Feat: Implement SSH Host Key verification

app/src/main/AndroidManifest.xml

@@ -295,6 +295,11 @@
             android:autoRemoveFromRecents="true"
             android:theme="@style/Theme.MaterialFiles.Translucent" />
 
+        <activity
+            android:name="me.zhanghai.android.files.storage.SftpHostKeyChangedDialogActivity"
+            android:autoRemoveFromRecents="true"
+            android:theme="@style/Theme.MaterialFiles.Translucent" />
+
         <activity
             android:name="me.zhanghai.android.files.viewer.saveas.SaveAsActivity"
             android:autoRemoveFromRecents="true"

app/src/main/java/me/zhanghai/android/files/provider/sftp/client/Authenticator.kt

@@ -7,4 +7,14 @@ package me.zhanghai.android.files.provider.sftp.client
 
 interface Authenticator {
     fun getAuthentication(authority: Authority): Authentication?
+
+    fun getHostKey(authority: Authority): String?
+
+    fun putHostKey(authority: Authority, hostKey: String)
+
+    fun confirmChangedHostKey(
+        authority: Authority,
+        oldHostKey: String,
+        newHostKey: String
+    ): Boolean = false
 }

app/src/main/java/me/zhanghai/android/files/provider/sftp/client/Client.kt

@@ -10,6 +10,8 @@ import me.zhanghai.android.files.provider.common.LocalWatchService
 import me.zhanghai.android.files.provider.common.NotifyEntryModifiedSeekableByteChannel
 import me.zhanghai.android.files.util.closeSafe
 import net.schmizz.sshj.SSHClient
+import net.schmizz.sshj.common.KeyType
+import net.schmizz.sshj.common.SecurityUtils
 import net.schmizz.sshj.sftp.FileAttributes
 import net.schmizz.sshj.sftp.FileMode
 import net.schmizz.sshj.sftp.OpenMode
@@ -18,9 +20,10 @@ import net.schmizz.sshj.sftp.Response
 import net.schmizz.sshj.sftp.SFTPClient
 import net.schmizz.sshj.sftp.SFTPException
 import net.schmizz.sshj.transport.TransportException
-import net.schmizz.sshj.transport.verification.PromiscuousVerifier
+import net.schmizz.sshj.transport.verification.HostKeyVerifier
 import net.schmizz.sshj.userauth.UserAuthException
 import java.io.IOException
+import java.security.PublicKey
 import java.util.Collections
 import java.util.WeakHashMap
 import java8.nio.file.Path as Java8Path
@@ -237,12 +240,13 @@ object Client {
             }
             val authentication = authenticator.getAuthentication(authority)
                 ?: throw ClientException("No authentication found for $authority")
-            val sshClient = SSHClient().apply { addHostKeyVerifier(PromiscuousVerifier()) }
+            val hostKeyVerifier = PersistentHostKeyVerifier(authority, authenticator)
+            val sshClient = SSHClient().apply { addHostKeyVerifier(hostKeyVerifier) }
             try {
                 sshClient.connect(authority.host, authority.port)
             } catch (e: IOException) {
                 sshClient.closeSafe()
-                throw ClientException(e)
+                throw ClientException(hostKeyVerifier.exception ?: e)
             }
             try {
                 sshClient.auth(authority.username, authentication.toAuthMethod())
@@ -259,6 +263,49 @@ object Client {
         }
     }
 
+    private class PersistentHostKeyVerifier(
+        private val authority: Authority,
+        private val authenticator: Authenticator
+    ) : HostKeyVerifier {
+        var exception: IOException? = null
+            private set
+
+        override fun verify(hostname: String, port: Int, key: PublicKey): Boolean {
+            val hostKey = key.toHostKey()
+            val knownHostKey = authenticator.getHostKey(authority)
+
+            return when {
+                knownHostKey == null -> {
+                    authenticator.putHostKey(authority, hostKey)
+                    true
+                }
+
+                knownHostKey == hostKey -> true
+
+                authenticator.confirmChangedHostKey(
+                    authority,
+                    knownHostKey,
+                    hostKey
+                ) -> true
+
+                else -> {
+                    exception = IOException(
+                        "Host key for $authority has changed: expected $knownHostKey, got $hostKey. " +
+                                "This may indicate a man-in-the-middle attack."
+                    )
+                    false
+                }
+            }
+        }
+
+        override fun findExistingAlgorithms(hostname: String, port: Int): List<String> =
+            authenticator.getHostKey(authority)?.substringBefore(' ')?.let { listOf(it) }
+                ?: emptyList()
+
+        private fun PublicKey.toHostKey(): String =
+            "${KeyType.fromKey(this)} ${SecurityUtils.getFingerprint(this)}"
+    }
+
     interface Path {
         val authority: Authority
         val remotePath: String

