diff --git a/Makefile.am b/Makefile.am
index abfdc6e..fb21633 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -20,8 +20,7 @@
 include vpnor/Makefile.am.include
 else
 mboxd_SOURCES += flash.c \
-	lpc_reset.c \
-	protocol_negotiate_version.c
+	lpc_reset.c
 endif
 
 mboxctl_SOURCES = mboxctl.c
diff --git a/flash.c b/flash.c
index e6058f0..6bd5c84 100644
--- a/flash.c
+++ b/flash.c
@@ -105,6 +105,13 @@
 
 /* Flash Functions */
 
+int flash_validate(struct mbox_context *context, uint32_t offset,
+		   uint32_t size, bool ro)
+{
+	/* Default behaviour is all accesses are valid */
+	return 0;
+}
+
 /*
  * flash_is_erased() - Check if an offset into flash is erased
  * @context:	The mbox context pointer
diff --git a/flash.h b/flash.h
index 44f5155..2760563 100644
--- a/flash.h
+++ b/flash.h
@@ -4,6 +4,7 @@
 #ifndef FLASH_H
 #define FLASH_H
 
+#include <stdbool.h>
 #include <stdint.h>
 
 #define FLASH_DIRTY	0x00
@@ -16,6 +17,8 @@
 
 int flash_dev_init(struct mbox_context *context);
 void flash_dev_free(struct mbox_context *context);
+int flash_validate(struct mbox_context *context, uint32_t offset,
+		   uint32_t size, bool ro);
 int64_t flash_copy(struct mbox_context *context, uint32_t offset, void *mem,
 		   uint32_t size);
 int flash_set_bytemap(struct mbox_context *context, uint32_t offset,
diff --git a/protocol.c b/protocol.c
index 2a04d89..76bc6ea 100644
--- a/protocol.c
+++ b/protocol.c
@@ -7,8 +7,9 @@
 
 #include "common.h"
 #include "flash.h"
-#include "mboxd.h"
 #include "lpc.h"
+#include "mboxd.h"
+#include "protocol.h"
 #include "windows.h"
 
 #define BLOCK_SIZE_SHIFT_V1		12 /* 4K */
@@ -76,15 +77,18 @@
 	return context->transport->clear_events(context, bmc_event, mask);
 }
 
-int protocol_v1_reset(struct mbox_context *context)
+static int protocol_v1_reset(struct mbox_context *context)
 {
 	/* Host requested it -> No BMC Event */
 	windows_reset_all(context);
 	return lpc_reset(context);
 }
 
-int protocol_v1_get_info(struct mbox_context *context,
-			 struct protocol_get_info *io)
+static int protocol_negotiate_version(struct mbox_context *context,
+				      uint8_t requested);
+
+static int protocol_v1_get_info(struct mbox_context *context,
+				struct protocol_get_info *io)
 {
 	uint8_t old_version = context->version;
 	int rc;
@@ -120,7 +124,7 @@
 	return lpc_map_memory(context);
 }
 
-int protocol_v1_get_flash_info(struct mbox_context *context,
+static int protocol_v1_get_flash_info(struct mbox_context *context,
 			       struct protocol_get_flash_info *io)
 {
 	io->resp.v1.flash_size = context->flash_size;
@@ -149,11 +153,18 @@
 	return lpc_addr >> context->block_size_shift;
 }
 
-int protocol_v1_create_window(struct mbox_context *context,
-			      struct protocol_create_window *io)
+static int protocol_v1_create_window(struct mbox_context *context,
+				     struct protocol_create_window *io)
 {
-	int rc;
 	uint32_t offset = io->req.offset << context->block_size_shift;
+	uint32_t size = io->req.size << context->block_size_shift;
+	int rc;
+
+	rc = flash_validate(context, offset, size, io->req.ro);
+	if (rc < 0) {
+		/* Backend does not allow window to be created. */
+		return rc;
+	}
 
 	/* Close the current window if there is one */
 	if (context->current) {
@@ -202,8 +213,8 @@
 	return 0;
 }
 
-int protocol_v1_mark_dirty(struct mbox_context *context,
-			   struct protocol_mark_dirty *io)
+static int protocol_v1_mark_dirty(struct mbox_context *context,
+				  struct protocol_mark_dirty *io)
 {
 	uint32_t offset = io->req.v1.offset;
 	uint32_t size = io->req.v1.size;
@@ -305,7 +316,8 @@
 				  WINDOW_CLEAN);
 }
 
