Add padding to the gzipped backup output.

app/src/main/java/org/thoughtcrime/securesms/backup/v2/stream/EncryptedBackupWriter.kt

@@ -13,7 +13,6 @@ import org.whispersystems.signalservice.api.backup.BackupKey
 import org.whispersystems.signalservice.api.push.ServiceId.ACI
 import java.io.IOException
 import java.io.OutputStream
-import java.util.zip.GZIPOutputStream
 import javax.crypto.Cipher
 import javax.crypto.CipherOutputStream
 import javax.crypto.Mac
@@ -33,7 +32,7 @@ class EncryptedBackupWriter(
   private val append: (ByteArray) -> Unit
 ) : BackupExportWriter {
 
-  private val mainStream: GZIPOutputStream
+  private val mainStream: PaddedGzipOutputStream
   private val macStream: MacOutputStream
 
   init {
@@ -48,13 +47,9 @@ class EncryptedBackupWriter(
     }
 
     macStream = MacOutputStream(outputStream, mac)
+    val cipherStream = CipherOutputStream(macStream, cipher)
 
-    mainStream = GZIPOutputStream(
-      CipherOutputStream(
-        macStream,
-        cipher
-      )
-    )
+    mainStream = PaddedGzipOutputStream(cipherStream)
   }
 
   override fun write(header: BackupInfo) {

app/src/main/java/org/thoughtcrime/securesms/backup/v2/stream/PaddedGzipOutputStream.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.backup.v2.stream
+
+import org.whispersystems.signalservice.internal.crypto.PaddingInputStream
+import java.io.FilterOutputStream
+import java.io.OutputStream
+import java.util.zip.GZIPOutputStream
+
+/**
+ * GZIPs the content of the provided [outputStream], but also adds padding to the end of the stream using the same algorithm as [PaddingInputStream].
+ * We do this to fit files into a smaller number of size buckets to avoid fingerprinting. And it turns out that bolting on zeros to the end of a GZIP stream is
+ * fine, because GZIP is smart enough to ignore it. This means readers of this data don't have to do anything special.
+ */
+class PaddedGzipOutputStream private constructor(private val outputStream: SizeObservingOutputStream) : GZIPOutputStream(outputStream) {
+
+  constructor(outputStream: OutputStream) : this(SizeObservingOutputStream(outputStream))
+
+  override fun finish() {
+    super.finish()
+
+    val totalLength = outputStream.size
+    val paddedSize: Long = PaddingInputStream.getPaddedSize(totalLength)
+    val paddingToAdd: Int = (paddedSize - totalLength).toInt()
+
+    outputStream.write(ByteArray(paddingToAdd))
+  }
+
+  /**
+   * We need to know the size of the *compressed* stream to know how much padding to add at the end.
+   */
+  private class SizeObservingOutputStream(val wrapped: OutputStream) : FilterOutputStream(wrapped) {
+
+    var size: Long = 0L
+      private set
+
+    override fun write(b: Int) {
+      wrapped.write(b)
+      size++
+    }
+
+    override fun write(b: ByteArray) {
+      wrapped.write(b)
+      size += b.size
+    }
+
+    override fun write(b: ByteArray, off: Int, len: Int) {
+      wrapped.write(b, off, len)
+      size += len
+    }
+  }
+}