Friend-courier store-and-forward: mutual favorites carry sealed messages to offline peers

When a private message has no reachable transport, the router now seals it
to the recipient's Noise static key (new one-way Noise X pattern) and hands
the envelope to up to three connected mutual favorites. Couriers store the
opaque ciphertext under strict quotas (20 total, 5 per depositor, 16 KiB,
24 h) and hand it over when the recipient's announce matches a rotating
HMAC recipient tag; the recipient opens it and the message flows through
the normal private-message pipeline, so dedup and delivery acks just work.

- CourierEnvelope TLV + courierEnvelope (0x04) message type in BitFoundation
- Noise X one-way pattern reusing the existing handshake machinery,
  domain-separated by a courier prologue; sender identity authenticated
  via the ss DH (no forward secrecy - documented tradeoff)
- CourierStore with eviction, file persistence, and panic-wipe integration
- Rotating recipient tags (HMAC over epoch day) so carried envelopes don't
  correlate for observers who don't already know the recipient's key
- New "carried" delivery status with figure.walk glyph; header indicator
  while carrying mail for others
- Three-node end-to-end test ferrying packets through real BLEService
  instances, plus codec/crypto/store/router suites (986 tests green)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

localPackages/BitFoundation/Sources/BitFoundation/CourierEnvelope.swift

@@ -0,0 +1,147 @@
+//
+// CourierEnvelope.swift
+// BitFoundation
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+import Foundation
+private import CryptoKit
+
+/// TLV payload for store-and-forward courier envelopes.
+///
+/// A courier envelope lets a mutual favorite physically carry an encrypted
+/// message to a peer who is currently offline. The envelope is opaque to the
+/// courier: the only routing information is a rotating recipient tag derived
+/// from the recipient's Noise static public key and the UTC day, so envelopes
+/// addressed to the same peer on different days do not correlate for
+/// observers who don't already know that peer's public key.
+public struct CourierEnvelope: Equatable {
+    /// Rotating recipient hint: HMAC-SHA256(recipient static key, context || epoch day), truncated.
+    public let recipientTag: Data
+    /// Milliseconds since epoch after which the envelope must be discarded.
+    public let expiry: UInt64
+    /// Opaque one-way Noise X ciphertext (sender identity rides inside).
+    public let ciphertext: Data
+
+    public static let tagLength = 16
+    /// Couriered messages are text-sized; media transfers are out of scope.
+    public static let maxCiphertextBytes = 16 * 1024
+    /// Matches the outbox retention policy in MessageRouter.
+    public static let maxLifetimeSeconds: TimeInterval = 24 * 60 * 60
+
+    private enum TLVType: UInt8 {
+        case recipientTag = 0x01
+        case expiry = 0x02
+        case ciphertext = 0x03
+    }
+
+    public init(recipientTag: Data, expiry: UInt64, ciphertext: Data) {
+        self.recipientTag = recipientTag
+        self.expiry = expiry
+        self.ciphertext = ciphertext
+    }
+
+    public var isExpired: Bool {
+        isExpired(at: Date())
+    }
+
+    public func isExpired(at date: Date) -> Bool {
+        UInt64(max(0, date.timeIntervalSince1970 * 1000)) >= expiry
+    }
+
+    public func encode() -> Data? {
+        guard recipientTag.count == Self.tagLength else { return nil }
+        guard !ciphertext.isEmpty, ciphertext.count <= Self.maxCiphertextBytes else { return nil }
+
+        func appendBE<T: FixedWidthInteger>(_ value: T, into data: inout Data) {
+            var big = value.bigEndian
+            withUnsafeBytes(of: &big) { data.append(contentsOf: $0) }
+        }
+
+        var encoded = Data()
+        encoded.reserveCapacity(3 * 3 + Self.tagLength + 8 + ciphertext.count)
+
+        encoded.append(TLVType.recipientTag.rawValue)
+        appendBE(UInt16(recipientTag.count), into: &encoded)
+        encoded.append(recipientTag)
+
+        encoded.append(TLVType.expiry.rawValue)
+        appendBE(UInt16(8), into: &encoded)
+        appendBE(expiry, into: &encoded)
+
+        encoded.append(TLVType.ciphertext.rawValue)
+        appendBE(UInt16(ciphertext.count), into: &encoded)
+        encoded.append(ciphertext)
+
+        return encoded
+    }
+
+    public static func decode(_ data: Data) -> CourierEnvelope? {
+        var cursor = data.startIndex
+        let end = data.endIndex
+
+        var recipientTag: Data?
+        var expiry: UInt64?
+        var ciphertext: Data?
+
+        while cursor < end {
+            let typeRaw = data[cursor]
+            cursor = data.index(after: cursor)
+
+            guard data.distance(from: cursor, to: end) >= 2 else { return nil }
+            let length = Int(data[cursor]) << 8 | Int(data[data.index(after: cursor)])
+            cursor = data.index(cursor, offsetBy: 2)
+            guard data.distance(from: cursor, to: end) >= length else { return nil }
+            let value = data[cursor..<data.index(cursor, offsetBy: length)]
+            cursor = data.index(cursor, offsetBy: length)
+
+            switch TLVType(rawValue: typeRaw) {
+            case .recipientTag:
+                guard length == tagLength else { return nil }
+                recipientTag = Data(value)
+            case .expiry:
+                guard length == 8 else { return nil }
+                expiry = value.reduce(UInt64(0)) { ($0 << 8) | UInt64($1) }
+            case .ciphertext:
+                guard length > 0, length <= maxCiphertextBytes else { return nil }
+                ciphertext = Data(value)
+            case nil:
+                // Unknown TLV: skip for forward compatibility.
+                continue
+            }
+        }
+
+        guard let recipientTag, let expiry, let ciphertext else { return nil }
+        return CourierEnvelope(recipientTag: recipientTag, expiry: expiry, ciphertext: ciphertext)
+    }
+
+    // MARK: - Recipient Tags
+
+    private static let tagContext = Data("bitchat-courier-tag-v1".utf8)
+
+    /// UTC day number used to rotate recipient tags.
+    public static func epochDay(for date: Date) -> UInt32 {
+        UInt32(max(0, date.timeIntervalSince1970) / 86_400)
+    }
+
+    /// Rotating recipient hint for a given day. Computable only by parties
+    /// who already know the recipient's Noise static public key.
+    public static func recipientTag(noiseStaticKey: Data, epochDay: UInt32) -> Data {
+        var message = tagContext
+        withUnsafeBytes(of: epochDay.bigEndian) { message.append(contentsOf: $0) }
+        let mac = HMAC<SHA256>.authenticationCode(for: message, using: SymmetricKey(data: noiseStaticKey))
+        return Data(mac).prefix(tagLength)
+    }
+
+    /// Tags to test when checking whether an envelope is addressed to a peer.
+    /// Covers the adjacent days so envelopes sealed near midnight (or across
+    /// modest clock skew) still match while being carried.
+    public static func candidateTags(noiseStaticKey: Data, around date: Date) -> [Data] {
+        let day = epochDay(for: date)
+        return [day == 0 ? 0 : day - 1, day, day + 1].map {
+            recipientTag(noiseStaticKey: noiseStaticKey, epochDay: $0)
+        }
+    }
+}

