Introduction-to-Cryptography Practice Questions

The Advanced Encryption Standard (AES), as defined by the National Institute of Standards and Technology (NIST) in FIPS PUB 197, specifies three possible key lengths for the algorithm. These are 128 bits, 192 bits, and 256 bits. The largest of these, and therefore the maximum key size supported by the standard, is 256 bits. While the underlying Rijndael algorithm could support other key and block sizes, the AES standard strictly defines these three options.

A. 128 bits is a valid and commonly used AES key size, but it is the minimum supported key length, not the maximum.

B. 192 bits is the intermediate AES key size specified in the standard, falling between the minimum and maximum options.

D. 512 bits is not a key size defined in the FIPS 197 standard for AES. It is not a valid option for AES.

1. National Institute of Standards and Technology (NIST). (2001). FIPS PUB 197: Advanced Encryption Standard (AES). Section 5, "Algorithm Specification," p. 13, Table 1.

2. Daemen, J., & Rijmen, V. (2002). The Design of Rijndael: AES - The Advanced Encryption Standard. Springer. Section 3.4, "Key-Block-Round Arrangements," p. 34.

A digital signature is a cryptographic mechanism used to demonstrate the authenticity, data integrity, and non-repudiation of a digital message or document. It is the digital equivalent of a handwritten signature. The process involves creating a message hash and then encrypting the hash with the sender's private key. The recipient can use the sender's public key to decrypt the hash and verify that it matches a hash of the received message. This confirms the sender's identity (authenticity) and that the message has not been altered (integrity).

A. Encryption provides confidentiality (secrecy), whereas a digital signature's primary goals are authenticity, integrity, and non-repudiation.

C. A digital signature is a cryptographic scheme or protocol that uses algorithms (e.g., hashing and asymmetric encryption), not an encryption algorithm itself.

D. A hash function creates a unique identifier (digest) for a file, which is a component of a digital signature, but not the complete signature.

1. National Institute of Standards and Technology (NIST). (2013). FIPS PUB 186-4: Digital Signature Standard (DSS). Section 1, "Introduction," p. 1.

2. Katz, J., & Lindell, Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. Chapter 12, "Digital Signatures," provides a formal definition and security properties. (A standard textbook in university cryptography courses).

3. Rivest, R. L., Shamir, A., & Adleman, L. (1978). A method for obtaining digital signatures and public-key cryptosystems. Communications of the ACM, 21(2), 120–126. https://doi.org/10.1145/359340.359342 (The foundational academic paper introducing the concept).

The Payment Card Industry Data Security Standard (PCI DSS) is a set of security standards designed to ensure that all companies that accept, process, store, or transmit credit card information maintain a secure environment. A core component of PCI DSS is the requirement to protect cardholder data. Specifically, Requirement 3 mandates the protection of stored account data, often through strong cryptography, and Requirement 4 requires the encryption of cardholder data during transmission over open, public networks. This makes it the primary regulation focused on credit card data protection.

A: CCPA is a broad consumer privacy law for California residents, not a standard specifically focused on credit card data security.

C: HIPAA mandates the protection of electronic protected health information (ePHI), not financial or credit card data.

D: GDPR is a comprehensive data privacy regulation for EU citizens, covering all personal data, not just credit card information.

1. PCI Security Standards Council. (2022). Payment Card Industry (PCI) Data Security Standard Requirements and Security Assessment Procedures, Version 4.0. Requirement 3, "Protect Stored Account Data," and Requirement 4, "Protect Cardholder Data with Strong Cryptography During Transmission Over Open, Public Networks." Document available at https://www.pcisecuritystandards.org/documentlibrary/

2. National Institute of Standards and Technology (NIST). (2016). Special Publication (SP) 800-53 Rev. 4, Security and Privacy Controls for Federal Information Systems and Organizations. Appendix J, "Privacy Control Catalog," references PCI DSS as a related security standard for financial data. https://doi.org/10.6028/NIST.SP.800-53r4

During the Transport Layer Security (TLS) handshake, the process begins with the client sending a ClientHello message to the server. This initial message contains critical information for establishing a secure session, including the version of TLS the client supports, a random number, and a comprehensive list of cipher suites it is configured to use. A cipher suite is a named combination of authentication, encryption, and message authentication code (MAC) algorithms. The client presents its supported suites in order of preference, allowing the server to choose one that it also supports to secure the communication channel.

A. Forward secrecy is a property of a key exchange protocol that ensures past session keys are not compromised even if a long-term secret key is compromised later.

B. The ServerHello message is the server's response; it selects a single cipher suite from the client's list, it does not send a list of its own.

D. An integrity check, such as a MAC, is a function used after the secure channel is established to ensure data has not been altered in transit.

