It wouldn't necessarily be hard, it would just be very out of the norm. The polie generally only train dogs for the most popular drugs because it equates to more busts for them.
Off the top of my head the main drugs dogs are trained to detect are Heroin, Amphetamines, Marijuana, Cocaine and MDMA. They are the main staples, but there have been knows to be variations.
None of this crap about coffee beans or deoderant or any other odour masking technique is right either. All bullshit that gets passed around by friends. There is no sure way to get them through, even if you do it carefully. You simply get lucky. That's all.
This man is right, dogs sniffing methods are different.
Watch Never Get Busted, and the guy in there explains how he knows it works.
And a few months ago i walked past a sniffer dog with 1g of some bud, the dog didn't stop or do anything, i just kept on walking and it was fine.
I was scared shitless though.