Introduction-to-Cryptography WGU Introduction to Cryptography HNO1 Questions and Answers

The value 51 mod 11 is the remainder after dividing 51 by 11. Modular arithmetic is widely used in cryptography to keep computations within a finite set of residues, such as in RSA where values are taken modulo n, or in Diffie–Hellman where exponents and group elements are reduced modulo a prime. To compute 51 mod 11, find the largest multiple of 11 less than or equal to 51. Multiples of 11 are 11, 22, 33, 44, 55. The closest without exceeding 51 is 44. Subtracting gives 51 − 44 = 7, so the remainder is 7. Therefore, 51 mod 11 = 7, matching option “07.” This remainder is always in the range 0 through 10 because the modulus is 11. Such residue computations underpin the “wraparound” behavior that makes modular exponentiation and inverse computations well-defined in cryptographic groups.

To ensure confidentiality so that only the intended recipient can decrypt an email, the sender must encrypt in a way that only the recipient can reverse. In public key cryptography, that means encrypting with the recipient’s public key. The recipient is the only party who should possess the matching private key, so only they can decrypt the ciphertext. This pattern is fundamental to PKI-based secure email systems such as S/MIME and OpenPGP: the sender looks up or is provided the recipient’s certificate/public key, encrypts the message (often by encrypting a randomly generated symmetric session key with the recipient’s public key), and the recipient uses their private key to recover the session key and decrypt the content. Encrypting with the sender’s private key would not provide confidentiality; it resembles signing because anyone with the sender’s public key could “decrypt” it. Encrypting with a private key of the recipient is also incorrect because private keys are not shared and should never leave the recipient’s control. Therefore, the correct key to encrypt the message so only the friend can decrypt it is the recipient’s public key.

A cryptographic hash function maps input data of arbitrary length to a fixed-length digest and is designed to be one-way: given a digest, it should be computationally infeasible to recover the original input (preimage resistance). This one-way property enables many security applications: file integrity checking (any change alters the digest), password storage (combined with salt and slow hashing), digital signatures (signing a digest rather than large data), and commitment schemes. Hash functions are not encryption because they do not support decryption; they intentionally discard information about the input. They also do not use public/private key pairs; those are features of asymmetric cryptography. “Reversible” is the opposite of the intended design goal. A well-designed hash also aims for collision resistance (hard to find two different inputs with the same digest) and second-preimage resistance. Among the answer choices, the defining characteristic that best captures what makes a hash function distinct is that it is one-way.

3DES (Triple DES) is a symmetric block cipher that retains DES’s 64-bit block size while increasing effective security by applying DES multiple times. The common “two-key 3DES” variant uses two independent 56-bit DES keys (K1 and K2) in an Encrypt–Decrypt–Encrypt (EDE) sequence: Encrypt with K1, Decrypt with K2, then Encrypt again with K1. Because each DES key is 56 bits (ignoring parity bits), the total keying material is 112 bits. This matches the question’s “112-bit key size and 64-bit block size.” Plain DES uses only a 56-bit effective key and a 64-bit block size, so it does not match the 112-bit key size. AES has a 128-bit block size and key sizes of 128/192/256. IDEA uses a 64-bit block size but has a 128-bit key. Therefore, the correct algorithm is 3DES. Although 3DES improved on DES, it is now considered legacy due to its small 64-bit block size (birthday-bound issues for large data volumes) and performance overhead compared to AES.

In digital forensics, investigators must preserve evidence integrity and demonstrate an unbroken chain of custody. Creating a cryptographic hash (such as SHA-256) of the original drive and then hashing the forensic bitstream image provides a strong mathematical assurance that the copy is an exact, bit-for-bit replica. Because secure hash functions are designed so that any tiny change in data produces a dramatically different digest, matching hashes indicate the image contains identical data to the source at the time of acquisition. This is critical in legal and investigative contexts: analysis is performed on the copy, not the original, to avoid altering evidence. If the hashes match, the investigator can testify that the evidence examined is identical to what was collected, supporting admissibility and credibility. Hashing does not prove who created files, nor does it directly show whether someone “opened the drive”; it specifically validates the integrity and equivalence of the captured image. Therefore, hashing both artifacts is done to verify that the original and the bitstream copy are identical.

