diff --git a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherOutputStream.java b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherOutputStream.java
deleted file mode 100644
index c82fea9dde..0000000000
--- a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherOutputStream.java
+++ /dev/null
@@ -1,108 +0,0 @@
-/*
- * Copyright (C) 2014-2017 Open Whisper Systems
- *
- * Licensed according to the LICENSE file in this repository.
- */
-
-package org.whispersystems.signalservice.api.crypto;
-
-import org.whispersystems.signalservice.internal.util.Util;
-
-import java.io.IOException;
-import java.io.OutputStream;
-import java.security.InvalidAlgorithmParameterException;
-import java.security.InvalidKeyException;
-import java.security.NoSuchAlgorithmException;
-
-import javax.crypto.BadPaddingException;
-import javax.crypto.Cipher;
-import javax.crypto.IllegalBlockSizeException;
-import javax.crypto.Mac;
-import javax.crypto.NoSuchPaddingException;
-import javax.crypto.spec.IvParameterSpec;
-import javax.crypto.spec.SecretKeySpec;
-
-public class AttachmentCipherOutputStream extends DigestingOutputStream {
-
-  private final Cipher cipher;
-  private final Mac    mac;
-
-  public AttachmentCipherOutputStream(byte[] combinedKeyMaterial,
-                                      byte[] iv,
-                                      OutputStream outputStream)
-      throws IOException
-  {
-    super(outputStream);
-    try {
-      this.cipher       = initializeCipher();
-      this.mac          = initializeMac();
-      byte[][] keyParts = Util.split(combinedKeyMaterial, 32, 32);
-
-      if (iv == null) {
-        this.cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keyParts[0], "AES"));
-      } else {
-        this.cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(keyParts[0], "AES"), new IvParameterSpec(iv));
-      }
-
-      this.mac.init(new SecretKeySpec(keyParts[1], "HmacSHA256"));
-
-      mac.update(cipher.getIV());
-      super.write(cipher.getIV());
-    } catch (InvalidKeyException | InvalidAlgorithmParameterException e) {
-      throw new AssertionError(e);
-    }
-  }
-
-  @Override
-  public void write(byte[] buffer) throws IOException {
-    write(buffer, 0, buffer.length);
-  }
-
-  @Override
-  public void write(byte[] buffer, int offset, int length) throws IOException {
-    byte[] ciphertext = cipher.update(buffer, offset, length);
-
-    if (ciphertext != null) {
-      mac.update(ciphertext);
-      super.write(ciphertext);
-    }
-  }
-
-  @Override
-  public void write(int b) throws IOException {
-    byte[] input = new byte[1];
-    input[0] = (byte) b;
-    write(input, 0, 1);
-  }
-
-  @Override
-  public void close() throws IOException {
-    try {
-      byte[] ciphertext = cipher.doFinal();
-      byte[] auth       = mac.doFinal(ciphertext);
-
-      super.write(ciphertext);
-      super.write(auth);
-
-      super.close();
-    } catch (IllegalBlockSizeException | BadPaddingException e) {
-      throw new AssertionError(e);
-    }
-  }
-
-  private Mac initializeMac() {
-    try {
-      return Mac.getInstance("HmacSHA256");
-    } catch (NoSuchAlgorithmException e) {
-      throw new AssertionError(e);
-    }
-  }
-
-  private Cipher initializeCipher() {
-    try {
-      return Cipher.getInstance("AES/CBC/PKCS5Padding");
-    } catch (NoSuchAlgorithmException | NoSuchPaddingException e) {
-      throw new AssertionError(e);
-    }
-  }
-}
diff --git a/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherOutputStream.kt b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherOutputStream.kt
new file mode 100644
index 0000000000..858104d7bf
--- /dev/null
+++ b/libsignal-service/src/main/java/org/whispersystems/signalservice/api/crypto/AttachmentCipherOutputStream.kt
@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2014-2017 Open Whisper Systems
+ *
+ * Licensed according to the LICENSE file in this repository.
+ */
+package org.whispersystems.signalservice.api.crypto
+
+import org.whispersystems.signalservice.internal.util.Util
+import java.io.IOException
+import java.io.OutputStream
+import javax.crypto.BadPaddingException
+import javax.crypto.Cipher
+import javax.crypto.IllegalBlockSizeException
+import javax.crypto.Mac
+import javax.crypto.spec.IvParameterSpec
+import javax.crypto.spec.SecretKeySpec
+
+/**
+ * An OutputStream for encrypting attachment data.
+ * The output stream writes the IV, ciphertext, and HMAC in sequence.
+ *
+ * @param combinedKeyMaterial The key material used for encryption and authentication. It is expected to be a byte array
+ *    containing two parts: the first half being the AES key and the second half being the HMAC key.
+ * @param iv The initialization vector (IV) for the cipher, or null to generate a random one.
+ * @param outputStream The underlying output stream to write the encrypted data to.
+ */
+class AttachmentCipherOutputStream(
+  combinedKeyMaterial: ByteArray,
+  iv: ByteArray?,
+  outputStream: OutputStream
+) : DigestingOutputStream(outputStream) {
+
+  private val cipher: Cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
+  private val mac: Mac = Mac.getInstance("HmacSHA256")
+
+  init {
+    val keyParts = Util.split(combinedKeyMaterial, 32, 32)
+
+    if (iv == null) {
+      cipher.init(Cipher.ENCRYPT_MODE, SecretKeySpec(keyParts[0], "AES"))
+    } else {
+      cipher.init(Cipher.ENCRYPT_MODE, SecretKeySpec(keyParts[0], "AES"), IvParameterSpec(iv))
+    }
+
+    mac.init(SecretKeySpec(keyParts[1], "HmacSHA256"))
+
+    mac.update(cipher.iv)
+    super.write(cipher.iv)
+  }
+
+  @Throws(IOException::class)
+  override fun write(buffer: ByteArray) {
+    write(buffer, 0, buffer.size)
+  }
+
+  @Throws(IOException::class)
+  override fun write(buffer: ByteArray, offset: Int, length: Int) {
+    val ciphertext = cipher.update(buffer, offset, length)
+
+    if (ciphertext != null) {
+      mac.update(ciphertext)
+      super.write(ciphertext)
+    }
+  }
+
+  @Throws(IOException::class)
+  override fun write(b: Int) {
+    val input = ByteArray(1)
+    input[0] = b.toByte()
+    write(input, 0, 1)
+  }
+
+  @Throws(IOException::class)
+  override fun close() {
+    try {
+      val ciphertext = cipher.doFinal()
+      val auth = mac.doFinal(ciphertext)
+
+      super.write(ciphertext)
+      super.write(auth)
+
+      super.close()
+    } catch (e: IllegalBlockSizeException) {
+      throw AssertionError(e)
+    } catch (e: BadPaddingException) {
+      throw AssertionError(e)
+    }
+  }
+}