CompTIA Security+ Question L-36

Which of the following types of attacks involves interception of authentication traffic in an attempt to gain unauthorized access to a wireless network?

A. Near field communication
B. IV attack
C. Evil twin
D. Replay attack

Answer: B

Explanation:
An initialization vector is a random number used in combination with a secret key as a means to encrypt data. This number is sometimes referred to as a nonce, or “number occurring once,” as an encryption program uses it only once per session. An initialization vector is used to avoid repetition during the data encryption process, making it impossible for hackers who use dictionary attack to decrypt the exchanged encrypted message by discovering a pattern. This is known as an IV attack. A particular binary sequence may be repeated more than once in a message, and the more it appears, the more the encryption method is discoverable. For example if a one-letter word exists in a message, it may be either “a” or “I” but it can’t be “e” because the word “e” is non-sensical in English, while “a” has a meaning and “I” has a meaning. Repeating the words and letters makes it possible for software to apply a dictionary and discover the binary sequence corresponding to each letter. Using an initialization vector changes the binary sequence corresponding to each letter, enabling the letter “a” to be represented by a particular sequence in the first instance, and then represented by a completely different binary sequence in the second instance.

WEP (Wireless Equivalent Privacy) is vulnerable to an IV attack. Because RC4 is a stream cipher, the same traffic key must never be used twice. The purpose of an IV, which is transmitted as plain text, is to prevent any repetition, but a 24-bit IV is not long enough to ensure this on a busy network. The way the IV was used also opened WEP to a related key attack. For a 24-bit IV, there is a 50% probability the same IV will repeat after 5000 packets.

CompTIA Security+ Question J-34

Timestamps and sequence numbers act as countermeasures against which of the following types of attacks?

A. Smurf
B. DoS
C. Vishing
D. Replay

Answer: D

Explanation:
A replay attack (also known as playback attack) is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack by IP packet substitution (such as stream cipher attack).

For example: Suppose Alice wants to prove her identity to Bob. Bob requests her password as proof of identity, which Alice dutifully provides (possibly after some transformation like a hash function); meanwhile, Eve is eavesdropping on the conversation and keeps the password (or the hash). After the interchange is over, Eve (posing as Alice) connects to Bob; when asked for a proof of identity, Eve sends Alice’s password (or hash) read from the last session, which Bob accepts thus granting access to Eve.

Countermeasures: A way to avoid replay attacks is by using session tokens: Bob sends a one­time token to Alice, which Alice uses to transform the password and send the result to Bob (e.g. computing a hash function of the session token appended to the password). On his side Bob performs the same computation; if and only if both values match, the login is successful. Now suppose Eve has captured this value and tries to use it on another session; Bob sends a different session token, and when Eve replies with the captured value it will be different from Bob’s computation. Session tokens should be chosen by a (pseudo-) random process. Otherwise Eve may be able to pose as Bob, presenting some predicted future token, and convince Alice to use that token in her transformation. Eve can then replay her reply at a later time (when the previously predicted token is actually presented by Bob), and Bob will accept the authentication. One-time passwords are similar to session tokens in that the password expires after it has been used or after a very short amount of time. They can be used to authenticate individual transactions in addition to sessions. The technique has been widely implemented in personal online banking systems. Bob can also send nonces but should then include a message authentication code (MAC), which Alice should check. Timestamping is another way of preventing a replay attack. Synchronization should be achieved using a secure protocol. For example Bob periodically broadcasts the time on his clock together with a MAC. When Alice wants to send Bob a message, she includes her best estimate of the time on his clock in her message, which is also authenticated. Bob only accepts messages for which the timestamp is within a reasonable tolerance. The advantage of this scheme is that Bob does not need to generate (pseudo-) random numbers, with the trade-off being that replay attacks, if they are performed quickly enough i.e. within that ‘reasonable’ limit, could succeed.

CompTIA Security+ Question I-65

Which of the following ciphers would be BEST used to encrypt streaming video?

A. RSA
B. RC4
C. SHA1
D. 3DES

Answer: B

Explanation:
In cryptography, RC4 is the most widely used software stream cipher and is used in popular Internet protocols such as Transport Layer Security (TLS). While remarkable for its simplicity and speed in software, RC4 has weaknesses that argue against its use in new systems. It is especially vulnerable when the beginning of the output keystream is not discarded, or when nonrandom or related keys are used; some ways of using RC4 can lead to very insecure protocols such as WEP.

