After /tool/signup the client switches to SID-only headers (no UDID), so
the next request's body can't be decrypted unless the server already
knows the SID's UDID. ShadowverseSessionService now mirrors the client's
Cute/Cryptographer.MakeMd5(viewerId + udid) formula (salt
"r!I@ws8e5i="), and ToolController.Signup prestores the mapping at the
end. Verified against a live signup capture: viewerId=1 +
udid=62747917-93bc-454c-abb4-ef423b3c9317 produces the captured SID
dc4aac79d35fe15dfb6262e0071bb03c.
Note: this only fixes the fresh-signup path. Clients restarting with a
cached viewer_id (which skip /tool/signup entirely) still hit the same
issue — separate follow-up.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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