212-81問題集と練習テスト（200問題） [Q32-Q55]

（2023)212-81問題集と練習テスト（200問題)

ガイド（2023年最新)リアルなEC-COUNCIL 212-81試験問題

質問 # 32
Which of the following encryption algorithms relies on the inability to factor large prime numbers?

• A. MQV
• B. AES
• C. RSA
• D. EC

正解：C

解説：
Correct answers: RSA
https://en.wikipedia.org/wiki/RSA_(cryptosystem)
RSA (Rivest-Shamir-Adleman) is a public-key cryptosystem that is widely used for secure data transmission. It is also one of the oldest. The acronym RSA comes from the surnames of Ron Rivest, Adi Shamir, and Leonard Adleman, who publicly described the algorithm in 1977. An equivalent system was developed secretly, in 1973 at GCHQ (the British signals intelligence agency), by the English mathematician Clifford Cocks. That system was declassified in 1997.
In a public-key cryptosystem, the encryption key is public and distinct from the decryption key, which is kept secret (private). An RSA user creates and publishes a public key based on two large prime numbers, along with an auxiliary value. The prime numbers are kept secret. Messages can be encrypted by anyone, via the public key, but can only be decoded by someone who knows the prime numbers.
The security of RSA relies on the practical difficulty of factoring the product of two large prime numbers, the "factoring problem". Breaking RSA encryption is known as the RSA problem. Whether it is as difficult as the factoring problem is an open question. There are no published methods to defeat the system if a large enough key is used.
Incorrect answers:
EC - Elliptic-curve cryptography (ECC) is an approach to public-key cryptography based on the algebraic structure of elliptic curves over finite fields. ECC allows smaller keys compared to non-EC cryptography (based on plain Galois fields) to provide equivalent security.
AES - 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.
MQV - (Menezes-Qu-Vanstone) is an authenticated protocol for key agreement based on the Diffie-Hellman scheme. Like other authenticated Diffie-Hellman schemes, MQV provides protection against an active attacker. The protocol can be modified to work in an arbitrary finite group, and, in particular, elliptic curve groups, where it is known as elliptic curve MQV (ECMQV).

質問 # 33
Symmetric algorithm. Designed by James Massey and Xuejia Lai. Operates on 64 bit blocks and has a 128 bit key. Consists of 8 identical transformations each round and an output transformation.

• A. RSA
• B. DES
• C. CAST
• D. IDEA

正解：D

解説：
IDEA
https://en.wikipedia.org/wiki/International_Data_Encryption_Algorithm
In cryptography, the International Data Encryption Algorithm (IDEA), originally called Improved Proposed Encryption Standard (IPES), is a symmetric-key block cipher designed by James Massey of ETH Zurich and Xuejia Lai and was first described in 1991. The algorithm was intended as a replacement for the Data Encryption Standard (DES). IDEA is a minor revision of an earlier cipher Proposed Encryption Standard (PES).
The cipher was designed under a research contract with the Hasler Foundation, which became part of Ascom-Tech AG. The cipher was patented in a number of countries but was freely available for non-commercial use. The name "IDEA" is also a trademark. The last patents expired in 2012, and IDEA is now patent-free and thus completely free for all uses.
IDEA was used in Pretty Good Privacy (PGP) v2.0 and was incorporated after the original cipher used in v1.0, BassOmatic, was found to be insecure. IDEA is an optional algorithm in the OpenPGP standard.
Incorrect answers:
CAST - symmetric algorithm. Created in 1996 by Carlisle Adams and Stafford Tavares. Operates on 64 bit blocks and has a key size of 40-128 bits. Typically used in GPG and PGP. Encryption is either 12 or 16 rounds.
DES - The Data Encryption Standard is a symmetric-key algorithm for the encryption of digital data. Although its short key length of 56 bits makes it too insecure for applications, it has been highly influential in the advancement of cryptography.
RSA - a public-key cryptosystem that is widely used for secure data transmission. It is also one of the oldest. The acronym RSA comes from the surnames of Ron Rivest, Adi Shamir, and Leonard Adleman, who publicly described the algorithm in 1977

質問 # 34
During the process of encryption and decryption, what keys are shared?

• A. User passwords
• B. Private keys
• C. Public keys
• D. Public and private keys

正解：C

解説：
Public keys
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.
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.
Alice and Bob do not need to plan anything ahead of time to communicate securely: they generate their public-private key pairs independently, and happily broadcast their public keys to the world at large. Alice can rest assured that only Bob can decrypt the message she sends because she has encrypted it with his public key.

質問 # 35
Which algorithm was U. S. Patent 5,231,668, filed on july 26, 1991, attributed to David W. Kravitz, and adopted by the U. S. government in 1993 with FIPS 186?

