stream ciphers and block ciphers
Post on 31-Dec-2015
101 Views
Preview:
DESCRIPTION
TRANSCRIPT
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.
• Typical Application : General-purpose block-oriented transmission
• Authentication
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.
• Typical Application : General-purpose stream-oriented transmission
• Authentication
Block Cipher Modes of Operation Cipher Feedback (CFB)
Block Cipher Modes of Operation
• Mode : Output Feedback (OFB)• Description : Similar to CFB, except that the
input to the encryption algorithm is the preceding DES output.
• Typical Application : Stream-oriented transmission over noisy channel (e.g., satellite communication)
Block Cipher Modes of Operation Output Feedback (OFB)
top related