security: fix all review findings, bump to 1.0.19 (build 20)

CRITICAL
- SftpProvider: replace PromiscuousVerifier with TofuHostKeyVerifier
  (trust-on-first-use; stores SHA-256 fingerprints in EncryptedSharedPreferences;
  rejects key changes on subsequent connections)

HIGH
- GoogleDriveProvider: replace raw string interpolation with buildJsonObject
  in uploadFile, createDirectory, and moveFile to prevent JSON injection
- DropboxProvider: replace all raw JSON strings and Dropbox-API-Arg headers
  with buildJsonObject for the same reason
- OAuthHelper: add cryptographically random state parameter to Dropbox and
  OneDrive authorization URLs (stored alongside the PKCE verifier)
- OAuthRedirectActivity: validate returned state against stored value before
  exchanging the authorization code (CSRF protection)

MEDIUM
- WebDavProvider: block cross-host redirects in the manual redirect interceptor
  so Authorization headers are never forwarded to a different server
- AccountSetupScreen: set FLAG_SECURE on the window while credential fields
  are visible to prevent screenshots and screen-recording capture
- libs.versions.toml: security-crypto alpha06 → stable 1.0.0;
  biometric-ktx alpha05 → biometric 1.1.0 (stable, non-ktx artifact matches
  the BiometricManager/BiometricPrompt API actually used in MainActivity)
- CredentialStore: migrate to security-crypto 1.0.0 API (MasterKeys.getOrCreate
  + positional create() args); add saveHostKey/getHostFingerprint for SFTP TOFU

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/com/syncflow/data/providers/ProviderFactory.kt

@@ -7,19 +7,20 @@ import com.syncflow.data.providers.owncloud.OwnCloudProvider
 import com.syncflow.data.providers.onedrive.OneDriveProvider
 import com.syncflow.data.providers.sftp.SftpProvider
 import com.syncflow.data.providers.webdav.WebDavProvider
+import com.syncflow.data.security.CredentialStore
 import com.syncflow.domain.model.CloudAccount
 import com.syncflow.domain.model.ProviderType
 import javax.inject.Inject
 import javax.inject.Singleton
 
 @Singleton
-class ProviderFactory @Inject constructor() {
+class ProviderFactory @Inject constructor(private val credentialStore: CredentialStore) {
     fun create(account: CloudAccount): CloudProvider = when (account.providerType) {
         ProviderType.GOOGLE_DRIVE -> GoogleDriveProvider(account)
         ProviderType.DROPBOX -> DropboxProvider(account)
         ProviderType.ONEDRIVE -> OneDriveProvider(account)
         ProviderType.WEBDAV -> WebDavProvider(account)
-        ProviderType.SFTP -> SftpProvider(account)
+        ProviderType.SFTP -> SftpProvider(account, credentialStore)
         ProviderType.NEXTCLOUD -> NextcloudProvider(account)
         ProviderType.OWNCLOUD -> OwnCloudProvider(account)
         ProviderType.SFTPGO -> WebDavProvider(account)     // SFTPGo exposes WebDAV

app/src/main/kotlin/com/syncflow/data/providers/dropbox/DropboxProvider.kt

@@ -18,9 +18,9 @@ class DropboxProvider(private val account: CloudAccount) : CloudProvider {
     }
     private val client = OkHttpClient()
 
-    private fun apiReq(url: String, bodyJson: String): Request =
+    private fun apiReq(url: String, argJson: JsonObject): Request =
         Request.Builder().url(url)
-            .post(bodyJson.toRequestBody("application/json".toMediaType()))
+            .post(argJson.toString().toRequestBody("application/json".toMediaType()))
             .header("Authorization", "Bearer $token")
             .build()
 
@@ -33,7 +33,8 @@ class DropboxProvider(private val account: CloudAccount) : CloudProvider {
 
     override suspend fun listFiles(remotePath: String): Result<List<RemoteFile>> = runCatching {
         val path = if (remotePath == "/" || remotePath.isBlank()) "" else remotePath
-        val req = apiReq("https://api.dropboxapi.com/2/files/list_folder", """{"path":"$path","recursive":false}""")
+        val arg = buildJsonObject { put("path", path); put("recursive", false) }
+        val req = apiReq("https://api.dropboxapi.com/2/files/list_folder", arg)
         client.newCall(req).execute().use { resp ->
             val body = resp.body?.string() ?: throw Exception("HTTP ${resp.code}")
             if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}: $body")
@@ -44,11 +45,15 @@ class DropboxProvider(private val account: CloudAccount) : CloudProvider {
 
