Stream Ciphers and Block Ciphers •A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream ciphers are the auto keyed Vigenère cipher and the Vernam cipher. • 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.
Stream Ciphers and Block Ciphers. A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream ciphers are the auto keyed Vigenère cipher and the Vernam cipher. - PowerPoint PPT Presentation
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Stream Ciphers and Block Ciphers
• A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream ciphers are the auto keyed Vigenère cipher and the Vernam cipher.
• 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.
Modern Block CiphersModern Block Ciphers
• Will now look at modern block ciphers
• One of the most widely used types of cryptographic algorithms
• Provide secrecy and/or authentication services
• In particular will introduce DES (Data Encryption Standard)
Claude Shannon and Substitution-Claude Shannon and Substitution-Permutation CiphersPermutation Ciphers
• In 1949 Claude Shannon introduced idea of substitution-permutation (S-P) networks– modern substitution-transposition product cipher
• These form the basis of modern block ciphers • S-P networks are based on the two primitive
cryptographic operations we have seen before: – substitution (S-box)– permutation (P-box)
• Provide confusion and diffusion of message
Confusion and DiffusionConfusion and Diffusion
• Cipher needs to completely obscure statistical properties of original message
• A one-time pad does this• More practically Shannon suggested combining
elements to obtain:• Diffusion – dissipates statistical structure of plaintext
over bulk of ciphertext, i.e., each plaintext digit affect the value of many cipher text digits.
• Confusion – makes relationship between ciphertext and key as complex as possible
Block Cipher PrinciplesBlock Cipher Principles
• Most symmetric block ciphers are based on a Feistel Cipher Structure
• Needed since must be able to decrypt ciphertext to recover messages efficiently
• Block ciphers look like an extremely large substitution
• Would need table of 264 entries for a 64-bit block • Using idea of a product cipher
Block Cipher Modes of Operation
• Mode : Electronic Codebook (ECB)• Description : Each block of 64 plaintext bits is
encoded independently using the same key.• Typical Application : Secure transmission of
single values (e.g., an encryption key)
Block Cipher Modes of Operation Electronic Codebook (ECB)
Block Cipher Modes of Operation
• Mode : Cipher Block Chaining (CBC)• Description : The input to the encryption
algorithm is the XOR of the next 64 bits of plaintext and the preceding 64 bits of cipher text.
Block Cipher Modes of Operation Cipher Block Chaining (CBC)
Block Cipher Modes of Operation
• Mode : Cipher Feedback (CFB)• Description : Input is processed j bits at a time.
Preceding cipher text is used as input to the encryption algorithm to produce pseudorandom output, which is XORed with plaintext to produce next unit of cipher text.