Explanation/Reference:
Explanation:
A digital signature is a hash value that was encrypted with the sender's private key.
Digital signatures are based on public key cryptography, also known as asymmetric cryptography. Using a public key algorithm such as RSA, one can generate two keys that are mathematically linked: one private and one public. To create a digital signature, signing software (such as an email program) creates a one- way hash of the electronic data to be signed. The private key is then used to encrypt the hash. The encrypted hash -- along with other information, such as the hashing algorithm -- is the digital signature.
The reason for encrypting the hash instead of the entire message or document is that a hash function can convert an arbitrary input into a fixed length value, which is usually much shorter. This saves time since hashing is much faster than signing.
Incorrect Answers:
B: The hash value is signed with the sender's private key, not the public key to prove that the message came from the sender and has not been altered in transit.
C: A session key is not used to encrypt the hash value in a digital signature.
D: A digital signature is not a sender's signature signed and scanned in a digital format.
References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, p. 829
http://searchsecurity.techtarget.com/definition/digital-signature