diff --git a/plugin/src/widevine_cas_api.cpp b/plugin/src/widevine_cas_api.cpp
index 483737f..0362366 100644
--- a/plugin/src/widevine_cas_api.cpp
+++ b/plugin/src/widevine_cas_api.cpp
@@ -2,11 +2,11 @@
 
 #include <openssl/sha.h>
 
-#include "ca_descriptor.pb.h"
 #include "cas_events.h"
 #include "cas_util.h"
 #include "license_protocol.pb.h"
 #include "log.h"
+#include "media_cas.pb.h"
 #include "string_conversions.h"
 #include "widevine_cas_session_map.h"
 
diff --git a/plugin/src/widevine_media_cas_plugin.cpp b/plugin/src/widevine_media_cas_plugin.cpp
index fceb421..5c013a6 100644
--- a/plugin/src/widevine_media_cas_plugin.cpp
+++ b/plugin/src/widevine_media_cas_plugin.cpp
@@ -36,16 +36,20 @@ static const char kLicenseFileExtension[] = ".lic";
 // int32_t and the underlying type can be changed.
 WvCasSessionId androidSessionIdToWidevine(
     const CasSessionId& android_session_id) {
-  std::string session_id_str(android_session_id.begin(),
-                             android_session_id.end());
-  WvCasSessionId wv_session_id = std::atoi(session_id_str.c_str());
+  WvCasSessionId wv_session_id = 0;
+  // CasSessionId is expected to be in little endian order.
+  for (int i = android_session_id.size() - 1; i >= 0; --i) {
+    wv_session_id = (wv_session_id << 8) | android_session_id[i];
+  }
   return wv_session_id;
 }
 
 CasSessionId widevineSessionIdToAndroid(const WvCasSessionId& wv_session_id) {
-  auto wv_session_id_begin = reinterpret_cast<const uint8_t*>(&wv_session_id);
-  CasSessionId android_session_id(wv_session_id_begin,
-                                  wv_session_id_begin + sizeof(wv_session_id));
+  CasSessionId android_session_id;
+  // CasSessionId is expected to be in little endian order.
+  for (int i = 0; i < static_cast<int>(sizeof(WvCasSessionId)); ++i) {
+    android_session_id.push_back((wv_session_id >> (8 * i)) & 0xff);
+  }
   return android_session_id;
 }
 
@@ -101,8 +105,11 @@ status_t WidevineCasPlugin::openSession(CasSessionId* sessionId) {
   if (!status.ok()) {
     return android::ERROR_CAS_SESSION_NOT_OPENED;
   }
-  std::string session_id_str = std::to_string(new_session_id);
-  *sessionId = CasSessionId(session_id_str.begin(), session_id_str.end());
+
+  const CasSessionId& android_session_id =
+      widevineSessionIdToAndroid(new_session_id);
+  sessionId->assign(android_session_id.begin(), android_session_id.end());
+
   // Not a session event, so CasSessionId in callback is null.
   CallBack(app_data_, CAS_SESSION_ID, new_session_id, nullptr, 0, nullptr);
   return OK;
diff --git a/protos/Android.bp b/protos/Android.bp
index 151ed5e..62bfaa7 100644
--- a/protos/Android.bp
+++ b/protos/Android.bp
@@ -10,7 +10,7 @@ cc_library_static {
   proprietary: true,
 
   srcs: [
-    "ca_descriptor.proto",
+    "media_cas.proto",
     "device_files.proto",
     "license_protocol.proto",
   ],
diff --git a/protos/ca_descriptor.proto b/protos/media_cas.proto
similarity index 100%
rename from protos/ca_descriptor.proto
rename to protos/media_cas.proto