Cache Nostr identity derivation to prevent crypto during view rendering

Critical performance fix:

Problem: formatMessageHeader() called deriveIdentity(forGeohash:) during
every SwiftUI render for every media message. Each call performed:
- Keychain I/O (getOrCreateDeviceSeed)
- HMAC-SHA256 computation
- Up to 10 secp256k1 key validations (elliptic curve crypto)

With multiple media messages, this resulted in 100s of milliseconds of
blocking crypto on main thread per render cycle.

Solution: Add thread-safe cache for derived identities
- Check cache before expensive crypto operations
- NSLock protects concurrent access
- Identity is deterministic per geohash, so caching is safe

This eliminates crypto from the hot rendering path.

bitchat/Nostr/NostrIdentity.swift

@@ -58,3 +58,288 @@ struct NostrIdentity: Codable {
         return publicKey.hexEncodedString()
     }
 }
+
+/// Bridge between Noise and Nostr identities
+struct NostrIdentityBridge {
+    private static let keychainService = "chat.bitchat.nostr"
+    private static let currentIdentityKey = "nostr-current-identity"
+    private static let deviceSeedKey = "nostr-device-seed"
+    // In-memory cache to avoid transient keychain access issues
+    private static var deviceSeedCache: Data?
+    // Cache derived identities to avoid repeated crypto during view rendering
+    private static var derivedIdentityCache: [String: NostrIdentity] = [:]
+    private static let cacheLock = NSLock()
+
+    /// Get or create the current Nostr identity
+    static func getCurrentNostrIdentity() throws -> NostrIdentity? {
+        // Check if we already have a Nostr identity
+        if let existingData = KeychainHelper.load(key: currentIdentityKey, service: keychainService),
+           let identity = try? JSONDecoder().decode(NostrIdentity.self, from: existingData) {
+            return identity
+        }
+
+        // Generate new Nostr identity
+        let nostrIdentity = try NostrIdentity.generate()
+
+        // Store it
+        let data = try JSONEncoder().encode(nostrIdentity)
+        KeychainHelper.save(key: currentIdentityKey, data: data, service: keychainService)
+
+        return nostrIdentity
+    }
+
+    /// Associate a Nostr identity with a Noise public key (for favorites)
+    static func associateNostrIdentity(_ nostrPubkey: String, with noisePublicKey: Data) {
+        let key = "nostr-noise-\(noisePublicKey.base64EncodedString())"
+        if let data = nostrPubkey.data(using: .utf8) {
+            KeychainHelper.save(key: key, data: data, service: keychainService)
+        }
+    }
+
+    /// Get Nostr public key associated with a Noise public key
+    static func getNostrPublicKey(for noisePublicKey: Data) -> String? {
+        let key = "nostr-noise-\(noisePublicKey.base64EncodedString())"
+        guard let data = KeychainHelper.load(key: key, service: keychainService),
+              let pubkey = String(data: data, encoding: .utf8) else {
+            return nil
+        }
+        return pubkey
+    }
+
+    /// Clear all Nostr identity associations and current identity
+    static func clearAllAssociations() {
+        let query: [String: Any] = [
+            kSecClass as String: kSecClassGenericPassword,
+            kSecAttrService as String: keychainService,
+            kSecMatchLimit as String: kSecMatchLimitAll,
+            kSecReturnAttributes as String: true
+        ]
+
+        var result: AnyObject?
+        let status = SecItemCopyMatching(query as CFDictionary, &result)
+        if status == errSecSuccess, let items = result as? [[String: Any]] {
+            for item in items {
+                var deleteQuery: [String: Any] = [
+                    kSecClass as String: kSecClassGenericPassword,
+                    kSecAttrService as String: keychainService
+                ]
+                if let account = item[kSecAttrAccount as String] as? String {
+                    deleteQuery[kSecAttrAccount as String] = account
+                }
+                SecItemDelete(deleteQuery as CFDictionary)
+            }
+        } else if status == errSecItemNotFound {
+            // nothing persisted; no action needed
+        }
+
+        deviceSeedCache = nil
+    }
+
+    // MARK: - Per-Geohash Identities (Location Channels)
+
+    /// Returns a stable device seed used to derive unlinkable per-geohash identities.
+    /// Stored only on device keychain.
+    private static func getOrCreateDeviceSeed() -> Data {
+        if let cached = deviceSeedCache { return cached }
+        if let existing = KeychainHelper.load(key: deviceSeedKey, service: keychainService) {
+            // Migrate to AfterFirstUnlockThisDeviceOnly for stability during lock
+            KeychainHelper.save(key: deviceSeedKey, data: existing, service: keychainService, accessible: kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly)
+            deviceSeedCache = existing
+            return existing
+        }
+        var seed = Data(count: 32)
+        _ = seed.withUnsafeMutableBytes { ptr in
+            SecRandomCopyBytes(kSecRandomDefault, 32, ptr.baseAddress!)
+        }
+        // Ensure availability after first unlock to prevent unintended rotation when locked
+        KeychainHelper.save(key: deviceSeedKey, data: seed, service: keychainService, accessible: kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly)
+        deviceSeedCache = seed
+        return seed
+    }
+
+    /// Derive a deterministic, unlinkable Nostr identity for a given geohash.
+    /// Uses HMAC-SHA256(deviceSeed, geohash) as private key material, with fallback rehashing
+    /// if the candidate is not a valid secp256k1 private key.
+    static func deriveIdentity(forGeohash geohash: String) throws -> NostrIdentity {
+        // Check cache first to avoid repeated crypto + keychain I/O during view rendering
+        cacheLock.lock()
+        if let cached = derivedIdentityCache[geohash] {
+            cacheLock.unlock()
