Cybersecurity Cryptography Syllabus

Cryptography syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Cryptography in Cybersecurity, not a summary of it.

1. 1

   Cryptography Foundations

   4 topics

   • History and Goals of Cryptography
   • Encryption, Decryption, and Keys
   • Substitution and Transposition Ciphers
   • Kerckhoffs's Principle
2. 2

   Symmetric Cryptography

   4 topics

   • Block vs Stream Ciphers
   • DES and 3DES
   • AES
   • Modes of Operation
3. 3

   Asymmetric Cryptography

   4 topics

   • Public and Private Key Pairs
   • RSA
   • Diffie-Hellman Key Exchange
   • Elliptic Curve Cryptography (ECC)
4. 4

   Hashing and Integrity

   4 topics

   • Hash Functions
   • Message Authentication Codes (HMAC)
   • Salting, Peppering, and Key Stretching
   • Collision and Birthday Attacks
5. 5

   Public Key Infrastructure (PKI)

   4 topics

   • Digital Certificates and X.509
   • Certificate Authorities and Chain of Trust
   • Digital Signatures
   • Certificate Revocation
6. 6

   Applied Cryptography and Attacks

   4 topics

   • TLS/SSL Handshake
   • Cryptographic Attacks
   • Steganography and Obfuscation
   • Post-Quantum and Quantum Cryptography

Cryptography flashcards for Cybersecurity

25 of 51 cards from the Cryptography deck — real questions with worked answers.

1. What is the primary goal of cryptography, and what are the four core security services it aims to provide?

   Cryptography secures communication in the presence of adversaries. Its four core goals are: confidentiality (secrecy of data), integrity (data not altered), authentication (verifying identity/origin), and non-repudiation (sender cannot deny having sent the message).

2. Historically, what is the difference between a code and a cipher?

   A code replaces whole words or phrases with other words, numbers, or symbols using a codebook. A cipher operates at the level of individual letters or bits, transforming them via an algorithm and key regardless of meaning.

3. Define cryptography, cryptanalysis, and cryptology.

   Cryptography is the science of designing secure ciphers/schemes. Cryptanalysis is the science of breaking them (recovering plaintext or keys without authorization). Cryptology is the overarching field encompassing both.

4. In cryptographic terms, what are plaintext, ciphertext, encryption, and decryption?

   Plaintext is the original readable message. Ciphertext is the scrambled, unreadable output. Encryption is the process $E_k(P) = C$ converting plaintext to ciphertext with key $k$. Decryption is the reverse $D_k(C) = P$.

5. What is a cryptographic key and why is it essential to a cipher?

   A key is a secret parameter that controls the encryption/decryption transformation. It is essential because the algorithm is assumed public; the key alone provides security, so only key holders can decrypt.

6. State Kerckhoffs's Principle.

   A cryptosystem should remain secure even if everything about it, except the key, is public knowledge. Security must depend solely on the secrecy of the key, not the obscurity of the algorithm.

7. What modern maxim, attributed to Claude Shannon, restates Kerckhoffs's Principle?

   "The enemy knows the system" (Shannon's maxim). One should design assuming the adversary has full knowledge of the algorithm, so only the key is secret.

8. Why is "security through obscurity" considered a weak approach?

   It relies on keeping the algorithm secret rather than the key. Once the algorithm leaks or is reverse-engineered, the system is broken. It cannot be publicly reviewed for flaws and violates Kerckhoffs's Principle.

9. What is a substitution cipher? Give the classic example.

   A substitution cipher replaces each plaintext unit (usually a letter) with another symbol according to a fixed mapping. The classic example is the Caesar cipher, which shifts each letter by a fixed amount, e.g. $C = (P + 3) \bmod 26$.

10. What is a transposition cipher, and how does it differ from a substitution cipher?

    A transposition cipher rearranges the positions of the plaintext characters (permutation) without changing the characters themselves. A substitution cipher changes the characters but keeps their positions.

11. How can a simple monoalphabetic substitution cipher be broken?

    By frequency analysis: the ciphertext preserves the language's letter-frequency distribution, so matching the most common ciphertext letters to common plaintext letters (e.g. 'e', 't', 'a' in English) reveals the mapping.

