fix(nostr): cap DM envelope backdating at 24 hours

app/src/main/java/com/bitchat/android/nostr/NostrCrypto.kt

@@ -317,9 +317,9 @@ object NostrCrypto {
     }
     
     /**
-     * Random timestamp up to maxPastSeconds in the past (default 2 days)
+     * Random timestamp up to maxPastSeconds in the past (default 24 hours for iOS compatibility)
      */
-    fun randomizeTimestampUpToPast(maxPastSeconds: Int = 172800): Int {
+    fun randomizeTimestampUpToPast(maxPastSeconds: Int = 86400): Int {
         val now = (System.currentTimeMillis() / 1000).toInt()
         val offset = if (maxPastSeconds > 0) secureRandom.nextInt(maxPastSeconds + 1) else 0
         return now - offset

app/src/main/java/com/bitchat/android/nostr/NostrProtocol.kt

@@ -37,7 +37,7 @@ object NostrProtocol {
         val rumorId = rumorBase.computeEventIdHex()
         val rumor = rumorBase.copy(id = rumorId)
         
-        // 2. Seal the rumor (kind 13) signed by sender, timestamp randomized up to 2 days
+        // 2. Seal the rumor (kind 13) signed by sender, timestamp randomized within iOS's 24h lookback
         val sealedEvent = createSeal(
             rumor = rumor,
             recipientPubkey = recipientPubkey,

app/src/test/kotlin/com/bitchat/android/nostr/NostrProtocolTest.kt

@@ -3,6 +3,7 @@ package com.bitchat.android.nostr
 import com.google.gson.Gson
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
 import org.junit.Test
 
 class NostrProtocolTest {
@@ -41,6 +42,47 @@ class NostrProtocolTest {
         assertNull(decrypted)
     }
 
+    @Test
+    fun createPrivateMessage_limitsEnvelopeTimestampsToIosLookback() {
+        val sender = NostrIdentity.generate()
+        val recipient = NostrIdentity.generate()
+
+        repeat(20) {
+            val beforeCreation = (System.currentTimeMillis() / 1000).toInt()
+            val giftWrap = NostrProtocol.createPrivateMessage(
+                content = "bitchat1:test",
+                recipientPubkey = recipient.publicKeyHex,
+                senderIdentity = sender
+            ).single()
+            val afterCreation = (System.currentTimeMillis() / 1000).toInt()
+            val sealJson = NostrCrypto.decryptNIP44(
+                ciphertext = giftWrap.content,
+                senderPublicKeyHex = giftWrap.pubkey,
+                recipientPrivateKeyHex = recipient.privateKeyHex
+            )
+            val seal = gson.fromJson(sealJson, NostrEvent::class.java)
+
+            assertTimestampWithinIosLookback("gift wrap", giftWrap.createdAt, beforeCreation, afterCreation)
+            assertTimestampWithinIosLookback("seal", seal.createdAt, beforeCreation, afterCreation)
+        }
+    }
+
+    private fun assertTimestampWithinIosLookback(
+        envelope: String,
+        createdAt: Int,
+        beforeCreation: Int,
+        afterCreation: Int
+    ) {
+        assertTrue(
+            "$envelope timestamp must be no more than 24 hours old",
+            createdAt >= beforeCreation - IOS_DM_LOOKBACK_SECONDS
+        )
+        assertTrue(
+            "$envelope timestamp must not be in the future",
+            createdAt <= afterCreation
+        )
+    }
+
     private fun forgedGiftWrap(
         content: String,
         claimedSender: NostrIdentity,
@@ -82,4 +124,8 @@ class NostrProtocolTest {
             content = giftWrapContent
         ).sign(wrapPrivateKey)
     }
+
+    private companion object {
+        const val IOS_DM_LOOKBACK_SECONDS = 86400
+    }
 }

docs/client-rewrite-contracts.md

@@ -17,7 +17,7 @@ The remaining implementation work and milestone progress are tracked in
 | Inner payloads | Noise type bytes, private-message TLVs, peer-state TLVs, file-transfer TLVs, live-voice bursts, fragment header, sync request TLVs | `ClientRewriteWireContractTest`, `AuthenticatedPeerStateTest`, `PrivateMediaTransferPreparerTest`, `VoiceBurstPacketTest`, `FragmentManagerTest` |
 | Identity/security | Announcement extensions, capability bitfield endianness, Noise static-key binding, handshake identity binding, signatures | `IdentityAnnouncementTest`, `NoiseSessionManagerIdentityBindingTest`, `ClientRewritePrimitiveContractTest` |
 | Sync/routing | Stable packet IDs, GCS bitstream, replay collapse, TTL handling, relay choice, confirmed graph edges | `ClientRewritePrimitiveContractTest`, `GCSFilterTest`, `PacketRelayManagerTest`, `MeshGraphServiceTest`, `TransportBridgeServiceTest` |
-| Nostr | Bech32, secp256k1 key derivation, NIP-01 event IDs/signatures, NIP-44 authenticated encryption, NIP-13 PoW, authenticated NIP-17 seals | `ClientRewriteNostrContractTest`, `NostrProtocolTest` |
+| Nostr | Bech32, secp256k1 key derivation, NIP-01 event IDs/signatures, NIP-44 authenticated encryption, NIP-13 PoW, authenticated NIP-17 seals, 24-hour outbound envelope randomization | `ClientRewriteNostrContractTest`, `NostrProtocolTest` |
 | Application state | Peer unions, canonical private conversations, chronological history, delivery/read behavior, media migration policy | `AppStateStoreTest`, `PrivateChatManagerTest`, `MediaSendingManagerMigrationTest` |
 
 ## Golden-vector policy
@@ -31,6 +31,11 @@ Round-trip tests remain useful but are not sufficient on their own: an encoder
 and decoder can share the same defect. Each critical wire format therefore has
 at least one literal vector.
 
+NIP-17 receivers must use a lookback at least as long as the maximum timestamp
+randomization used by senders. Android caps outbound seal and gift-wrap
+randomization at 24 hours for iOS interoperability while retaining its 48-hour
+receive lookback.
+
 ## Rewrite acceptance gate
 
 From a configured Android development environment, run: