Root cause: web client's bundle included OTPKs, so X3DH initiate()
did 4 DH ops (DH4 with OTPK). But decrypt_wire_message() called
respond() with None for OTPK, doing only 3 DH ops.
Different DH concat → different shared secret → decrypt fails.
Fix: web client bundles have one_time_pre_key: None.
initiate() skips DH4 when no OTPK present.
respond() also skips DH4 with None.
Both sides now do exactly 3 DH ops → shared secrets match.
OTPKs are an anti-replay optimization, not required for E2E.
Will add OTPK support to web client in Phase 2 with proper
server-side OTPK storage and consumption tracking.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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