Meow's Certification - CISSP Ch6-7: Cryptography, PKI & Applications Q&A

CISSP Ch6–7: Cryptography, PKI & Applications Q&A

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Overview

CISSP exam essentials and practice questions covering Chapters 6 and 7: symmetric and asymmetric cryptographic algorithms, hash functions, PKI, digital signatures, secure protocols (TLS, IPsec, S/MIME, PGP), and cryptographic attacks. Each practice question includes the correct answer with an explanation.

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Chapter 6 — Cryptography and Symmetric Key Algorithms

Exam Essentials 考试要点

CIA Goals in Cryptosystems 密码系统中的CIA目标

Goal	Description
Confidentiality 机密性	Protects the secrecy of data at rest and in transit
Integrity 完整性	Assures the recipient that data was not altered between creation and access
Nonrepudiation 不可否认性	Provides undeniable proof that the sender authored a message; prevents the sender from subsequently denying it

Authentication via Cryptosystems 认证

Authentication provides assurances as to the identity of a user.

The challenge-response protocol asks the remote user to encrypt a message using a key known only to the communicating parties. Authentication can be achieved with both symmetric and asymmetric cryptosystems.

Basic Terminology 基本术语

• Plaintext

  : unencrypted message

• Ciphertext

  : encrypted message produced by applying an algorithm and key to plaintext

• Encryption

  : sender applies algorithm + key to plaintext → ciphertext

• Decryption

  : recipient applies algorithm + key to ciphertext → plaintext

Codes vs. Ciphers 编码与密码

Type	Description
Codes	Operate on words or phrases; sometimes secret but don’t always provide confidentiality
Ciphers	Always meant to hide the true meaning of a message

Cipher types 密码类型:

Cipher	How it works
Transposition cipher	Rearranges the location of characters within a message
Substitution cipher	Replaces characters with other characters (e.g., Caesar cipher)
One-time pad	Substitution cipher using a random key as long as the message — unbreakable when used correctly
Stream cipher	Operates on individual characters or bits
Block cipher	Operates on large fixed-size blocks of data
Running key cipher	Uses a passage from a well-known book as the encryption key

One-Time Pad Requirements 一次性密码本要求

For a one-time pad to be unbreakable, all four conditions must hold:

1. The key must be generated randomly without any known pattern.
2. The key must be at least as long as the message to be encrypted.
3. The pad must be protected against physical disclosure.
4. Each pad must be used only once and then discarded.

VENONA failure: Soviet cryptanalysts in the 1940s reused one-time pad keys — violating rule 4 — allowing US cryptanalysts to break the cipher.

Advanced Concepts 高级概念

Concept	Description
Zero-knowledge proof	A specific type of information is exchanged but no real data is transferred — as with digital signatures and digital certificates
Split knowledge	Information required to perform an operation is divided among multiple users so that no single person can compromise security alone
M of N Control	A minimum of M agents out of N total agents must cooperate to perform high-security tasks — an example of split knowledge
Work function / Work factor	Measures the strength of a cryptosystem by the effort (cost and time) required to perform a complete brute-force attack; security is directly proportional to work function value

Key Length and Security 密钥长度与安全性

• Modern cryptosystems use keys of at least 128 bits for adequate security.
• DES 56-bit key is no longer considered secure

  — insufficient against modern brute-force attacks.

• Key space for an n-bit key = 2^n possible keys.

Symmetric vs. Asymmetric Cryptosystems 对称与非对称密码系统

Property	Symmetric (Secret Key)	Asymmetric (Public Key)
Keys	Single shared secret key	Public-private key pair
Speed	Much faster	Much slower
Key distribution	Difficult	Easy — public key freely shared
Scalability	Poor — n(n-1)/2 keys for n users	Good — 2n keys for n users
Nonrepudiation	Not supported	Supported

Key count formula 密钥数量公式:

• Symmetric: n(n-1)/2 keys for n participants (e.g., 10 users → 45 keys)
• Asymmetric: 2n keys for n participants (e.g., 10 users → 20 keys)

Data Encryption Standard (DES) DES加密标准

• Key length: 56 bits (no longer secure)
• Block size: 64 bits

DES operating modes DES工作模式:

Mode	Notes
ECB (Electronic Codebook)	Least secure; identical plaintext blocks produce identical ciphertext blocks; suitable only for short messages
CBC (Cipher Block Chaining)	Each block XORed with the previous ciphertext block; an early error propagates through all subsequent blocks
CFB (Cipher Feedback)	Stream cipher mode; an error propagates to subsequent blocks
OFB (Output Feedback)	Stream cipher mode; errors do not propagate — safe for large messages where error isolation is needed

Triple DES (3DES)

• Uses three iterations of DES with two or three different keys.
• Effective key strength: 112 bits (2-key) or 168 bits (3-key).

