Chapter 19 — Identity and cryptography | Engineering Cryptography and trust services

Open, free learning for anyone who wants to protect people and technology. No account is required to read this text or its worked answers.

Chapter 19 — Identity and cryptography

Lesson map

Lesson 19 published. Map, example, EX055.

flowchart TD
  N0["Learning outcomes"]
  N1["19.1 Device identity"]
  N2["Lab — Message authentication"]
  N3["19.2 Reading the HMAC run"]
  N4["19.3 People and revocation"]
  N5["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5
  N5 --> EX["Worked example"]
  EX --> Q["Exercise EX055"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["19.1 Device identity"]
  N2["Lab — Message authentication"]
  N3["19.2 Reading the HMAC run"]
  N4["19.3 People and revocation"]
  N5["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> Q["EX055"]

[Learning outcomes]
    |
    v
[19.1 Device identity]
    |
    v
[Lab — Message authentication]
    |
    v
[19.2 Reading the HMAC run]
    |
    v
[19.3 People and revocation]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX055]

Learning outcomes

You will (1) authenticate devices and people with keys and certificates; (2) use HMAC for message authentication; (3) relate identities to certificates and revocation.

19.1 Device identity

Each Ridgeway meter needs an identity that others can verify. Aiyana compares two approaches. Symmetric identity uses a per-meter secret key shared with the key service; it is cheap but the service must protect every key. Asymmetric identity gives each meter a key pair and a certificate signed by Ridgeway's device authority, following the X.509 profile in RFC 5280; verifiers need only public keys. She recommends symmetric HMAC keys for readings, where volume is high, and certificates for collectors, which authenticate to the gateway over TLS.

Lab — Message authentication

"""Authenticate synthetic meter readings with HMAC-SHA256 (RFC 2104) and constant-time compare."""
import hmac, hashlib
KEY = bytes(range(32))   # demo key; real keys come from the utility's key service
def tag(msg): return hmac.new(KEY, msg, hashlib.sha256).hexdigest()

def main():
    msg = b"meter=M-0417;litres=18342;ts=2026-04-01T06:00Z"
    t = tag(msg)
    print("tag", t[:24], "...")
    print("genuine verifies:", hmac.compare_digest(t, tag(msg)))
    forged = msg.replace(b"18342", b"08342")
    print("edited reading verifies:", hmac.compare_digest(t, tag(forged)))

if __name__ == "__main__":
    main()

Run with python3 lab_hmac.py. Verified output:

tag bc7de0101afc1c79bfc23747 ...
genuine verifies: True
edited reading verifies: False

19.2 Reading the HMAC run

The lab tags a canonical reading with HMAC-SHA256 and verifies it. The genuine reading verifies and the edited reading, with one digit changed, fails, because without the key nobody can compute a valid tag for a modified message. Verification uses hmac.compare_digest to avoid timing leaks. The demonstration key is a fixed byte sequence only so the output is reproducible; real keys come from the key service. Aiyana stresses that HMAC gives integrity and origin authentication between parties sharing the key, but not non-repudiation, because either party could have produced the tag.

19.3 People and revocation

People at Ridgeway authenticate with hardware security keys and short-lived certificates for administrative access. When someone leaves, their access must end quickly. Aiyana documents revocation for each identity type: meter keys are disabled in the key service; collector certificates are added to a revocation list or simply allowed to expire, since they are short-lived; staff certificates are revoked and their tokens disabled. She notes that short lifetimes reduce dependence on revocation checking, which clients perform unevenly. Revocation steps are rehearsed twice a year so that staff know them under pressure.

Worked example

Aiyana chooses HMAC per-meter keys for readings and certificates for collectors and staff, demonstrates tag verification, and documents revocation per identity type.

Synthetic case. Identities and the demo key are invented; RFC 5280 and RFC 2104 are real.

Errors and limits

HMAC cannot prove to a third party which side created a message. Long-lived certificates make revocation essential and hard.

Exercises

EX055 — Symmetric or asymmetric. Why use HMAC keys for readings but certificates for collectors? Rubric: 3 = high-volume readings suit cheap symmetric tags; collectors use TLS with public verification;; 2 = reasons partial;; 1 = random;; 0 = none.
EX056 — Edited reading. What did the lab print for the edited reading, and why? Rubric: 3 = False, since a valid tag cannot be computed without the key;; 2 = False;; 1 = True;; 0 = none.
EX057 — Non-repudiation. Why does HMAC not give non-repudiation? Rubric: 3 = both parties share the key, so either could have made the tag;; 2 = shared key;; 1 = it does;; 0 = none.

Synthesis

Identity combines keys, certificates and revocation; HMAC authenticates high-volume messages between key holders, while certificates scale verification.

Continue studying — chapter 19 of 50

← Chapter 18 (draft) · Course chapter index · Chapter 20 (draft) →