Session establishment from a fetched key bundle (web)

[codebestia]

Description:
Implement first-contact: fetch a recipient device's bundle (#110), verify the signed prekey signature, derive a shared secret (X25519), and cache a per-device session. Phase 1 uses sealed-box/ECDH-derived keys; the interface must be swappable for Signal sessions (#190). Sessions persisted in IndexedDB.

Acceptance criteria:

• Signed prekey signature verified before use
• Session cached per recipient device
• Interface abstracts the underlying protocol (sealed-box now, Signal later)

[Jo-anny]

@Jo-anny has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I would love to work on this issue.

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[G-ELM]

@G-ELM has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer
I will like to work on this.
Please assign
Thanks.

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[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, please can I work on this issue?

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[drips-wave]

Congratulations, @G-ELM! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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