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  • Computer and Data Security

    Lecture 4 Block Cipher Operation

  • Modes of Operation

    l  Block cipher: operates on fixed length b-bit input to produce b-bit ciphertext

    l  What about encrypting plaintext longer than b bits? l  Break plaintext into b-bit blocks (padding if

    necessary) and apply cipher on each block l  Security issues arise: different modes of operation

    have been developed

  • ECB Encryption

  • ECB Decryption

  • Summary

    l  Each block of 64 plaintext bits is encoded independently using same key

    l  Typical applications: secure transmission of single values (e.g. encryption key)

    l  Problem: with long message, repetition in plaintext may cause repetition in ciphertext

  • CBC Encryption

  • CBC Decryption

  • CBC Summary

    l  Input to encryption algorithm is XOR of next 64-bits plaintext and preceding 64-bits ciphertext

    l  Typical applications: General-purpose block-oriented transmission; authentication

    l  Initialisation Vector (IV) must be known by sender/ receiver, but secret from attacker

  • CFB Encryption

  • CFB Decryption

  • CFB Summary

    l  Converts block cipher into stream cipher Ø  No need to pad message to integral number of blocks Ø  Operate in real-time: each character encrypted and

    transmitted immediately

    l  Input processed s bits at a time l  Preceding ciphertext used as input to cipher to

    produce pseudo-random output l  XOR output with plaintext to produce ciphertext l  Typical applications: General-purpose stream-

    oriented transmission; authentication

  • OFB Encryption

  • OFB Decryption

  • OFB Summary

    l  Converts block cipher into stream cipher l  Similar to CFB, except input to encryption algorithm is

    preceding encryption output l  Typical applications: stream-oriented transmission over

    noisy channels (e.g. satellite communications) l  Advantage compared to OFB: bit errors do not

    propagate l  Disadvantage: more vulnerable to message stream

    modification attack

  • CTR Encryption

  • CTR Decryption

  • CTR Summary

    l  Converts block cipher into stream cipher l  Each block of plaintext XORed with encrypted

    counter l  Typical applications: General-purpose block-oriented

    transmission; useful for high speed requirements l  Efficient hardware and software implementations l  Simple and secure