Migrate private media without silent downgrades

2026-07-25 02:05:20 +00:00 · 2026-07-12 10:21:07 -04:00
parent b7f0b33d3a
commit 55f85d920c
36 changed files with 2430 additions and 303 deletions
@@ -0,0 +1,105 @@
+package com.bitchat.android.mesh
+
+import android.content.Context
+import com.bitchat.android.identity.SecureIdentityStateManager
+import com.bitchat.android.model.PeerCapabilities
+import java.security.MessageDigest
+
+internal interface PrivateMediaCapabilityPinStore {
+    fun contains(fingerprint: String): Boolean
+    fun insert(fingerprint: String)
+}
+
+internal class SecurePrivateMediaCapabilityPinStore(context: Context) :
+    PrivateMediaCapabilityPinStore {
+    private val identityState = SecureIdentityStateManager(context.applicationContext)
+
+    override fun contains(fingerprint: String): Boolean =
+        identityState.isPrivateMediaCapable(fingerprint)
+
+    override fun insert(fingerprint: String) {
+        identityState.markPrivateMediaCapable(fingerprint)
+    }
+}
+
+internal sealed interface PrivateMediaPolicyDecision {
+    data object Encrypted : PrivateMediaPolicyDecision
+    data object RequiresLegacyConsent : PrivateMediaPolicyDecision
+    data object NeedsHandshake : PrivateMediaPolicyDecision
+    data class Blocked(val reason: String) : PrivateMediaPolicyDecision
+}
+
+/**
+ * Binds an advertised private-media capability to a live Noise remote-static
+ * key and persists an HSTS-style pin by that authenticated key's SHA-256
+ * fingerprint. Announcements alone can never create a pin.
+ */
+internal class PrivateMediaSecurityController(
+    private val peerInfoProvider: (String) -> PeerInfo?,
+    private val authenticatedRemoteStaticProvider: (String) -> ByteArray?,
+    private val pinStore: PrivateMediaCapabilityPinStore
+) {
+    fun refreshAuthenticatedCapability(peerID: String): Boolean {
+        val remoteStatic = authenticatedRemoteStaticProvider(peerID)
+            ?.takeIf { it.size == 32 }
+            ?: return false
+        val peer = peerInfoProvider(peerID) ?: return false
+        if (!peer.hasVerifiedAnnouncement) return false
+        val announcedStatic = peer.verifiedAnnouncementNoisePublicKey ?: return false
+        if (!announcedStatic.contentEquals(remoteStatic)) return false
+        if (peer.capabilities?.contains(PeerCapabilities.PRIVATE_MEDIA) != true) return false
+
+        pinStore.insert(fingerprint(remoteStatic))
+        return true
+    }
+
+    fun sendPolicy(peerID: String): PrivateMediaPolicyDecision {
+        val remoteStatic = authenticatedRemoteStaticProvider(peerID)
+            ?.takeIf { it.size == 32 }
+            ?: return PrivateMediaPolicyDecision.NeedsHandshake
+        val authenticatedFingerprint = fingerprint(remoteStatic)
+
+        // Once a fingerprint has authenticated encrypted media support, never
+        // downgrade it because a later announce omits or clears the bit.
+        if (pinStore.contains(authenticatedFingerprint)) {
+            return PrivateMediaPolicyDecision.Encrypted
+        }
+
+        val peer = peerInfoProvider(peerID)
+            ?: return PrivateMediaPolicyDecision.Blocked(
+                "No signature-verified identity announcement is available"
+            )
+        if (!peer.hasVerifiedAnnouncement) {
+            return PrivateMediaPolicyDecision.Blocked(
+                "The peer identity announcement has not been verified"
+            )
+        }
+        val announcedStatic = peer.verifiedAnnouncementNoisePublicKey
+            ?: return PrivateMediaPolicyDecision.Blocked(
+                "The verified announcement did not contain a Noise identity"
+            )
+        if (!announcedStatic.contentEquals(remoteStatic)) {
+            return PrivateMediaPolicyDecision.Blocked(
+                "The authenticated Noise identity does not match the verified announcement"
+            )
+        }
+
+        return if (peer.capabilities?.contains(PeerCapabilities.PRIVATE_MEDIA) == true) {
+            pinStore.insert(authenticatedFingerprint)
+            PrivateMediaPolicyDecision.Encrypted
+        } else {
+            // Both a missing TLV and an explicitly empty bitfield are legacy.
+            PrivateMediaPolicyDecision.RequiresLegacyConsent
+        }
+    }
+
+    internal fun authenticatedFingerprint(peerID: String): String? =
+        authenticatedRemoteStaticProvider(peerID)
+            ?.takeIf { it.size == 32 }
+            ?.let(::fingerprint)
+
+    private fun fingerprint(publicKey: ByteArray): String =
+        MessageDigest.getInstance("SHA-256")
+            .digest(publicKey)
+            .joinToString("") { "%02x".format(it) }
+}