Because RC4 is a stream cipher, it is more malleable than common block ciphers. If not used together with a strong message authentication code (MAC), then encryption is vulnerable to a bit-flipping attack. The cipher is also vulnerable to a stream cipher attack if not implemented correctly.

Furthermore, inadvertent double encryption of a message with the same key may accidentally output plaintext rather than ciphertext because the involutory nature of the XOR function would result in the second operation reversing the first. It is noteworthy, however, that RC4, being a stream cipher, was for a period of time the only common cipher that was immune to the 2011 BEAST attack on TLS 1.0. The attack exploits a known weakness in the way cipher block chaining mode is used with all of the other ciphers supported by TLS 1.0, which are all block ciphers.

CompTIA Security+ Question E-35

Which of the following functions provides an output which cannot be reversed and converts data into a string of characters?

A. Hashing
B. Stream ciphers
C. Steganography
D. Block ciphers

Answer: A

Explanation:
Hashing refers to the hash algorithms used in cryptography. It is used to store data, such as hash tables one of its characteristics is that it must be one-way – it is not reversible.

CompTIA Security+ Question D-97

A network stream needs to be encrypted. Emily, the network administrator, has selected a cipher which will encrypt 8 bits at a time before sending the data across the network. Which of the following has Emily selected?

A. Block cipher
B. Stream cipher
C. CRC
D. Hashing algorithm

Answer: A

Explanation:
With a block cipher the algorithm works on chunks of data—encrypting one and then moving to the next. Example: Blowfish is an encryption system that performs a 64-bit block cipher at very fast speeds.

CompTIA Security+ Question D-92

Which of the following can use RC4 for encryption? (Select TWO).

A. CHAP
B. SSL
C. WEP
D. AES
E. 3DES

Answer: B,C

Explanation:
B: In cryptography, RC4 (Rivest Cipher 4 also known as ARC4 or ARCFOUR meaning Alleged RC4) is the most widely used software stream cipher and is used in popular Internet protocols such as Secure Sockets Layer (SSL) and Transport Layer Security (TLS).

C: WEP also uses RC4, however WEP is still unsecure.

CompTIA Security+ Question C-65

Emily, a security administrator, is noticing a slow down in the wireless network response. Emily launches a wireless sniffer and sees a large number of ARP packets being sent to the AP. Which of the following type of attacks is underway?

A. IV attack
B. Interference
C. Blue jacking
D. Packet sniffing

Answer: A

Explanation:
In this question, it’s likely that someone it trying to crack the wireless network security. An initialization vector is a random number used in combination with a secret key as a means to encrypt data. This number is sometimes referred to as a nonce, or “number occurring once,” as an encryption program uses it only once per session. An initialization vector is used to avoid repetition during the data encryption process, making it impossible for hackers who use dictionary attack to decrypt the exchanged encrypted message by discovering a pattern. This is known as an IV attack. A particular binary sequence may be repeated more than once in a message, and the more it appears, the more the encryption method is discoverable. For example if a one-letter word exists in a message, it may be either “a” or “I” but it can’t be “e” because the word “e” is non-sensical in English, while “a” has a meaning and “I” has a meaning. Repeating the words and letters makes it possible for software to apply a dictionary and discover the binary sequence corresponding to each letter. Using an initialization vector changes the binary sequence corresponding to each letter, enabling the letter “a” to be represented by a particular sequence in the first instance, and then represented by a completely different binary sequence in the second instance.

WEP (Wireless Equivalent Privacy) is vulnerable to an IV attack. Because RC4 is a stream cipher, the same traffic key must never be used twice. The purpose of an IV, which is transmitted as plain text, is to prevent any repetition, but a 24-bit IV is not long enough to ensure this on a busy network. The way the IV was used also opened WEP to a related key attack. For a 24-bit IV, there is a 50% probability the same IV will repeat after 5000 packets.

CompTIA Security+ Question B-89

A network analyst received a number of reports that impersonation was taking place on the network. Session tokens were deployed to mitigate this issue and defend against which of the following attacks?

A. Replay
B. DDoS
C. Smurf
D. Ping of Death

Answer: A

Explanation:
A replay attack (also known as playback attack) is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack by IP packet substitution (such as stream cipher attack).