12. What is the Vigenère cipher and why is it stronger than a Caesar cipher?

    The Vigenère cipher is a polyalphabetic substitution using a repeating keyword to apply different Caesar shifts to successive letters: $C_i = (P_i + K_{i \bmod m}) \bmod 26$. Multiple shifts flatten single-letter frequencies, resisting simple frequency analysis.

13. What is a one-time pad, and what conditions make it perfectly secure (unbreakable)?

    A one-time pad XORs plaintext with a truly random key of equal length. It achieves perfect secrecy (Shannon) if the key is truly random, at least as long as the message, never reused, and kept secret.

14. Distinguish symmetric-key from asymmetric-key cryptography.

    Symmetric (secret-key) cryptography uses the same shared key for encryption and decryption (e.g. AES). Asymmetric (public-key) cryptography uses a mathematically linked key pair: a public key and a private key (e.g. RSA), where one encrypts and the other decrypts.

15. What is the key-distribution problem, and which type of cryptography solves it?

    The key-distribution problem is securely sharing a secret key over an insecure channel. Asymmetric (public-key) cryptography solves it: the public key can be freely distributed, so no secret needs to be pre-shared.

16. Compare block ciphers and stream ciphers.

    A block cipher encrypts fixed-size blocks of bits (e.g. 128 bits) at a time, often needing padding and a mode of operation (e.g. AES, DES). A stream cipher encrypts one bit or byte at a time by XOR-ing plaintext with a pseudorandom keystream (e.g. RC4, ChaCha20).

17. For a block cipher, what is a block and what is padding?

    A block is the fixed number of bits the cipher processes at once (e.g. 64 or 128 bits). Padding adds extra bytes to the final plaintext block so its length is an exact multiple of the block size (e.g. PKCS#7).

18. Describe the DES algorithm's key parameters and structure.

    DES (Data Encryption Standard) is a block cipher with a 64-bit block and a 56-bit effective key (64 bits with 8 parity bits). It uses a 16-round Feistel network.

19. Why is DES considered insecure today?

    Its 56-bit key gives only $2^{56}$ possible keys, which is brute-forceable with modern hardware (demonstrated in hours/days). The small key space is the fatal weakness.

20. What is 3DES (Triple DES) and how does it work? Give its effective security.

    3DES applies DES three times with the EDE scheme: $C = E_{k_3}(D_{k_2}(E_{k_1}(P)))$. With three independent keys it has a 168-bit key length but only about 112 bits of effective security due to meet-in-the-middle attacks. Using $k_1=k_3$ gives two-key 3DES.

21. Why does 3DES use Encrypt-Decrypt-Encrypt (EDE) rather than Encrypt-Encrypt-Encrypt?

    EDE provides backward compatibility: setting all three keys equal ($k_1=k_2=k_3$) makes 3DES reduce to single DES ($E(D(E(P))) = E(P)$), so it can interoperate with legacy DES systems.

22. What is AES, and what block/key sizes does it support?

    AES (Advanced Encryption Standard) is the current symmetric block-cipher standard (Rijndael). It has a fixed 128-bit block and supports key sizes of 128, 192, or 256 bits, using 10, 12, or 14 rounds respectively.

23. AES is based on which cipher structure, and name its main round operations.

    AES uses a substitution-permutation network (SPN), not a Feistel network. Each round performs SubBytes (S-box substitution), ShiftRows (row permutation), MixColumns (column mixing), and AddRoundKey (XOR with round key); the final round omits MixColumns.

24. Why do block ciphers need a "mode of operation"?

    A block cipher only encrypts a single fixed-size block. A mode of operation defines how to securely encrypt messages longer than one block, chaining or combining block operations while preventing patterns and enabling features like randomization.

25. What is ECB mode and why is it insecure?

    Electronic Codebook (ECB) mode encrypts each block independently with the same key. It is insecure because identical plaintext blocks yield identical ciphertext blocks, leaking patterns (e.g. the famous encrypted-penguin image).

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