diff --git a/flash-ipmi.hpp b/flash-ipmi.hpp
index 56c32da..7a63fc3 100644
--- a/flash-ipmi.hpp
+++ b/flash-ipmi.hpp
@@ -106,6 +106,14 @@
      * Called to indicate the host is done sending the flash bytes.
      */
     virtual bool flashFinish() = 0;
+
+    /**
+     * Prepare to receive a BMC image signature.
+     *
+     * @param[in] length - length of signature in bytes.
+     * @return true on success, false otherwise.
+     */
+    virtual bool startHash(uint32_t length) = 0;
 };
 
 class FlashUpdate : public UpdateInterface
@@ -124,6 +132,8 @@
 
     bool flashFinish() override;
 
+    bool startHash(uint32_t length) override;
+
   private:
     /**
      * Tries to close out and delete anything staged.