Chapter 6-Block Cipher Operation

块密码和五种模式，块密码（Block Cipher）是对称加密的一种常见形式，通常用于加密大段数据。其基本原理是将明文分成多个等长的块（blocks），然后使用同一个密钥对每个块进行加密，得到密文块。

Chapter 6-Block Cipher Operation

• double-DES
  • 
  • issue of reduction to single stage
  • have "meet-in middle" attack
    • encrypting P with all keys and store
    • since there is an 
    • decrypt C with all keys and match X value
    • can show takes O(2^56) steps
• Triple DES
  • with 2 keys
    • structure
      • use 2 keys with E-D-E sequence
    • security
      • if k1=k2 then can work with single DES
      • no current known practical attacks, but several proposed impractical attacks might become basis of future attacks
  • with 3 keys
    • structure
    • application
      • has been adopted by some Internet applications
        • PGP
        • S
        • MIME
• Modes of Operation
  • intro
    • block ciphers encrypt fixed size blocks: eg.DES encrypts 63 bits blocks with 56-bits key
  • NIST SP 800-38A defines 5 modes
    • Block Modes
      • Electronic Codebook Book(ECB)
        • uses
          • sending a few blocks of data
        • Limitations
          • 现象
            • message repetition may show in ciphertext
            • particularly with data such graphics
          • 原因
            • due to the encrypted message blocks independent
        • structure
          • 
      • Cipher Block Chaining(CBC)
        • structure
          • use Initial Vector(IV to start process)
          • 
        • uses
          • bulk data encryption,authentication
        • advantages
          • 密文块的依赖性
            • a ciphertext block depends on all blocks before it
          • 变动的影响
            • any change to a block affects all following ciphertext blocks
        • limitation
          • need Initiation Vector(IV)
            • must be known to sender & reciever
            • if sent in clear, attack can change bits of first block, and change IV to compensate(抵消)
            • sent encrypted in ECB mode
        • Message Padding(信息填充)
          • def
            • 当信息末尾不足以构成一个加密块是需要采取措施处理它
          • 方法
            • pad either with known non-data valus(eg nulls)
            • pad last block along with count od pad size
              • 
      • XTS-AES Mode
        • intro
          • for block oriented storage use
          • 加密函数
            • T is tweak(调整值)，H is a Hash function
    • Stream Modes
      • Cipher Feedback(CFB)
        • 
        • uses
          • stream data encryption, authentication
        • process
          • 1. message is treated as a stream of bits
          • 2. added to the output of the block ciphert
          • 3. result is fed back for next stage
        • standard allows any number of bit(1,8,64,128 etc)
        • structure
        • advantages
          • appropriate when data arrives in bit/bytes
          • block cipher is used in encryption mode at both ends to yield ps-random bitstream
          • errors propagate for several blocks after the error(but not indefinitely遥遥无期地)
        • Limitation
          • need to stall(暂停) while do block encryption after every-bits
      • Output Feed Back(OFB)
        • intro
          • outpu tof cipher is added to message
          • output is then feedback
          • feedback is independent of message
          • can be computed in advance
        • uses
          • stream encryption on noisy channels
        • structure
        • advantages
          • bit errors do not propagate
          • more vulnerable to message stream modification
        • Limitation
          • needs an IV which is unique for each use if ever reuse attacker can recover propagate
          • sender& reciever must remian in sync
          • Only full block feedback(ie CFB-64 or CFB-128) should be used
      • Counter(CTR)
        • intro
          • 与OFB相似
            • similar to OFB
          • 加密计数器值
            • but enctypts counter value rather than any feedback value
          • 每块需要不同的counter value 和 key value，且不重用
          • 加密方程
            • 
        • uses
          • high-speed network encryptions
        • structure
        • advantages
          • efficiency
            • can do parallel encryptions in h/w or s/w
            • can preprocess in advance of need
            • good for bursty high speed links
          • allow random access to encrypted data blocks
          • provable security(good as other modes)
        • Limitation
          • ensure never reuse key/counter values, otherwise could break(cf. OFB)
  • Feedback Characteristics