Symmetric cryptography uses a single shared secret key for both encryption and decryption. This contrasts with asymmetric cryptography, which uses a key pair (public/private). Symmetric algorithms (like AES, ChaCha20) are efficient and well-suited for bulk data encryption, but they require a secure method for key distribution because both parties must possess the same secret. Hashing is not a keyed operation by default (though HMAC is keyed); it maps arbitrary data to a fixed-size digest and is primarily used for integrity checking, fingerprints, and password hashing constructions. Padding is a data formatting technique (e.g., PKCS#7) used to align plaintext to a block size; it is not a cryptographic “operation” that uses a key. Therefore, the cryptographic operation characterized by using one key shared between parties is symmetric encryption. In real systems, symmetric encryption is frequently combined with asymmetric methods for key exchange and with MACs/AEAD for integrity, producing the standard hybrid approach used in protocols like TLS and IPsec.

IPsec’s initial key exchange is commonly performed using IKE (Internet Key Exchange), which negotiates Security Associations (SAs), authenticates peers, and establishes shared keys for ESP/AH protection. The traditional and default transport for IKEv1 and IKEv2 is UDP port 500. During negotiation, peers exchange proposals (crypto suites), perform Diffie–Hellman to derive key material, and authenticate using pre-shared keys, certificates, or EAP methods. If a firewall blocks UDP 500, the IKE negotiation cannot begin, preventing IPsec tunnels from forming. In many real deployments, NAT traversal is also used; in that case, traffic typically shifts to UDP 4500 (NAT-T) after detection of NAT, but UDP 500 is still required for the initial exchange and NAT detection in many configurations. TCP 500 is not standard for IKE. Port 443 is associated with HTTPS/TLS and some SSL VPNs, not IPsec IKE. Therefore, among the options provided, the firewall must allow UDP 500 for IPsec key exchange to succeed.

CTR mode turns a block cipher (like AES) into a stream-like construction by generating a keystream from successive encryptions of a changing input block. Specifically, CTR forms input blocks using a nonce (unique per message) combined with an increasing counter. Each nonce||counter block is encrypted with the block cipher under the shared key, producing a pseudorandom output block. That output is then XORed with plaintext to yield ciphertext (and XORed with ciphertext to recover plaintext). This design enables parallelization (blocks can be generated independently), efficient random access decryption, and avoids chaining dependencies seen in modes like CBC. Option B describes CFB-like behavior; option C describes ECB; option D describes CBC. CTR’s security critically depends on never reusing the same nonce/counter sequence with the same key, because reuse would repeat keystream blocks and expose plaintext relationships. Therefore, the correct description is that CTR converts the block cipher into a stream cipher using a counter value and a nonce.

Asymmetric encryption (also called public key cryptography) uses a pair of mathematically related keys: a public key and a private key. One key is used to encrypt, and the other is used to decrypt, which is the defining “different keys” property asked in the question. In the common confidentiality use case, a sender encrypts a message using the recipient’s public key, and only the recipient can decrypt it using their private key. This solves the key distribution problem inherent in symmetric encryption, where both parties must securely share the same secret key in advance. Asymmetric systems also enable digital signatures: the private key signs (creates a signature) and the public key verifies it, providing authenticity and integrity. Symmetric encryption, by contrast, uses the same shared key for both encryption and decryption (even though internal round keys may exist, it is still one shared secret). “Private key” alone is not a full encryption type, and “secure” is a generic description rather than a cryptographic category. Therefore, the correct answer is D. Asymmetric.

Large prime numbers are crucial because they enable cryptosystems where certain operations are easy to perform, but reversing them is computationally hard without secret information. In RSA, security is based on the difficulty of factoring a large composite number that is the product of two large primes; multiplying primes is easy, but factoring the product is believed to be hard at sufficient sizes. In Diffie–Hellman and related systems, primes define finite groups (often modulo a large prime) where exponentiation is efficient but the discrete logarithm problem is hard. Primes also help ensure desirable group properties—such as having a large cyclic subgroup—reducing vulnerabilities from small subgroups or weak structure. The value of “large” is that it makes brute-force and known algorithmic attacks infeasible with current computing resources. Large primes do not primarily make encryption faster, nor do they make decryption easier; they are chosen to maximize security margins. While primes can be involved in encoding steps, their importance is security: they form the mathematical foundation for hardness assumptions used by major public-key algorithms. Therefore, the best answer is that they provide security in encryption algorithms.

