From 939f2f82b1045d72043a95dc6ca4cc14b281a20e Mon Sep 17 00:00:00 2001
From: Greyson Parrelli <greyson@signal.org>
Date: Tue, 1 Jul 2025 08:42:39 -0400
Subject: [PATCH] Fix crash when skipping in BetterCipherInputStream.

---
 .../api/crypto/BetterCipherInputStream.kt     | 15 -----------
 .../api/crypto/AttachmentCipherTest.kt        | 26 +++++++++++++++++++
 2 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/BetterCipherInputStream.kt b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/BetterCipherInputStream.kt
index 1755f23861..1bfc15af3a 100644
--- a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/BetterCipherInputStream.kt
+++ b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/BetterCipherInputStream.kt
@@ -58,21 +58,6 @@ class BetterCipherInputStream(
 
   override fun markSupported(): Boolean = false
 
-  @Throws(IOException::class)
-  override fun skip(byteCount: Long): Long {
-    val buffer = ByteArray(4096)
-    var skipped = 0L
-
-    while (skipped < byteCount) {
-      val remaining = byteCount - skipped
-      val read = read(buffer, 0, remaining.toInt())
-
-      skipped += read.toLong()
-    }
-
-    return skipped
-  }
-
   @Throws(IOException::class)
   private fun readIncremental(outputBuffer: ByteArray, originalOffset: Int, originalLength: Int): Int {
     var offset = originalOffset
diff --git a/libsignal-service/src/test/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherTest.kt b/libsignal-service/src/test/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherTest.kt
index fa1645341d..f892410153 100644
--- a/libsignal-service/src/test/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherTest.kt
+++ b/libsignal-service/src/test/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherTest.kt
@@ -101,6 +101,32 @@ class AttachmentCipherTest {
     cipherFile.delete()
   }
 
+  @Test
+  fun attachment_encryptDecrypt_skipAll_manyFileSizes(){
+    for (i in 0..99) {
+      attachment_encryptDecrypt_skipAll(incremental = false, fileSize = MEBIBYTE + Random().nextInt(1, 64 * 1024))
+    }
+  }
+
+  private fun attachment_encryptDecrypt_skipAll(incremental: Boolean, fileSize: Int) {
+    val key = Util.getSecretBytes(64)
+    val plaintextInput = Util.getSecretBytes(fileSize)
+    val plaintextHash = MessageDigest.getInstance("SHA-256").digest(plaintextInput)
+
+    val encryptResult = encryptData(plaintextInput, key, incremental)
+    val cipherFile = writeToFile(encryptResult.ciphertext)
+
+    val integrityCheck = IntegrityCheck(
+      encryptedDigest = encryptResult.digest,
+      plaintextHash = plaintextHash
+    )
+    val inputStream = AttachmentCipherInputStream.createForAttachment(cipherFile, plaintextInput.size.toLong(), key, integrityCheck, encryptResult.incrementalDigest, encryptResult.chunkSizeChoice)
+    while(inputStream.skip(cipherFile.length()) > 0) {
+      // Empty body, just skipping
+    }
+    cipherFile.delete()
+  }
+
   private fun attachment_encryptDecrypt_plaintextHash(incremental: Boolean, fileSize: Int) {
     val key = Util.getSecretBytes(64)
     val plaintextInput = Util.getSecretBytes(fileSize)