Q-Day is a probability, not a date
Q-Day has no established date. Migration planning can consider system lifetimes, the EU critical-infrastructure roadmap and NIST transition guidance. These sources have different scopes and statuses; they do not establish one binding deadline for XDC or all users.
Why Q-Day has no date
"Q-Day" is shorthand for the moment a cryptographically-relevant quantum computer (CRQC) can run Shor's algorithm at the scale needed to recover a private key from a public one. The honest answer to "when?" is that nobody knows — and anyone who gives you a single year is selling certainty that doesn't exist. What we have instead is a distribution of expert estimates that clusters in the early-to-mid 2030s and, crucially, keeps shifting earlier as the algorithms improve. The most-cited recent example: the estimated qubit count to factor RSA-2048 fell roughly 20× between 2019 and 2025 — a software result, not a hardware one. (We unpack that in Reading the 2025 Gidney result.)
Treating a moving probability as a fixed deadline is the trap. You can't build a migration plan around a date that doesn't exist — so you build it around the dates that do.
The deadlines that are fixed
While Q-Day floats, the readiness deadlines are concrete, regulatory, and already on the calendar:
Notice that the last one isn't set by a regulator — it's set by your own data. A signed letter of credit can be contested in court decades after issuance, and the ECDSA signature is the proof. That makes document lifetime, not Q-Day, the binding constraint.
Mosca's inequality
Michele Mosca gave the quantum transition its planning rule, and it needs no precise Q-Day to be useful. If the time your data must stay secure (X) plus the time it takes to migrate (Y) is greater than the time until a CRQC arrives (Z), you are already late.
X (document lifetime, 20–30 yrs) + Y (migration, 4–7 yrs) > Z (Q-Day, ~8–12 yrs out). X alone can exceed most Q-Day estimates — so the inequality is already breached for documents being signed right now.
What XDC plans to
Because the threat date is uncertain but the readiness dates are not, XDC's XDSS-PQ standard is built as a hybrid — classical ECDSA and NIST-selected post-quantum signatures coexist through the transition, with compatibility and verification behavior subject to implementation and testing. The roadmap is paced to the deadlines above (consensus migration targeted for 2030, the EU window), not to a guess about when a quantum computer will arrive. The full plan lives in the technical deep-dive, the XDSS-PQ specification, and the live Q-Day countdown.
Frequently asked
When is Q-Day?
There is no fixed date. Q-Day is a probability distribution, not a calendar entry. Commonly-cited estimates land in the early-to-mid 2030s, but they are estimates that keep moving earlier as algorithms improve — not a scheduled event.
If Q-Day has no date, what deadline should a blockchain plan to?
Use the rules and transition guidance applicable to the specific organization and system, together with its confidentiality and authenticity requirements. The EU roadmap and draft NIST IR 8547 are planning references, not proof of a universal XDC deadline.
What is Mosca's inequality?
If the time your data must stay secure (X) plus the time to migrate (Y) exceeds the time until a quantum computer arrives (Z), you are already late. For 30-year trade documents, X alone can exceed most Q-Day estimates, which is why migration must begin before any working attack exists.
Sources
- NIST Post-Quantum Cryptography — FIPS 203/204 and the IR 8547 transition timeline (deprecate ECDSA 2030, retire 2035). csrc.nist.gov/projects/post-quantum-cryptography
- Gidney, C. "How to factor 2048-bit RSA integers with less than a million noisy qubits." 2025 — the falling-estimate trajectory. arxiv.org/abs/2505.15917
- XDC — Quantum threats to XDC (CRQC timeline, Mosca's theorem, harvest-now-decrypt-later) and the live Q-Day countdown.
Related reading
About this source
Publisher: XDC Innovation Labs. Contact: pq@xdcindia.com. This source-specific contact is separate from the replacement website operator.
Repository snapshot d2345a8 · Source content and historical measurements are attributed to XDC Innovation Labs. Links to independently operated services remain with their providers.