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

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Secure Communication using Cryptography

● Same Key for Encryption and Decryption ⇒ Symmetric Key Cipher Alice Bob C = E K (M) ⇒ M = D K (C) ● Different Keys for Encryption and Decryption ⇒ Asymmetric Key Cipher Alice Bob C = E KPB (M) ⇒ M = D KSB (C)

Classification of Symmetric Ciphers ● Stream Ciphers: ● Block Ciphers: Input bits are divided Encryption/Decryption is done on one into fixed sized blocks and each symbol at a time block is encrypted/decrypted Block Size = 1 separately. Ex: RC4 Input: 1010010101011100 Block Size: 3 Input: 1010010101011100…. Padding Key Stream Key Generator 101 001 010 101 110 000 ...0011101010100101 ...10110111010110001

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

Classes of Block Ciphers ● Fiestel Ciphers Non-Fiestel ciphers ●

Fiestel Ciphers ● Encryption Input L i = R i-1 L0 R0 R i = L i-1 ⊕ f (R i-1 , K i ) ● Decryption K 1 Function r e . l u R i-1 = L i d e h c S L i-1 = R i ⊕ f (L i , K i ) y e K L1 R1

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) E K (E K (M)) ○ ● Semi-weak Keys ○ Generate only 2 distinct round keys ○ Semi-weak key pairs: Generate same(2) round keys with reverse order E K1 (E K2 (M)) ○ ○ There are 6 such key pairs

2DES Encryption Decryption Meet-In-the-middle attack C= E K2 (E K1 (P)) P= D K1 (D K2 (C)) Given a pair (P,C) P E K (P) Number of possible key combinations: 2 56 *2 56 C1 D K (C) C

3DES(Triple DES) Encryption Decryption C= E K3 (D K2 (E K1 (P))) P= D K1 (E K2 ((D K3 (C))) Effective Key Strength: 2 112 ● ● 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 Fiestel Ciphers Ex: DES Block Ciphers Symmetric Ciphers Non- Fiestel Ciphers Ex: AES Stream Ciphers Asymmetric Ciphers

Major Drawback? Key Distribution

References 1. Menezes, Alfred J., Paul C. Van Oorschot, and Scott A. Vanstone. Handbook of applied cryptography . CRC press, 1996. 2. Stallings, William. Cryptography and network security: principles and practices . Pearson Education India, 2006.

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