Invisible ink

Someone back home told me he read in the newspaper this week that during the first World War British spies used cum as invisible ink but it had to be fresh cum because stale cum smells do you think this is true?

http://www.lostiempos.com/diario/actualidad/vida-y-futuro/20091027/media_recortes/2009/10/27/72642_gd.jpg
Keith Jeffrey, MI6: The History of the Secret Intelligence Service 1909-1949, page 66.

http://www.guardian.co.uk/books/2010/se ... ce-service (http://www.guardian.co.uk/books/2010/sep/26/mi6-history-secret-intelligence-service)
http://www.telegraph.co.uk/news/uknews/ ... e-ink.html (http://www.telegraph.co.uk/news/uknews/8015180/MI6-used-bodily-fluids-as-invisible-ink.html)

http://en.wikipedia.org/wiki/Semen#Semen_in_espionage