-int protocol_v1_flush(struct mbox_context *context, struct protocol_flush *io)
+static int protocol_v1_flush(struct mbox_context *context,
+			     struct protocol_flush *io)
 {
 	int rc;
 
@@ -331,7 +343,8 @@
 	return generic_flush(context);
 }
 
-int protocol_v1_close(struct mbox_context *context, struct protocol_close *io)
+static int protocol_v1_close(struct mbox_context *context,
+			     struct protocol_close *io)
 {
 	int rc;
 
@@ -355,7 +368,8 @@
 	return 0;
 }
 
-int protocol_v1_ack(struct mbox_context *context, struct protocol_ack *io)
+static int protocol_v1_ack(struct mbox_context *context,
+			   struct protocol_ack *io)
 {
 	return protocol_events_clear(context,
 				     (io->req.flags & BMC_EVENT_ACK_MASK));
@@ -380,8 +394,8 @@
 	return ret;
 }
 
-int protocol_v2_get_info(struct mbox_context *context,
-			 struct protocol_get_info *io)
+static int protocol_v2_get_info(struct mbox_context *context,
+				struct protocol_get_info *io)
 {
 	uint8_t old_version = context->version;
 	int rc;
@@ -415,8 +429,8 @@
 	return lpc_map_memory(context);
 }
 
-int protocol_v2_get_flash_info(struct mbox_context *context,
-			       struct protocol_get_flash_info *io)
+static int protocol_v2_get_flash_info(struct mbox_context *context,
+				      struct protocol_get_flash_info *io)
 {
 	io->resp.v2.flash_size =
 		context->flash_size >> context->block_size_shift;
@@ -426,8 +440,8 @@
 	return 0;
 }
 
-int protocol_v2_create_window(struct mbox_context *context,
-			      struct protocol_create_window *io)
+static int protocol_v2_create_window(struct mbox_context *context,
+				     struct protocol_create_window *io)
 {
 	int rc;
 
@@ -442,8 +456,8 @@
 	return 0;
 }
 
-int protocol_v2_mark_dirty(struct mbox_context *context,
-			   struct protocol_mark_dirty *io)
+static int protocol_v2_mark_dirty(struct mbox_context *context,
+				  struct protocol_mark_dirty *io)
 {
 	if (!(context->current && context->current_is_write)) {
 		MSG_ERR("Tried to call mark dirty without open write window\n");
@@ -458,8 +472,8 @@
 				  io->req.v2.size, WINDOW_DIRTY);
 }
 
-int protocol_v2_erase(struct mbox_context *context,
-		      struct protocol_erase *io)
+static int protocol_v2_erase(struct mbox_context *context,
+			     struct protocol_erase *io)
 {
 	size_t start, len;
 	int rc;
@@ -487,7 +501,8 @@
 	return 0;
 }
 
-int protocol_v2_flush(struct mbox_context *context, struct protocol_flush *io)
+static int protocol_v2_flush(struct mbox_context *context,
+			     struct protocol_flush *io)
 {
 	if (!(context->current && context->current_is_write)) {
 		MSG_ERR("Tried to call flush without open write window\n");
@@ -497,7 +512,8 @@
 	return generic_flush(context);
 }
 
-int protocol_v2_close(struct mbox_context *context, struct protocol_close *io)
+static int protocol_v2_close(struct mbox_context *context,
+			     struct protocol_close *io)
 {
 	int rc;
 
