Introduction-to-Cryptography Questions and Answers

In AES, the number of rounds is explicitly tied to the key length. AES-128 uses 10 rounds, AES-192 uses 12 rounds, and AES-256 uses 14 rounds. The purpose of additional rounds is to increase diffusion and confusion, strengthening resistance against cryptanalysis as the key schedule and state transformations iterate more times. Although key length primarily affects brute-force resistance, AES’s designers and standardization parameters link longer keys with more rounds to maintain security margins across variants, especially considering differences in the key schedule structure. Thus, as key length increases from 128 to 192 to 256 bits, the number of rounds increases correspondingly from 10 to 12 to 14. This relationship is fixed by the AES specification and does not vary dynamically at runtime. Therefore, the correct correlation is that the number of rounds increases as the key length increases.

The Caesar cipher is the classic substitution cipher that encrypts by shifting letters of the alphabet by a fixed number of positions (e.g., shift by 3: A→D, B→E, etc.). It is a monoalphabetic cipher because a single shift value is applied uniformly across the entire message, making it simple and vulnerable to frequency analysis and brute force (only 25 meaningful shifts in the Latin alphabet). Vigenère also involves shifting, but it uses a repeating keyword to vary the shift per character (polyalphabetic), whereas the question’s phrasing typically points to the fundamental “shift cipher,” which is Caesar. SHA-1 is a cryptographic hash function, not a cipher. Bifid is a fractionation cipher combining Polybius square coordinates and transposition, not a direct shifting method. Therefore, the cipher that uses shifting letters of the alphabet for encryption is the Caesar cipher.

OTP systems (such as HOTP and TOTP) generate a sequence of passwords using a shared secret and a moving factor (counter or time). The critical secret that underpins the ability to compute past or future OTP values is the seed (also called the shared secret key). In HOTP, the seed is used with an HMAC function and an incrementing counter; in TOTP, the seed is used with HMAC and a time-step value. If an attacker obtains the seed and knows the algorithm and moving factor, they can compute future OTPs. The “function” and “encryption algorithm” are typically standardized and public; security relies on keeping the seed secret. An initialization vector is not a standard OTP component in HOTP/TOTP generation. Therefore, the component needed to predict future OTP values is the seed. Protecting the seed is essential: it should be stored securely (e.g., hardware token secure storage) and transmitted only through controlled provisioning processes. If compromised, OTP becomes predictable and no longer serves as a strong second factor.

With IPsec ESP in transport mode, the payload of the original IP packet (typically the transport-layer segment and higher) is encrypted and integrity-protected between the two endpoints—here, the corporate server and the remote client. Because encryption is applied by the sending endpoint and removed only by the receiving endpoint, intermediate routers, switches, and monitoring devices in either network cannot view the protected payload while it is in transit. They may see outer IP headers and certain metadata needed for routing, but not the encrypted content protected by ESP. As a result, the packet’s contents are inspectable only at the endpoints: before encryption on the sender (plaintext exists in memory/stack before IPsec processing) and after decryption on the receiver (plaintext is restored for the application). This is true whether the traffic traverses internal networks or the Internet; the cryptographic boundary is between the endpoints participating in the IPsec SA. Therefore, inspection of the actual content is possible only on the devices at headquarters and offsite, before sending and after receiving, not by in-transit networks.

In classic WEP deployments, RC4 was used with what is commonly called “40-bit WEP” (also labeled “64-bit WEP” because it combines a 40-bit secret key with a 24-bit IV to form a 64-bit RC4 seed). The key attribute emphasized in many foundational descriptions of WEP is this 40-bit shared secret length, which was originally chosen due to export restrictions and legacy constraints. Although “104-bit WEP” (sometimes called “128-bit WEP,” again counting the 24-bit IV) also existed, the option set here points to the historically standard and widely referenced attribute: a 40-bit key when RC4 is used in WEP. Importantly, WEP’s security failure is not only about key size; the 24-bit IV is too small and repeats frequently, and WEP’s key scheduling vulnerabilities combined with IV reuse allow attackers to recover the secret key with enough captured frames. Still, among the given options, the correct attribute is the 40-bit key.

802.1X is a port-based network access control framework that enables centralized authentication using an authentication server (commonly RADIUS). EAP (Extensible Authentication Protocol) runs within 802.1X to support many credential types (password-based methods like PEAP, certificate-based methods like EAP-TLS, and others). WPA-Enterprise is the wireless security mode that explicitly uses 802.1X + EAP with an authentication server to perform per-user/per-device authentication and to derive dynamic session keys. By contrast, WPA-PSK uses a pre-shared key without an external authentication server; all users share the same PSK, which is weaker for enterprise identity management. WEP is an older mechanism using static keys and does not provide modern 802.1X/EAP enterprise authentication in the WPA-Enterprise sense. TKIP is an encryption/integrity protocol used under WPA, not the full authentication “standard” involving an authentication server. Therefore, the correct choice is WPA-Enterprise.

The expression 23 mod 6 asks for the remainder when 23 is divided by 6. Modular arithmetic is foundational in cryptography, especially in public-key systems (RSA, Diffie–Hellman, ECC) where operations occur in finite rings or fields. To compute 23 mod 6, identify the largest multiple of 6 that does not exceed 23. Multiples of 6 are 6, 12, 18, 24. Since 24 is greater than 23, the largest valid multiple is 18. Subtract: 23 − 18 = 5, so the remainder is 5. Therefore, 23 mod 6 = 5, which corresponds to option “05.” Modular reduction keeps numbers within a fixed range (0 to modulus−1), enabling stable arithmetic under wraparound behavior. In cryptographic protocols, this wraparound property is essential for defining groups and ensuring operations remain bounded and consistent.

