Deciding
The exposure window and Mosca's inequality
How to combine confidentiality lifetime and migration time into an exposure window, using Mosca's inequality as a planning heuristic, not a measurement.
6 min read · 6 min 21 sec film
What you'll be able to do
- State Mosca's inequality and what each variable (X, Y, Z) represents.
- Explain why Z cannot be measured and why the inequality is a heuristic, not a prediction.
- Compute whether a scenario is already late, given X, Y, and Z.
Lesson film
Watch: The exposure window and Mosca's inequality
Two of the three terms are estimable and one never will be — which turns out to be fine, because the comparison between flows does the work.
6 min 21 sec · 8 scenes
6 min 21 sec, narrated by your browser's voice. Captions and a full transcript are below — nothing here needs sound.
The four variables answer whether a specific piece of data is exposed at all.
With the player focused: Space plays or pauses, ← → seek ten seconds, Home restarts, M toggles narration.
Full transcript
1. From exposure to urgency
2. The three terms
3. Four steps
4. What a verdict looks like
5. Heuristic, not measurement
6. Comparing flows
7. X and Y do the work
8. Takeaway
The written lesson
The four variables from Module 2 answer whether a specific piece of data is exposed at all. This lesson adds the timing question: given that exposure, how urgently does it need to be fixed? Michele Mosca's formulation gives a simple way to reason about that: act now if X + Y > Z.
- X
- Required secrecy lifetime — the confidentiality lifetime from Module 2, measured forward from today.
- Y
- Migration time — how long it will actually take to identify, replace, and verify the protecting cryptography for this data flow.
- Z
- Time until a cryptographically relevant quantum computer exists — unknown, and not something this course, or anyone, can measure.
- 01
Estimate X
Take the confidentiality lifetime established in Module 2, measured from today, not from when the data was created.
- 02
Estimate Y
Ask the team that would actually do the migration how long discovery, replacement, and verification would take for this specific flow — not a generic program estimate.
- 03
Treat Z as unknown
No credible source can state a date for a cryptographically relevant quantum computer. Use Z as a labeled, contested planning input, never as a fact plugged into a spreadsheet.
- 04
Compare, and date the verdict
Compare X + Y to explicitly hypothetical values or ranges for Z, and explain when the conclusion changes. Record the date, capture path, and confidence behind X and Y. The inequality helps decide what to investigate and migrate; it does not establish a measured deadline.
X and Y are variables an organization can investigate and sometimes shorten; Z remains uncertain. Long confidentiality lifetimes and slow migration paths can increase urgency under a range of plausible Z scenarios. A decision should also state whether the data and relevant key artifacts are actually capturable and what impact their disclosure would have.
| Hypothetical input | Years from now | Interpretation |
|---|---|---|
| Existing record secrecy ends | 5 | A record captured today may no longer be sensitive at year 10 |
| Classical flow migration finishes | 8 | New records may continue to be captured until then |
| Assumed capable machine appears | 10 | A record captured in year 8 with five-year secrecy can still matter |
The inequality compares a period of continued exposure and the protection lifetime of information introduced during that period. It does not prove that today’s record was captured, or supply a probability for a quantum computer. Compare several explicitly hypothetical Z values and migration estimates; identify which conclusions survive the uncertainty.
Primary reading and claim sources
Publication status checked on the date shown. Draft guidance can change.
- Cybersecurity in an Era with Quantum Computers: Will We Be Ready?Michele Mosca · Research paper · checked 2026-09-26
- TR 104 016: A Repeatable Framework for Quantum-Safe MigrationsETSI · Published technical report · checked 2026-09-26
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