diff --git a/alts/src/main/java/io/grpc/alts/internal/AltsChannelCrypter.java b/alts/src/main/java/io/grpc/alts/internal/AltsChannelCrypter.java
index 5e4c6fec30..e47433ff03 100644
--- a/alts/src/main/java/io/grpc/alts/internal/AltsChannelCrypter.java
+++ b/alts/src/main/java/io/grpc/alts/internal/AltsChannelCrypter.java
@@ -17,10 +17,10 @@
 package io.grpc.alts.internal;
 
 import static com.google.common.base.Preconditions.checkArgument;
-import static com.google.common.base.Verify.verify;
 
 import com.google.common.annotations.VisibleForTesting;
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
 import java.nio.ByteBuffer;
 import java.security.GeneralSecurityException;
 import java.util.List;
@@ -56,61 +56,72 @@ final class AltsChannelCrypter implements ChannelCrypterNetty {
 
   @Override
   public void encrypt(ByteBuf outBuf, List<ByteBuf> plainBufs) throws GeneralSecurityException {
-    checkArgument(outBuf.nioBufferCount() == 1);
-    // Copy plaintext buffers into outBuf for in-place encryption on single direct buffer.
-    ByteBuf plainBuf = outBuf.slice(outBuf.writerIndex(), outBuf.writableBytes());
-    plainBuf.writerIndex(0);
-    for (ByteBuf inBuf : plainBufs) {
-      plainBuf.writeBytes(inBuf);
+    byte[] tempArr = new byte[outBuf.writableBytes()];
+
+    // Copy plaintext into tempArr.
+    {
+      ByteBuf tempBuf = Unpooled.wrappedBuffer(tempArr, 0, tempArr.length - TAG_LENGTH);
+      tempBuf.resetWriterIndex();
+      for (ByteBuf plainBuf : plainBufs) {
+        tempBuf.writeBytes(plainBuf);
+      }
     }
 
-    verify(outBuf.writableBytes() == plainBuf.readableBytes() + TAG_LENGTH);
-    ByteBuffer out = outBuf.internalNioBuffer(outBuf.writerIndex(), outBuf.writableBytes());
-    ByteBuffer plain = out.duplicate();
-    plain.limit(out.limit() - TAG_LENGTH);
+    // Encrypt into tempArr.
+    {
+      ByteBuffer out = ByteBuffer.wrap(tempArr);
+      ByteBuffer plain = ByteBuffer.wrap(tempArr, 0, tempArr.length - TAG_LENGTH);
 
-    byte[] counter = incrementOutCounter();
-    int outPosition = out.position();
-    aeadCrypter.encrypt(out, plain, counter);
-    int bytesWritten = out.position() - outPosition;
-    outBuf.writerIndex(outBuf.writerIndex() + bytesWritten);
-    verify(!outBuf.isWritable());
+      byte[] counter = incrementOutCounter();
+      aeadCrypter.encrypt(out, plain, counter);
+    }
+    outBuf.writeBytes(tempArr);
   }
 
   @Override
-  public void decrypt(ByteBuf out, ByteBuf tag, List<ByteBuf> ciphertextBufs)
+  public void decrypt(ByteBuf outBuf, ByteBuf tagBuf, List<ByteBuf> ciphertextBufs)
       throws GeneralSecurityException {
+    // There is enough space for the ciphertext including the tag in outBuf.
+    byte[] tempArr = new byte[outBuf.writableBytes()];
 
-    ByteBuf cipherTextAndTag = out.slice(out.writerIndex(), out.writableBytes());
-    cipherTextAndTag.writerIndex(0);
-
-    for (ByteBuf inBuf : ciphertextBufs) {
-      cipherTextAndTag.writeBytes(inBuf);
+    // Copy ciphertext and tag into tempArr.
+    {
+      ByteBuf tempBuf = Unpooled.wrappedBuffer(tempArr);
+      tempBuf.resetWriterIndex();
+      for (ByteBuf ciphertextBuf : ciphertextBufs) {
+        tempBuf.writeBytes(ciphertextBuf);
+      }
+      tempBuf.writeBytes(tagBuf);
     }
-    cipherTextAndTag.writeBytes(tag);
 
-    decrypt(out, cipherTextAndTag);
+    decryptInternal(outBuf, tempArr);
   }
 
   @Override
-  public void decrypt(ByteBuf out, ByteBuf ciphertextAndTag) throws GeneralSecurityException {
-    int bytesRead = ciphertextAndTag.readableBytes();
-    checkArgument(bytesRead == out.writableBytes());
+  public void decrypt(
+      ByteBuf outBuf, ByteBuf ciphertextAndTagDirect) throws GeneralSecurityException {
+    byte[] tempArr = new byte[ciphertextAndTagDirect.readableBytes()];
 
-    checkArgument(out.nioBufferCount() == 1);
-    ByteBuffer outBuffer = out.internalNioBuffer(out.writerIndex(), out.writableBytes());
+    // Copy ciphertext and tag into tempArr.
+    {
+      ByteBuf tempBuf = Unpooled.wrappedBuffer(tempArr);
+      tempBuf.resetWriterIndex();
+      tempBuf.writeBytes(ciphertextAndTagDirect);
+    }
 
-    checkArgument(ciphertextAndTag.nioBufferCount() == 1);
-    ByteBuffer ciphertextAndTagBuffer =
-        ciphertextAndTag.nioBuffer(ciphertextAndTag.readerIndex(), bytesRead);
+    decryptInternal(outBuf, tempArr);
+  }
 
-    byte[] counter = incrementInCounter();
-    int outPosition = outBuffer.position();
-    aeadCrypter.decrypt(outBuffer, ciphertextAndTagBuffer, counter);
-    int bytesWritten = outBuffer.position() - outPosition;
-    out.writerIndex(out.writerIndex() + bytesWritten);
-    ciphertextAndTag.readerIndex(out.readerIndex() + bytesRead);
-    verify(out.writableBytes() == TAG_LENGTH);
+  private void decryptInternal(ByteBuf outBuf, byte[] tempArr) throws GeneralSecurityException {
+    // Perform in-place decryption on tempArr.
+    {
+      ByteBuffer ciphertextAndTag = ByteBuffer.wrap(tempArr);
+      ByteBuffer out = ByteBuffer.wrap(tempArr);
+      byte[] counter = incrementInCounter();
+      aeadCrypter.decrypt(out, ciphertextAndTag, counter);
+    }
+
+    outBuf.writeBytes(tempArr, 0, tempArr.length - TAG_LENGTH);
   }
 
   @Override