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Modes of operation

Mode of operation is the configuration used around a block cipher

  • Most messages don't come in convenient 128-bit block lengths
  • Need to run a block cipher repeatedly on consecutive blocks
  • Stream Cipher have been proven harder to implement. ChaCha20 is pretty good. Have been massively broken

Padding​

  • ECB and some other modes require message length to be a multiple of the block size
  • Public Key Cryptography Standards (PKCS7) is a common padding scheme:
  1. Padding bytes are always added to the plaintext before it is encrypted
  2. Each padding byte had a value equal to the total number of padding bytes that are added
  3. The total number of padding bytes is at least one

Electronic Code Block (ECB)​

Just encrypt each block one after another (Never do this)

Weakness​

  • ECB allows an attacker to infer information on the plaintext
  • ECB divulges whenever messages or blocks are the same

Deterministic vs Probabilistic Encryption​

  • An encryption scheme is deterministic if some plaintext is mapped to a fixed ciphertext if the key is unchanged
  • ECB is deterministic, but most modern modes of operation are probabilistic
  • Probabilistic encryption schemes add randomness to the encryption process to achieve a non-deterministic generation of the cipher text

Cipher Block Chaining (CBC)​

  • XOR the output of each cipher block with the next input
  • IV: Initialisation Vector

Decryption​

  • Similar, but the XOR now takes place after decryption
  • Easier to parallelise

Weakness​

  • Was the primary method of encryption for many years, now less common

Padding Oracles​

  • Oracle is a system we can query and it will tell us if, once decrypted, some text has valid padding
  • System is unlikely to tell you directly, but it might give away some clues

Attacks​

  • Attack is essentially the same for multiple blocks, just one at a time
  • Attack the last block, which contains padding

Counter Mode (CTR)​

Encrypt a nonce + counter and use this to mask the plaintext with XOR

  • This is very easily parallelised
  • Each block is encrypted differently, avoiding the issue with ECB mode

Galois Counter Mode​

  • Extends the counter mode to add authenticity
    • Sender definitely sent that message, and it hasn't changed
  • Very similar to counter mode, but adds an authentication tag
    • Uses multiplication in a Galois Finite field ....
  • Extremely parallelisable
  • Robust to message alteration