Session Reuse, Not a Cheaper Cipher, Makes TLS Affordable

Ada re-derives this chapter’s own numbers step by step, at full precision

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Ada ADA · CALCULATION AUDIT

Session Reuse, Not a Cheaper Cipher, Makes TLS Affordable

Turning on TLS adds a one-time handshake worth 0.72 mAs every time a sensor connects, while each message it sends costs about 0.4 mAs. Hold one connection open for a day of 1440 messages and that handshake is spread almost to nothing; reconnect for every reading and it is paid in full each time. This audit amortises the same 0.72 mAs both ways to test the claim in its title: is it session reuse, not a cheaper cipher, that makes TLS affordable?

Companion to the chapter MQTT Security Fundamentals — every number here comes from that chapter.

Ada: The callout and calculator both say the TLS handshake energy “falls away” over a session. Let me make that precise, because the same 0.72 mAs is either negligible or dominant depending entirely on how often you reconnect.

The handshake energy is a fixed, one-time cost per connection:

  • E_handshake = 6 packets x (8 mA x 15 ms) = 6 x 120 mA-ms = 720 mA-ms = 0.72 mAs

Amortise it over a one-day session of N = 1440 messages, each costing about 0.4 mAs to send:

  • Handshake share per message: 0.72 / 1440 = 0.0005 mAs
  • Total per message: 0.0005 + 0.4 = 0.4005 mAs
  • Handshake as a fraction of energy: 0.0005 / 0.4005 = 0.12%

Now reconnect for every single message instead, and each message pays the full handshake:

  • Per message: 0.72 + 0.4 = 1.12 mAs, of which the handshake is 0.72 / 1.12 = 64.3%

The cipher and the 0.72 mAs never changed; only the number of messages sharing one handshake did. Keeping the connection persistent drives the handshake from 64% of per-message energy down to 0.12%, so the design lever that makes TLS practical on a battery is session reuse, not a lighter TLS version, and the one-time 100 ms (TLS 1.3) or 200 ms (TLS 1.2) latency is paid once rather than on every reading.

Every number above is taken from the chapter’s own material and re-derived step by step.