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The trivial mode of operation, ECB (Electronic Code Book), is insecure, as it maps repeated plaintext block always to the same ciphertext block.

Better encryption modes include:

  • CBC (Cipher-Block Chaining, which feeds the previous ciphertext block back into next plaintext),
  • PCBC (Propagating Cipher-Block Chaining, which feeds previous ciphertext and plaintext blocks back into the next plaintext),
  • CFB (Cipher Feedback, which feeds the previous ciphertext block into the cipher, then combines with the plaintext),
  • OFB (Output Feedback, which feeds the output of the cipher as the input for the next block, before combining with plaintext)
  • CTR (Counter, which encrypts a nounce+counter and combines the result with the plaintext)

There are also some modes which support authenticated encryption (AE), or authenticated encryption with associated data (AEAD):

  • OCB (offset codebook - adds a counter-like value to each plaintext block before and after encryption, and encrypts a checksum of the plaintext for authentication)
  • CCM (Counter with CBC-MAC, combines CTR mode with a CBC-bases MAC)
  • EAX (which combines CTR mode with OMAC for authentication and creation of an initialization vector from a nonce)
  • GCM (Galois/Counter mode, combines CTR with a new authentication based on a finite field), and its variant SGCM (Sophie Germain Counter mode, which uses a different field).
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