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.

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Presentation 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.stream 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.block cipher

Modern 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- Permutation 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 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 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)