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 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 SubstitutionPermutation 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 blockoriented 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 streamoriented 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)