@@ -521,6 +537,51 @@
 	return 0;
 }
 
+static const struct protocol_ops protocol_ops_v1 = {
+	.reset = protocol_v1_reset,
+	.get_info = protocol_v1_get_info,
+	.get_flash_info = protocol_v1_get_flash_info,
+	.create_window = protocol_v1_create_window,
+	.mark_dirty = protocol_v1_mark_dirty,
+	.erase = NULL,
+	.flush = protocol_v1_flush,
+	.close = protocol_v1_close,
+	.ack = protocol_v1_ack,
+};
+
+static const struct protocol_ops protocol_ops_v2 = {
+	.reset = protocol_v1_reset,
+	.get_info = protocol_v2_get_info,
+	.get_flash_info = protocol_v2_get_flash_info,
+	.create_window = protocol_v2_create_window,
+	.mark_dirty = protocol_v2_mark_dirty,
+	.erase = protocol_v2_erase,
+	.flush = protocol_v2_flush,
+	.close = protocol_v2_close,
+	.ack = protocol_v1_ack,
+};
+
+static const struct protocol_ops *protocol_ops_map[] = {
+	[0] = NULL,
+	[1] = &protocol_ops_v1,
+	[2] = &protocol_ops_v2,
+};
+
+static int protocol_negotiate_version(struct mbox_context *context,
+				      uint8_t requested)
+{
+	/* Check we support the version requested */
+	if (requested < API_MIN_VERSION)
+		return -EINVAL;
+
+	context->version = (requested > API_MAX_VERSION) ?
+				API_MAX_VERSION : requested;
+
+	context->protocol = protocol_ops_map[context->version];
+
+	return context->version;
+}
+
 int protocol_init(struct mbox_context *context)
 {
 	protocol_negotiate_version(context, API_MAX_VERSION);
diff --git a/protocol.h b/protocol.h
index 5a678c6..f571fe1 100644
--- a/protocol.h
+++ b/protocol.h
@@ -119,8 +119,6 @@
 int protocol_init(struct mbox_context *context);
 void protocol_free(struct mbox_context *context);
 
-int protocol_negotiate_version(struct mbox_context *context, uint8_t requested);
-
 /* Sneaky reset: Don't tell the host */
 int __protocol_reset(struct mbox_context *context);
 
@@ -132,32 +130,4 @@
 int protocol_events_set(struct mbox_context *context, uint8_t bmc_event);
 int protocol_events_clear(struct mbox_context *context, uint8_t bmc_event);
 
-/* Protocol v1 */
-int protocol_v1_reset(struct mbox_context *context);
-int protocol_v1_get_info(struct mbox_context *context,
-			 struct protocol_get_info *io);
-int protocol_v1_get_flash_info(struct mbox_context *context,
-			       struct protocol_get_flash_info *io);
-int protocol_v1_create_window(struct mbox_context *context,
-			      struct protocol_create_window *io);
-int protocol_v1_mark_dirty(struct mbox_context *context,
-			   struct protocol_mark_dirty *io);
-int protocol_v1_flush(struct mbox_context *context, struct protocol_flush *io);
-int protocol_v1_close(struct mbox_context *context, struct protocol_close *io);
-int protocol_v1_ack(struct mbox_context *context, struct protocol_ack *io);
-
-/* Protocol v2 */
-int protocol_v2_get_info(struct mbox_context *context,
-			 struct protocol_get_info *io);
-int protocol_v2_get_flash_info(struct mbox_context *context,
-			       struct protocol_get_flash_info *io);
-int protocol_v2_create_window(struct mbox_context *context,
-			      struct protocol_create_window *io);
-int protocol_v2_mark_dirty(struct mbox_context *context,
-			   struct protocol_mark_dirty *io);
-int protocol_v2_erase(struct mbox_context *context,
-		      struct protocol_erase *io);
-int protocol_v2_flush(struct mbox_context *context, struct protocol_flush *io);
-int protocol_v2_close(struct mbox_context *context, struct protocol_close *io);
-
 #endif /* PROTOCOL_H */
diff --git a/protocol_negotiate_version.c b/protocol_negotiate_version.c
deleted file mode 100644
index 16b8e64..0000000
--- a/protocol_negotiate_version.c
+++ /dev/null
@@ -1,53 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright (C) 2018 IBM Corp.
-#include "config.h"
-
-#include <errno.h>
-
-#include "mboxd.h"
-#include "protocol.h"
-
-static const struct protocol_ops protocol_ops_v1 = {
-	.reset = protocol_v1_reset,
-	.get_info = protocol_v1_get_info,
-	.get_flash_info = protocol_v1_get_flash_info,
-	.create_window = protocol_v1_create_window,
-	.mark_dirty = protocol_v1_mark_dirty,
-	.erase = NULL,
-	.flush = protocol_v1_flush,
-	.close = protocol_v1_close,
-	.ack = protocol_v1_ack,
-};
-
-static const struct protocol_ops protocol_ops_v2 = {
-	.reset = protocol_v1_reset,
-	.get_info = protocol_v2_get_info,
-	.get_flash_info = protocol_v2_get_flash_info,
-	.create_window = protocol_v2_create_window,
-	.mark_dirty = protocol_v2_mark_dirty,
-	.erase = protocol_v2_erase,
-	.flush = protocol_v2_flush,
-	.close = protocol_v2_close,
-	.ack = protocol_v1_ack,
-};
-
-static const struct protocol_ops *protocol_ops_map[] = {
-	[0] = NULL,
-	[1] = &protocol_ops_v1,
-	[2] = &protocol_ops_v2,
-};
-
-int protocol_negotiate_version(struct mbox_context *context,
-				   uint8_t requested)
-{
-	/* Check we support the version requested */
-	if (requested < API_MIN_VERSION)
-		return -EINVAL;
-
-	context->version = (requested > API_MAX_VERSION) ?
-				API_MAX_VERSION : requested;
-
-	context->protocol = protocol_ops_map[context->version];
-
-	return context->version;
-}
diff --git a/test/Makefile.am.include b/test/Makefile.am.include
index de1bfce..195e451 100644
--- a/test/Makefile.am.include
+++ b/test/Makefile.am.include
@@ -26,8 +26,7 @@
 	lpc_reset.c \
 	common.c \
 	flash.c \
-	protocol.c \
-	protocol_negotiate_version.c
+	protocol.c
 