The string format $2a$08$... is a well-known identifier for the bcrypt password hashing scheme. In common password-hash notation, the prefix indicates the algorithm and parameters: “$2a$” denotes bcrypt (version 2a), and “08” indicates the cost factor (work factor) controlling how computationally expensive hashing is. bcrypt is designed specifically for password storage: it includes a built-in salt and is intentionally slow and adaptive, making brute-force and GPU attacks far more expensive than fast general-purpose hashes like MD5 or SHA-512. NTLM and MD5 are obsolete for secure password storage due to speed and known weaknesses. SHA-512, while cryptographically strong as a hash, is still too fast for password hashing unless used in a dedicated password-hashing construction (e.g., PBKDF2, scrypt, Argon2) with appropriate parameters and salts. Since the given hash clearly matches bcrypt’s encoding, the correct algorithm is bcrypt, which incorporates salting and cost-based key stretching as part of its design.

DER (Distinguished Encoding Rules) is a binary encoding format used to represent ASN.1 structures in a canonical, unambiguous way. X.509 certificates are defined using ASN.1, and DER provides a strict subset of BER (Basic Encoding Rules) that guarantees a single, unique encoding for any given data structure. That “unique encoding” property is important for cryptographic operations such as hashing and digital signatures, because different encodings of the same abstract data could otherwise produce different hashes and break signature verification. In contrast, PEM is not a binary encoding; it is essentially a Base64-encoded text wrapper around DER data, bounded by header/footer lines (e.g., “BEGIN CERTIFICATE”). PKI is an overall framework for certificate issuance, trust, and lifecycle management—not an encoding. RSA is an asymmetric algorithm used for encryption/signing, not a certificate encoding format. Therefore, the binary-based certificate encoding process among the options is DER.

Lightweight cryptography is designed for constrained environments—devices with limited CPU, memory, storage, bandwidth, and power (battery). Examples include IoT sensors, smart locks, RFID tags, embedded controllers, and industrial devices. Administrators choose lightweight algorithms and protocols to maintain reasonable security while fitting strict resource budgets and real-time constraints. The goal is not “weaker security because data is unimportant,” but rather efficient security that can still meet threat models under constraints. Option B captures this: embedded systems often cannot afford the computational cost of heavy cryptographic primitives (large key sizes, complex modes, frequent handshakes) or may struggle with latency and energy consumption. Option A is irrelevant because physical security of a desktop doesn’t remove the need for cryptography in communications or storage. Option C is the opposite of lightweight design. Option D is a poor justification; security design should be based on risk, and lightweight cryptography is not merely for “minimal protection,” but for practical deployability under constraints. Therefore, the correct reason is limited resources on embedded systems.

Skipjack is a symmetric block cipher historically associated with the Clipper chip initiative. Its defining parameters match the question: it operates on 64-bit blocks and uses an 80-bit key. The other options do not fit those exact sizes. Twofish is a 128-bit block cipher with key sizes up to 256 bits. Blowfish is a 64-bit block cipher, but its key size is variable from 32 up to 448 bits and is not fixed at 80 bits as a defining property. Camellia is a 128-bit block cipher with key sizes of 128, 192, or 256 bits. Skipjack’s smaller key size and legacy design make it unsuitable for modern security needs, but the question is purely about identifying the algorithm that matches an 80-bit key and 64-bit blocks. Therefore, the correct answer is Skipjack.

WPA with TKIP was designed as an interim improvement over WEP while still using the RC4 stream cipher for compatibility with legacy hardware. TKIP addresses WEP’s major weaknesses by introducing per-packet key mixing, a message integrity mechanism (“Michael”), and replay protection. In TKIP, the encryption key used with RC4 is 128 bits. Practically, TKIP derives a per-packet RC4 key from a 128-bit temporal key (TK), the transmitter’s MAC address, and a sequence counter (TKIP Sequence Counter, TSC) to avoid the simple IV reuse patterns that made WEP easy to break. Even with these improvements, TKIP has known weaknesses and is deprecated in favor of WPA2/WPA3 using AES-based CCMP/GCMP. But strictly for the question asked, TKIP’s RC4 keying material is based on a 128-bit key size, not 40/56-bit legacy sizes and not 256-bit.

One-way server authentication is the standard model used by most TLS-enabled web services to prove the server’s identity to a client. In this model, the server presents an X.509 certificate during the TLS handshake. The client validates the certificate chain to a trusted root CA, checks hostname binding (CN/SAN), validates validity dates, and may check revocation status. If validation succeeds, the client gains cryptographic assurance that it is communicating with the holder of the private key corresponding to the server certificate’s public key, and that the certificate is issued to the expected domain/identity. This proves the server’s identity to the customer without requiring the customer to present a certificate. Mutual authentication would require both client and server to authenticate each other using certificates (commonly in certain enterprise APIs), but the question asks specifically about the web service proving its identity to the customer, which is satisfied by server-only authentication. One-way client authentication is the opposite direction (client proves identity to server). “End-to-end authentication” is a broader concept and not the specific TLS identity proof mechanism described here. Thus, one-way server authentication is the correct choice.