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)