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212-81問題集でリアル試験問題でテストエンジン問題集でトレーニング
質問 89
A simple algorithm that will take the initial key and from that generate a slightly different key each round.
- A. Feistel Network
- B. Key Schedule
- C. Diffie-Helman
- D. SHA-2
正解: B
解説:
Key Schedule
https://en.wikipedia.org/wiki/Key_schedule
In cryptography, the so-called product ciphers are a certain kind of cipher, where the (de-)ciphering of data is typically done as an iteration of rounds. The setup for each round is generally the same, except for round-specific fixed values called a round constant, and round-specific data derived from the cipher key called a round key. A key schedule is an algorithm that calculates all the round keys from the key.
Incorrect answers:
Feistel Network - (also known as Luby-Rackoff block cipher) is a symmetric structure used in the construction of block ciphers, named after the German-born physicist and cryptographer Horst Feistel who did pioneering research while working for IBM (USA).
SHA-2 - (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001. They are built using the Merkle-Damgard structure, from a one-way compression function itself built using the Davies-Meyer structure from a specialized block cipher.
Diffie-Hellman - key exchange is a method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols as conceived by Ralph Merkle and named after Whitfield Diffie and Martin Hellman.
質問 90
If you use substitution alone, what weakness is present in the resulting cipher text?
- A. It is easily broken with modern computers
- B. It is too simple
- C. It maintains letter and word frequency
- D. It is the same length as the original text
正解: C
解説:
It maintains letter and word frequency
https://en.wikipedia.org/wiki/Frequency_analysis
Frequency analysis (also known as counting letters) is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers.
Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and combinations of letters occur with varying frequencies. Moreover, there is a characteristic distribution of letters that is roughly the same for almost all samples of that language. For instance, given a section of English language, E, T, A and O are the most common, while Z, Q, X and J are rare. Likewise, TH, ER, ON, and AN are the most common pairs of letters (termed bigrams or digraphs), and SS, EE, TT, and FF are the most common repeats. The nonsense phrase "ETAOIN SHRDLU" represents the 12 most frequent letters in typical English language text.
In some ciphers, such properties of the natural language plaintext are preserved in the ciphertext, and these patterns have the potential to be exploited in a ciphertext-only attack.
質問 91
Software for maintaining an on-the-fly-encrypted volume. Data is automatically encrypted right before it is saved, then decrypted right after it is loaded, all w/o user intervention.
- A. VeraCrypt
- B. VPN
- C. Cryptool
- D. PGP
正解: A
解説:
VeraCrypt
https://en.wikipedia.org/wiki/VeraCrypt
VeraCrypt is a source-available freeware utility used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file or encrypt a partition or (in Windows) the entire storage device with pre-boot authentication.
Incorrect answers:
PGP - designed by Phil Zimmerman as a freeware e-mail security program and was released in 1991. It was the first widespread public key encryption program.
VPN - A virtual private network (VPN) extends a private network across a public network and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running across a VPN may therefore benefit from the functionality, security, and management of the private network. Encryption is a common, although not an inherent, part of a VPN connection Cryptool - an open-source project that focuses on the free e-learning software CrypTool illustrating cryptographic and cryptanalytic concepts. According to "Hakin9", CrypTool is worldwide the most widespread e-learning software in the field of cryptology.
質問 92
Which of the following is assured by the use of a hash?
- A. Authentication
- B. Integrity
- C. Confidentiality
- D. Availability
正解: B
解説:
Integrity
https://en.wikipedia.org/wiki/Cryptographic_hash_function#Verifying_the_integrity_of_messages_and_files An important application of secure hashes is verification of message integrity. Comparing message digests (hash digests over the message) calculated before, and after, transmission can determine whether any changes have been made to the message or file.
質問 93
The mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext.
- A. Output feedback (OFB)
- B. Cipher-block chaining (CBC)
- C. Cipher feedback (CFB)
- D. Electronic codebook (ECB)
正解: A
解説:
Output feedback (OFB)
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Output_feedback_(OFB) The output feedback (OFB) mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext. Just as with other stream ciphers, flipping a bit in the ciphertext produces a flipped bit in the plaintext at the same location. This property allows many error-correcting codes to function normally even when applied before encryption.
Incorrect answers:
Cipher feedback (CFB) - mode, a close relative of CBC, makes a block cipher into a self-synchronizing stream cipher.
Electronic codebook (ECB) - the simplest of the encryption modes (named after conventional physical codebooks). The message is divided into blocks, and each block is encrypted separately.
Cipher-block chaining (CBC) - Ehrsam, Meyer, Smith and Tuchman invented the cipher block chaining (CBC) mode of operation in 1976. In CBC mode, each block of plaintext is XORed with the previous ciphertext block before being encrypted. This way, each ciphertext block depends on all plaintext blocks processed up to that point. To make each message unique, an initialization vector must be used in the first block.