 TEST_MOCK_SRCS = %reldir%/tmpf.c %reldir%/mbox.c %reldir%/system.c
 
diff --git a/vpnor/Makefile.am.include b/vpnor/Makefile.am.include
index 75bd92e..36e3c50 100644
--- a/vpnor/Makefile.am.include
+++ b/vpnor/Makefile.am.include
@@ -2,9 +2,7 @@
 	%reldir%/mboxd_pnor_partition_table.cpp \
 	%reldir%/flash.cpp \
 	%reldir%/pnor_partition.cpp \
-	%reldir%/lpc_reset.cpp \
-	%reldir%/protocol.cpp \
-	%reldir%/protocol_negotiate_version.cpp
+	%reldir%/lpc_reset.cpp
 
 mboxd_LDFLAGS += -lstdc++fs \
 	$(SDBUSPLUS_LIBS) \
diff --git a/vpnor/flash.cpp b/vpnor/flash.cpp
index cd2a37f..847e6a5 100644
--- a/vpnor/flash.cpp
+++ b/vpnor/flash.cpp
@@ -237,3 +237,39 @@
     }
     return 0;
 }
+
+static bool vpnor_partition_is_readonly(const pnor_partition& part)
+{
+    return part.data.user.data[1] & PARTITION_READONLY;
+}
+
+int flash_validate(struct mbox_context* context, uint32_t offset,
+                   uint32_t size __attribute__((unused)), bool ro)
+{
+    /* All reads are allowed */
+    if (ro)
+    {
+        return 0;
+    }
+
+    /* Only allow write windows on regions mapped by the ToC as writeable */
+    try
+    {
+        const pnor_partition& part = context->vpnor->table->partition(offset);
+        if (vpnor_partition_is_readonly(part))
+        {
+            return -EPERM;
+        }
+    }
+    catch (const openpower::virtual_pnor::UnmappedOffset& e)
+    {
+        /*
+         * Writes to unmapped areas are not meaningful, so deny the request.
+         * This removes the ability for a compromised host to abuse unused
+         * space if any data was to be persisted (which it isn't).
+         */
+        return -EACCES;
+    }
+
+    return 0;
+}
diff --git a/vpnor/protocol.cpp b/vpnor/protocol.cpp
deleted file mode 100644
index c3f4fca..0000000
--- a/vpnor/protocol.cpp
+++ /dev/null
@@ -1,65 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright (C) 2018 IBM Corp.
-#include "config.h"
-
-extern "C" {
-#include "mboxd.h"
-#include "protocol.h"
-#include "vpnor/protocol.h"
-}
-
-#include "vpnor/pnor_partition_table.hpp"
-
-/* XXX: Maybe this should be a method on a class? */
-static bool vpnor_partition_is_readonly(const pnor_partition& part)
-{
-    return part.data.user.data[1] & PARTITION_READONLY;
-}
-
-typedef int (*create_window_fn)(struct mbox_context* context,
-                                struct protocol_create_window* io);
-
-static int generic_vpnor_create_window(struct mbox_context* context,
-                                       struct protocol_create_window* io,
-                                       create_window_fn create_window)
-{
-    if (io->req.ro)
-    {
-        return create_window(context, io);
-    }
-
-    /* Only allow write windows on regions mapped by the ToC as writeable */
-    size_t offset = io->req.offset;
-    offset <<= context->block_size_shift;
-    try
-    {
-        const pnor_partition& part = context->vpnor->table->partition(offset);
-        if (vpnor_partition_is_readonly(part))
-        {
-            return -EPERM;
-        }
-    }