1. Dierks, T., & Rescorla, E. (2008). The Transport Layer Security (TLS) Protocol Version 1.2. IETF. RFC 5246. Section 7.4.1.2. "Client Hello". Retrieved from https://datatracker.ietf.org/doc/html/rfc5246#section-7.4.1.2

2. Rescorla, E. (2018). The Transport Layer Security (TLS) Protocol Version 1.3. IETF. RFC 8446. Section 4.1.2. "Client Hello". Retrieved from https://datatracker.ietf.org/doc/html/rfc8446#section-4.1.2

3. Boneh, D. (n.d.). Lecture 11: TLS. Stanford University CS255 Introduction to Cryptography. Retrieved from https://crypto.stanford.edu/~dabo/courses/CS255/lectures/11-TLS.pdf (Slide 10 describes the ClientHello message content).

Code signing applies a digital signature to software executables, scripts, and other code artifacts. The primary purpose is to provide assurances of authenticity and integrity. Authenticity verifies the identity of the software publisher, confirming that the code originates from a known and trusted source. Integrity guarantees that the code has not been altered, corrupted, or tampered with since it was signed by the publisher. This mechanism is a critical defense against malware distribution, as it allows operating systems and users to distinguish between legitimate software and potentially malicious code.

A. To generate random numbers for encryption

Random number generation is a separate cryptographic primitive and is not the purpose of code signing.

C. To compress data for efficient storage

Data compression is a technique to reduce file size and is unrelated to the security functions of code signing.

D. To authenticate users during data transmission

Code signing authenticates the software's publisher (the author), not the end-user who is running the software.

1. Microsoft Corporation. (2023). Introduction to Code Signing. Microsoft Docs. "Code signing is the process of using a certificate-based digital signature to sign executables and scripts in order to verify the author's identity and ensure that the code has not been changed or corrupted since it was signed."

2. National Institute of Standards and Technology (NIST). (2020). SP 800-175B Rev. 1: Guideline for Using Cryptographic Standards in the Federal Government. Section 5.3, "Digital Signatures," Page 23. This section explains that digital signatures provide data integrity and source authentication, which are the core principles of code signing.

3. Paar, C., & Pelzl, J. (2010). Understanding Cryptography: A Textbook for Students and Practitioners. Springer. Chapter 11, "Digital Signatures," explains the underlying technology that provides integrity and authenticity.

The expression "51 mod 11" asks for the remainder when 51 is divided by 11. This is a standard modular arithmetic operation. To calculate this, we find the largest multiple of 11 that is less than or equal to 51. In this case, 11 multiplied by 4 is 44. We then subtract this multiple from 51 to find the remainder: 51 - 44 = 7. Therefore, the value of 51 mod 11 is 7.

A. The remainder is not 4. 11 4 + 4 = 48, not 51.

B. The remainder is not 5. 11 4 + 5 = 49, not 51.

D. The result of a modulo n operation must be an integer in the range [0, n-1]. 11 is outside the range [0, 10].

1. Rosen, K. H. (2019). Discrete Mathematics and Its Applications (8th ed.). McGraw-Hill. Chapter 4, Section 4.1, "Divisibility and Modular Arithmetic," Definition 4, p. 271.

2. Paar, C., & Pelzl, J. (2010). Understanding Cryptography: A Textbook for Students and Practitioners. Springer. Chapter 4, Section 4.2, "The Integers," pp. 88-89.

3. MIT OpenCourseWare. (2015). Mathematics for Computer Science (6.042J). Fall 2010. Lecture 10: Number Theory I, Section 2.1 "The Modulo Operator."

The process of verifying a digital signature involves using the signer's public key. The organization first creates a hash (digest) of the software and then encrypts this digest with its own private key; this encrypted digest is the digital signature. The customer, upon receiving the software, uses the organization's publicly available key to decrypt the signature. This reveals the original digest, which the customer compares to a new digest they compute from the received software. If they match, the software's integrity and origin are verified.

A. The customer’s private key is secret to the customer and is used for signing their own communications, not for verification.

C. The organization’s private key is used to create the signature and is kept secret; it is never shared with customers.

D. The customer’s public key would be used by others to verify the customer's signature, not the organization's.

- Paar, C., & Pelzl, J. (2010). Understanding Cryptography: A Textbook for Students and Practitioners. Springer. Chapter 10.1, "The Digital Signature Scheme RSA," details the verification process where the verifier uses the sender's public key.

- Katz, J., & Lindell, Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. Section 12.2, "Digital Signatures," defines the verification algorithm Vrfy as taking the public key, a message, and a signature as input.

