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Unit 4 of 4 · B.Sc IT Sem 5

Unit 4: Cryptography

Information Security notes · PTU syllabus (BSIT501)

3 min read9 topics10 exam questions
On this page
  1. Unit summary
  2. Cryptography concepts
  3. Classical techniques: substitution and transposition
  4. Symmetric and asymmetric key cryptography
  5. Steganography
  6. DES: structure and analysis
  7. AES: structure and analysis
  8. Public key cryptosystems: principles
  9. The RSA algorithm
  10. Digital signatures
  11. Key terms
  12. Quick revision
  13. Important questions

Unit summary

Cryptography is the mathematical core of information security. This unit covers cryptography concepts and techniques, symmetric and asymmetric key cryptography, steganography, the DES and AES symmetric ciphers, public key cryptosystem principles, the RSA algorithm and digital signatures.

After this unit you can

  • Explain cryptography concepts and classical techniques
  • Distinguish symmetric and asymmetric cryptography and steganography
  • Explain the structure of DES and AES
  • Apply RSA and explain digital signatures

PTU syllabus topics

  • Cryptography concepts and techniques
  • symmetric and asymmetric key cryptography
  • steganography
  • symmetric key ciphers (DES, AES structure and analysis)
  • asymmetric key ciphers (public key cryptosystem principles, RSA algorithm)
  • digital signatures
ComparisonDES vs AES vs RSA
Type
Key size

DES

Symmetric block cipher

56 bits: now insecure

AES

Symmetric block cipher

128, 192 or 256 bits

RSA

Asymmetric, public key

Typically 2048 bits or more

1

Topic 1

Cryptography concepts

ProcessEncryption model
  1. 1Plaintext

    Readable message

  2. 2Encryption algorithm with key
  3. 3Ciphertext

    Unreadable message sent

  4. 4Decryption algorithm with key
  5. 5Plaintext recovered
Key termsCryptography terms
Cryptography
Science of secret writing
Cryptanalysis
Breaking ciphers without the key
Cryptology
Cryptography and cryptanalysis together
Key
Secret value controlling encryption
Brute-force attack
Trying every possible key
2

Topic 2

Classical techniques: substitution and transposition

ComparisonClassical ciphers
Method
Example

Caesar cipher

Shift each letter by k positions

k = 3: HELLO → KHOOR

Monoalphabetic

Each letter replaced by a fixed other letter

26! possible keys, but frequency analysis breaks it

Playfair

Digrams encrypted using a 5 × 5 key square

Used in World War I

Vigenère (polyalphabetic)

Shift varies with a keyword

Key LEMON

Rail fence (transposition)

Write in zigzag rows, read row by row

HELLOWORLD with 2 rails → HLOOLELWRD

Columnar transposition

Write in rows, read columns in key order

Key 3-1-2

  • Substitution replaces symbols; transposition rearranges them; modern ciphers combine both in many rounds (product ciphers).
3

Topic 3

Symmetric and asymmetric key cryptography

ComparisonSymmetric vs asymmetric encryption
Symmetric (secret key)
Asymmetric (public key)

Keys

One shared secret key

Pair — public key and private key

Speed

Fast

Slower

Key distribution

Difficult — key must be shared securely

Easy — public key can be published

Examples

AES, DES, 3DES

RSA, ECC

Use

Bulk data encryption

Key exchange, digital signatures

  • Hybrid approach (SSL/TLS): asymmetric encryption to exchange a session key, then symmetric encryption for data.
  • Public/private key pair: what one key encrypts only the other can decrypt — encrypt with the receiver's public key for confidentiality; sign with the sender's private key for authentication.
  • Hash functions (SHA-256) produce fixed-length digests for integrity checks; AES is the common symmetric standard; RSA and elliptic-curve cryptography are common asymmetric methods.
4

Topic 4

Steganography

  • Steganography: hiding the existence of a message inside another medium — image, audio, video or text — rather than making it unreadable.
  • Techniques: least significant bit (LSB) substitution in image pixels, invisible ink, hidden text, audio echo hiding; digital watermarking marks ownership.
ComparisonCryptography and steganography
Cryptography
Steganography

Goal

Makes the message unreadable

Hides that a message exists

Visibility

Ciphertext is obvious

Cover file looks normal

Attack

Cryptanalysis

Steganalysis

Best practice

Combine both: encrypt, then hide

Combine both: encrypt, then hide

5

Topic 5

DES: structure and analysis

  • Data Encryption Standard (DES, 1977): a symmetric block cipher based on the Feistel structure; 64-bit block, 56-bit effective key (64 with parity), 16 rounds, 48-bit round keys.
ProcessDES structure
  1. 1Initial permutation of the 64-bit block
  2. 2Split into left and right 32-bit halves
  3. 316 Feistel rounds

    Right half expanded to 48 bits, XORed with round key, passed through 8 S-boxes, permuted, XORed with left half; halves swapped

