I'm trying to get my head around MimbleWimble transactions and I have a few questions in relation to parameters.

A MimbleWimble transaction consists of two Pedersen commitments (i.e. money-in & money-out) and below is the basic algorithm (I think).

(amount-sent x bignumber1) + (key1 x bignumber2) 
                       + 
(amount-received x bignumber1) + (key2 x bignumber2)

I'm unclear on the below points:

• If key1 and key2 are private keys for each user, are these the blinding factor?
• Where do bignumber1 and bignumber2 come from?