     override suspend fun uploadFile(localStream: InputStream, remotePath: String, sizeBytes: Long, onProgress: (Long) -> Unit): Result<RemoteFile> = runCatching {
         val bytes = localStream.readBytes()
-        val argHeader = """{"path":"${remotePath.normalizeDropbox()}","mode":"overwrite","autorename":false}"""
+        val arg = buildJsonObject {
+            put("path", remotePath.normalizeDropbox())
+            put("mode", "overwrite")
+            put("autorename", false)
+        }
         val req = Request.Builder().url("https://content.dropboxapi.com/2/files/upload")
             .post(bytes.toRequestBody("application/octet-stream".toMediaType()))
             .header("Authorization", "Bearer $token")
-            .header("Dropbox-API-Arg", argHeader).build()
+            .header("Dropbox-API-Arg", arg.toString()).build()
         client.newCall(req).execute().use { resp ->
             val body = resp.body?.string() ?: throw Exception("HTTP ${resp.code}")
             if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}: $body")
@@ -58,11 +63,11 @@ class DropboxProvider(private val account: CloudAccount) : CloudProvider {
     }
 
     override suspend fun downloadFile(remotePath: String, destination: OutputStream, onProgress: (Long) -> Unit): Result<Unit> = runCatching {
-        val argHeader = """{"path":"${remotePath.normalizeDropbox()}"}"""
+        val arg = buildJsonObject { put("path", remotePath.normalizeDropbox()) }
         val req = Request.Builder().url("https://content.dropboxapi.com/2/files/download")
             .post("".toRequestBody())
             .header("Authorization", "Bearer $token")
-            .header("Dropbox-API-Arg", argHeader).build()
+            .header("Dropbox-API-Arg", arg.toString()).build()
         client.newCall(req).execute().use { resp ->
             if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}")
             var total = 0L
@@ -75,17 +80,20 @@ class DropboxProvider(private val account: CloudAccount) : CloudProvider {
     }
 
     override suspend fun deleteFile(remotePath: String): Result<Unit> = runCatching {
-        val req = apiReq("https://api.dropboxapi.com/2/files/delete_v2", """{"path":"${remotePath.normalizeDropbox()}"}""")
+        val arg = buildJsonObject { put("path", remotePath.normalizeDropbox()) }
+        val req = apiReq("https://api.dropboxapi.com/2/files/delete_v2", arg)
         client.newCall(req).execute().use { resp -> if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}") }
     }
 
     override suspend fun createDirectory(remotePath: String): Result<Unit> = runCatching {
-        val req = apiReq("https://api.dropboxapi.com/2/files/create_folder_v2", """{"path":"${remotePath.normalizeDropbox()}"}""")
+        val arg = buildJsonObject { put("path", remotePath.normalizeDropbox()) }
+        val req = apiReq("https://api.dropboxapi.com/2/files/create_folder_v2", arg)
         client.newCall(req).execute().use { resp -> if (!resp.isSuccessful && resp.code != 409) throw Exception("HTTP ${resp.code}") }
     }
 
     override suspend fun getFileMetadata(remotePath: String): Result<RemoteFile> = runCatching {
-        val req = apiReq("https://api.dropboxapi.com/2/files/get_metadata", """{"path":"${remotePath.normalizeDropbox()}"}""")
+        val arg = buildJsonObject { put("path", remotePath.normalizeDropbox()) }
+        val req = apiReq("https://api.dropboxapi.com/2/files/get_metadata", arg)
         client.newCall(req).execute().use { resp ->
             val body = resp.body?.string() ?: throw Exception("HTTP ${resp.code}")
             Json.parseToJsonElement(body).jsonObject.toRemoteFile()
@@ -93,8 +101,11 @@ class DropboxProvider(private val account: CloudAccount) : CloudProvider {
     }
 
     override suspend fun moveFile(fromPath: String, toPath: String): Result<Unit> = runCatching {
-        val req = apiReq("https://api.dropboxapi.com/2/files/move_v2",
-            """{"from_path":"${fromPath.normalizeDropbox()}","to_path":"${toPath.normalizeDropbox()}"}""")
+        val arg = buildJsonObject {
+            put("from_path", fromPath.normalizeDropbox())
+            put("to_path", toPath.normalizeDropbox())
+        }
+        val req = apiReq("https://api.dropboxapi.com/2/files/move_v2", arg)
         client.newCall(req).execute().use { resp -> if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}") }
     }
 

app/src/main/kotlin/com/syncflow/data/providers/google/GoogleDriveProvider.kt

@@ -44,9 +44,11 @@ class GoogleDriveProvider(private val account: CloudAccount) : CloudProvider {
         val name = remotePath.substringAfterLast('/')
         val parentId = if ('/' in remotePath.dropLast(1)) remotePath.substringBeforeLast('/') else "root"
 
