The Real Cost of DTLS on a Coin Cell

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

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Ada ADA · CALCULATION AUDIT

The Real Cost of DTLS on a Coin Cell

The chapter’s hospital-wearable example adds DTLS to a CoAP link and reports it costs only about 15% more energy — dropping battery life from 550 to 480 days — even though it grows every message from 85 to 114 bytes. A 34% larger message that costs only 15% more energy is a paradox worth checking on the 2,376 J coin cell. This audit rebuilds the budget to find the real cost of DTLS on a coin cell.

Companion to the chapter CoAP Security and Applications — every number here comes from that chapter.

Ada: The hospital wearable example above claims DTLS adds only about 15% energy – dropping battery life from 550 to 480 days – even though it grows every message from 85 to 114 bytes. A 34% larger message that costs only 15% more energy deserves a check, so let me rebuild the budget from the CR2032 spec and the per-message figures the section states.

  • Coin-cell energy: 0.22 Ah x 3 V x 3600 s = 2376 J – the chapter’s 2,376 J.
  • Handshake amortization: 120 + 140 + 40 + 60 = 360 handshake bytes spread over 2880 messages/day is 360 / 2880 = 0.125 bytes per message – negligible.
  • Per-message size: 13 (DTLS record) + 85 (encrypted CoAP) + 16 (MAC) = 114 bytes versus 85 bytes plain, a 34% growth.
  • Application-layer AES daily energy: 2880 x 1.5 mJ = 4320 mJ = 4.32 J/day, giving 2376 / 4.32 = 550.0 days.
  • DTLS-PSK daily energy: 2880 x 1.72 mJ = 4953.6 mJ = 4.9536 J/day, giving 2376 / 4.9536 = 479.7 days, the chapter’s ~480 days.
  • Energy overhead: 4.9536 / 4.32 = 1.14667, i.e. +14.7% (the chapter’s “15% more energy”).

The audit resolves the apparent paradox: the message grows 34% in bytes but only 14.7% in energy because radio time is not the whole per-message cost – the app-layer option was already paying for AES, and the one-time handshake amortizes to 0.125 bytes each. The design meaning is that transport-layer encryption of the entire message, metadata included, costs about two months of a 550-day battery, which is why the honest release record prices HIPAA-grade confidentiality at “about 15% energy and 70 fewer days,” not “free.”

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