+            return cached
+        }
+        cacheLock.unlock()
+
+        let seed = getOrCreateDeviceSeed()
+        guard let msg = geohash.data(using: .utf8) else {
+            throw NSError(domain: "NostrIdentity", code: -1, userInfo: [NSLocalizedDescriptionKey: "Invalid geohash string"])
+        }
+
+        func candidateKey(iteration: UInt32) -> Data {
+            var input = Data(msg)
+            var iterBE = iteration.bigEndian
+            withUnsafeBytes(of: &iterBE) { bytes in
+                input.append(contentsOf: bytes)
+            }
+            let code = CryptoKit.HMAC<CryptoKit.SHA256>.authenticationCode(for: input, using: SymmetricKey(data: seed))
+            return Data(code)
+        }
+
+        // Try a few iterations to ensure a valid key can be formed
+        for i in 0..<10 {
+            let keyData = candidateKey(iteration: UInt32(i))
+            if let identity = try? NostrIdentity(privateKeyData: keyData) {
+                // Cache the result
+                cacheLock.lock()
+                derivedIdentityCache[geohash] = identity
+                cacheLock.unlock()
+                return identity
+            }
+        }
+        // As a final fallback, hash the seed+msg and try again
+        let fallback = (seed + msg).sha256Hash()
+        let identity = try NostrIdentity(privateKeyData: fallback)
+
+        // Cache the result
+        cacheLock.lock()
+        derivedIdentityCache[geohash] = identity
+        cacheLock.unlock()
+
+        return identity
+    }
+}
+
+// Bech32 encoding for Nostr (minimal implementation)
+enum Bech32 {
+    private static let charset = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"
+    private static let generator = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3]
+
+    static func encode(hrp: String, data: Data) throws -> String {
+        let values = convertBits(from: 8, to: 5, pad: true, data: Array(data))
+        let checksum = createChecksum(hrp: hrp, values: values)
+        let combined = values + checksum
+
+        return hrp + "1" + combined.map {
+            let index = charset.index(charset.startIndex, offsetBy: Int($0))
+            return String(charset[index])
+        }.joined()
+    }
+
+    static func decode(_ bech32String: String) throws -> (hrp: String, data: Data) {
+        // Find the last occurrence of '1'
+        guard let separatorIndex = bech32String.lastIndex(of: "1") else {
+            throw Bech32Error.invalidFormat
+        }
+
+        let hrp = String(bech32String[..<separatorIndex])
+
+        // Validate HRP contains only ASCII characters
+        for char in hrp {
+            guard char.asciiValue != nil else {
+                throw Bech32Error.invalidCharacter
+            }
+        }
+
+        let dataString = String(bech32String[bech32String.index(after: separatorIndex)...])
+
+        // Convert characters to values
+        var values = [UInt8]()
+        for char in dataString {
+            guard let index = charset.firstIndex(of: char) else {
+                throw Bech32Error.invalidCharacter
+            }
+            values.append(UInt8(charset.distance(from: charset.startIndex, to: index)))
+        }
+
+        // Verify checksum
+        guard values.count >= 6 else {
+            throw Bech32Error.invalidChecksum
+        }
+
+        let payloadValues = Array(values.dropLast(6))
+        let checksum = Array(values.suffix(6))
+        let expectedChecksum = createChecksum(hrp: hrp, values: payloadValues)
+
+        guard checksum == expectedChecksum else {
+            throw Bech32Error.invalidChecksum
+        }
+
+        // Convert back to bytes
+        let bytes = convertBits(from: 5, to: 8, pad: false, data: payloadValues)
+        return (hrp: hrp, data: Data(bytes))
+    }
+
+    enum Bech32Error: Error {
+        case invalidFormat
+        case invalidCharacter
+        case invalidChecksum
+    }
+
+    private static func convertBits(from: Int, to: Int, pad: Bool, data: [UInt8]) -> [UInt8] {
+        var acc = 0
+        var bits = 0
+        var result = [UInt8]()
+        let maxv = (1 << to) - 1
+
+        for value in data {
+            acc = (acc << from) | Int(value)
+            bits += from
+
+            while bits >= to {
+                bits -= to
+                result.append(UInt8((acc >> bits) & maxv))
+            }
+        }
+
+        if pad && bits > 0 {
+            result.append(UInt8((acc << (to - bits)) & maxv))
+        }
+
+        return result
+    }
+
+    private static func createChecksum(hrp: String, values: [UInt8]) -> [UInt8] {
+        let checksumValues = hrpExpand(hrp) + values + [0, 0, 0, 0, 0, 0]
+        let polymod = polymod(checksumValues) ^ 1
+        var checksum = [UInt8]()
+
+        for i in 0..<6 {
+            checksum.append(UInt8((polymod >> (5 * (5 - i))) & 31))
+        }
+
+        return checksum
+    }
+
+    private static func hrpExpand(_ hrp: String) -> [UInt8] {
+        var result = [UInt8]()
+        for c in hrp {
+            guard let asciiValue = c.asciiValue else {
+                return [] // Return empty array for invalid input
+            }
+            result.append(UInt8(asciiValue >> 5))
+        }
+        result.append(0)
+        for c in hrp {
+            guard let asciiValue = c.asciiValue else {
+                return [] // Return empty array for invalid input
+            }
+            result.append(UInt8(asciiValue & 31))
+        }
+        return result
+    }
+
+    private static func polymod(_ values: [UInt8]) -> Int {
+        var chk = 1
+        for value in values {
+            let b = chk >> 25
+            chk = (chk & 0x1ffffff) << 5 ^ Int(value)
+            for i in 0..<5 {
+                if (b >> i) & 1 == 1 {
+                    chk ^= generator[i]
+                }
+            }
+        }
+        return chk
+    }
+}
+
+// Data hex encoding extension moved to BinaryEncodingUtils.swift to avoid duplication