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A digital signature is incorrect because it is a form of authenticator. It is decrypted using the secret decryption key and sent to the receiver. The receiver may encrypt, using the public key, and may verify the signature, but the signature cannot be forged because only the sender knows the secret decryption key. Nonpublic key algorithms can also be used for digital signatures. The basic difference between the message authentication code and the digital signature is that although message authentication codes require a secret (private) key to verify, digital signatures are verifiable with a public key, that is, a published value. Message authentication codes are used to exchange information between two parties, where both have knowledge of the secret key. A digital signature does not require any secret key to be verified.

A cyclic redundancy check (CRC) is incorrect because it uses an algorithm for generating error detection bits, and the receiving station performs the same calculation as the transmitting station. If the results differ, then one or more bits are in error. Both message authentication codes and digital signatures operate with keys (whether public or private), are based on cryptography, and are hard to attack by intruders. On the other hand, data checksums and cyclic redundancy checks operate on algorithms, are not based on cryptography, and are easily attacked by intruders.

82. For security protection mechanisms for cryptographic data in storage, the encryption mechanism should not be easier to recover the key encrypting key than it is to recover the key being encrypted is a part of which of the following cryptographic service?

a. Confidentiality

b. Availability

c. Integrity

d. Labels

82. a. For confidentiality service, encryption with an approved algorithm is needed for the cryptographic module. Moreover, the encryption mechanism should not be an easier way to recover the key encrypting key than it is to recover the key being encrypted. In other words, recovering the key being encrypted should be relatively easier and recovering the key encrypting key should be difficult.

83. Which of the following is least effective in verifying against malicious tampering?

a. Message authentication code

b. Digital signatures

c. Message digests

d. Cyclic redundancy code

83. d. Checksums are of two types: a cryptographic checksum and a noncryptographic checksum. A cyclic redundancy code is a noncryptographic checksum, which is designed to detect random bit changes, not purposeful alterations or malicious tampering. These checksums are good at finding a few bits changed at random.

The other three incorrect choices are based on cryptographic checksum techniques. Message authentication code is a message digest with a password attached to it. The intent is that someone cannot re-create the code with the same input unless that person also knows the secret key (password). A digital signature is a message digest encrypted with someone’s private key to certify the contents. Digital signatures perform three important functions: integrity, authentication, and nonrepudiation. A message digest is a hash code produced by a mathematical function. It takes variable length input and reduces it to a small value, and a small change in the input results in a significant change in the output.

Secure hash algorithms create a short message digest. The message digest is then used, with the sender’s private key and the algorithm specified in digital signature standard, to produce a message-specific signature. Verifying the digital signature standard involves a mathematical operation on the signature and message digest, using the sender’s public key and the hash standard.

84. What is password hashing?

a. Storing a hash of the password

b. Storing the password in a clear text and encrypting it as needed

c. Guessing a password

d. Cracking a password

84. a. Password hashing requires storing a password in its hash form, which is better than storing an unencrypted password. When a password is supplied, it computes the password’s hash and compares it with the stored value. If they match, the password is correct. An attacker cannot derive the password from the hashes. It is good to hide the hashed password list.

The other three incorrect choices are weak forms of handling a password. Encrypting passwords leads to judgmental errors. A password can be easily guessed if the user selects the password from a word dictionary. An exhaustive search may then “crack” the password.