Network security practice questions.

A clinic finds that blood-test values in its patient records were altered by a member of staff who had no right to change them, although the records stayed readable and available to everyone who needed them. Which security principle has been broken?

• Confidentiality
• Integrity
• Availability
• Non-repudiation

Answer: Integrity

Integrity is about data staying accurate and unaltered by anyone without the right to change it. Nothing was read by an outsider and nothing became unreachable, so neither confidentiality nor availability was affected.

Anya wants to send a file across the internet so that only Ben is able to read it. Which key does she use to encrypt the file?

• Ben's public key, because only Ben's private key can undo it
• Anya's private key, so that Ben is able to check who sent the file
• Ben's private key, which he must send to Anya beforehand
• Anya's public key, which she has published on her website

Answer: Ben's public key, because only Ben's private key can undo it

In asymmetric encryption the key pair works one way round: what a public key locks, only the matching private key opens, and that private key never leaves its owner. Encrypting with your own private key proves authorship instead of hiding the content.

A login system stores a random salt next to each password hash. What does the salt achieve?

• It allows the original password to be recovered if a user forgets it
• It shortens the stored hash so that the table needs less storage
• It encrypts the password instead of hashing it
• It makes identical passwords produce different stored hashes

Answer: It makes identical passwords produce different stored hashes

Without a salt two users who choose the same password store the same value, so one precomputed table breaks both accounts at once. A different salt per account forces an attacker to work on every account separately.

A supplier attaches a digital signature to an order confirmation. What can the customer check with it?

• That nobody except the customer is able to read the message
• That the message will be delivered within a guaranteed time limit
• That the message is unchanged and really came from the supplier
• That the supplier's network has been scanned for weaknesses

Answer: That the message is unchanged and really came from the supplier

The signature is a hash of the message encrypted with the sender's private key, so recomputing the hash shows the content is unaltered and opening it with the public key shows who sent it. A signature does not hide the message from anyone.

A teacher signs in with the correct password but the finance folder still will not open. Which statement describes what happened?

• Authentication succeeded but authorisation was refused
• Authentication failed because the identity could not be established
• Encryption failed, so the folder contents stayed unreadable
• The stored password hash was decrypted and then rejected

Answer: Authentication succeeded but authorisation was refused

Authentication settles who the user is, while authorisation decides what that identity is allowed to do. A correct password answers the first question only, so the two steps must be kept apart.

A cafe replaces its open wireless network with one protected by WPA2. What is the main security benefit for its customers?

• The bandwidth available to each wireless device is doubled
• Data sent over the air can no longer be read without the network key
• Web pages load with lower latency because the packets are smaller
• Customers no longer need a password to reach the internet

Answer: Data sent over the air can no longer be read without the network key

An open wireless network sends frames in the clear, so any device in range can read them. WPA2 encrypts the wireless link, so the frames are meaningless to anyone who does not hold the key, while capacity and delay are unchanged.