Breaches resulting from poor cryptographic practices, such as using weak algorithms, hardcoded keys, or improper key storage, are fundamentally failures in risk management. A comprehensive risk assessment is the formal process for identifying, analyzing, and evaluating such risks. The key lesson from these breaches is the critical importance of proactively conducting these assessments to discover cryptographic vulnerabilities and other weaknesses before they can be exploited by attackers. This allows an organization to prioritize and apply appropriate controls to mitigate the identified risks, thereby preventing potential breaches.

B. Many cryptographic failures stem from human error, making employee training on secure practices highly relevant and necessary.

C. The lesson is the opposite; breaches often occur because systems were not regularly audited or updated to fix known vulnerabilities.

D. This mindset is a cause of breaches; the lesson is that integrating security with business objectives is essential for long-term viability.

1. National Institute of Standards and Technology (NIST). (2012). NIST Special Publication 800-30 Revision 1: Guide for Conducting Risk Assessments. Section 2.1, "Purpose and Applicability," p. 4, states that risk assessments are a key component in "making risk-based decisions to protect information and information systems."

2. Goel, S., & Chen, V. (2005). Information security: The role of risk analysis. In Proceedings of the 2nd annual conference on Information security curriculum development (InfoSecCD '05). Association for Computing Machinery, New York, NY, USA, 134–137. https://doi.org/10.1145/1107622.1107655. (This academic paper highlights risk analysis as foundational to information security).

3. Bauer, L. (2017). 15-330/630: Introduction to Computer Security, Fall 2017 Course Notes. Carnegie Mellon University. Lecture 1, "Intro, Threat Models, and Risk Management," pp. 10-12, introduces risk analysis as the process of identifying assets, threats, and vulnerabilities to decide which countermeasures to deploy.

Counter (CTR) mode is a block cipher mode of operation that effectively transforms a block cipher into a stream cipher. It does not encrypt the plaintext directly. Instead, it generates a keystream by encrypting successive values of a "counter." This counter is typically initialized with a nonce (a number used once) to ensure each stream is unique. The resulting encrypted counter blocks (the keystream) are then XORed with the plaintext blocks to produce the ciphertext. This process allows for parallel encryption and decryption.

B. This description is more aligned with Cipher Feedback (CFB) mode, which can be self-synchronizing. CTR mode is not self-synchronizing.

C. This describes Electronic Codebook (ECB) mode, where each block is encrypted independently without any chaining or feedback mechanism.

D. This describes Cipher Block Chaining (CBC) mode, where the ciphertext from the previous block is used to encrypt the next plaintext block.

1. National Institute of Standards and Technology (NIST). (2001). SP 800-38A: Recommendation for Block Cipher Modes of Operation. Section 6.5, "The Counter (CTR) Mode," page 13.

2. Boneh, D., & Shoup, V. (2017). A Graduate Course in Applied Cryptography. Version 0.5. Chapter 4, "Symmetric Encryption," Section 4.5, "Modes of Operation," page 101. (Available at https://crypto.stanford.edu/~dabo/cryptobook/)

Triple DES (3DES) is a symmetric-key block cipher that applies the Data Encryption Standard (DES) algorithm three times to each data block. It has two primary keying options defined by NIST. The most common option, Two-Key 3DES, uses two independent 56-bit keys (K1 and K2), where the process is Encrypt-Decrypt-Encrypt (EDE) with the key sequence K1, K2, K1. This results in an effective key size of 112 bits (56 bits + 56 bits). While a three-key option exists with a 168-bit key, its effective strength is also considered to be 112 bits due to meet-in-the-middle attacks.

A. 128-bit is a standard key size for the Advanced Encryption Standard (AES), not 3DES.

B. 2,048-bit is a common key size for asymmetric algorithms like RSA, not for a symmetric block cipher like 3DES.

C. 56-bit is the key size for the original, single DES algorithm, which 3DES was designed to replace due to its vulnerability to brute-force attacks.

1. National Institute of Standards and Technology (NIST). (2017). Special Publication 800-67 Revision 2: Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher. Section 3, "TDEA Keying Options," p. 4. Retrieved from https://doi.org/10.6028/NIST.SP.800-67r2

2. Katz, J., & Lindell, Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. Chapter 6, "Constructions of Pseudorandom Permutations," Section 6.2.4, "Stronger Block Ciphers from Weaker Ones," pp. 193-195.

3. Rivest, R. L. (1998). Lecture 4: The Data Encryption Standard (DES). MIT 6.857 Computer and Network Security. Retrieved from https://ocw.mit.edu/courses/6-857-computer-and-network-security-spring-2017/resources/lecture-4-the-data-encryption-standard-des/ (Discusses DES key size and mentions Triple DES).