212-81 Online Practice Questions and Answers

Correct Answer: A

The Lagged Fibonacci generator

https://en.wikipedia.org/wiki/FISH_(cipher)

The FISH (FIbonacci SHrinking) stream cipher is a fast software based stream cipher using Lagged Fibonacci generators, plus a concept from the shrinking generator cipher. It was published by Siemens in 1993. FISH is quite fast in software

and has a huge key length. However, in the same paper where he proposed Pike, Ross Anderson showed that FISH can be broken with just a few thousand bits of known plaintext.

Correct Answer: C

Instance Deduction

https://en.wikipedia.org/wiki/Cryptanalysis

The results of cryptanalysis can also vary in usefulness. For example, cryptographer Lars Knudsen (1998) classified various types of attack on block ciphers according to the amount and quality of secret information that was discovered:

Total break -- the attacker deduces the secret key. Global deduction -- the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key. Instance (local) deduction -- the attacker discovers

additional plaintexts (or ciphertexts) not previously known.

Information deduction -- the attacker gains some Shannon information about plaintexts (or ciphertexts) not previously known.

Distinguishing algorithm -- the attacker can distinguish the cipher from a random permutation.

Correct Answer: B

multi-alphabet encryption https://en.wikipedia.org/wiki/Polyalphabetic_cipher Two different shifts create two different alphabets. For +1 and +2 Plaintext alphabet A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 2 ciphertext alphabets B C D E F G H I J K L M N O P Q R S T U V W X Y Z A C D E F G H I J K L M N O P Q R S T U V W X Y Z A B

Correct Answer: C

SHA-1 https://en.wikipedia.org/wiki/SHA-1 SHA-1 (Secure Hash Algorithm 1) is a cryptographic hash function which takes an input and produces a 160-bit (20-byte) hash value known as a message digest ?typically rendered as a hexadecimal number, 40 digits long. It was designed by the United States National Security Agency, and is a U.S. Federal Information Processing Standard. SHA-1 produces a message digest based on principles similar to those used by Ronald L. Rivest of MIT in the design of the MD2, MD4 and MD5 message digest algorithms, but generates a larger hash value (160 bits vs. 128 bits).

Correct Answer: A

Substitution box where input bits are replaced https://en.wikipedia.org/wiki/S-box In cryptography, an S-box (substitution-box) is a basic component of symmetric key algorithms which performs substitution. In block ciphers, they are typically used to obscure the relationship between the key and the ciphertext -- Shannon's property of confusion.

Correct Answer: A

DES https://en.wikipedia.org/wiki/Data_Encryption_Standard DES is the archetypal block cipher--an algorithm that takes a fixed-length string of plaintext bits and transforms it through a series of complicated operations into another ciphertext bitstring of the same length. In the case of DES, the block size is 64 bits. DES also uses a key to customize the transformation, so that decryption can supposedly only be performed by those who know the particular key used to encrypt. The key ostensibly consists of 64 bits; however, only 56 of these are actually used by the algorithm. Eight bits are used solely for checking parity, and are thereafter discarded. Hence the effective key length is 56 bits.

Correct Answer: D

Correct Answer: A

https://en.wikipedia.org/wiki/Public-key_cryptography Public-key cryptography, or asymmetric cryptography, is a cryptographic system that uses pairs of keys: public keys, which may be disseminated widely, and private keys, which are known only to the owner. The generation of such keys depends on cryptographic algorithms based on mathematical problems to produce one-way functions. Effective security only requires keeping the private key private; the public key can be openly distributed without compromising security.

In such a system, any person can encrypt a message using the receiver's public key, but that encrypted message can only be decrypted with the receiver's private key.

Correct Answer: C

Alice's public key https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange In asymmetric (public key) cryptography, both communicating parties (i.e. both Alice and Bob) have two keys of their own -- just to be clear, that's four keys total. Each party has their own public key, which they share with the world, and their own private key which they ... well, which they keep private, of course but, more than that, which they keep as a closely guarded secret. The magic of public key cryptography is that a message encrypted with the public key can only be decrypted with the private key. Alice will encrypt her message with Bob's public key, and even though Eve knows she used Bob's public key, and even though Eve knows Bob's public key herself, she is unable to decrypt the message. Only Bob, using his secret key, can decrypt the message ... assuming he's kept it secret, of course.

Correct Answer: D

Password salting https://en.wikipedia.org/wiki/Salt_(cryptography)#Benefits Salts also combat the use of hash tables and rainbow tables for cracking passwords. A hash table is a large list of pre-computed hashes for commonly used passwords. For a password file without salts, an attacker can go through each entry and look up the hashed password in the hash table or rainbow table. If the look-up is considerably faster than the hash function (which it often is), this will considerably speed up cracking the file. However, if the password file is salted, then the hash table or rainbow table would have to contain "salt . password" pre-hashed. If the salt is long enough and sufficiently random, this is very unlikely. Unsalted passwords chosen by humans tend to be vulnerable to dictionary attacks since they have to be both short and meaningful enough to be memorized. Even a small dictionary (or its hashed equivalent, a hash table) is significant help cracking the most commonly used passwords. Since salts do not have to be memorized by humans they can make the size of the rainbow table required for a successful attack prohibitively large without placing a burden on the users.