Contents Introduction Classification of Symmetric Ciphers - - PowerPoint PPT Presentation

Contents

• Introduction
• Classification of Symmetric Ciphers
• Types of Attacks
• Properties of Secure Ciphers
• Components of Block Ciphers
• Classes of Block Ciphers
• DES
• AES

Secure Communication using Cryptography

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Secure Communication using Cryptography

• Same Key for Encryption and Decryption ⇒ Symmetric Key Cipher

Alice Bob C = EK(M) ⇒ M = DK(C)

• Different Keys for Encryption and Decryption ⇒ Asymmetric Key Cipher

Alice Bob C = EKPB(M) ⇒ M = DKSB(C)

Classification of Symmetric Ciphers

• Block Ciphers: Input bits are divided

into fixed sized blocks and each block is encrypted/decrypted separately. Input: 1010010101011100 Block Size: 3

• Stream Ciphers:

Encryption/Decryption is done on one symbol at a time Block Size = 1 Ex: RC4 Input: 1010010101011100…. 101 001 010 101 110 000

Padding

...0011101010100101

Key Stream Generator

...10110111010110001

Key

Common Types of Attacks

• Ciphertext Only Attacks

○ Attacker has: Ciphertext

• Known Plaintext Attacks

○ Attacker has: Ciphertext, known plaintexts and corresponding ciphertexts

• Chosen Plaintext Attacks

○ Attacker has: Ciphertext, chosen plaintexts and corresponding ciphertexts

• Chosen Ciphertext Attacks

○ Attacker has: Ciphertext, chosen ciphertexts and corresponding plaintexts

Properties of Secure Ciphers

• Confusion: The cipher should not reveal any information regarding the

relationship between ciphertext and key

• Diffusion: The cipher should not reveal any information regarding the

relationship between ciphertext and plaintext

Components of Block Ciphers

P-Box

• Performs permutation on input bits
• When input size = output size

1011010 0111001

Reveals the number of 0’s and 1’s

• Can also be used for expansion and compression

Components of Block Ciphers

S-Box

• Performs substitution using table
• Input and output sizes can be different
• Provides non-linearity

00 01 10 11 00 1011 1000 1101 0011 01 1100 1110 0110 0101 10 0010 1001 1010 0100 11 0000 0001 0111 1111 High Low

Classes of Block Ciphers

• Fiestel Ciphers
• Non-Fiestel ciphers

Fiestel Ciphers

• Encryption

Li = Ri-1 Ri = Li-1 ⊕ f(Ri-1, Ki)

• Decryption

Ri-1 = Li Li-1 = Ri ⊕ f(Li , Ki)

. Input R0 L0 Function L1 R1 K e y S c h e d u l e r K1

Fiestel Ciphers

Security Depends On

• Block Size
• Key Size
• Number of Rounds
• Function
• Key Scheduling

DES

• Developed by IBM and NSA in 1970’s
• Adopted as a standard in 1977
• Encryption and Decryption use same algorithm with round keys used in

different order

• Specifications:

○ Input Block Size: 64 bits ○ Key Size: 56 bits ○ Number of Rounds: 16

DES Single Round

DES Substitution

DES: Drawbacks

• Small key size
• Weak Keys

○ Generate same round key for all the rounds ○ There are 4 such keys(00000000000000, FFFFFFFFFFFFFF, 0000000FFFFFFF, FFFFFFF0000000) ○ EK(EK(M))

• Semi-weak Keys

○ Generate only 2 distinct round keys ○ Semi-weak key pairs: Generate same(2) round keys with reverse order ○ EK1(EK2(M)) ○ There are 6 such key pairs

2DES

Encryption Decryption C= EK2(EK1(P)) P= DK1(DK2(C)) Number of possible key combinations: 256*256 Meet-In-the-middle attack Given a pair (P,C)

P EK(P) C1 C DK(C)

3DES(Triple DES)

Encryption Decryption C= EK3(DK2(EK1(P))) P= DK1(EK2((DK3(C)))

• Effective Key Strength: 2112
• Drawback: Too slow

Non-Fiestel Ciphers

• Substitution Permutation Networks
• Use invertible S-box
• Use Different Algorithm for encryption and Decryption
• Decryption uses the round keys in the reverse order

AES

• NIST called for proposals for AES in 1997
• 15 ciphers were submitted
• 5 were shortlisted after the first round
• Finally, Rijndael(developed by Joan Daemen and Vincent Rijmen) was chosen

in 2001

• Provides high security and performance
• Block size is 128 bits
• 3 possible key sizes: 128, 198, 256
• Depending on the key size, 3 possible number of rounds: 10, 12, 14

AES

• Operations are performed on 4×4 bytes matrices
• Round Operations

○ Byte-Substitution: Done using S-Boxes ○ Shift Rows: Simple shift operation ○ Mix Columns: Provides permutation ○ Add Round Key: Performs XOR with the round key

Wrapping-Up

Symmetric Ciphers Asymmetric Ciphers Block Ciphers Stream Ciphers Fiestel Ciphers Ex: DES Non- Fiestel Ciphers Ex: AES

Major Drawback?

Key Distribution

References

1. Menezes, Alfred J., Paul C. Van Oorschot, and Scott A. Vanstone. Handbook

• f applied cryptography. CRC press, 1996.

2. Stallings, William. Cryptography and network security: principles and

• practices. Pearson Education India, 2006.

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