Asymmetric encryption is defined by using a key pair: a public key that can be shared widely and a private key that remains secret to its owner. The keys are mathematically related so that data encrypted with one key can be decrypted with the other (in confidentiality use cases, encryption with the recipient’s public key and decryption with the recipient’s private key). This design solves key distribution challenges: anyone can encrypt to a recipient without first sharing a secret key securely. It also enables digital signatures, where the private key signs and the public key verifies—supporting authenticity and integrity. Option B describes symmetric cryptography, not asymmetric. Option C is not a defining property; both symmetric and asymmetric algorithms can involve rounds or repeated operations. Option D is incorrect because asymmetric encryption is reversible for the intended holder of the private key; “irreversible” describes hashing, not encryption. Therefore, the characteristic feature of asymmetric encryption is the use of both a public and private key.

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.

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.

For protecting credit card data, the primary compliance framework is PCI DSS (Payment Card Industry Data Security Standard). PCI DSS is an industry standard created by major card brands and administered through the PCI Security Standards Council. It sets requirements for organizations that store, process, or transmit cardholder data, including controls around network security, access control, monitoring, and cryptography. PCI DSS explicitly addresses encryption and protection of cardholder data (for example, protecting stored cardholder data and encrypting transmission over open, public networks, and using strong cryptography and secure protocols). CCPA and GDPR are privacy regulations focused on personal data rights and governance, and while they may encourage security measures, they are not specifically the card-industry security standard for payment data. HIPAA applies to protected health information, not payment card data. Therefore, the correct answer is PCI DSS.

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.

Auditing improves cryptographic practice by systematically evaluating whether cryptographic controls are correctly selected, implemented, configured, and maintained. Through audits, an organization can discover weak algorithms (e.g., deprecated hashes), improper key lengths, unsafe modes (e.g., unauthenticated CBC), missing integrity controls, poor certificate validation, and operational problems such as key reuse, weak randomness sources, inadequate rotation, or overly permissive access to key material. Audits also assess compliance with internal policy and external standards, ensuring crypto is used consistently across systems and that exceptions are documented and risk-managed. Importantly, auditing does not guarantee that incidents will never happen; it reduces risk by finding gaps before attackers do. It also does not eliminate the need for updates—audits often reveal that policies must evolve as threats and best practices change. Employee training can be recommended as an outcome of auditing, but audits do not automatically ensure training. Thus, the most accurate benefit is that auditing identifies weaknesses and drives corrective action, strengthening cryptographic posture over time.

Modular arithmetic is the mathematics of working with remainders after division by a fixed number called the modulus. In cryptography, it underpins many core constructions because it defines arithmetic in finite sets (rings and fields) where values “wrap around,” enabling stable, repeatable operations with bounded results. Public-key systems like RSA rely on modular exponentiation (raising integers to powers modulo a composite number), while Diffie–Hellman and many elliptic-curve schemes operate in groups defined by modular arithmetic properties. Encryption and key exchange use modular operations because they allow efficient computation forward (e.g., exponentiation modulo a large number) while making certain inverse problems computationally hard without secret information (e.g., factoring or discrete logarithms). Modular reduction also helps keep intermediate values manageable and supports group properties needed for proofs of security. Although modular arithmetic is not “encryption by itself,” it is a foundational method used inside encryption algorithms and protocols. Therefore, among the options, describing it as a method used for encryption via modular operations best matches cryptographic usage.

In the TLS handshake, the ClientHello message is the client’s opening negotiation message and includes the client’s supported cryptographic capabilities. A key part of ClientHello is the offered cipher suites list, which advertises combinations of key exchange, authentication, encryption, and integrity/AEAD algorithms the client is willing to use. The server responds with ServerHello, selecting one of the offered cipher suites (in TLS 1.2 and earlier) and confirming protocol parameters. Forward secrecy is a property achieved by using ephemeral key exchange (e.g., (EC)DHE), not a specific message that “sends a list.” “Integrity check” is a security goal/mechanism, not the negotiation step. While TLS 1.3 changes the structure of negotiation (cipher suite list still appears in ClientHello but only covers AEAD and hash; key exchange is negotiated via extensions), the fundamental idea remains: the client proposes supported cipher suites in ClientHello, and the server picks compatible parameters. Therefore, the process that sends the list of supported cipher suites is the ClientHello.

Digital signatures provide integrity, authenticity, and typically non-repudiation by using an asymmetric key pair. The signer uses the private key to create a signature over a message (usually over a hash/digest of the message). Because the private key is kept secret, only the legitimate signer should be able to produce a valid signature. Anyone who has the corresponding public key can then validate the signature: they verify that the signature matches the message digest under the public key and that the signed data has not been altered. This is why the public key can be widely distributed (often inside an X.509 certificate) while the private key must be protected by the signer. If a public key were used to sign, anyone could forge signatures; if a private key were required for validation, only the signer could validate, defeating the purpose of public verifiability. Therefore, the correct key roles are private key for signing and public key for signature validation.

