One administrator command, and Windows negotiated post-quantum key exchange with the Internet
Schannel ships the hybrid ML-KEM group disabled. Enabling it took one elevated PowerShell line and no reboot; the far end, a production endpoint, reported X25519MLKEM768 from its side.
The objective was to establish whether a current Windows client can perform post-quantum key exchange, and to establish it by the book's standard: not that a setting was applied, but that the far end of a real connection reports post-quantum key agreement. The measurement was made on a Windows 11 machine at build 26200, on 4 September 2026.
Windows exposes its TLS groups through the elliptic-curve cmdlets, and the hybrid group is accepted by them under a name that is not a curve — an inversion the chapter records on its own account. From an Administrator PowerShell:
Enable-TlsEccCurve x25519_mlkem768
Get-TlsEccCurve # the hybrid now listed after the three classical defaultsNo reboot. Then Windows's own curl.exe, which is built on Schannel rather than on OpenSSL, was pointed at Cloudflare's post-quantum test endpoint, which reports the negotiated parameters as the server saw them:
tls=TLSv1.3
kex=X25519MLKEM768That is the evidence chain the book requires: the server, not the client, says the key exchange was post-quantum. It is also the cheapest post-quantum migration step measured anywhere in the book — a servicing update already installed, plus one elevated command — and it ships disabled, so the finding is an instruction rather than an observation. The fetched page's own text, captured in the transcript, warned that the larger post-quantum ClientHello can break buggy middleboxes; the largest post-quantum deployment's operator restating the book's middlebox caution in the book's own evidence was a pleasing coincidence.
The chapter's scoreboard for Windows at that point read: ML-DSA-87 certificates parsed and verified by CryptoAPI; OpenSSL-minted ML-DSA material ingested, trusted and used to sign; the hybrid negotiated against production infrastructure. The one layer that could not be tested is the one the platform does not yet provide — Schannel presenting an ML-DSA server certificate — and it stands as the chapter's per-servicing-update re-test. A reader with a Windows estate should take from this that confidentiality is a flag away today, and authentication is waiting on Redmond, not on them.
One test artifact was deliberately left in place: the machine's hybrid group stayed enabled, so that from that day it performs post-quantum key exchange with every capable peer. Everything else — test roots, test certificates — was removed, and the removal is in the transcript too.
In the book: Chapter 16 carries the full transcript, the figures and the runbook. In the apparatus: the fix is in the repository, pinned to the versions that were measured. In your estate: if this is the problem you are looking at, this is what an engagement does about it.