[Ans: A]

Public key cryptography gives a reliable method for digital signing and signature verification based on public/private key pairs. A person can sign a given digital message (file, document, e-mail, and so forth) with his private key.

From a technical point of view, the digital signing of a message is performed in two steps:

1) In the first step of the process, a hash-value of the message (often called the message digest) is calculated by applying some cryptographic hashing algorithm (for example, MD2, MD4, MD5, SHA1, or other). The calculated hash-value of a message is a sequence of bits, usually with a fixed length, extracted in some manner from the message.

2) In the second step of digitally signing a message, the information obtained in the first step hash-value of the message (the message digest) is encrypted with the private key of the person who signs the message and thus an encrypted hash-value, also called digital signature, is obtained.