On Thu, Jul 19, 2018 at 8:16 AM, Erik Aronesty via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

>  you can't birthday attack something where there's only a single variable
> that you can modify.
>

When engaging in a multiparty signature, the attacker can more than one
variable to modify.  When you are party to a multi-party signature (for
example, in some sort of coin-join protocol) it could be that every other
participant in the multi-party signature is, in fact, the same single
attacker representing themselves as multiple participants.  This is how the
attacker gets their hands on multiple variables.