質問 94
Basic information theory is the basis for modern symmetric ciphers. Understanding the terminology of information theory is, therefore, important. Changes to one character in the plaintext affect multiple characters in the ciphertext. What is this referred to?
- A. Avalanche
- B. Scrambling
- C. Diffusion
- D. Confusion
正解: C
解説:
Diffusion
https://en.wikipedia.org/wiki/Confusion_and_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. 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.
The idea of diffusion is to hide the relationship between the ciphertext and the plain text.
This will make it hard for an attacker who tries to find out the plain text and it increases the redundancy of plain text by spreading it across the rows and columns; it is achieved through transposition of algorithm and it is used by block ciphers only Incorrect answers:
Confusion
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.
Avalanche https://en.wikipedia.org/wiki/Avalanche_effect
An 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.
If a block cipher or cryptographic hash function does not exhibit the avalanche effect to a significant degree, then it has poor randomization, and thus a cryptanalyst can make predictions about the input, being given only the output. This may be sufficient to partially or completely break the algorithm. Thus, the avalanche effect is a desirable condition from the point of view of the designer of the cryptographic algorithm or device.
Constructing a cipher or hash to exhibit a substantial avalanche effect is one of the primary design objectives, and mathematically the construction takes advantage of the butterfly effect. This is why most block ciphers are product ciphers. It is also why hash functions have large data blocks. Both of these features allow small changes to propagate rapidly through iterations of the algorithm, such that every bit of the output should depend on every bit of the input before the algorithm terminates.
質問 95
Which of the following is the standard for digital certificates?
- A. X.509
- B. 509
https://en.wikipedia.org/wiki/X.509 - C. CRL
- D. CA
- E. RFC 2298
- F. 509 is a standard defining the format of public key certificates. X.509 certificates are used in many Internet protocols, including TLS/SSL, which is the basis for HTTPS, the secure protocol for browsing the web. They are also used in offline applications, like electronic signatures. An X.509 certificate contains a public key and an identity (a hostname, or an organization, or an individual), and is either signed by a certificate authority or self-signed. When a certificate is signed by a trusted certificate authority, or validated by other means, someone holding that certificate can rely on the public key it contains to establish secure communications with another party, or validate documents digitally signed by the corresponding private key.
正解: A
質問 96
Which of the following statements is most true regarding binary operations and encryption?
- A. They are only useful as a teaching method
- B. They can form a part of viable encryption methods
- C. They can provide secure encryption
- D. They are completely useless
正解: B
解説:
They can form a part of viable encryption methods
for example - https://en.wikipedia.org/wiki/XOR_cipher
The XOR operator is extremely common as a component in more complex ciphers. By itself, using a constant repeating key, a simple XOR cipher can trivially be broken using frequency analysis. If the content of any message can be guessed or otherwise known then the key can be revealed. Its primary merit is that it is simple to implement, and that the XOR operation is computationally inexpensive. A simple repeating XOR (i.e. using the same key for xor operation on the whole data) cipher is therefore sometimes used for hiding information in cases where no particular security is required. The XOR cipher is often used in computer malware to make reverse engineering more difficult.
質問 97
A non-secret binary vector used as the initializing input algorithm for encryption of a plaintext block sequence to increase security by introducing additional cryptographic variance.
- A. L2TP
- B. Salt
- C. IV
- D. Nonce
正解: C
解説:
IV
https://en.wikipedia.org/wiki/Initialization_vector
In cryptography, an initialization vector (IV) or starting variable (SV) is a fixed-size input to a cryptographic primitive that is typically required to be random or pseudorandom. Randomization is crucial for encryption schemes to achieve semantic security, a property whereby repeated usage of the scheme under the same key does not allow an attacker to infer relationships between segments of the encrypted message. For block ciphers, the use of an IV is described by the modes of operation. Randomization is also required for other primitives, such as universal hash functions and message authentication codes based thereon.
Incorrect answers:
L2TP - PPTP combined with L2F (Layer 2 Forwarding) (Cisco proprietary protocol) - Uses EAP, CHAP, MS-CHAP, PAP, or S-PAP for authentication. IPSec is used to provide encryption.
Salt - random bits of data intermixed with the message that is to be hashed.
Nonce - an arbitrary number that can be used just once in a cryptographic communication. It is similar in spirit to a nonce word, hence the name. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in replay attacks. They can also be useful as initialization vectors and in cryptographic hash functions.
質問 98
Which of the following is a type of encryption that has two different keys. One key can encrypt the message and the other key can only decrypt it?
- A. Stream cipher
- B. Symmetric
- C. Asymmetric
- D. Block cipher
正解: C
解説:
Asymmetric
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.
