Proverb Assistance: Enemy's Enemy

This tiny article at Wikipedia gives the Chinese credit for the proverb:

"The enemy of my enemy is my friend."

as well as:

"It is good to strike the serpent's head with your enemy's hand."

But there are no references for either. I'd like to learn the Chinese for one or both of those.So can anyone confirm that those are actually Chinese idioms by giving us the hanzi for either?On a more general note, this little wèntí 问题 clearly shows one the of the gaps in the Chinese-learning resources market: a complete and easily searchable proverb and idiom dictionary for Chinese and English. Does anyone have a favorite they'd like to recommend?