add explicit offset methods

Author: banister

test/fake_code_signer.rb

@@ -28,19 +28,17 @@ def initialize(input:, output:, padding: 4)
   def sign
     if pe_header.security_size !=0
       raise "Binary already signed, nothing to do!"
-    elsif @input == @output
-      raise "input and output files must be different!"
     end
 
     # Below we access an instance of the IMAGE_DATA_DIRECTORY struct.
     # This instance is called IMAGE_DIRECTORY_ENTRY_SECURITY and it contains information about the digital signature
     # see: https://msdn.microsoft.com/en-us/library/windows/desktop/ms680305(v=vs.85).aspx
 
     # write the offset (address) of the digital signature to the security header (VirtualAddress field)
-    @image[pe_header.security_offset, 4] = raw_bytes(@image.size + @padding)
+    @image[pe_header.security_address_offset, PEHeader::DWORD_SIZE] = raw_bytes(@image.size + @padding)
 
     # write the size of the digital signature to the security header (Size field)
-    @image[pe_header.security_offset + 4, 4] = raw_bytes(FAKE_SIG.size)
+    @image[pe_header.security_size_offset, PEHeader::DWORD_SIZE] = raw_bytes(FAKE_SIG.size)
 
     # append the "digital signature" to the end of the executable, complete with padding
     @image << padding_string << FAKE_SIG

test/fake_code_signer/pe_header.rb

@@ -39,14 +39,20 @@ def security_offset
       image_data_directory_offset + DATA_DIRECTORY_ENTRY_SIZE * 4
     end
 
+    alias security_address_offset security_offset
+
+    def security_size_offset
+      security_offset + DWORD_SIZE
+    end
+
     # location of the digital signature
     def security_address
-      deref(security_offset)
+      deref(security_address_offset)
     end
 
     # size of the digital signature
     def security_size
-      deref(security_offset + DWORD_SIZE)
+      deref(security_size_offset)
     end
 
     private