Some Simple Ciphers

• Caesar Cipher
  • Rotation of alphabet (substitution cipher)
  • Caesar used a fixed rotation of 3. (Computer ↔Frpsxwhu)
  • Variant is when this rotation is variable.
• Mono-alphabetic Substitution Cipher
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Breaking a Cipher Scheme

• Various kinds of attacks are possible.
  • Cipher-text only.
    • The attacker has access to cipher-text only but not the plaintext.
  • Known plaintext.
    • The attacker has access to few cipher-text and corresponding plaintext pairs.
  • Chosen plaintext.
    • The attacker can run his plaintext to get the corresponding cipher-text.
  • Chosen Cipher-text.
    • Same as chosen plaintext (but on the decryption algorithm)