localPackages/BitFoundation/Sources/BitFoundation/DeliveryStatus.swift

@@ -11,17 +11,20 @@ import struct Foundation.Date
 public enum DeliveryStatus: Codable, Equatable, Hashable {
     case sending
     case sent  // Left our device
+    case carried  // Sealed envelope handed to a courier; best-effort physical delivery
     case delivered(to: String, at: Date)  // Confirmed by recipient
     case read(by: String, at: Date)  // Seen by recipient
     case failed(reason: String)
     case partiallyDelivered(reached: Int, total: Int)  // For rooms
-    
+
     public var displayText: String {
         switch self {
         case .sending:
             return "Sending..."
         case .sent:
             return "Sent"
+        case .carried:
+            return "Carried by a friend"
         case .delivered(let nickname, _):
             return "Delivered to \(nickname)"
         case .read(let nickname, _):

localPackages/BitFoundation/Sources/BitFoundation/MessageType.swift

@@ -12,8 +12,9 @@
 public enum MessageType: UInt8 {
     // Public messages (unencrypted)
     case announce = 0x01        // "I'm here" with nickname
-    case message = 0x02         // Public chat message  
+    case message = 0x02         // Public chat message
     case leave = 0x03           // "I'm leaving"
+    case courierEnvelope = 0x04 // Store-and-forward envelope carried by a trusted peer
     case requestSync = 0x21     // GCS filter-based sync request (local-only)
     
     // Noise encryption
@@ -29,6 +30,7 @@ public enum MessageType: UInt8 {
         case .announce: return "announce"
         case .message: return "message"
         case .leave: return "leave"
+        case .courierEnvelope: return "courierEnvelope"
         case .requestSync: return "requestSync"
         case .noiseHandshake: return "noiseHandshake"
         case .noiseEncrypted: return "noiseEncrypted"