Symmetric Cryptography(Con’t)

Stream Ciphers

  1. A5/1 Cipher (GSM)

    • Produces keystream bit at each step
    • Easy to implement in hardware

  2. RC4 Cipher

    • Produces keystream byte at each step
    • One of the most widely used software stream ciphers
      • in older implementations of SSL /TLS; WEP(!)
      • removed from major browsers in 2015

Block Ciphers

  1. Feistel Cipher
    • key will change every round
    • function F must be reversible
    • multiple “rounds”

Block Cipher Modes

  1. Electronic Code Book (ECB) Mode
    • 將plain text切成 blocks
    • 分別對每個block做Block Ciphers
    • 缺點:同樣的block會被加密成同樣的cipher
    • diffusion較差
  2. Cipher Block Chaining (CBC) Mode
    • 跟ECB類似
    • 在做完Block Ciphers，會在跟前一個block的結果做xor，成為最後加密的結果
    • 改善diffusion
    • 最後一個block可以當MAC blocks(包含整個plain text的資訊)

  3. Comparison

    ECB V.S CBC

AES Cipher

  • most popular nowadays

Symmetric Key Ciphers and Key Reuse

  • Depth Attacks : Giving attackers deeper information about your plaintext or key

    • 例如之前的one-time pad ，當key重複使用時，攻擊者可以得知P1 ⊕ P2

  • Replay Attacks:

    • 當攻擊者監聽到一個Ciphers，傳送給對應的人，一樣可以得到正確的回覆

Using a Nonce(one-time use random bytes)

  • Countering Depth Attacks :
    當key重複使用時，還是可以因為nonce不同，而加密成不同的cipher

  • Countering Replay Attacks :
    當攻擊者監聽到一個Ciphers，想要重送時，會因為Nonce不同，而被拒絕。

Hashing

  • No key from the user (built-in key)
  • One-way function only (cannot reverse/decrypt)
  • Compresses data to fixed length (e.g., 128-bits)
  • Collisions exist, but should be extremely rare

Non-cryptographic Hashing

  • Cyclic Redundancy Check (CRC)

    • 👍 Data Lookup
    • 👎 Data integrity

    pro

    • simple to perform in hardware
    • can catch common, unintentional errors

    cons

    • cannot catch intentional manipulation
    • easy to revers
    • crc(x⊕y⊕z) = crc(x) ⊕ crc(y) ⊕ crc(z)
      well-known design flaws of the Wired Equivalent Privacy (WEP) protocol

Cryptographic Hashing

  • Sha1 sha2
  • md5
  • …

Data Integrity: MAC blocks(Message Authentication Code)

  • protects both a message’s data integrity as well as its authenticity, by allowing verifiers to detect any changes to the message content.
  • 在解密前先驗證資料來源及正確性(有沒有被變更)
  • 沒有先驗證就解密的話，攻擊者能從更改後的cipher跟plaintext中得到額外的訊息

Approaches to Authenticated Encryption

  1. MAC-and-Encrypt
  • encrypy the plaintextthen calculate MAC on plaintext
  1. MAC-then-Encrypt (MtE)
  • calculate MAC on plaintext and encrypt plaintext + MAC
  • vulnerable to Decryption Oracle Attack
  • ∵ you have to perform decryption before Authenticated
  1. Encrypt-then-MAC
  • encrypy the plaintext then calculate MAC on cipher

Coding time ~

So many things to care about

  • which algorithm to apply(must be not broken yet)
  • implements a algorithm wihout making mistake
  • generate different keys
  • use nonce
  • Authenticated Encryption order

##Developers should know about cryptography!

##But only cryptographers should write ciphers!

NaCl

  • NaCl : a new easy-to-use highspeed software library for network communication, encryption, decryption, signatures, etc.

Implemented in C

  • libSodium : portable, cross-compilable, installable, packageable fork of NaCl

Implemented in C

  • RbNaCl : Ruby binding to the Networking and Cryptography (NaCl) library

Implemented in Ruby, but you have to install libSodium yourself

  • rbnacl-libsodium : packages the libsodium library as a Ruby Gem

Implemented in Ruby, the only thing to do is
gem install rbnacl-libsodium

Document of RbNaCl

灑花(￣▽￣)/‧☆“‘ *-.,,.-*‘`”*-.,☆