-    catch (const openpower::virtual_pnor::UnmappedOffset& e)
-    {
-        /*
-         * Writes to unmapped areas are not meaningful, so deny the request.
-         * This removes the ability for a compromised host to abuse unused
-         * space if any data was to be persisted (which it isn't).
-         */
-        return -EACCES;
-    }
-
-    return create_window(context, io);
-}
-
-int protocol_v1_vpnor_create_window(struct mbox_context* context,
-                                    struct protocol_create_window* io)
-{
-    return generic_vpnor_create_window(context, io, protocol_v1_create_window);
-}
-
-int protocol_v2_vpnor_create_window(struct mbox_context* context,
-                                    struct protocol_create_window* io)
-{
-    return generic_vpnor_create_window(context, io, protocol_v2_create_window);
-}
diff --git a/vpnor/protocol_negotiate_version.cpp b/vpnor/protocol_negotiate_version.cpp
deleted file mode 100644
index 12593ea..0000000
--- a/vpnor/protocol_negotiate_version.cpp
+++ /dev/null
@@ -1,58 +0,0 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright (C) 2018 IBM Corp.
-#include "config.h"
-
-#include <errno.h>
-
-extern "C" {
-#include "mboxd.h"
-#include "protocol.h"
-#include "vpnor/protocol.h"
-}
-
-/* We need to hijack create_window for vpnor */
-
-static const struct protocol_ops protocol_ops_v1 = {
-    .reset = protocol_v1_reset,
-    .get_info = protocol_v1_get_info,
-    .get_flash_info = protocol_v1_get_flash_info,
-    .create_window = protocol_v1_vpnor_create_window,
-    .mark_dirty = protocol_v1_mark_dirty,
-    .erase = NULL,
-    .flush = protocol_v1_flush,
-    .close = protocol_v1_close,
-    .ack = protocol_v1_ack,
-};
-
-static const struct protocol_ops protocol_ops_v2 = {
-    .reset = protocol_v1_reset,
-    .get_info = protocol_v2_get_info,
-    .get_flash_info = protocol_v2_get_flash_info,
-    .create_window = protocol_v2_vpnor_create_window,
-    .mark_dirty = protocol_v2_mark_dirty,
-    .erase = protocol_v2_erase,
-    .flush = protocol_v2_flush,
-    .close = protocol_v2_close,
-    .ack = protocol_v1_ack,
-};
-
-static const struct protocol_ops* protocol_ops_map[] = {
-    [0] = NULL,
-    [1] = &protocol_ops_v1,
-    [2] = &protocol_ops_v2,
-};
-
-int protocol_negotiate_version(struct mbox_context* context, uint8_t requested)
-{
-    /* Check we support the version requested */
-    if (requested < API_MIN_VERSION)
-        return -EINVAL;
-
-    uint8_t version =
-        (requested > API_MAX_VERSION) ? API_MAX_VERSION : requested;
-    context->version = static_cast<enum api_version>(version);
-
-    context->protocol = protocol_ops_map[context->version];
-
-    return context->version;
-}
diff --git a/vpnor/test/Makefile.am.include b/vpnor/test/Makefile.am.include
index f3f4fc2..5ff4dcc 100644
--- a/vpnor/test/Makefile.am.include
+++ b/vpnor/test/Makefile.am.include
@@ -14,8 +14,6 @@
 	vpnor/flash.cpp \
 	vpnor/pnor_partition.cpp \
 	vpnor/pnor_partition_table.cpp \
-	vpnor/protocol.cpp \
-	vpnor/protocol_negotiate_version.cpp \
 	%reldir%/tmpd.cpp
 
 VPNOR_LDADD = -lstdc++fs \