Crypto Algorithm Question - Perfect Secrecy

I'm not sure where to start this problem:
Consider the following definition of perfect secrecy for the en- cryption of two messages. An encryption scheme (Gen, Enc, Dec) over a message space M is perfectly-secret for two messages if for all distributions over M, all m,m′ ∈M, and all c,c′ ∈C with Pr[C=c∧C′ =c′]>0:
Pr[M=m∧M′ =m′ |C=c∧C′ =c′]=Pr[M=m∧M′ =m′ |M̸=M′],
where m and m′ are sampled independently from the same distribution over M. Show an encryption scheme that provably satisfies this definition. How long are the keys in terms of the length of a message?
Hint. The encryption scheme you propose need not be “efficient”.
ANY help would be great!

Google "perfect secrecy" and click on the first result (it should be a Wikipedia page).

Ever tried to read Paul Syverson's books?

Or Roger Dingledine's.