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>

bitchatTests/NoiseCourierTests.swift

@@ -0,0 +1,107 @@
+//
+// NoiseCourierTests.swift
+// bitchat
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+import Testing
+import Foundation
+@testable import bitchat
+
+/// One-way Noise X envelopes: encryption to a known static key without an
+/// interactive handshake, used by the courier store-and-forward path.
+struct NoiseCourierTests {
+
+    @Test func sealAndOpenRoundTrip() throws {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let bob = NoiseEncryptionService(keychain: MockKeychain())
+
+        let payload = Data("meet at the north gate".utf8)
+        let sealed = try alice.sealCourierPayload(payload, recipientStaticKey: bob.getStaticPublicKeyData())
+
+        let opened = try bob.openCourierPayload(sealed)
+        #expect(opened.payload == payload)
+        // The X pattern authenticates the sender: Bob learns Alice's real static key.
+        #expect(opened.senderStaticKey == alice.getStaticPublicKeyData())
+    }
+
+    @Test func wrongRecipientCannotOpen() throws {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let bob = NoiseEncryptionService(keychain: MockKeychain())
+        let carol = NoiseEncryptionService(keychain: MockKeychain())
+
+        let sealed = try alice.sealCourierPayload(Data("secret".utf8), recipientStaticKey: bob.getStaticPublicKeyData())
+
+        #expect(throws: (any Error).self) {
+            _ = try carol.openCourierPayload(sealed)
+        }
+    }
+
+    @Test func tamperedEnvelopeFailsToOpen() throws {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let bob = NoiseEncryptionService(keychain: MockKeychain())
+
+        var sealed = try alice.sealCourierPayload(Data("secret".utf8), recipientStaticKey: bob.getStaticPublicKeyData())
+        sealed[sealed.count - 1] ^= 0x01
+
+        #expect(throws: (any Error).self) {
+            _ = try bob.openCourierPayload(sealed)
+        }
+    }
+
+    @Test func senderIdentityCannotBeForged() throws {
+        // The encrypted static key inside the envelope is bound by the ss DH;
+        // splicing one envelope's ephemeral prefix onto another must fail.
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let mallory = NoiseEncryptionService(keychain: MockKeychain())
+        let bob = NoiseEncryptionService(keychain: MockKeychain())
+
+        let bobKey = bob.getStaticPublicKeyData()
+        let fromAlice = try alice.sealCourierPayload(Data("hi".utf8), recipientStaticKey: bobKey)
+        let fromMallory = try mallory.sealCourierPayload(Data("hi".utf8), recipientStaticKey: bobKey)
+
+        // e (32 bytes) from Mallory's envelope + rest from Alice's.
+        let spliced = fromMallory.prefix(32) + fromAlice.dropFirst(32)
+        #expect(throws: (any Error).self) {
+            _ = try bob.openCourierPayload(Data(spliced))
+        }
+    }
+
+    @Test func sealRejectsInvalidRecipientKey() {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        #expect(throws: (any Error).self) {
+            _ = try alice.sealCourierPayload(Data("x".utf8), recipientStaticKey: Data(repeating: 0, count: 32))
+        }
+        #expect(throws: (any Error).self) {
+            _ = try alice.sealCourierPayload(Data("x".utf8), recipientStaticKey: Data(repeating: 1, count: 8))
+        }
+    }
+
+    @Test func emptyAndLargePayloadsRoundTrip() throws {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let bob = NoiseEncryptionService(keychain: MockKeychain())
+        let bobKey = bob.getStaticPublicKeyData()
+
+        let empty = try alice.sealCourierPayload(Data(), recipientStaticKey: bobKey)
+        #expect(try bob.openCourierPayload(empty).payload.isEmpty)
+
+        let large = Data((0..<8192).map { UInt8($0 % 251) })
+        let sealed = try alice.sealCourierPayload(large, recipientStaticKey: bobKey)
+        #expect(try bob.openCourierPayload(sealed).payload == large)
+    }
+
+    @Test func envelopesAreNotLinkableAcrossSends() throws {
+        // Fresh ephemeral per seal: same payload to the same recipient must
+        // produce entirely different ciphertexts.
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let bob = NoiseEncryptionService(keychain: MockKeychain())
+        let payload = Data("same message".utf8)
+
+        let a = try alice.sealCourierPayload(payload, recipientStaticKey: bob.getStaticPublicKeyData())
+        let b = try alice.sealCourierPayload(payload, recipientStaticKey: bob.getStaticPublicKeyData())
+        #expect(a != b)
+        #expect(a.prefix(32) != b.prefix(32))
+    }
+}