1. 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

2. 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)

3. 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

4. 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

5. 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

6. 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)

7. Block Cipher Modes of Operation Electronic Codebook (ECB)

8. 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

9. Block Cipher Modes of Operation Cipher Block Chaining (CBC)

10. 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

11. Block Cipher Modes of Operation Cipher Feedback (CFB)

12. 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)

13. Block Cipher Modes of Operation Output Feedback (OFB)