• A. RSA
• B. AES
• C. DSA
• D. RC4

正解：C

解説：
DSA
https://en.wikipedia.org/wiki/Digital_Signature_Algorithm
DSA is covered by U.S. Patent 5,231,668 , filed July 26, 1991 and now expired, and attributed to David
W. Kravitz, a former NSA employee. This patent was given to "The United States of America as represented by the Secretary of Commerce, Washington, D.C.", and NIST has made this patent available worldwide royalty-free. Claus
P. Schnorr claims that his U.S. Patent 4,995,082 (also now expired) covered DSA; this claim is disputed.

質問 # 36
The most widely used asymmetric encryption algorithm is what?

• A. Caesar Cipher
• B. Vigenere
• C. RSA
• D. DES

正解：C

解説：
RSA
The RSA encryption algorithm is one of the most widely used public key encryption algorithms that have ever been invented. It was created by the three scientists Ronald Rivest, Adi Shamir, and Leonard Adleman in 1977, and today it is increasingly being used in the network area.
Incorrect answers:
Caesar Cipher - Monoalphabetic cipher where letters are shifted one or more letters in either direction. The method is named after Julius Caesar, who used it in his private correspondence.
Vigenere - Multi alphabet cipher Invented by Giovan Battista Bellaso in middle 1553. Vigenere created a stronger version of the cipher. Combining/Weaving Caesar cipher. Not cracked until late 1800s. Widely used from 16th century to early 20th century. It is a cipher square with A to Z across all the columns and rows. You then use a keyword to encrypt the message DES - The Data Encryption Standard is a symmetric-key algorithm for the encryption of digital data.

質問 # 37
In a ______ the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key.

• A. Global deduction
• B. Instance deduction
• C. Information deduction
• D. Total break

正解：A

解説：
Global deduction
https://en.wikipedia.org/wiki/Cryptanalysis
Global deduction - the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key.
Incorrect answers:
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.
Total break - the attacker deduces the secret key.

質問 # 38
In 2007, this wireless security algorithm was rendered useless by capturing packets and discovering the passkey in a matter of seconds. This security flaw led to a network invasion of TJ Maxx and data theft through a technique known as wardriving.
Which Algorithm is this referring to?

• A. Wi-Fi Protected Access (WPA)
• B. Wired Equivalent Privacy (WEP)
• C. Temporal Key Integrity Protocol (TKIP)
• D. Wi-Fi Protected Access 2 (WPA2)

正解：B

解説：
Wired Equivalent Privacy (WEP)
https://en.wikipedia.org/wiki/Wired_Equivalent_Privacy#Weak_security
In 2007, Erik Tews, Andrei Pychkine, and Ralf-Philipp Weinmann were able to extend Klein's 2005 attack and optimize it for usage against WEP. With the new attack it is possible to recover a 104-bit WEP key with probability 50% using only 40,000 captured packets. For 60,000 available data packets, the success probability is about 80% and for 85,000 data packets about 95%. Using active techniques like deauth and ARP re-injection, 40,000 packets can be captured in less than one minute under good conditions. The actual computation takes about 3 seconds and 3 MB of main memory on a Pentium-M 1.7 GHz and can additionally be optimized for devices with slower CPUs. The same attack can be used for 40-bit keys with an even higher success probability.

質問 # 39
Message hidden in unrelated text. Sender and receiver have pre-arranged to use a pattern to remove certain letters from the message which leaves only the true message behind.

• A. Playfair Cipher
• B. Null Ciphers
• C. Caesar Cipher
• D. Vigenere Cipher

正解：B

解説：
Null Ciphers
https://en.wikipedia.org/wiki/Null_cipher
A null cipher, also known as concealment cipher, is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher material. Today it is regarded as a simple form of steganography, which can be used to hide ciphertext.
Incorrect answers:
Caesar Cipher - Monoalphabetic cipher where letters are shifted one or more letters in either direction. The method is named after Julius Caesar, who used it in his private correspondence.
Vigenere - method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It employs a form of polyalphabetic substitution.
Playfair Cipher - manual symmetric encryption technique and was the first literal digram substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of Lord Playfair for promoting its use.

質問 # 40
Which of the following equations is related to EC?

• A. y^2 = x^3 + Ax + B
• B. P = Cd%n
• C. Me%n
• D. Let m = (p-1)(q-1)

正解：A

解説：
y^2 = x^3 + Ax + B
https://en.wikipedia.org/wiki/Elliptic-curve_cryptography
For current cryptographic purposes, an elliptic curve is a plane curve over a finite field (rather than the real numbers) which consists of the points satisfying the equation:

質問 # 41
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. Twofish
• B. PIKE
• C. RC4
• D. Blowfish

正解：C

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 # 42
How did the ATBASH cipher work?