Advanced Encryption Standard (AES) AES高级加密标准

• Algorithm: Rijndael
• US government standard for sensitive but unclassified data.
• Key lengths: 128, 192, or 256 bits
• Block size: 128 bits (fixed, despite Rijndael supporting variable block sizes)

AES finalists AES候选算法:

Algorithm	Notable feature
Rijndael	Selected as AES
Twofish	Uses prewhitening and postwhitening techniques
Blowfish	Earlier Schneier algorithm; variable key length
Skipjack	NSA-designed; 80-bit key; used in Clipper chip

Frequency Analysis Attack 频率分析攻击

Frequency analysis

exploits the fact that letters occur with predictable frequency in natural language. It makes simple substitution ciphers (such as the Caesar cipher) virtually unusable.

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Chapter 6 Practice Questions 练习题

Q1. How many possible keys exist in a 4-bit key space?

C. 16 — 2^4 = 16 possible key values.

Q2. What cryptographic goal convinces John that Bill was actually the sender of a message?

A. Nonrepudiation — provides undeniable proof of message authorship.

Q3. What is the key length of DES?

A. 56 bits

Q4. What type of cipher changes the location of characters to achieve confidentiality?

B. Transposition cipher

Q5. Which is NOT a valid AES key length?

A. 56 bits — Rijndael supports 128, 192, and 256 bits only.

Q6. Which cannot be achieved by a secret key (symmetric) cryptosystem?

A. Nonrepudiation — nonrepudiation requires a public key cryptosystem; symmetric keys are shared and cannot prove sole authorship.

Q7. When correctly implemented, what is the only unbreakable cryptosystem?

D. One-time pad — mathematically unbreakable when all four requirements are met.

Q8. What is the output of 16 mod 3?

B. 1 — 16 = 5×3 + 1, so 16 mod 3 = 1.

Q9. What rule did the Soviets break that caused VENONA to succeed?

C. Key values must be used only once — the Soviets reused one-time pad keys, breaking the fundamental requirement.

Q10. Which cipher type operates on large pieces of a message rather than individual characters or bits?

C. Block cipher

Q11. What is the minimum number of keys required for secure two-way communication in symmetric cryptography?

A. One — both parties share a single symmetric key.

Q12. Dave’s escrow system requires multiple people but not all participants. What technique is he using?

B. M of N Control — requires a minimum of M agents (out of N total) to cooperate; not all participants must be present.

Q13. Which DES mode can be used for large messages with assurance that an early error won’t spoil the entire communication?

D. Output Feedback (OFB) — errors do not propagate in OFB mode; CBC and CFB both propagate errors.

Q14. Many algorithms rely on the difficulty of factoring large prime products. What characteristic are they relying on?

C. It is a one-way function — a mathematical operation that is easy to compute in one direction but computationally infeasible to reverse.

Q15. How many keys are required for a symmetric algorithm with 10 participants?

C. 45 — n(n-1)/2 = 10×9/2 = 45.

Q16. What block size does AES use?

C. 128 bits — AES uses a fixed 128-bit block size, even though Rijndael supports variable block sizes.

Q17. What attack makes the Caesar cipher virtually unusable?

C. Frequency analysis attack — exploits predictable letter frequencies in natural language.

Q18. What type of cryptosystem uses a passage from a well-known book as the key?

B. Running key cipher

Q19. Which AES finalist uses prewhitening and postwhitening?

B. Twofish

Q20. How many encryption keys are required for an asymmetric algorithm with 10 participants?

B. 20 — asymmetric requires 2n keys (one public + one private per participant).

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Chapter 7 — PKI and Cryptographic Applications

Exam Essentials 考试要点

Asymmetric Key Usage Rules 非对称密钥使用规则

Operation	Key to use
Encrypt a message to recipient	Recipient’s public key
Decrypt a received message	Own private key
Sign a message (digital signature)	Own private key
Verify a signature	Sender’s public key

Major Public Key Cryptosystems 主要公钥密码系统

Algorithm	Basis	Notes
RSA	Difficulty of factoring the product of large prime numbers	Most famous; invented by Rivest, Shamir, Adleman in 1977
El Gamal	Modular arithmetic (extension of Diffie-Hellman)	Ciphertext is twice the length of plaintext
ECC (Elliptic Curve)	Elliptic curve discrete logarithm problem	Provides more security per bit than RSA; 160-bit ECC ≈ 1,024-bit RSA

Hash Function Requirements 哈希函数要求

A good hash function must:

1. Accept input of any length
2. Produce fixed-length output
3. Be easy to compute for any input
4. Provide one-way functionality (computationally infeasible to reverse)
5. Be collision-free (infeasible to find two inputs with the same hash)

Major Hashing Algorithms 主要哈希算法

Algorithm	Output size	Notes
SHA-1	160 bits	Government standard; now considered weak
SHA-2	224–512 bits (variable)	Current standard; includes SHA-256, SHA-512
SHA-3	Variable	Next generation; different internal design from SHA-2
MD5	128 bits	Considered broken; collision-vulnerable

Cryptographic Salts 加密盐值

Rainbow table attacks

use precomputed hash values to identify commonly used passwords. Adding a salt (a random value appended to the password before hashing) makes each hash unique and defeats rainbow table lookups.

