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ICS 454: Principles of Cryptography. Block Ciphers & DES Sultan Almuhammadi. Outline. (Stallings 3.1-3.2) Stream cipher vs. Block cipher Motivation Reversible vs. Irreversible mapping Ideal Block Cipher DES. Stream Cipher & Block Cipher.

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ics 454 principles of cryptography
ICS 454:Principles of Cryptography

Block Ciphers & DES

Sultan Almuhammadi

outline
Outline

(Stallings 3.1-3.2)

  • Stream cipher vs. Block cipher
  • Motivation
  • Reversible vs. Irreversible mapping
  • Ideal Block Cipher
  • DES
stream cipher block cipher
Stream Cipher & Block Cipher
  • stream cipher: encrypts data stream one bit or one byte at a time. E.g.:
    • Caesar shift cipher (one letter at time)
    • XOR-scheme (one bit at a time)
  • block cipher: a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length.
    • Block size (typically): 64 or 128 bits
    • e.g. Feistel cipher and DES
motivation
Motivation
  • A block cipher operates on a block of n bits.
  • It produces a ciphertext block of n bits.
  • There are 2n possible different plaintext/ciphertext blocks.
  • The encryption must be reversible. i.e.
    • decryption to be possible.
    • each plaintext must produce a unique ciphertext block. (one-to-one correspondence)
reversible vs irreversible
Reversible Mapping

Plaintext Ciphertext

00 11

01 10

10 00

11 01

Irreversible Mapping

Plaintext Ciphertext

00 11

01 10

10 01

11 01

Reversible vs. Irreversible
problems with ideal cipher
Problems with Ideal Cipher
  • If a small block size, such as n = 4, is used, then the system is equivalent to a classical substitution cipher  Easy attack (statistical analysis of the plaintext)
  • If large block size is usednot practical (for implementation and performance)
    • Huge encryption/decryption tables
    •  Huge key:
      • for n = 4, key size = 4 bits x 16 rows = 64 bits
      • for n = 64, key size = 64 x 264 = 270 = 1021 bits
data encryption standard des
Data Encryption Standard (DES)
  • Widely used encryption scheme.
  • Adopted by National Bureau of Standards in 1977.
  • The algorithm itself is called Data Encryption Algorithm (DEA).
  • Data are encrypted in 64-bit blocks using a 56-bit key.
des decryption
DES Decryption
  • Decryption uses the same algorithm as encryption, except that the application of the subkeys is reversed.