-        // Multipart upload
-        val metaPart = """{"name":"$name","parents":["$parentId"]}"""
-            .toRequestBody("application/json".toMediaType())
+        // Multipart upload — use JSON builder to avoid injection via filenames with special chars
+        val metaPart = buildJsonObject {
+            put("name", name)
+            put("parents", buildJsonArray { add(parentId) })
+        }.toString().toRequestBody("application/json".toMediaType())
         val dataPart = bytes.toRequestBody("application/octet-stream".toMediaType())
         val multipart = MultipartBody.Builder()
             .setType(MultipartBody.FORM)
@@ -86,8 +88,11 @@ class GoogleDriveProvider(private val account: CloudAccount) : CloudProvider {
     override suspend fun createDirectory(remotePath: String): Result<Unit> = runCatching {
         val name = remotePath.substringAfterLast('/')
         val parentId = if ('/' in remotePath.dropLast(1)) remotePath.substringBeforeLast('/') else "root"
-        val body = """{"name":"$name","mimeType":"application/vnd.google-apps.folder","parents":["$parentId"]}"""
-            .toRequestBody("application/json".toMediaType())
+        val body = buildJsonObject {
+            put("name", name)
+            put("mimeType", "application/vnd.google-apps.folder")
+            put("parents", buildJsonArray { add(parentId) })
+        }.toString().toRequestBody("application/json".toMediaType())
         val req = auth(Request.Builder().url("https://www.googleapis.com/drive/v3/files").post(body)).build()
         client.newCall(req).execute().use { resp -> if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}") }
     }
@@ -102,7 +107,8 @@ class GoogleDriveProvider(private val account: CloudAccount) : CloudProvider {
 
     override suspend fun moveFile(fromPath: String, toPath: String): Result<Unit> = runCatching {
         val newName = toPath.substringAfterLast('/')
-        val body = """{"name":"$newName"}""".toRequestBody("application/json".toMediaType())
+        val body = buildJsonObject { put("name", newName) }.toString()
+            .toRequestBody("application/json".toMediaType())
         val req = auth(Request.Builder().url("https://www.googleapis.com/drive/v3/files/$fromPath").patch(body)).build()
         client.newCall(req).execute().use { resp -> if (!resp.isSuccessful) throw Exception("HTTP ${resp.code}") }
     }

app/src/main/kotlin/com/syncflow/data/providers/sftp/SftpProvider.kt

@@ -1,6 +1,7 @@
 package com.syncflow.data.providers.sftp
 
 import com.syncflow.data.providers.CloudProvider
+import com.syncflow.data.security.CredentialStore
 import com.syncflow.domain.model.CloudAccount
 import com.syncflow.domain.model.RemoteFile
 import kotlinx.serialization.json.Json
@@ -8,13 +9,12 @@ import kotlinx.serialization.json.jsonObject
 import kotlinx.serialization.json.jsonPrimitive
 import net.schmizz.sshj.SSHClient
 import net.schmizz.sshj.sftp.SFTPClient
-import net.schmizz.sshj.transport.verification.PromiscuousVerifier
 import net.schmizz.sshj.xfer.InMemorySourceFile
 import java.io.InputStream
 import java.io.OutputStream
 import java.time.Instant
 
-class SftpProvider(private val account: CloudAccount) : CloudProvider {
+class SftpProvider(private val account: CloudAccount, private val credentialStore: CredentialStore) : CloudProvider {
 
     private val creds = Json.parseToJsonElement(account.credentialJson).jsonObject
     private val host = account.serverUrl ?: "localhost"
@@ -25,7 +25,7 @@ class SftpProvider(private val account: CloudAccount) : CloudProvider {
 
     private fun <T> withSftp(block: (SFTPClient) -> T): T {
         val ssh = SSHClient()
-        ssh.addHostKeyVerifier(PromiscuousVerifier()) // TODO: replace with proper key pinning
+        ssh.addHostKeyVerifier(TofuHostKeyVerifier(credentialStore))
         ssh.connect(host, port)
         try {
             if (!privateKey.isNullOrBlank()) {

