Merge pull request #166 from synonymdev/sync-backup

Synchronous backups

backup-server/.env.template

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+HOST=0.0.0.0
+PORT=3003
+# npm run create-keypair
+SECRET_KEY=
+PUBLIC_KEY=

backup-server/.gitignore

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+local-storage
+node_modules
+.env

backup-server/README.md

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+# react-native-ldk backup server
+
+This a server allows apps using the react-native-ldk to persist all node state remotely and can be restored only using the seed.
+
+## Running the server
+``` bash
+npm i
+
+cp .env.template .env
+
+npm run create-keypair
+
+#Paste new key pair in .env
+
+npm start
+```
+** Remember to update wallet env with new backup server pub key
+
+## Clients
+[Swift](https://github.com/synonymdev/react-native-ldk/blob/master/lib/ios/Classes/BackupClient.swift)
+[Kotlin](https://github.com/synonymdev/react-native-ldk/blob/master/lib/android/src/main/java/com/reactnativeldk/classes/BackupClient.kt)
+[NodeJS](https://github.com/synonymdev/react-native-ldk/blob/master/backup-server/src/test.js)
+
+## Persiting
+All message signing/verifying is done using [LDK's node signing](https://docs.rs/lightning/latest/lightning/util/message_signing/fn.sign.html) on the client and [ln-verifymessagejs](https://github.com/SeverinAlexB/ln-verifymessagejs) on the server.
+
+1. Payload is encrypted using using standard AES/GCM encryption with the encryption key being the node secret.
+2. Client creates a hash of encrypted backup and signs it.
+3. Client creates unique challenge in this format: `sha256_hash(pubkey+timestamp)`
+4. Client uploads encrypted bytes along with node pubkey, signed hash and challenge in request header.
+5. Server hashes received payload and validates client's signed hash was actually signed by provided pubkey.
+6. Server stores encrypted bytes to disk.
+7. Server signs client's challenge and returns signature in response.
+8. Client validate that the bytes were stored by the correct server by checking the signature in the response was signed by the server pubkey hard coded in the client.
+
+## Retrieving
+Retieving or querying a backup requires a bearer token first done by a fairly standard challenge/response using the same node signing.
+
+1. Client fetches challenge from server by posting timestamp (nonce) and signed (signed timestamp) in body with pubkey in the header.
+2. Server validates signature and returns challenge (32 bytes hex string).
+3. Client signs challenge.
+4. Client posts signed challenge with pubkey in the header.
+5. Server validates signature.
+6. On success server returns bearer token with 5min expiry. A long expiry isn't needed as token is only used briefly to perform a restore.
+7. Client uses bearer token to pull list of backed up files.
+8. Client iterates through list and downloads each file and persists to disk.

backup-server/index.js

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+const start = require('./src/server');
+require('dotenv').config();
+
+const HOST = process.env.HOST;
+const PORT = process.env.PORT;
+const SECRET_KEY = process.env.SECRET_KEY;
+const PUBLIC_KEY = process.env.PUBLIC_KEY;
+
+if (!HOST || !PORT || !SECRET_KEY || !PUBLIC_KEY) {
+    console.error('HOST, PORT, SECRET_KEY or PUBLIC_KEY environment variable is not set');
+    process.exit(1);
+}
+
+start({host: HOST, port: PORT, keypair: {secretKey: SECRET_KEY, publicKey: PUBLIC_KEY}})
+    .then(() => {
+    console.log("Server started");
+})
+    .catch((error) => {
+        console.error(error);
+        process.exit(1);
+    });