Incorrect answers:
Symmetric - Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both encryption of plaintext and decryption of ciphertext.
Block cipher - A block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks. It uses an unvarying transformation, that is, it uses a symmetric key.
Stream cipher - A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream.
質問 99
If you wished to see a list of revoked certificates from a CA, where would you look?
- A. CRL
- B. RA
- C. CA
- D. RFC
正解: A
解説:
CRL
https://ru.wikipedia.org/wiki/Certificate_Revocation_List
Certificate Revocation List (or CRL) is "a list of digital certificates that have been revoked by the issuing certificate authority (CA) before their scheduled expiration date and should no longer be trusted".
Incorrect answers:
RA - Used to proxy the certificate requests on behalf of the user and validate whether or not they are legitimate instead of having the user go directly to the CA. The RA talks to the subordinate CA on behalf of the user, which makes it harder for the actor to get directly to the certificate authority and do harm.
RFC - Request for Comments (RFC) is a publication from the Internet Society (ISOC) and its associated bodies, most prominently the Internet Engineering Task Force (IETF), the principal technical development and standards-setting bodies for the Internet.
CA - certificate authority or certification authority is an entity that issues digital certificates
質問 100
Original, unencrypted information is referred to as ____.
- A. cleartext
- B. plaintext
- C. text
- D. ciphertext
正解: B
解説:
plaintext
https://en.wikipedia.org/wiki/Plaintext
In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. Cleartext usually refers to data that is transmitted or stored unencrypted ("in clear").
質問 101
Which of the following are valid key sizes for AES (choose three)?
- A. 0
- B. 1
- C. 2
- D. 3
- E. 4
- F. 5
正解: B,C,F
解説:
Correct answers: 128, 192, 256
https://en.wikipedia.org/wiki/Advanced_Encryption_Standard
The Advanced Encryption Standard (AES), also known by its original name Rijndael, is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001.
AES is a subset of the Rijndael block cipher developed by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, who submitted a proposal to NIST during the AES selection process. Rijndael is a family of ciphers with different key and block sizes. For AES, NIST selected three members of the Rijndael family, each with a block size of 128 bits, but three different key lengths: 128, 192 and 256 bits.
質問 102
Which of the following is the successor of SSL?
- A. TLS
- B. IPSec
- C. GRE
- D. RSA
正解: A
解説:
TLS
https://en.wikipedia.org/wiki/Transport_Layer_Security#History_and_development TLS 1.0 was first defined in RFC 2246 in January 1999 as an upgrade of SSL Version 3.0, and written by Christopher Allen and Tim Dierks of Consensus Development. As stated in the RFC, "the differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough to preclude interoperability between TLS 1.0 and SSL 3.0". Tim Dierks later wrote that these changes, and the renaming from "SSL" to "TLS", were a face-saving gesture to Microsoft, "so it wouldn't look [like] the IETF was just rubberstamping Netscape's protocol".
質問 103
Which one of the following is an algorithm that uses variable length key from 1 to 256 bytes, which constitutes a state table that is used for subsequent generation of pseudorandom bytes and then a pseudorandom string of bits, which is XORed with the plaintext to produce the ciphertext?
- A. Blowfish
- B. PIKE
- C. Twofish
- D. RC4
正解: D
解説:
RC4
https://en.wikipedia.org/wiki/RC4
RC4 (Rivest Cipher 4 also known as ARC4 or ARCFOUR meaning Alleged RC4, see below) is a stream cipher. While it is remarkable for its simplicity and speed in software, multiple vulnerabilities have been discovered in RC4, rendering it insecure. It is especially vulnerable when the beginning of the output keystream is not discarded, or when nonrandom or related keys are used. Particularly problematic uses of RC4 have led to very insecure protocols such as WEP.
The key-scheduling algorithm is used to initialize the permutation in the array "S". "keylength" is defined as the number of bytes in the key and can be in the range 1 ≤ keylength ≤ 256, typically between 5 and 16, corresponding to a key length of 40 - 128 bits. First, the array "S" is initialized to the identity permutation. S is then processed for 256 iterations in a similar way to the main PRGA, but also mixes in bytes of the key at the same time.
Incorrect answers:
Blowfish - has a 64-bit block size and a variable key length from 32 bits up to 448 bits. It is a 16-round Feistel cipher and uses large key-dependent S-boxes. In structure it resembles CAST-128, which uses fixed S-boxes.
Twofish - is a symmetric key block cipher with a block size of 128 bits and key sizes up to 256 bits.
PIKE - stream cipher was invented by Ross Anderson to be a "leaner and meaner" version of FISH after he broke FISH in 1994. Its name is supposed to be a humorous allusion to the pike fish.
質問 104
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