Stream Ciphers 2

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Stream Ciphers

• Introduction to Applied Cryptography
• Instructor Inam Ul Haq
• Inam.bth_at_gmail.com
• BSIT-7th

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Stream Ciphers

• Stream ciphers, like block ciphers, break message
  into fixed length blocks, but use a sequence of
  keys to encrypt the blocks.
• That means, different key for each block of
  plaintext
• Examples Vigenère cipher, RC4, SEAL, SALSA20,
  SOSEMANUK
• Block and Stream are symmetric ciphers

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Formal Definition of a Stream Cipher

• Let E be an encipherment algorithm, and let Ek(b)
  be the encipherment of the message b with key k.
• Let a message mb1b2...where each bi is of a
  fixed length, and let k k1k2....
• A stream cipher is a cipher for which Ek(m) Ek1
  (b1)Ek2(b2)...

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How Stream Cipher Works

• Generate a pseudo-random key stream XOR to the
  plaintext.
• Encrypts bit by bit. Starts from right.
• Uses bitwise XOR operator
• Here A is plaintext, B is key.
• Example. Alice wants to send letter A to Bob.
• ASCII code for A 1000001 and we need 7-bit key
  (lets suppose we have 1101100 key), now lets do
  bit by bit.
• It means its 7-bit stream cipher

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Synchronous Stream Ciphers

• The one-time pad is also a stream cipher, and can
  be implemented by exclusive-oring each bit of the
  key with one bit of the message.
• A synchronous stream cipher generates the key
  string from a source other than the message
  itself.
• A source for the key could come from an randomly
  generated binary sequence.

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Block Ciphers

• Block ciphers break messages into fixed length
  blocks, and encrypt each block using the same
  key.
• The Data Encryption Standard (DES) is an example
  of a block cipher, where blocks of 64 bits are
  encrypted using a 56-bit key.

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Formal Definition of a Block Cipher

• Let E be an encipherment algorithm, and let Ek(b)
  be the encipherment of the message b with key k.
• Let a message mb1b2...where each bi is of a
  fixed length.
• A block cipher is a cipher for which Ek(m)
  Ek(b1)Ek(b2)...

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Self-Synchronous Stream Ciphers

• Self-synchronous stream ciphers obtain a key from
  the message itself.
• An autokey cipher is a self-synchronous cipher
  that uses the message itself as a key.
• Reference Matt Bishop, Computer Security,
  Addison Wesley, 2003.

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Exercise