Stellar stealth cryptography deep-dive page

[truthixify]

Labels: Stellar Wave, stellar, docs, crypto, drips, help-wanted
Tier: M (2–4 days)
Type: writing

Context

The existing docs/sdk/chains/stellar.mdx shows how to call the SDK. It does not explain why the Stellar implementation looks the way it does — why ed25519 instead of secp256k1, why X25519 ECDH, why the specific domain-separation strings ("wraith:scalar:", "wraith:tag:"), why we have a custom signWithScalar.

Without this context, contributors can't safely extend the protocol, and security reviewers (see sdk/01) have to reverse-engineer intent. We want one definitive page.

Scope

Write docs/architecture/stellar-cryptography.mdx:

1. Choice of curve — why ed25519. Comparison to secp256k1 (the EVM/CKB choice). Verifier ecosystem on Stellar.
2. ECDH via X25519 — why we convert Edwards → Montgomery for the shared secret, why this is safe, and a citation chain to RFC 7748 / Curve25519 birational equivalence.
3. Domain separation — list every prefix used ("wraith:spending:", "wraith:viewing:", "wraith:scalar:", "wraith:tag:"), explain what each is separating from what, and link to the file/line where each is used.
4. View tag — how it's computed, what false-positive rate it achieves, what it costs (one extra SHA-256 vs. saved ECDH on rejected announcements).
5. signWithScalar — why standard ed25519 isn't enough (we don't have a seed, we have a derived scalar), how the custom signing path stays compatible with standard verifiers, link to the RFC 8032 audit (sdk/02).
6. Encoding — how meta-addresses are encoded as st:xlm: prefix + StrKey, address derivation as StrKey of the stealth public key.

Style

• Diagrams welcome — keep them in the existing SVG style (use the assets/diagrams/ neighbors as a starting point).
• Every cryptographic claim must cite the line in the SDK where it's implemented.
• Mathematical notation: prefer plain text "P_stealth = K_spend + s_h * G" over LaTeX unless absolutely necessary. Mintlify renders LaTeX inconsistently.

Acceptance criteria

• stellar-cryptography.mdx committed.
• Added to docs.json under Architecture.
• At least 2 new diagrams (key derivation, scanning flow with view tag).
• Reviewed by someone who has not implemented the code (e.g., the Stellar SDK audit assignee from sdk/01).

Why this matters

This is the page we'll point every external auditor at. Investing in it now saves dozens of repetitive walkthroughs later.