Add Sign in with Zcash use case page

site/Zcash_Use_Cases/Sign_in_With_Zcash.md

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+# <img src="/content-images/icons8-lock-2f8e221321.svg" width="24" height="24" alt="lock icon"/> Sign in with Zcash
+
+<span className="inline-flex items-center gap-[6px]"><span className="inline-block w-[12px] h-[12px] bg-green-500 rounded-full"></span>Intermediate - 7 min</span>
+
+## TL;DR
+
+- Log in by proving you control a Zcash address, instead of using a password
+- Two designs are in use: **signing a challenge**, or **sending a shielded payment with a code in the memo**
+- Because shielded addresses hide balance and history, proving control does not expose your finances
+- The pattern is early. There is no ratified standard yet, and implementations do not interoperate
+
+<br/>
+
+## <img src="/content-images/user-svgrepo-com-21adf62b7c.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="user icon"/> Who is this for?
+
+- Developers who want passwordless login without collecting personal data
+- Users who would rather not hand an email address to every site
+- Anyone who wants to log in without linking their financial history to an account
+
+<br/>
+
+## <img src="/content-images/warning-error-svgrepo-com-b7ea8a50da.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="warning icon"/> The Problem
+
+Most login options leak something:
+
+- **Passwords and email** create an account tied to your identity, and both end up in breach dumps
+- **Social sign-in** tells the identity provider every place you log in and when
+- **Wallet sign-in on transparent chains** is worse than it looks. Connecting a wallet can hand the site your entire balance and transaction history, permanently
+
+You are usually choosing between convenience and disclosure.
+
+<br/>
+
+## <img src="/content-images/celebration-spark-svgrepo-com-bc98dec7c1.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="spark icon"/> Why Zcash?
+
+Zcash separates *proving control* from *revealing finances*:
+
+- **Shielded addresses** keep balances and transaction history private, so proving you hold one says nothing about what you hold
+- **Encrypted memos** can carry a one time login code privately inside a transaction
+- **Viewing keys** allow selective disclosure, so an app can be given read access to exactly what it needs and nothing more
+
+<br/>
+
+## <img src="/content-images/ladder-svgrepo-com-7232bf46ed.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="step icon"/> How It Works
+
+Two approaches have emerged. Both end with the app holding a stable identifier for you and no password.
+
+### Approach 1: Sign a challenge
+
+1. The app generates a random, single use challenge
+2. Your wallet signs that challenge with the key behind your address
+3. The app verifies the signature and logs you in
+
+Nothing is broadcast, so there is no fee and no waiting for a block. The relevant specification is [ZIP 304, Sapling Address Signatures](https://zips.z.cash/zip-0304), which is still a draft, so wallet support for message signing varies.
+
+### Approach 2: Prove it with a shielded payment
+
+1. The app generates a one time code and shows a payment request
+2. You send a small shielded transaction with that code in the memo
+3. The app watches for the memo, matches the code, and logs you in
+
+This works with wallets that already support memos today, which is most of them. The tradeoff is that it costs a network fee and you wait for confirmation.
+
+### Keeping the address private
+
+An app does not have to store your address to recognise you. Some implementations hash it together with an application specific value, so each site sees a different, stable identifier for the same user. That prevents sites from comparing notes to link your accounts.
+
+<br/>
+
+## <img src="/content-images/icons8-toolbox-9bebbb1619.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="toolbox icon"/> Trade-offs
+
+Worth understanding before you build on it or rely on it.
+
+| | Signed challenge | Shielded payment |
+|---|---|---|
+| Cost | Free | Network fee per login |
+| Speed | Instant | Waits for confirmation |
+| Wallet support | Limited, ZIP 304 is a draft | Broad, only needs memos |
+| Leaves a chain record | No | Yes, a transaction exists |
+
+Shared limitations:
+
+- **No account recovery by default.** Losing the key means losing the account, unless the app designs a recovery path
+- **Address reuse can link you.** Using the same address across many sites recreates the tracking problem, which is why app specific identifiers matter
+- **No ratified standard.** Each project has its own scheme, so a login built for one does not work with another
+- **Not anonymity on its own.** It hides your finances from the app, but the app can still profile what you do once you are inside
+
+<br/>
+
+## <img src="/content-images/icons8-cancel-7f786be3c1.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="cancel icon"/> Common Mistakes to Avoid
+
+- Reusing a challenge code. Every code should be single use and expire quickly, or a captured one can be replayed
+- Asking users to send a meaningful amount to log in. The payment is a proof, so the amount should be trivial
+- Storing the raw address when an application specific identifier would do the same job
+- Assuming message signing works everywhere. Check the wallets your users actually have
+- Treating a memo as secret after the fact. It proves the sender acted, it is not a password
+
+<br/>
+
+## <img src="/content-images/checked-checkbox-svgrepo-com-7ea19022da.svg" width="28" height="28" className="inline-block align-middle mr-1 p-[2px]" alt="done icon"/> Projects Exploring This
+
+These were built for the **Zcash Login** track of [ZecHub Hackathon 3.0](https://zechub.wiki/hackathon). They are experiments rather than finished products, and they show how differently the same idea can be built.
+
+- **ZecAuth** - a wallet connection protocol for Zcash, in the spirit of what WalletConnect does elsewhere. The app shows a QR code or `zecauth://` link carrying a challenge plus the capabilities it is asking for, such as sign in, payment requests, or viewing access. No transaction, no fee, no chain interaction. It ships a written protocol specification alongside the code
+- **ZShield** - turns a shielded address into a W3C DID and an OpenID Connect identity. The browser generates a keypair, the server issues a nonce over a ZIP 304 style interface, the wallet signs it, and the server returns a JWT. Because the result is OIDC compatible, existing apps can consume it without bespoke integration
+- **ZecPass** - proves ownership through a signed memo, and is built so the app never learns the user's address at all. It derives an application scoped hash to use as a stable identifier, keeps challenges single use and time bound, and ships a drop in React button with a Node verification library
+- **Portal** - login by sending a shielded transaction with a one time code in the memo, running on mainnet. The same flow is reused to unlock paid content and to send or receive money from a link
+- **ZcashMe** - uses a shielded payment as the proof of identity, and focuses on the desktop to mobile gap so that signing in on a laptop does not require a browser extension
+- **ZBooks** - an accounting and payouts tool that treats sign in with Zcash as a reusable auth primitive rather than the product itself, and reads treasury data through a Unified Full Viewing Key
+
+<br/>
+
+## <img src="/content-images/chain-for-links-svgrepo-com-117ee0dec1.svg" width="24" height="24" className="inline-block align-middle mr-1 p-[2px]" alt="chain-links icon"/> Related pages
+
+- [Memos](/using-zcash/memos) - how encrypted memos work, and how a login code travels inside one
+- [Viewing Keys](/zcash-tech/viewing-keys) - granting read only access without handing over spending power
+- [Keeping Records With Shielded ZEC](/zcash-use-cases/keeping-records-with-shielded-zec) - the same selective disclosure idea, applied to accounting
+- [Send Money Without Linking Identity](/zcash-use-cases/send-money-without-linking-identity) - why address reuse undermines privacy
+
+<br/>