CYS406 · Chapter 3 · Annotated SlidesCryptography Basics: DES and AES
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Cryptography Basics: DES and AES

Cryptography and encryption, symmetric vs. asymmetric encryption, government access to keys, ciphers (block and stream), DES and its Feistel structure, and AES (Rijndael): key expansion and the four round transformations.

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Every original slide is shown unchanged. Yellow marks on a slide point to a numbered note: hover or tap a mark to jump to its note, and hover a note to see its marks. Questions tagged Past exam appeared on a real paper; Prediction questions are likely angles, not confirmed. Pick an answer to each question: a wrong pick tells you why it is wrong, so you can try again.

All 38 slides
  1. 1 Slide 1
  2. 2 Module Objectives
  3. 3 Module Flow
  4. 4 Cryptography
  5. 5 Encryption
  6. 6 Symmetric Encryption
  7. 7 Asymmetric Encryption
  8. 8 Government Access to Keys (GAK)
  9. 9 Module Flow
  10. 10 Ciphers are algorithms used to encrypt or decrypt the data
  11. 11 Secret Key algorithms
  12. 12 Secret Key algorithms ● Block ciphers (e.g., DES, AES)
  13. 13 Data Encryption
  14. 14 Slide 14
  15. 15 Slide 15
  16. 16 Slide 16
  17. 17 Slide 17
  18. 18 Slide 18
  19. 19 Slide 19
  20. 20 Advanced Encryption Standard (AES)
  21. 21 2. The Advanced Encryption Standard (AES)
  22. 22 2.The AES Cipher - Rijndael
  23. 23 AES Structure
  24. 24 Slide 24
  25. 25 AES Structure-State
  26. 26 AES Structure- Key Expansion
  27. 27 AES Structure- Key Expansion
  28. 28 AES Structure- Round
  29. 29 AES Structure- SubBytes
  30. 30 AES Structure- SubBytes
  31. 31 AES Structure- Shift Rows
  32. 32 AES Structure-Shift Rows
  33. 33 AES Structure- Mix Columns
  34. 34 AES Structure- Add Round Key
  35. 35 AES Structure- Add Round Key
  36. 36 Slide 36
  37. 37 AES Summary
  38. 38 Module Summary
Slide 1 / 38

Slide 1

Slide 1: Slide 1
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Module Objectives

Slide 2: Module Objectives
Slide 3 / 38

Module Flow

Slide 3: Module Flow
Slide 4 / 38

Cryptography

Slide 4: Cryptography
Slide 5 / 38

Encryption

Slide 5: Encryption
Slide 6 / 38

Symmetric Encryption

Slide 6: Symmetric Encryption

Notes

  1. Symmetric: ONE shared secret key for encryption and decryption. Fast; the problem is key distribution.

Exam angle

PredictionQ1

How many keys do n users need for pairwise symmetric encryption?

Slide 7 / 38

Asymmetric Encryption

Slide 7: Asymmetric Encryption

Notes

  1. Asymmetric: a public key and a private key. Slower; solves key distribution and enables digital signatures.

Slide 8 / 38

Government Access to Keys (GAK)

Slide 8: Government Access to Keys (GAK)
Slide 9 / 38

Module Flow

Slide 9: Module Flow
Slide 10 / 38

Ciphers are algorithms used to encrypt or decrypt the data

Slide 10: Ciphers are algorithms used to encrypt or decrypt the data

Notes

  1. Block ciphers encrypt fixed-size blocks (DES 64-bit, AES 128-bit); stream ciphers encrypt bit by bit (RC4).

Exam angle

PredictionQ1

AES encrypts blocks of:

Slide 11 / 38

Secret Key algorithms

Slide 11: Secret Key algorithms
Slide 12 / 38

Secret Key algorithms ● Block ciphers (e.g., DES, AES)

Slide 12: Secret Key algorithms ● Block ciphers (e.g., DES, AES)
Slide 13 / 38

Data Encryption

Slide 13: Data Encryption

Notes

  1. DES: 64-bit blocks, 56-bit effective key, 16 Feistel rounds. Broken by brute force; 3DES applies it three times.

Exam angle

PredictionQ1

DES’s effective key length is:

Slide 14 / 38

Slide 14

Slide 14: Slide 14
Slide 15 / 38

Slide 15

Slide 15: Slide 15
Slide 16 / 38

Slide 16

Slide 16: Slide 16
Slide 17 / 38

Slide 17

Slide 17: Slide 17
Slide 18 / 38

Slide 18

Slide 18: Slide 18
Slide 19 / 38

Slide 19

Slide 19: Slide 19
Slide 20 / 38

Advanced Encryption Standard (AES)

Slide 20: Advanced Encryption Standard (AES)

Notes

  1. AES (Rijndael) replaced DES: 128-bit block; 128/192/256-bit keys with 10/12/14 rounds.

Exam angle

PredictionQ1

AES-256 uses how many rounds?

Slide 21 / 38

2. The Advanced Encryption Standard (AES)

Slide 21: 2. The Advanced Encryption Standard (AES)
Slide 22 / 38

2.The AES Cipher - Rijndael

Slide 22: 2.The AES Cipher - Rijndael
Slide 23 / 38

AES Structure

Slide 23: AES Structure
Slide 24 / 38

Slide 24

Slide 24: Slide 24
Slide 25 / 38

AES Structure-State

Slide 25: AES Structure-State
Slide 26 / 38

AES Structure- Key Expansion

Slide 26: AES Structure- Key Expansion
Slide 27 / 38

AES Structure- Key Expansion

Slide 27: AES Structure- Key Expansion
Slide 28 / 38

AES Structure- Round

Slide 28: AES Structure- Round

Notes

  1. Each round: SubBytes (S-box substitution), ShiftRows (permutation), MixColumns (mixing), AddRoundKey (XOR with the round key). The last round skips MixColumns.

  2. Memory trick Sub, Shift, Mix, Add.

Exam angle

PredictionQ1

Which AES step is omitted in the final round?

Slide 29 / 38

AES Structure- SubBytes

Slide 29: AES Structure- SubBytes
Slide 30 / 38

AES Structure- SubBytes

Slide 30: AES Structure- SubBytes
Slide 31 / 38

AES Structure- Shift Rows

Slide 31: AES Structure- Shift Rows
Slide 32 / 38

AES Structure-Shift Rows

Slide 32: AES Structure-Shift Rows
Slide 33 / 38

AES Structure- Mix Columns

Slide 33: AES Structure- Mix Columns
Slide 34 / 38

AES Structure- Add Round Key

Slide 34: AES Structure- Add Round Key
Slide 35 / 38

AES Structure- Add Round Key

Slide 35: AES Structure- Add Round Key
Slide 36 / 38

Slide 36

Slide 36: Slide 36
Slide 37 / 38

AES Summary

Slide 37: AES Summary

Notes

  1. AES is not a Feistel cipher: every round processes the whole block.

Slide 38 / 38

Module Summary

Slide 38: Module Summary