app/src/main/java/me/zhanghai/android/files/storage/SftpHostKeyChangedDialogActivity.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2026 Material Files contributors
+ * SPDX‑License‑Identifier: GPL-3.0-or-later
+ */
+
+package me.zhanghai.android.files.storage
+
+import android.os.Bundle
+import android.view.View
+import androidx.fragment.app.commit
+import me.zhanghai.android.files.app.AppActivity
+import me.zhanghai.android.files.util.args
+import me.zhanghai.android.files.util.putArgs
+
+class SftpHostKeyChangedDialogActivity : AppActivity() {
+    private val args by args<SftpHostKeyChangedDialogFragment.Args>()
+
+    private lateinit var fragment: SftpHostKeyChangedDialogFragment
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+
+        // Calls ensureSubDecor().
+        findViewById<View>(android.R.id.content)
+        if (savedInstanceState == null) {
+            fragment = SftpHostKeyChangedDialogFragment().putArgs(args)
+            supportFragmentManager.commit {
+                add(fragment, SftpHostKeyChangedDialogFragment::class.java.name)
+            }
+        } else {
+            fragment = supportFragmentManager.findFragmentByTag(
+                SftpHostKeyChangedDialogFragment::class.java.name
+            ) as SftpHostKeyChangedDialogFragment
+        }
+    }
+
+    override fun onDestroy() {
+        super.onDestroy()
+
+        if (isFinishing) {
+            fragment.onFinish()
+        }
+    }
+}

app/src/main/java/me/zhanghai/android/files/storage/SftpHostKeyChangedDialogFragment.kt

@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2026 Material Files contributors
+ * SPDX‑License‑Identifier: GPL-3.0-or-later
+ */
+
+package me.zhanghai.android.files.storage
+
+import android.app.Dialog
+import android.content.Context
+import android.content.DialogInterface
+import android.os.Bundle
+import android.os.Parcel
+import androidx.appcompat.app.AppCompatDialogFragment
+import com.google.android.material.dialog.MaterialAlertDialogBuilder
+import kotlinx.parcelize.Parceler
+import kotlinx.parcelize.Parcelize
+import kotlinx.parcelize.WriteWith
+import me.zhanghai.android.files.R
+import me.zhanghai.android.files.util.ParcelableArgs
+import me.zhanghai.android.files.util.RemoteCallback
+import me.zhanghai.android.files.util.args
+import me.zhanghai.android.files.util.finish
+import me.zhanghai.android.files.util.getArgs
+import me.zhanghai.android.files.util.putArgs
+import me.zhanghai.android.files.util.readParcelable
+
+class SftpHostKeyChangedDialogFragment : AppCompatDialogFragment() {
+    private val args by args<Args>()
+
+    private var isListenerNotified = false
+
+    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog =
+        MaterialAlertDialogBuilder(requireContext(), theme)
+            .setTitle(R.string.storage_sftp_host_key_changed_title)
+            .setMessage(
+                getMessage(args.authority, args.oldHostKey, args.newHostKey, requireContext())
+            )
+            .setPositiveButton(R.string.storage_sftp_host_key_changed_trust) { _, _ ->
+                notifyListenerOnce(true)
+                finish()
+            }
+            .setNegativeButton(android.R.string.cancel) { _, _ ->
+                notifyListenerOnce(false)
+                finish()
+            }
+            .create()
+            .apply { setCanceledOnTouchOutside(false) }
+
+    override fun onCancel(dialog: DialogInterface) {
+        super.onCancel(dialog)
+
+        notifyListenerOnce(false)
+        finish()
+    }
+
+    fun onFinish() {
+        notifyListenerOnce(false)
+    }
+
+    private fun notifyListenerOnce(trust: Boolean) {
+        if (isListenerNotified) {
+            return
+        }
+        args.listener(trust)
+        isListenerNotified = true
+    }
+
+    companion object {
+        fun getMessage(authority: String, context: Context): String =
+            context.getString(R.string.storage_sftp_host_key_changed_message_format, authority)
+
+        private fun getMessage(
+            authority: String,
+            oldHostKey: String,
+            newHostKey: String,
+            context: Context
+        ): String = context.getString(
+            R.string.storage_sftp_host_key_changed_message_with_keys_format,
+            authority,
+            oldHostKey,
+            newHostKey
+        )
+    }
+
+    @Parcelize
+    class Args(
+        val authority: String,
+        val oldHostKey: String,
+        val newHostKey: String,
+        val listener: @WriteWith<ListenerParceler>() (Boolean) -> Unit
+    ) : ParcelableArgs {
+        object ListenerParceler : Parceler<(Boolean) -> Unit> {
+            override fun create(parcel: Parcel): (Boolean) -> Unit =
+                parcel.readParcelable<RemoteCallback>()!!.let {
+                    { trust -> it.sendResult(Bundle().putArgs(ListenerArgs(trust))) }
+                }
+
+            override fun ((Boolean) -> Unit).write(parcel: Parcel, flags: Int) {
+                parcel.writeParcelable(RemoteCallback {
+                    val args = it.getArgs<ListenerArgs>()
+                    this(args.trust)
+                }, flags)
+            }
+
+            @Parcelize
+            private class ListenerArgs(val trust: Boolean) : ParcelableArgs
+        }
+    }
+}

