Replace visible collection IDs with steganographic encoding

app/src/main/kotlin/at/bitfire/davdroid/resource/LocalAddressBookStore.kt

@@ -22,6 +22,8 @@ import at.bitfire.davdroid.settings.SettingsManager
 import at.bitfire.davdroid.sync.account.SystemAccountUtils
 import at.bitfire.davdroid.sync.account.setAndVerifyUserData
 import at.bitfire.davdroid.util.DavUtils.lastSegment
+import at.bitfire.davdroid.util.MARKER
+import at.bitfire.davdroid.util.encodeNumber
 import com.google.common.base.CharMatcher
 import dagger.hilt.android.qualifiers.ApplicationContext
 import kotlinx.coroutines.channels.awaitClose
@@ -74,7 +76,7 @@ class LocalAddressBookStore @Inject constructor(
             sb.append(" (${service.accountName})")
         }
         // Add the collection ID for uniqueness
-        sb.append(" #${info.id}")
+        sb.append(MARKER + encodeNumber(info.id))
         return sb.toString()
     }
 

app/src/main/kotlin/at/bitfire/davdroid/sync/account/SystemAccountUtils.kt

@@ -8,6 +8,8 @@ import android.accounts.Account
 import android.accounts.AccountManager
 import android.content.Context
 import android.os.Bundle
+import at.bitfire.davdroid.util.MARKER
+import at.bitfire.davdroid.util.decodeWithMarker
 import java.util.logging.Logger
 
 object SystemAccountUtils {
@@ -67,4 +69,9 @@ fun AccountManager.setAndVerifyUserData(account: Account, key: String, value: St
         Thread.sleep(100)
     }
     Logger.getGlobal().warning("AccountManager failed to set $account user data $key := $value")
-}
+}
+
+fun Account.nameWithNumber(): String {
+    val number = decodeWithMarker(name) ?: return name
+    return name.substringBeforeLast(MARKER) + " (#" + number + ")"
+}

app/src/main/kotlin/at/bitfire/davdroid/sync/adapter/SyncFrameworkIntegration.kt

@@ -12,6 +12,9 @@ import android.os.Bundle
 import androidx.annotation.WorkerThread
 import at.bitfire.davdroid.resource.LocalAddressBookStore
 import at.bitfire.davdroid.sync.SyncDataType
+import at.bitfire.davdroid.sync.account.nameWithNumber
+import at.bitfire.davdroid.util.MARKER
+import at.bitfire.davdroid.util.decodeWithMarker
 import dagger.Lazy
 import dagger.hilt.android.qualifiers.ApplicationContext
 import kotlinx.coroutines.ExperimentalCoroutinesApi
@@ -256,7 +259,7 @@ class SyncFrameworkIntegration @Inject constructor(
     private fun anyPendingSync(accounts: List<Account>, authority: String): Boolean =
         accounts.any { account ->
             ContentResolver.isSyncPending(account, authority).also { pending ->
-                logger.finer("Sync pending($account, $authority) = $pending")
+                logger.finer("Sync pending(name=${account.nameWithNumber()}, type=${account.type}, $authority) = $pending")
             }
         }
 

app/src/main/kotlin/at/bitfire/davdroid/util/StringUtils.kt

@@ -13,4 +13,40 @@ fun String.withTrailingSlash() =
     if (this.endsWith('/'))
         this
     else
-        "$this/"
+        "$this/"
+
+const val MARKER = '\uFEFF'   // Start of hidden payload
+const val ZERO = '\u200B'     // Zero Width Space for 0
+const val ONE = '\u200C'      // Zero Width Non-Joiner for 1
+
+/**
+ * Encodes a number using our own steganographic encoding scheme to hide a number in UTF-8 invisible characters.
+ */
+fun encodeNumber(num: Long): String {
+    require(num >= 0) { "Only non-negative integers are supported" }
+    if (num == 0L) return ZERO.toString() // Represent zero as a single zero-width space
+
+    val binary = num.toString(2) // e.g., 8 -> "1000"
+    val builder = StringBuilder()
+    for (bit in binary) {
+        builder.append(
+            if (bit == '0') ZERO  // Zero Width Space
+            else ONE              // Zero Width Non-Joiner
+        )
+    }
+    return builder.toString()
+}
+
+fun decodeWithMarker(text: String): Int? {
+    val markerIndex = text.indexOf(MARKER)
+    if (markerIndex == -1) return null // No hidden payload
+    val hiddenPart = text.substring(markerIndex + 1)
+    val binary = hiddenPart.map {
+        when (it) {
+            ZERO -> '0'
+            ONE -> '1'
+            else -> throw IllegalArgumentException("Invalid hidden character")
+        }
+    }.joinToString("")
+    return binary.toInt(2)
+}