For example: Suppose Alice wants to prove her identity to Bob. Bob requests her password as proof of identity, which Alice dutifully provides (possibly after some transformation like a hash function); meanwhile, Eve is eavesdropping on the conversation and keeps the password (or the hash). After the interchange is over, Eve (posing as Alice) connects to Bob; when asked for a proof of identity, Eve sends Alice’s password (or hash) read from the last session, which Bob accepts thus granting access to Eve.

Countermeasures: A way to avoid replay attacks is by using session tokens: Bob sends a one­time token to Alice, which Alice uses to transform the password and send the result to Bob (e.g. computing a hash function of the session token appended to the password). On his side Bob performs the same computation; if and only if both values match, the login is successful. Now suppose Eve has captured this value and tries to use it on another session; Bob sends a different session token, and when Eve replies with the captured value it will be different from Bob’s computation. Session tokens should be chosen by a (pseudo-) random process. Otherwise Eve may be able to pose as Bob, presenting some predicted future token, and convince Alice to use that token in her transformation. Eve can then replay her reply at a later time (when the previously predicted token is actually presented by Bob), and Bob will accept the authentication. One-time passwords are similar to session tokens in that the password expires after it has been used or after a very short amount of time. They can be used to authenticate individual transactions in addition to sessions. The technique has been widely implemented in personal online banking systems. Bob can also send nonces but should then include a message authentication code (MAC), which Alice should check. Timestamping is another way of preventing a replay attack. Synchronization should be achieved using a secure protocol. For example Bob periodically broadcasts the time on his clock together with a MAC. When Alice wants to send Bob a message, she includes her best estimate of the time on his clock in her message, which is also authenticated. Bob only accepts messages for which the timestamp is within a reasonable tolerance. The advantage of this scheme is that Bob does not need to generate (pseudo-) random numbers, with the trade-off being that replay attacks, if they are performed quickly enough i.e. within that ‘reasonable’ limit, could succeed.

CompTIA Security+ Question B-42

Which of the following attacks could be used to initiate a subsequent man-in-the-middle attack?

A. ARP poisoning
B. DoS
C. Replay
D. Brute force

Answer: C

Explanation:
A replay attack (also known as playback attack) is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack by IP packet substitution (such as stream cipher attack).

For example: Suppose Alice wants to prove her identity to Bob. Bob requests her password as proof of identity, which Alice dutifully provides (possibly after some transformation like a hash function); meanwhile, Eve is eavesdropping on the conversation and keeps the password (or the hash). After the interchange is over, Eve (posing as Alice) connects to Bob; when asked for a proof of identity, Eve sends Alice’s password (or hash) read from the last session, which Bob accepts thus granting access to Eve.

Countermeasures: A way to avoid replay attacks is by using session tokens: Bob sends a one­time token to Alice, which Alice uses to transform the password and send the result to Bob (e.g. computing a hash function of the session token appended to the password). On his side Bob performs the same computation; if and only if both values match, the login is successful. Now suppose Eve has captured this value and tries to use it on another session; Bob sends a different session token, and when Eve replies with the captured value it will be different from Bob’s computation. Session tokens should be chosen by a (pseudo-) random process. Otherwise Eve may be able to pose as Bob, presenting some predicted future token, and convince Alice to use that token in her transformation. Eve can then replay her reply at a later time (when the previously predicted token is actually presented by Bob), and Bob will accept the authentication. One-time passwords are similar to session tokens in that the password expires after it has been used or after a very short amount of time. They can be used to authenticate individual transactions in addition to sessions. The technique has been widely implemented in personal online banking systems. Bob can also send nonces but should then include a message authentication code (MAC), which Alice should check. Timestamping is another way of preventing a replay attack. Synchronization should be achieved using a secure protocol. For example Bob periodically broadcasts the time on his clock together with a MAC. When Alice wants to send Bob a message, she includes her best estimate of the time on his clock in her message, which is also authenticated. Bob only accepts messages for which the timestamp is within a reasonable tolerance. The advantage of this scheme is that Bob does not need to generate (pseudo-) random numbers, with the trade-off being that replay attacks, if they are performed quickly enough i.e. within that ‘reasonable’ limit, could succeed.