app/src/main/java/me/zhanghai/android/files/storage/SftpServerAuthenticator.kt

@@ -5,14 +5,27 @@
 
 package me.zhanghai.android.files.storage
 
+import androidx.core.content.edit
+import kotlinx.coroutines.runBlocking
+import me.zhanghai.android.files.R
+import me.zhanghai.android.files.app.BackgroundActivityStarter
+import me.zhanghai.android.files.app.application
+import me.zhanghai.android.files.app.defaultSharedPreferences
 import me.zhanghai.android.files.provider.sftp.client.Authentication
 import me.zhanghai.android.files.provider.sftp.client.Authenticator
 import me.zhanghai.android.files.provider.sftp.client.Authority
 import me.zhanghai.android.files.settings.Settings
+import me.zhanghai.android.files.util.createIntent
+import me.zhanghai.android.files.util.putArgs
 import me.zhanghai.android.files.util.valueCompat
+import kotlin.coroutines.resume
+import kotlin.coroutines.suspendCoroutine
 
 object SftpServerAuthenticator : Authenticator {
+    private const val HOST_KEY_PREFIX = "sftp_host_key_"
+
     private val transientServers = mutableSetOf<SftpServer>()
+    private val hostKeyLock = Any()
 
     override fun getAuthentication(authority: Authority): Authentication? {
         val server = synchronized(transientServers) {
@@ -23,11 +36,50 @@ object SftpServerAuthenticator : Authenticator {
         return server?.authentication
     }
 
+    override fun getHostKey(authority: Authority): String? = synchronized(hostKeyLock) {
+        defaultSharedPreferences.getString(authority.toHostKeyPreferenceKey(), null)
+    }
+
+    override fun putHostKey(authority: Authority, hostKey: String) {
+        synchronized(hostKeyLock) {
+            defaultSharedPreferences.edit { putString(authority.toHostKeyPreferenceKey(), hostKey) }
+        }
+    }
+
+    override fun confirmChangedHostKey(
+        authority: Authority,
+        oldHostKey: String,
+        newHostKey: String
+    ): Boolean = runBlocking {
+        suspendCoroutine { continuation ->
+            val authorityString = authority.toString()
+            val onTrustDecision: (Boolean) -> Unit = { trust ->
+                if (trust) {
+                    putHostKey(authority, newHostKey)
+                }
+                continuation.resume(trust)
+            }
+
+            BackgroundActivityStarter.startActivity(
+                SftpHostKeyChangedDialogActivity::class.createIntent().putArgs(
+                    SftpHostKeyChangedDialogFragment.Args(
+                        authorityString, oldHostKey, newHostKey, onTrustDecision
+                    )
+                ),
+                application.getString(R.string.storage_sftp_host_key_changed_title),
+                SftpHostKeyChangedDialogFragment.getMessage(authorityString, application),
+                application
+            )
+        }
+    }
+
     fun addTransientServer(server: SftpServer) {
         synchronized(transientServers) { transientServers += server }
     }
 
     fun removeTransientServer(server: SftpServer) {
         synchronized(transientServers) { transientServers -= server }
     }
+
+    private fun Authority.toHostKeyPreferenceKey(): String = "$HOST_KEY_PREFIX$this"
 }

app/src/main/res/values/strings.xml

@@ -613,6 +613,10 @@
     <string name="storage_edit_sftp_server_private_key_password_error_invalid">Invalid private key password</string>
     <string name="storage_edit_sftp_server_connect_and_add">Connect and add</string>
     <string name="storage_edit_sftp_server_add">Add</string>
+    <string name="storage_sftp_host_key_changed_title">SFTP host key changed</string>
+    <string name="storage_sftp_host_key_changed_message_format">The host key for %1$s has changed. This may indicate a man-in-the-middle attack. Only trust the new key if you expected this change.</string>
+    <string name="storage_sftp_host_key_changed_message_with_keys_format">The host key for %1$s has changed. This may indicate a man-in-the-middle attack. Only trust the new key if you expected this change.\n\nOld key:\n%2$s\n\nNew key:\n%3$s</string>
+    <string name="storage_sftp_host_key_changed_trust">Trust</string>
     <string name="storage_add_lan_smb_server_title" translatable="false">@string/storage_edit_smb_server_title_add</string>
     <string name="storage_add_lan_smb_server_loading">Searching for SMB servers…</string>
     <string name="storage_add_lan_smb_server_add">Add manually</string>