A hash function guarantees integrity of a message. It guarantees that message has not be
Answer: C
Correct answer: Changed
A hash function gives integrity: any alteration to the message produces a different digest, so a matching digest shows the message has not been changed. It says nothing about who sent it or whether it was intercepted.
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To check integrity of a message, or document, receiver creates the
Answer: B
Correct answer: Finger Print
The receiver computes the hash of the document it has received - its fingerprint - and compares it with the fingerprint sent with the document. If the two agree, nothing has been altered.
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A digital signature needs a
Answer: C
Correct answer: Public-key system
A digital signature must be creatable by one party alone and verifiable by everyone, which only asymmetric cryptography allows: the sender signs with a private key and anyone can check the signature with the matching public key. A shared key would let either party forge the other's signature.
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One way to preserve integrity of a document is through use of a
Answer: B
Correct answer: Finger Print
A hash of the document, called its fingerprint, is short, fixed in length, and changes completely if a single bit of the document changes - which is what makes it a check on integrity.
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A session symmetric key between two parties is used
Answer: A
Correct answer: Only once
A session key is generated for a single exchange and discarded afterwards. Using it once limits the amount of ciphertext produced under any one key and means that a key compromised later cannot decrypt earlier traffic.
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Encryption and decryption provide secrecy, or confidentiality, but not
Answer: B
Correct answer: Integrity
Encryption hides the content but does not protect it from alteration: an attacker who cannot read the ciphertext may still change bits in it, and the receiver would decrypt something different without knowing. Integrity therefore needs a hash or a MAC in addition to the encryption.
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MAC stands for
Answer: A
Correct answer: Message authentication code
A MAC is a hash computed with a secret key, so it proves both that the message is unaltered and that it came from someone holding the key - integrity and authentication together.
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Digest created by a hash function is normally called a
Answer: A
Correct answer: Modification detection code (MDC)
A plain hash, computed without a key, is a modification detection code: it detects change but proves nothing about origin, since anyone can recompute it. Adding a key turns it into a MAC.
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Message confidentiality is using
Answer: D
Correct answer: Asymmetric-Key
In an asymmetric system the sender encrypts with the receiver's public key, so only the holder of the matching private key can read the message. That gives confidentiality without the two parties ever having shared a secret in advance.
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A sender must not be able to deny sending a message that was sent, is known as
Answer: A
Correct answer: Message Nonrepudiation
Nonrepudiation means the sender cannot later deny having sent the message. A digital signature provides it, because only the sender's private key could have produced the signature.
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