  4. 432-bit swap
  5. 5Final permutation (inverse of the initial) gives the ciphertext
  • Analysis: the 56-bit key is too short — brute-forced in 1998 (EFF's DES Cracker) in days; avalanche effect is strong; S-boxes resist differential cryptanalysis. Triple DES (encrypt–decrypt–encrypt with two or three keys) extended its life but is now deprecated.
6

Topic 6

AES: structure and analysis

  • Advanced Encryption Standard (AES, 2001): the Rijndael cipher selected by NIST; 128-bit block; key sizes 128, 192 or 256 bits with 10, 12 or 14 rounds; not a Feistel cipher (substitution–permutation network).
ProcessAES round (on a 4 × 4 byte state)
  1. 1SubBytes

    Each byte replaced using the S-box

  2. 2ShiftRows

    Rows shifted cyclically left by 0, 1, 2, 3

  3. 3MixColumns

    Columns mixed by matrix multiplication (omitted in the last round)

  4. 4AddRoundKey

    State XORed with the round key

  • Analysis: no practical attack on full AES is known; fast in hardware and software (AES-NI instructions); used in Wi-Fi WPA2/WPA3, TLS, disk encryption and VPNs.
ComparisonDES and AES
DES
AES

Block size

64 bits

128 bits

Key size

56 bits

128, 192, 256 bits

Rounds

16

10, 12, 14

Structure

Feistel network

Substitution–permutation network

Security today

Insecure

Secure

7

Topic 7

Public key cryptosystems: principles

  • Proposed by Diffie and Hellman (1976). Each user has a public key (published) and a private key (secret); deriving the private key from the public key must be computationally infeasible (based on hard problems — factoring, discrete logarithms).
Key termsUses of public key cryptography
Encryption and decryption
Encrypt with receiver's public key; only the private key decrypts
Digital signature
Sign with sender's private key; anyone verifies with the public key
Key exchange
Agree a shared secret session key (Diffie–Hellman)
8

Topic 8

The RSA algorithm

ProcessRSA key generation, encryption and decryption
  1. 1

    Choose two large primes p and q

  2. 2

    Compute n = p × q and φ(n) = (p − 1)(q − 1)

  3. 3

    Choose e with 1 < e < φ(n) and gcd(e, φ(n)) = 1

  4. 4

    Compute d such that d × e mod φ(n) = 1

  5. 5

    Public key (e, n); private key (d, n)

  6. 6

    Encrypt C = M^e mod n; decrypt M = C^d mod n

Example

p = 3, q = 11 → n = 33, φ(n) = 20. Choose e = 3 (gcd(3, 20) = 1). d = 7, since 3 × 7 = 21 mod 20 = 1. Encrypt M = 4: C = 4³ mod 33 = 64 mod 33 = 31. Decrypt: 31⁷ mod 33 = 4.

  • Security: rests on the difficulty of factoring n; keys of 2048 bits or more are recommended.
9

Topic 9

Digital signatures

  • Digital signature: an electronic signature using asymmetric cryptography and a hash function to authenticate the signer and ensure integrity.
ProcessSigning and verifying
  1. 1Hash the document
  2. 2Encrypt the hash with the signer's private key
  3. 3Send document, signature and certificate
  4. 4Receiver decrypts the signature with the public key
  5. 5Compare with a fresh hash — match means authentic and unaltered
  • Digital Signature Certificate (DSC): classes based on verification level (Class 3 and document signer certificates now in use); used for MCA filings, income-tax returns, GST, e-tenders.
  • Aadhaar e-Sign: online electronic signature using Aadhaar OTP or biometric authentication.
  • Properties: authentication of the signer, integrity of the message, non-repudiation. Standards: RSA signatures, DSA (Digital Signature Standard), ECDSA; legally valid in India under the IT Act, 2000.

Key terms

Ciphertext
Encrypted, unreadable message
Steganography
Hiding the existence of a message
Feistel cipher
Structure splitting blocks into halves processed over rounds
AES
Current symmetric block cipher standard
RSA
Public key algorithm based on factoring large numbers

Quick revision

  • Plaintext, ciphertext, key, cryptanalysis; substitution and transposition ciphers.
  • Symmetric vs asymmetric; hybrid systems; hashing.
  • Steganography vs cryptography.
  • DES: 64-bit block, 56-bit key, 16 rounds; AES: 128-bit block, 10/12/14 rounds.
  • Public key principles; RSA steps; digital signatures.

Important exam questions

Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).

Short-answer questions

  1. Q1.Encrypt "CAB" with the Caesar cipher (k = 3).
  2. Q2.Distinguish substitution and transposition ciphers.
  3. Q3.What is steganography?
  4. Q4.State the block and key sizes of DES.
  5. Q5.Name the four steps of an AES round.
  6. Q6.On what mathematical problem is RSA based?

Long-answer questions

  1. Q1.Explain classical encryption techniques with examples.
  2. Q2.Explain the structure of DES and AES and compare them.
  3. Q3.Explain the principles of public key cryptosystems and the RSA algorithm with an example.
  4. Q4.Explain digital signatures.

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