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KeyExchange: signed_version, the server's signature over its version

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The echo catches an advertisement lowered in the clear only once a tagged
frame arrives, so the initiator's first frame has already gone out under
the lower version by then. A server that signs its version instead sets
bit 255 of the ephemeral X25519 key inside the signed keys[0] and puts
sign("rdkx-ver" || key || version LE) here. RFC 7748 masks that bit, so
the box opens as before on either side, and a legacy server, whose keys
are canonical, never sets it. A client that sees the bit, or the field,
verifies before trusting the version, so the pair settles it at the
handshake rather than one frame pair later.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns
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rustdesk
2026-09-22 13:18:45 +08:00
co-authored by Claude Fable 5.1
parent 1266b56150
commit d9f519e915
1 changed files with 3 additions and 0 deletions
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protos/rendezvous.proto
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@@ -238,6 +238,9 @@ message KeyExchange {
// From the server: the newest key exchange version it speaks. From the client: the version it
// picked, the highest both speak. Absent, i.e. 0, is the original scheme.
uint32 version = 2;
// Server signature of "rdkx-ver" || on-wire ephemeral public key || little-endian uint32 version.
// Bit 255 of the signed X25519 public key requires this field; legacy keys leave it clear.
bytes signed_version = 3;
}
message HealthCheck {