diff --git a/ClosedXML/Excel/XLSheetProtection.cs b/ClosedXML/Excel/XLSheetProtection.cs
index 9fd5108..9166280 100644
--- a/ClosedXML/Excel/XLSheetProtection.cs
+++ b/ClosedXML/Excel/XLSheetProtection.cs
@@ -16,7 +16,7 @@
         {
             set
             {
-                PasswordHash = StringExtensions.GetPasswordHash(value);
+                PasswordHash = GetPasswordHash(value);
             }
         }
 
@@ -66,7 +66,7 @@
             else
             {
                 Protected = true;
-                PasswordHash = StringExtensions.GetPasswordHash(password);
+                PasswordHash = GetPasswordHash(password);
             }
             return this;
         }
@@ -80,7 +80,7 @@
         {
             if (Protected)
             {
-                String hash = StringExtensions.GetPasswordHash(password);
+                String hash = GetPasswordHash(password);
                 if (hash != PasswordHash)
                     throw new ArgumentException("Invalid password");
                 else
@@ -92,5 +92,21 @@
 
             return this;
         }
+
+        private String GetPasswordHash(String password)
+        {
+            Int32 pLength = password.Length;
+            Int32 hash = 0;
+            if (pLength == 0) return String.Empty;
+
+            for (Int32 i = pLength - 1; i >= 0; i--)
+            {
+                hash ^= password[i];
+                hash = hash >> 14 & 0x01 | hash << 1 & 0x7fff;
+            }
+            hash ^= 0x8000 | 'N' << 8 | 'K';
+            hash ^= pLength;
+            return hash.ToString("X");
+        }
     }
 }
diff --git a/ClosedXML/Excel/XLWorkbook.cs b/ClosedXML/Excel/XLWorkbook.cs
index 173ff20..ed6398f 100644
--- a/ClosedXML/Excel/XLWorkbook.cs
+++ b/ClosedXML/Excel/XLWorkbook.cs
@@ -870,7 +870,7 @@
         {
             if (workbookPassword != null)
             {
-                var hashPassword = StringExtensions.GetPasswordHash(workbookPassword);
+                var hashPassword = GetPasswordHash(workbookPassword);
                 if (LockPassword != null)
                 {
                     if (LockPassword != hashPassword)
@@ -939,5 +939,21 @@
         {
             Protect(false, false, workbookPassword);
         }
+
+        private String GetPasswordHash(String password)
+        {
+            Int32 pLength = password.Length;
+            Int32 hash = 0;
+            if (pLength == 0) return String.Empty;
+
+            for (Int32 i = pLength - 1; i >= 0; i--)
+            {
+                hash ^= password[i];
+                hash = hash >> 14 & 0x01 | hash << 1 & 0x7fff;
+            }
+            hash ^= 0x8000 | 'N' << 8 | 'K';
+            hash ^= pLength;
+            return hash.ToString("X");
+        }
     }
 }
\ No newline at end of file
diff --git a/ClosedXML/Extensions.cs b/ClosedXML/Extensions.cs
index f9b9a10..7655168 100644
--- a/ClosedXML/Extensions.cs
+++ b/ClosedXML/Extensions.cs
@@ -132,22 +132,6 @@
 
             return value.Substring(0, 1).ToUpper() + value.Substring(1);
         }
-
-        public static String GetPasswordHash(String password)
-        {
-            Int32 pLength = password.Length;
-            Int32 hash = 0;
-            if (pLength == 0) return String.Empty;
-
-            for (Int32 i = pLength - 1; i >= 0; i--)
-            {
-                hash ^= password[i];
-                hash = hash >> 14 & 0x01 | hash << 1 & 0x7fff;
-            }
-            hash ^= 0x8000 | 'N' << 8 | 'K';
-            hash ^= pLength;
-            return hash.ToString("X");
-        }
     }
 
     public static class DateTimeExtensions