QUESTION: 69

Which of the following is generally true about key sizes?

Larger key sizes increase security
Key size is irrelevant to security
Key sizes must be more than 256 bits to be secure
Smaller key sizes increase security

Answer(s): A

Explanation:

Larger key sizes increase security https://en.wikipedia.org/wiki/Key_size
Key length defines the upper-bound on an algorithm's security (i.e. a logarithmic measure of the fastest known attack against an algorithm), since the security of all algorithms can be violated by brute-force attacks. Ideally, the lower-bound on an algorithm's security is by design equal to the key length (that is, the security is determined entirely by the keylength, or in other words, the algorithm's design doesn't detract from the degree of security inherent in the key length). Indeed, most symmetric-key algorithms are designed to have security equal to their key length. However, after design, a new attack might be discovered. For instance, Triple DES was designed to have a 168 bit key, but an attack of complexity 2112 is now known (i.e. Triple DES now only has 112 bits of security, and of the 168 bits in the key the attack has rendered 56 'ineffective' towards security). Nevertheless, as long as the security (understood as 'the amount of effort it would take to gain access') is sufficient for a particular application, then it doesn't matter if key length and security coincide. This is important for asymmetric-key algorithms, because no such algorithm is known to satisfy this property; elliptic curve cryptography comes the closest with an effective security of roughly half its key length.

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QUESTION: 70

The next number is derived from adding together the prior two numbers (1, 1, 2, 3, 5, 8, 13, 21, 34,
55, 89).

Odd numbers
Fibonacci Sequence
Fermat pseudoprime
Prime numbers

Answer(s): B

Explanation:

Fibonacci Sequence
https://en.wikipedia.org/wiki/Fibonacci_number
In mathematics, the Fibonacci numbers, commonly denoted Fn, form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is, F0 = 0, F1=1, Fn = Fn-1 + Fn-2; for n > 1.
The beginning of the sequence is thus:
0,1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144…

Incorrect answers:
Prime numbers - numbers that have only 2 factors: 1 and themselves. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47…
Fermat numbers - a positive integer of the form Fn = 2^2^n +1; where n is a non-negative integer. The first few Fermat numbers are: 3, 5, 17, 257, 65537, 4294967297, 18446744073709551617, …
Odd numbers - any number which cannot be divided by two 1, 3, 5, 7, 9, 11, 13, 15 …

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QUESTION: 71

In a Feistel cipher, the two halves of the block are swapped in each round. What does this provide?

Diffusion
Confusion
Avalanche
Substitution

Answer(s): B

Explanation:

Confusion https://en.wikipedia.org/wiki/Confusion_and_diffusion#Definition
Confusion means that each binary digit (bit) of the ciphertext should depend on several parts of the key, obscuring the connections between the two.
The property of confusion hides the relationship between the ciphertext and the key.
This property makes it difficult to find the key from the ciphertext and if a single bit in a key is changed, the calculation of the values of most or all of the bits in the ciphertext will be affected.
Confusion increases the ambiguity of ciphertext and it is used by both block and stream ciphers.

Incorrect answer:
Avalanche - The avalanche effect is the desirable property of cryptographic algorithms, typically block ciphers and cryptographic hash functions, wherein if an input is changed slightly (for example, flipping a single bit), the output changes significantly (e.g., half the output bits flip). In the case of high-quality block ciphers, such a small change in either the key or the plaintext should cause a drastic change in the ciphertext. The actual term was first used by Horst Feistel, although the concept dates back to at least Shannon's diffusion.
Diffusion - Diffusion means that if we change a single bit of the plaintext, then (statistically) half of the bits in the ciphertext should change, and similarly, if we change one bit of the ciphertext, then approximately one half of the plaintext bits should change.[2] Since a bit can have only two states, when they are all re-evaluated and changed from one seemingly random position to another, half of the bits will have changed state.
Substitution - Substitution technique is a classical encryption technique where the characters present in the original message are replaced by the other characters or numbers or by symbols.

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QUESTION: 72

What size key does Skipjack use?

128 bit
56 bit
80 bit
256 bit

Answer(s): C

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