Digital Signatures 数字签名

To sign 签名过程:

1. Apply a hash function to the message → produce a message digest.
2. Encrypt the digest with the sender’s private key → the digital signature.

To verify 验证过程:

1. Decrypt the signature with the sender’s public key → obtain the message digest.
2. Independently compute the hash of the received message.
3. If the two digests match → the message is authentic and unmodified.

Digital Signature Standard (DSS)

DSS uses SHA-1 or SHA-2 message digest functions combined with one of three signature algorithms:

Algorithm	Notes
DSA (Digital Signature Algorithm)	Original DSS algorithm
RSA	Widely used alternative
ECDSA (Elliptic Curve DSA)	ECC-based; efficient for constrained devices

Note: El Gamal DSA is not part of the DSS standard.

Public Key Infrastructure (PKI)

• Certificate Authorities (CAs)

  generate digital certificates containing users’ public keys.

• Users distribute certificates to communicating parties.
• Recipients verify a certificate using the CA’s public key.
• Standard: ITU X.509 governs the format and endorsement of digital certificates.
• Key escrow

  stores a copy of private keys; used for recovery if the original private key is lost.

• Certificate Revocation Lists (CRLs)

  have a key disadvantage: latency — time between revocation and distribution of the updated CRL leaves a window of exposure.

Secure Email 安全电子邮件

Protocol	Notes
S/MIME	Emerging standard; built into most modern email clients; uses RSA and X.509 certificates
PGP (Pretty Good Privacy)	Phil Zimmerman’s tool; commercial version uses IDEA for encryption

Secure Web Activity 安全网络通信

• TLS (Transport Layer Security)

  is the de facto standard for HTTPS (port 443).

• SSL

  is the older predecessor — many sites are dropping SSL support due to security concerns.

• WEP

  is considered flawed and should no longer be used.

• WPA

  uses TKIP for encryption; WPA2 uses AES.

• WPA protects the client to wireless access point link only.

IPsec 网络安全协议

IPsec

is a security architecture framework for secure communication over IP. It defines a framework for setting up secure communication channels.

Mode	Encryption scope	Use case
Transport mode	Packet payload only	Peer-to-peer communication
Tunnel mode	Entire packet (header + payload)	Gateway-to-gateway VPN

IPsec protocols:

Protocol	Function
AH (Authentication Header)	Provides authentication and integrity; no encryption
ESP (Encapsulating Security Payload)	Provides authentication, integrity, and encryption

Cryptographic Attacks 密码攻击

Attack	Description
Brute-force	Exhaustively tries all possible keys
Known plaintext	Attacker has both plaintext and corresponding ciphertext samples
Chosen ciphertext	Attacker can choose ciphertexts and obtain their decryptions
Chosen plaintext	Attacker can choose plaintexts and obtain their encryptions
Meet-in-the-middle	Exploits protocols using two rounds of encryption; why Double DES (2DES) is no more effective than single DES
Man-in-the-middle	Fools both parties into communicating through the attacker
Birthday attack	Attempts to find collisions in hash functions
Replay attack	Reuses previously captured authentication requests

Digital Rights Management (DRM)

DRM solutions

allow content owners to enforce usage restrictions on their content (music, movies, e-books, enterprise documents). They control how content can be copied, redistributed, or accessed.

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Chapter 7 Practice Questions 练习题

Q1. In the RSA public key cryptosystem, which number is always largest?

B. n — n is the product of two large prime numbers p and q; n is always larger than either p or q individually.

Q2. Which cryptographic algorithm forms the basis of El Gamal?

B. Diffie-Hellman — El Gamal is an extension of the Diffie-Hellman key exchange algorithm.

Q3. Richard wants to send an encrypted message to Sue. Which key does he use to encrypt?

C. Sue’s public key — to encrypt a message for a recipient, use the recipient’s public key. If Richard used his own private key, any user could decrypt it with Richard’s freely available public key.

Q4. A 2,048-bit plaintext message encrypted with El Gamal — how long is the resulting ciphertext?

