Comment by legalcorrection

In the US, the Uniform Electronic Transactions Act, passed by most states, clarifies that basically any sound or symbol or process is a valid electronic signature. This is in line with general contract law, under which any manifestation, written or verbal or even non-verbal, that would reasonably be understood as assent, is sufficient to form a contract. Of course, if you want a court to enforce that contract, you're going to have to prove that the other party did provide assent.