Block Cipher

• Encrypts and Decrypts messages in fixed-length groups of bits (blocks).
• Requires Block Modes and Padding to handle messages that don’t fit perfectly into fixed blocks.
• Examples
  • DES, 3DES
  • AES (Advanced Encryption Standard, NIST 2001)
  • Serpent, Twofish
  • Ascon (Lightweight, e.g.: for IoT)
• Padding is used to fill empty space in a block (e.g., PKCS#7 / RFC5652).
• Modes of Operation:
  • ECB (bad, because ECB encrypts identical plaintext blocks to identical ciphertext blocks, so repeated plaintext patterns remain visible in the ciphertext. This leaks structure (e.g., image outlines, repeated fields).)
  • CBC
  • CTR
  • etc.
• Block Length
  • AES: Uses a 128-bit block length.
  • 3DES: Uses a 64-bit block length
    • which is considered vulnerable to the Sweet32 Attack (2016).

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Relevant Note(s): Stream Cipher