For Bitcoin, transactions are confirmed by inclusion in blocks, and the network targets an average block interval of about 10 minutes. That means transactions are “added” to the Bitcoin blockchain approximately every 10 minutes in the sense that a new block containing a batch of transactions is appended at that cadence. The 10-minute target is achieved by a difficulty adjustment mechanism that recalibrates mining difficulty roughly every 2016 blocks, aiming to keep the average interval stable despite changes in total network hash power. It is important to note that this is an average: blocks can be found faster or slower in the short term due to the probabilistic nature of proof-of-work mining. Other blockchains have different block times (seconds to minutes), but the question’s options and typical curriculum context align with Bitcoin’s 10-minute design. Therefore, the correct choice is approximately every 10 minutes.

WEP (Wired Equivalent Privacy) uses the RC4 stream cipher and combines a per-packet Initialization Vector (IV) with a shared secret key to form the RC4 seed for that packet’s keystream. The IV in WEP is 24 bits long and is transmitted in the clear as part of the 802.11 frame so the receiver can reconstruct the same per-packet RC4 key stream. The short IV space (2²⁴ possible values) is a major design weakness: on a busy network, IVs repeat frequently, causing keystream reuse. Because RC4 is a stream cipher, keystream reuse enables attackers to derive relationships between plaintexts and recover keys with statistical attacks (notably the Fluhrer, Mantin, and Shamir (FMS) family of attacks and related improvements). WEP also uses a CRC-32 integrity check (ICV) that is not cryptographically strong and is vulnerable to modification attacks. The 24-bit IV length is therefore a key reason WEP is considered insecure and has been replaced by WPA/WPA2 mechanisms that use stronger key mixing, larger nonces/IVs, and robust integrity protection.

Frequency analysis is a classical cryptanalysis technique that exploits predictable statistical patterns in natural language. In English, certain letters (like E, T, A, O, I, N) occur more frequently than others, and common digrams/trigrams (TH, HE, IN, ER) appear with recognizable distribution. When a cipher preserves character boundaries (as in many substitution ciphers), the ciphertext will also show frequency patterns—though mapped to different symbols. The analyst counts ciphertext character occurrences, compares the distribution to expected English letter frequencies, and infers likely plaintext mappings. “Spotting variations” refers to observing differences in how often symbols appear and using that to plot relationships between ciphertext and standard English. Brute force instead tries all keys; known-plaintext attacks rely on having plaintext–ciphertext pairs; chosen-ciphertext attacks involve decrypting attacker-selected ciphertexts. Those are different attack models. Frequency analysis is specifically about statistical correlation between ciphertext symbols and language characteristics, which is why it is effective against monoalphabetic substitution and weak polyalphabetic schemes with short periods.

A rainbow table attack uses large, precomputed tables that link hash outputs back to likely original inputs (typically passwords). Instead of storing every password→hash pair directly (which would be huge), rainbow tables store chains created by alternating hash operations with reduction functions, allowing attackers to reconstruct candidate plaintexts that produce a given hash. This makes cracking fast , if the target hashes are unsalted and use a known, fast hash function. Salt defeats rainbow tables because the attacker would need separate tables for each salt value, which becomes infeasible when salts are unique and sufficiently large. A dictionary at tack is related but typically computes hashes on the fly from a wordlist rather than using precomputed chain structures. A birthday attack targets collisions, not mapping to original data. Brute-force tries all candidates without precomputation. Because th e question explicitly describes mapping hashed values back to original data via a precomputed approach, the correct choice is Rainbow table.

A true random number generator (TRNG) produces outputs derived from nondeterministic physical processes (e.g., thermal noise, oscillator jitter, radioactive decay, or other hardware entropy sources). Because the underlying phenomenon is not algorithmically determined by an input seed in the same way as a PRNG, repeated “inputs” (or identical conditions from a software perspective) do not yield the same sequence; the outputs vary unpredictably. By contrast, a pseudorandom number generator (PRNG) is deterministic: given the same seed and internal state, it produces the same output sequence, which is useful for repeatability but means security depends on seed secrecy and proper seeding. “Prime” is not a generator type, and “sequence” is too generic and does not imply nondeterminism. In cryptographic systems, TRNGs (or hardware entropy sources) are often used to seed cryptographically secure PRNGs (CSPRNGs), combining high-quality entropy with efficient generation. Therefore, the generator that can produce different results for the “same input data” is a true random number generator.