OT: utility account transfer frauds

[Peter Tomlinson]

David Howe wrote:
> I am just waiting for this to fail, and fail badly in court. the obvious
> defense is to prove (by example) that two documents, with different
> content, can have the same digital signature - something the birthday
> attack is ideal for.  This of course leaves aside the problems of key
> theft and security generally.

Have I missed something? Could someone explain the 'birthday attack'?
(privately if I did miss something)

Peter