plaintext → 874 B protected

   …
5. 05 · RECEIVER

   Device HMAC-SHA-256

   per-device key over header + ciphertext

   …
6. 06 · RECEIVER

   Sequence counter

   seq > last accepted (EasyFlash)

   …
7. 07 · SENDER

   Authenticated ACK

   CoAP 0x44 2.04 Changed

   …

Illustration of the firmware's check order using measured sizes (22 → 874 → 890 B). No cryptography runs in the browser.