DPoP-SK: Negotiated Symmetric Session Keys for DPoP-Bound Requests

An unofficial editor's draft (W3C Solid CG-shaped, no standing) specifying DPoP-SK — a negotiated, optional fast path for Solid-OIDC resource servers that preserves proof-of-possession while amortizing DPoP's per-request asymmetric signature verification:

1. Establish once: the client presents one ordinary RFC 9449 DPoP proof to a session establishment endpoint; both sides obtain a 32-byte symmetric session key — derived from TLS Exported Keying Material (RFC 8446 §7.5, exporter label EXPERIMENTAL-dpop-sk-v1) for native clients, or server-generated and TLS-delivered once for browsers.
2. Attest per request: an RFC 9421 HTTP Message Signature (hmac-sha256, tag dpop-sk) over ("@method" "@target-uri" "authorization") plus a counter — no fresh asymmetric operation.
3. Anti-replay: an RFC 4303 §3.4.3-style sliding window (verify-then-mark, atomic), tolerant of HTTP/2 out-of-order completion.
4. Never a downgrade: DPoP remains the mandatory, always-accepted Solid-OIDC baseline; the profile is advertised via RFC 9728 Protected Resource Metadata, is invisible without opt-in, and a token is never accepted bare.

The spec is index.html (ReSpec; view rendered at https://jeswr.github.io/dpop-sk-spec/ once published, or open locally — ReSpec loads from w3.org). It graduates §4–§6 of the reviewed design proposal high-throughput-pop-auth into normative text, with an adversarial review pass recorded in the spec's Appendix B — which found and fixed one genuine design-basis flaw (the proposal derived the session key from the RFC 9266 channel-binding value, which RFC 9266 itself forbids using as a secret key; the spec mints a dedicated exporter label instead).

Status

• No implementations exist. This document deliberately precedes code (the design proposal's own graduation condition). The reference implementation is the planned solid-server-rs Tier-2 work item.
• Not a W3C Solid CG work item; published to solicit review.
• The https://w3id.org/jeswr/dpop-sk/v1 profile URI needs a w3id redirect (pending).

Repository contents

File	What
index.html	The specification (ReSpec)
DECISIONS.md	Numbered design decisions + rationale, incl. divergences from the design proposal
tools/compute-examples.mjs	Generates every computed value in the spec's worked example (node tools/compute-examples.mjs; Node ≥ 20, stdlib only; self-checks the jkt against RFC 9449 §6.1's published value)
spec.statements.ttl	Machine-readable normative-statement companion (Turtle sidecar): every normative statement as an anchored spec:Requirement with a verbatim quote, RFC 2119 level, conformance-class binding, testability tag and honest test-gap accounting. COMPLEMENTARY — index.html remains the sole normative text. Format/shapes/validator: jeswr/spec-companion; validate with node <spec-companion>/tools/validate.mjs spec.statements.ttl --spec-html index.html

Provenance

Drafted with AI assistance (Claude Fable 5, Anthropic), from the cited design proposal and primary sources (RFC 9449, RFC 9421, RFC 8446/5705/9266, RFC 4303, RFC 9728, RFC 5869, Solid-OIDC 0.1.0), each verified against the primary text during drafting. Awaiting review by the human editor (Jesse Wright). See the spec's SOTD.