• A. By rotating text a given number of spaces
• B. By shifting each letter a certain number of spaces
• C. By Multi alphabet substitution
• D. By substituting each letter for the letter from the opposite end of the alphabet (i.e. A becomes Z, B becomes Y, etc.)

正解：D

解説：
By substituting each letter for the letter from the opposite end of the alphabet (i.e. A becomes Z, B becomes Y, etc.)
https://en.wikipedia.org/wiki/Atbash
The Atbash cipher is a particular type of monoalphabetic cipher formed by taking the alphabet (or abjad, syllabary, etc.) and mapping it to its reverse, so that the first letter becomes the last letter, the second letter becomes the second to last letter, and so on.

質問 # 43
A number that is used only one time, then discarded is called what?

• A. IV
• B. Nonce
• C. Chain
• D. Salt

正解：B

解説：
Nonce
https://en.wikipedia.org/wiki/Cryptographic_nonce
A nonce is 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.

質問 # 44
The art and science of writing hidden messages so that no one suspects the existence of the message, a type of security through obscurity. Message can be hidden in picture or audio file for example. Uses least significant bits in a file to store data.

• A. Cryptosystem
• B. Key Schedule
• C. Steganography
• D. Avalanche effect

正解：C

解説：
Steganography
https://en.wikipedia.org/wiki/Steganography
Steganography is the practice of concealing a file, message, image, or video within another file, message, image, or video.
The first recorded use of the term was in 1499 by Johannes Trithemius in his Steganographia, a treatise on cryptography and steganography, disguised as a book on magic. Generally, the hidden messages appear to be (or to be part of) something else: images, articles, shopping lists, or some other cover text. For example, the hidden message may be in invisible ink between the visible lines of a private letter. Some implementations of steganography that lack a shared secret are forms of security through obscurity, and key-dependent steganographic schemes adhere to Kerckhoffs's principle.
Incorrect answers:
Avalanche effect - 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.
Cryptosystem - a suite of cryptographic algorithms needed to implement a particular security service, most commonly for achieving confidentiality (encryption) Key Schedule - an algorithm for the key that calculates the subkeys for each round that the encryption goes through.

質問 # 45
Which of the following is a block cipher?

• A. RSA
• B. AES
• C. DH
• D. RC4

正解：B

解説：
AES
https://en.wikipedia.org/wiki/Advanced_Encryption_Standard
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 Incorrect answers:
RC4. RC4 (Rivest Cipher 4 also known as ARC4 or ARCFOUR meaning Alleged RC4, see below) is a stream cipher.
DH. 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.
RSA. RSA (Rivest-Shamir-Adleman) is one of the first public-key cryptosystems and is widely used for secure data transmission.

質問 # 46
A cipher is defined as what

• A. Encrypted text
• B. The key used to encrypt a message
• C. Any algorithm used in cryptography
• D. The algorithm(s) needed to encrypt and decrypt a message

正解：D

解説：
The algorithm(s) needed to encrypt and decrypt a message
https://en.wikipedia.org/wiki/Cipher
In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption-a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.

質問 # 47
Which of the following would be the fastest.

• A. RSA
• B. AES
• C. DH
• D. EC

正解：B

解説：
AES
https://en.wikipedia.org/wiki/Symmetric-key_algorithm
AES - symmetric cipher. Symmetric keys use the same key for both encryption and decryption. Both the sender and receiver of the data must know and share the secret key. For standard encrypt/decrypt functions, symmetric algorithms generally perform much faster than their asymmetrical counterparts. This is due to the fact that asymmetric cryptography is massively inefficient. Symmetric cryptography is designed precisely for the efficient processing of large volumes of data. In other words, symmetric encryption is generally used for speed and performance, e.g. when there's a large amount of data that needs to be encrypted/protected.
Incorrect answers:
RSA - asymmetric cipher,
DH - 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.
EC - Elliptic-curve cryptography (ECC) is an approach to public-key cryptography based on the algebraic structure of elliptic curves over finite fields.

質問 # 48
You are studying classic ciphers. You have been examining the difference between single substitution and multi-substitution. Which one of the following is an example of a multi-alphabet cipher?

• A. Vigenere
• B. Atbash
• C. Rot13
• D. Caesar

正解：A

解説：
Vigenere
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
The Vigenere cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It employs a form of polyalphabetic substitution.
First described by Giovan Battista Bellaso in 1553, the cipher is easy to understand and implement, but it resisted all attempts to break it until 1863, three centuries later. This earned it the description le chiffre indechiffrable (French for 'the indecipherable cipher'). Many people have tried to implement encryption schemes that are essentially Vigenere ciphers. In 1863, Friedrich Kasiski was the first to publish a general method of deciphering Vigenere ciphers.

質問 # 49
A ______ refers to a situation where two different inputs yield the same output.

