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A structural problem for digital signatures is that everyone has to agree to trust a central authority that knows everything and is trusted by everyone. The most logical candidates for central authority are the government, the banks, the accountants, and the lawyers. Unfortunately, none of these organizations inspire total confidence in all citizens. Hence, it would be nice if signing documents did not require a trusted authority. Fortunately, Public-key cryptography can make an important contribution to the improvement of digital signature security, allowing person A to send a digital signed message to person B without requiring a central authority.
Considering that person A has the private-key DA and the public-key EA and person B has the private-key DB and public-key EB, choose the alternative that shows the correct sequence of utilization of the keys when person A signs a message M, sends it to B and B checks he signature.