app/src/main/kotlin/com/syncflow/data/providers/sftp/TofuHostKeyVerifier.kt

@@ -0,0 +1,31 @@
+package com.syncflow.data.providers.sftp
+
+import com.syncflow.data.security.CredentialStore
+import net.schmizz.sshj.transport.verification.HostKeyVerifier
+import java.security.MessageDigest
+import java.security.PublicKey
+
+/**
+ * Trust-On-First-Use SSH host key verifier.
+ *
+ * First connection to a host: fingerprint is stored in EncryptedSharedPreferences and accepted.
+ * Subsequent connections: stored fingerprint must match — mismatch aborts (possible MITM).
+ */
+class TofuHostKeyVerifier(private val credentialStore: CredentialStore) : HostKeyVerifier {
+
+    override fun verify(hostname: String, port: Int, key: PublicKey): Boolean {
+        val fingerprint = sha256Fingerprint(key)
+        val stored = credentialStore.getHostFingerprint(hostname, port)
+        return if (stored == null) {
+            credentialStore.saveHostKey(hostname, port, fingerprint)
+            true
+        } else {
+            stored == fingerprint
+        }
+    }
+
+    private fun sha256Fingerprint(key: PublicKey): String {
+        val digest = MessageDigest.getInstance("SHA-256").digest(key.encoded)
+        return digest.joinToString(":") { "%02x".format(it) }
+    }
+}

app/src/main/kotlin/com/syncflow/data/providers/webdav/WebDavProvider.kt

@@ -10,6 +10,7 @@ import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.jsonObject
 import kotlinx.serialization.json.jsonPrimitive
 import okhttp3.*
+import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.RequestBody.Companion.toRequestBody
 import org.xmlpull.v1.XmlPullParser
@@ -38,9 +39,14 @@ open class WebDavProvider(protected val account: CloudAccount) : CloudProvider {
                     .header("Authorization", Credentials.basic(user, pass))
                     .build()
                 val resp = chain.proceed(req)
-                // follow redirect manually for WebDAV methods (OkHttp skips non-GET/HEAD redirects)
+                // Follow redirects for WebDAV methods (OkHttp skips non-GET/HEAD redirects).
+                // Only follow same-host redirects to prevent credential leakage to a different server.
                 if (resp.code in 301..308) {
                     val location = resp.header("Location") ?: return@addInterceptor resp
+                    val redirectHost = location.toHttpUrlOrNull()?.host
+                    if (redirectHost == null || redirectHost != req.url.host) {
+                        return@addInterceptor resp
+                    }
                     resp.close()
                     val redirectReq = req.newBuilder().url(location).build()
                     chain.proceed(redirectReq)

app/src/main/kotlin/com/syncflow/data/security/CredentialStore.kt

@@ -3,7 +3,7 @@ package com.syncflow.data.security
 import android.content.Context
 import android.content.SharedPreferences
 import androidx.security.crypto.EncryptedSharedPreferences
-import androidx.security.crypto.MasterKey
+import androidx.security.crypto.MasterKeys
 import dagger.hilt.android.qualifiers.ApplicationContext
 import javax.inject.Inject
 import javax.inject.Singleton
@@ -12,13 +12,11 @@ import javax.inject.Singleton
 class CredentialStore @Inject constructor(@ApplicationContext private val context: Context) {
 
     private val prefs: SharedPreferences by lazy {
-        val masterKey = MasterKey.Builder(context)
-            .setKeyScheme(MasterKey.KeyScheme.AES256_GCM)
-            .build()
+        val masterKeyAlias = MasterKeys.getOrCreate(MasterKeys.AES256_GCM_SPEC)
         EncryptedSharedPreferences.create(
-            context,
             "syncflow_credentials",
-            masterKey,
+            masterKeyAlias,
+            context,
             EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
             EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM,
         )
@@ -37,7 +35,7 @@ class CredentialStore @Inject constructor(@ApplicationContext private val contex
         prefs.edit().remove(credKey(accountId)).apply()
     }
 
-    // ── PKCE verifiers (OAuth flow) ───────────────────────────────────────────
+    // ── PKCE verifiers and OAuth state (OAuth flow) ───────────────────────────
 
     fun savePkceVerifier(provider: String, verifier: String) {
         prefs.edit().putString(pkceKey(provider), verifier).apply()
@@ -49,8 +47,18 @@ class CredentialStore @Inject constructor(@ApplicationContext private val contex
         prefs.edit().remove(pkceKey(provider)).apply()
     }
 
+    // ── SFTP host key fingerprints (TOFU) ─────────────────────────────────────
+
+    fun saveHostKey(host: String, port: Int, fingerprint: String) {
+        prefs.edit().putString(hostKey(host, port), fingerprint).apply()
+    }
+
+    fun getHostFingerprint(host: String, port: Int): String? =
+        prefs.getString(hostKey(host, port), null)
+
     // ── Key helpers ───────────────────────────────────────────────────────────
 
     private fun credKey(accountId: Long) = "cred_$accountId"
     private fun pkceKey(provider: String) = "pkce_$provider"
+    private fun hostKey(host: String, port: Int) = "sshhost_${host}_$port"
 }