Are Nikaahs seen as legal?

Assalamu Alaikum
i understand that our marriages are not legal yet, according to Civil Law.
Also that if we do legalise our marriage (according to civil law) we should get married out of community of property without accrual system.
Are muslim woman allowed to keep her maiden surname on her ID Document, is it a sin?
or can she keep both surnames (maiden and married surname)