Chapter 3

INTRODUCTION TO MODERN SYMMETRIC-KEY CIPHERS

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PREPARED BY:-SNEHA A. PADHIAR

Block Cipher

• A block cipher is one in which a block of plaintext is treated as a whole and used to produce a cipher text block of equal length
• Typically, a block size of 64 or 128 bits(8 or 16 bytes) is used
• Errors In transmitting one block generally do not affect other block
• Each block is enciphered independently, using the same key, identical plaintext block produce identical cipher text block.
• Ex playfair,hill cipher

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example

• Plaintext=227 bytes
• Block size= 16 bytes

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• 227/16
• 14 blocks plus 3 bytes

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• Algorithm encrypts 14 blocks and 3 bytes remain
• So encrypting last 3 bytes data padding is used.

Stream Cipher

• A Stream Cipher is one that encrypts a digital data stream one bit or one byte at a time.
• Examples are autokeyed Vigenere cipher and the Vernam cipher
• For practical reasons, the bit-stream generator must be implemented as an algorithmic procedure, so that the cryptographic bit stream can be produced by both users
• The bit-stream generator is a key controlled algorithm and must produce a bit stream that is cryptographically strong
• Ex ceaser cipher, one time pad, monoalphabetic cipher

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• Stream cipher algorithms are designed to accept a crypto key and a stream of plaintext to produce a stream of ciphertext.
• Stream cipher is similar to a one time pad
• A stream cipher encrypts smaller block of data, typically bits or bytes.
• Key stream is xored with a stream of plaintext bits p1,p2,p3…. To produce the stream of ciphertext bits.
• Ci=pi xor ki
• Pi=ci xor ki

Diffusion and Confusion

• Confusion is making the Output dependent on the key. Ideally every key bit influences every output bit.

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• Diffusion is making output dependent on previous input(Plain/cipher text). Ideally each output bit is influenced by every previous input bit.

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Modern Symmetric Key ciphers

Modern Block Cipher

Components of Modern block Cipher

Straight P-Box (Invertible for decryption)

Modern Stream Cipher

• In a modern stream cipher, encryption and decryption are done r bits at a time. We have a plaintext bit stream P = p_n…p₂ p₁, a ciphertext bit stream �C = c_n…c₂ c₁, and a key bit stream K = k_n…k₂ k₁, in which p_i , c_i , and k_i are r-bit words.

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Synchronous Stream Cipher

Nonsynchronous Stream Cipher