> Decentralization, n. The security assumption that a nineteen year old in Hangzhou and someone who is maybe in the UK and maybe not have not yet decided to collude with each other.
With parentheses:
> Decentralization, n. The security assumption that (a nineteen year old in Hangzhou and (someone who is maybe in the UK and maybe not)) have not yet decided to collude with each other.
I think it should be more like:
> Decentralization, n. The security assumption that a nineteen year old in Hangzhou and someone who is maybe in the UK (and maybe not) have not yet decided to collude with each other.
If you read your version using the rule that a parenthesised section represents an individual thought that can be removed, yours reads as:
> Decentralization, n. The security assumption that have not yet decided to collude with each other.
With parentheses:
> Decentralization, n. The security assumption that (a nineteen year old in Hangzhou and (someone who is maybe in the UK and maybe not)) have not yet decided to collude with each other.