diff --git a/cpp/src/arrow/filesystem/filesystem_test.cc b/cpp/src/arrow/filesystem/filesystem_test.cc
index 845c82a79edd5..df21d88e41c6d 100644
--- a/cpp/src/arrow/filesystem/filesystem_test.cc
+++ b/cpp/src/arrow/filesystem/filesystem_test.cc
@@ -59,7 +59,7 @@ TEST(CalculateSSECKeyMD5, Sanity) {
   ASSERT_FALSE(CalculateSSECKeyMD5("", lResult));  // invalid base64
   ASSERT_FALSE(CalculateSSECKeyMD5("%^H", lResult));  // invalid base64
   ASSERT_FALSE(CalculateSSECKeyMD5("INVALID", lResult));  // invalid base64
-  ASSERT_FALSE(CalculateSSECKeyMD5("MTIzNDU2Nzg5", lResult));  // not, match 32 bytes
+  ASSERT_FALSE(CalculateSSECKeyMD5("MTIzNDU2Nzg5", lResult));  // invalid, the input key size not match
   ASSERT_TRUE(CalculateSSECKeyMD5("1WH9aTJ0+Tn0NLbTMHZn9aCW3Li3ViAdBsoIldPCREw=", lResult));  // valid case
 }
 
diff --git a/cpp/src/arrow/filesystem/util_internal.h b/cpp/src/arrow/filesystem/util_internal.h
index 30402e28ae207..a9eef51607593 100644
--- a/cpp/src/arrow/filesystem/util_internal.h
+++ b/cpp/src/arrow/filesystem/util_internal.h
@@ -99,7 +99,12 @@ ARROW_EXPORT
 Result<FileInfoVector> GlobFiles(const std::shared_ptr<FileSystem>& filesystem,
                                  const std::string& glob);
 
-
+/// \brief Decode the Input SSE key,calculate the MD5
+/// \param base64_encoded_key is the input base64 encoded sse key
+/// \param md5_result, output resut
+/// \param expect_input_key_size, default 32
+/// \return true if the decode and calculate MD5 success, otherwise return false
+ARROW_EXPORT
 bool CalculateSSECKeyMD5(const std::string& base64_encoded_key, std::string& md5_result,
                          int expect_input_key_size = 32);