Symmetric Encryption

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Symmetric Encryption
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Content

• Block cipher
• DES

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Block cipher

• Definition a block cipher is a function which
  map n-bit plaintext blocks to n-bit ciphertext
• n is called blocklength.
• The function is parameterized by a k-bit key K.
• The function must be one-to-one (invertible)
• n gt 64 (larger than stream cipher)

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Adversaries Assumptions and Attacks

• Adversarys ability
• has access to all data transmitted over the
  ciphertext channel
• knows all details of the encryption function
  except the secret key
• Classses of attack (for a fixed key)
• Ciphertext-only
• Known-plaintext
• Chosen-plaintext (the attacker has the
  capability to choose arbitrary plaintexts to be
  encrypted and obtain the corresponding
  ciphertexts.)
• Adaptive chosen-plaintext (choosing subsequent
  plaintexts based on the information from the
  previous encryptions)

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Broken cipher

• A block cipher is totally broken if a key can be
  found,
• and partially broken if an adversary is able to
  recover part of the plaintext (but not the key)
  from ciphertext.

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Modes of operation

• A block cipher encrypts plaintext in fixed-size
  n-bit blocks (often n 64). For messages
  exceeding n bits, the simplest approach is to
  partition the message into n-bit blocks and
  encrypt each separately.
• electronic codebook (ECB)
• cipher-block chaining (CBC)
• Cipher feedback (CFB)
• output feedback (OFB)

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Multiple encryption
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Classical ciphers

• Transposition ciphers
• CAESAR
• Substitution ciphers
• simple substitution cipher or monoalphabetic
  substitution cipher
• affine cipher
• Polygram substitution
• Polyalphabetic substitutions and Vigenère ciphers
• Polyalphabetic cipher machines and rotors

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Vigenère ciphers

• Key ABCDEF AB CDEFA BCD EFABCDEFABCD
• Plaintext CRYPTO IS SHORT FOR CRYPTOGRAPHY
• Ciphertext CSASXT IT UKSWT GQU GWYQVRKWAQJB
• Key ABCDAB CD ABCDA BCD ABCDABCDABCD
• Plaintext CRYPTO IS SHORT FOR CRYPTOGRAPHY
• Ciphertext CSASTP KV SIQUT GQU CSASTPIUAQJB

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Polyalphabetic cipher machines and rotors
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Cryptanalysis of classical ciphers
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Cryptanalysis of classical ciphers
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Data Encryption Standard (DES)
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Example