• A. Convergence
• B. Collision
• C. Transposition
• D. Substitution

正解：B

解説：
Collision
https://en.wikipedia.org/wiki/Collision_(computer_science)
A collision or clash is a situation that occurs when two distinct pieces of data have the same hash value, checksum, fingerprint, or cryptographic digest.

質問 # 50
A protocol for key aggreement based on Diffie-Hellman. Created in 1995. Incorporated into the public key standard IEEE P1363.

• A. Elliptic Curve
• B. Euler's totient
• C. Blum Blum Shub
• D. Menezes-Qu-Vanstone

正解：D

解説：
Menezes-Qu-Vanstone
https://en.wikipedia.org/wiki/MQV
MQV (Menezes-Qu-Vanstone) is an authenticated protocol for key agreement based on the Diffie-Hellman scheme. Like other authenticated Diffie-Hellman schemes, MQV provides protection against an active attacker. The protocol can be modified to work in an arbitrary finite group, and, in particular, elliptic curve groups, where it is known as elliptic curve MQV (ECMQV).
MQV was initially proposed by Alfred Menezes, Minghua Qu and Scott Vanstone in 1995. It was modified with Law and Solinas in 1998.
Incorrect answers:
Elliptic Curve - an approach to public-key cryptography based on the algebraic structure of elliptic curves over finite fields. ECC allows smaller keys compared to non-EC cryptography (based on plain Galois fields) to provide equivalent security.
Euler's totient - function counts the positive integers up to a given integer n that are relatively prime to n.
Blum Blum Shub - a pseudorandom number generator proposed in 1986 by Lenore Blum, Manuel Blum and Michael Shub that is derived from Michael
O. Rabin's one-way function.

質問 # 51
What is the basis for the difficulty in breaking RSA?

• A. The birthday paradox
• B. Factoring numbers
• C. Equations that describe an elliptic curve
• D. Hashing

正解：B

解説：
Factoring numbers
https://en.wikipedia.org/wiki/RSA_(cryptosystem)
RSA (Rivest-Shamir-Adleman) is a public-key cryptosystem that is widely used for secure data transmission. It is also one of the oldest. The acronym RSA comes from the surnames of Ron Rivest, Adi Shamir, and Leonard Adleman, who publicly described the algorithm in 1977. An equivalent system was developed secretly, in 1973 at GCHQ (the British signals intelligence agency), by the English mathematician Clifford Cocks. That system was declassified in 1997.
In a public-key cryptosystem, the encryption key is public and distinct from the decryption key, which is kept secret (private). An RSA user creates and publishes a public key based on two large prime numbers, along with an auxiliary value. The prime numbers are kept secret. Messages can be encrypted by anyone, via the public key, but can only be decoded by someone who knows the prime numbers.

質問 # 52
In steganography, ________ is the data to be covertly communicated (in other words, it is the message you wish to hide).

• A. Carrier
• B. Channel
• C. Payload
• D. Signal

正解：C

解説：
Payload
https://en.wikipedia.org/wiki/Steganography
The payload is the data covertly communicated. The carrier is the signal, stream, or data file that hides the payload, which differs from the channel, which typically means the type of input, such as a JPEG image. The resulting signal, stream, or data file with the encoded payload is sometimes called the package, stego file, or covert message. The proportion of bytes, samples, or other signal elements modified to encode the payload is called the encoding density and is typically expressed as a number between 0 and 1.

質問 # 53
A measure of the uncertainty associated with a random variable.

• A. Diffusion
• B. Entropy
• C. Collision
• D. Whitening

正解：B

解説：
Entropy
https://en.wikipedia.org/wiki/Entropy_(information_theory)
In information theory, the entropy of a random variable is the average level of "information", "surprise", or "uncertainty" inherent in the variable's possible outcomes. The concept of information entropy was introduced by Claude Shannon in his 1948 paper "A Mathematical Theory of Communication".
Incorrect answers:
Diffusion - transposition processes used in encryption functions to increase randomness.
Whitening - technique intended to increase the security of an iterated block cipher. It consists of steps that combine the data with portions of the key.
Collision - situation where two different inputs yield the same output.

質問 # 54
Juanita has been assigned the task of selecting email encryption for the staff of the insurance company she works for. The various employees often use diverse email clients. Which of the following methods is available as an add-in for most email clients?

• A. RSA
• B. PGP
• C. Caesar cipher
• D. DES

正解：B

解説：
PGP
https://en.wikipedia.org/wiki/Pretty_Good_Privacy
Pretty Good Privacy (PGP) is an encryption program that provides cryptographic privacy and authentication for data communication. PGP is used for signing, encrypting, and decrypting texts, e-mails, files, directories, and whole disk partitions and to increase the security of e-mail communications. Phil Zimmermann developed PGP in 1991.

質問 # 55
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