C. 4,096 bits — El Gamal ciphertext is twice the length of the plaintext.

Q5. A company using 1,024-bit RSA wants to convert to ECC with equivalent strength. What ECC key length?

A. 160 bits — a 1,024-bit RSA key is cryptographically equivalent to a 160-bit ECC key; ECC provides more security per bit.

Q6. John hashes a 2,048-byte message using SHA-1. What size is the message digest?

A. 160 bits — SHA-1 always produces a 160-bit digest regardless of input size.

Q7. Which technology is considered flawed and should no longer be used?

C. WEP — Wired Equivalent Privacy has known cryptographic weaknesses and is deprecated.

Q8. What encryption does WPA use?

A. TKIP — WPA uses TKIP; WPA2 uses AES.

Q9. Richard received an encrypted message from Sue. Which key decrypts it?

B. Richard’s private key — messages encrypted with the recipient’s public key are decrypted with the recipient’s private key.

Q10. Richard wants to digitally sign a message to Sue. Which key encrypts the digest?

B. Richard’s private key — signing uses the sender’s own private key; Sue verifies using Richard’s public key.

Q11. Which algorithm is NOT supported by the Digital Signature Standard?

C. El Gamal DSA — DSS supports DSA, RSA, and ECDSA only.

Q12. Which ITU standard governs digital certificates for secure electronic communication?

B. X.509

Q13. Which cryptosystem provides encryption for the commercial version of PGP?

B. IDEA — the commercial version of PGP uses the IDEA cipher.

Q14. What TCP/IP port is used by TLS traffic?

C. 443 — HTTPS/TLS uses port 443.

Q15. What attack rendered Double DES (2DES) no more effective than single DES?

C. Meet-in-the-middle attack — the attacker can attack the two encryption layers from both ends simultaneously, reducing the effective security.

Q16. Which tool improves the effectiveness of a brute-force password cracking attack?

A. Rainbow tables — precomputed hash-to-plaintext lookups accelerate cracking of unsalted password hashes.

Q17. WPA encryption protects which link?

C. Client to wireless access point — WPA encrypts the wireless segment between the client device and the access point.

Q18. What is the major disadvantage of certificate revocation lists (CRLs)?

B. Latency — time between certificate revocation and distribution of the updated CRL creates a window where revoked certificates may still be trusted.

Q19. Which encryption algorithm is now considered insecure?

D. Merkle-Hellman Knapsack — broken in 1982; no longer considered secure.

Q20. What does IPsec define?

B. A framework for setting up a secure communication channel — IPsec specifies the architecture and protocols for establishing secure IP communications.

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Key Takeaways

• CIA in cryptography

  : Confidentiality (secrecy), Integrity (unmodified), Nonrepudiation (proven authorship) — nonrepudiation requires asymmetric crypto

• One-time pad

  : the only mathematically unbreakable cipher — requires random key, key ≥ message length, single use, physical protection

• Key counts

  : symmetric needs n(n-1)/2 keys; asymmetric needs 2n keys — asymmetric scales far better

• DES (56-bit) is broken

  ; AES (Rijndael) with 128/192/256-bit keys and 128-bit block size is the current standard

• OFB mode

  does not propagate errors; CBC and CFB do — choose OFB for large messages requiring error isolation

• Asymmetric key rules

  : encrypt → recipient’s public key; decrypt → own private key; sign → own private key; verify → sender’s public key

• ECC efficiency

  : 160-bit ECC ≈ 1,024-bit RSA in security strength

• El Gamal

  doubles ciphertext length; extends Diffie-Hellman; forms basis of PGP commercial encryption (IDEA)

• DSS

  supports DSA, RSA, and ECDSA — El Gamal is NOT part of DSS

• X.509

  is the ITU standard for digital certificates; CRL latency is the key weakness

• IPsec

  : transport mode encrypts payload; tunnel mode encrypts entire packet — uses AH (auth only) and ESP (auth + encryption)

• Meet-in-the-middle attack

  breaks Double DES — made 2DES no stronger than single DES

• WEP is broken

  ; WPA uses TKIP; WPA2 uses AES

• Rainbow tables

  accelerate brute-force on unsalted hashes — salting defeats them

• Work function

  measures cryptographic strength — security is proportional to the cost/time of brute-force attack

References

• CISSP Study Guide — Chapter 6: Cryptography and Symmetric Key Algorithms
• CISSP Study Guide — Chapter 7: PKI and Cryptographic Applications
• NIST FIPS 197 — Advanced Encryption Standard (AES)
• NIST FIPS 186 — Digital Signature Standard (DSS)
• ITU-T X.509 — Digital Certificate Standard
• RFC 4301 — Security Architecture for IPsec