Implementing-cryptography-using-python

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Chapter 5 ■ Stream Ciphers and Block Ciphers 165■■Encryption is decryption (just pass in the IV)■■We don’t have to pad the message, just use the bits we need (saves bandwidthon small messages)■■CTR can be efficiently parallelized both for encryption and decryptionWeaknesses:■■OFB might have short permutation cycles (more in a minute on that)■■Using the same IV twice is deadly (not great in CBC but worse here)■■Tampering with the ciphertext can lead to tampered plaintext (and notjust noise)DIY Block Cipher Using Feistel NetworksIn this next section, I want to show you a slick method to use one-way function,like hashes, to create block ciphers using the Feistel network. A Feistel cipher/Feistel network implements a series of iterative ciphers on a block of data andis generally designed for block ciphers that encrypt large quantities of data.A Feistel network works by splitting the data block into two equal pieces andapplying encryption in multiple rounds.The input block is split in the middle to create equal-sized sub-blocks. ExamineFigure 5.15; let’s call the initial left-hand side sub-block L 0 , and the initialright-hand side sub-block R 0 . Let the function F ki be the round function thatuses the i th round key, k i . These round keys are generated by the key schedule,which is unspecified. If we have r rounds, then for 1 ≤ i < r.L i = R i−1R i = L i−1 ⊕ F ki (R i−1 )and for i = rL r = R r−1 ⊕ F kr (L r−1 )R r = L r−1The idea is to split your message into two halves, with one half to makesomething that looks like noise. Then XOR the other half with the noise. Finally,pass the XOR’d result and the unmodified half. This gives you everything youneed to know to go backward, which is the same exact process. In Figure 5.16you will see an illustration of how the Feistel network decryption works.Next, convince yourself that reversing one step of the Feistel network is doableby hashing the part that passed in the clear, XORing with the other half, andreversing their spots. Now we want to have our output have the same bit lengthas our input, which was not true for a hash function.Use the super-hash as the one-way function in a Feistel network. Our outputis a 256-bit hexadecimal number. The input should be 512 bits. Eight bits isenough to encode one ASCII character. The secrets should be 64 characters long.
166 Chapter 5 ■ Stream Ciphers and Block CiphersplaintextL 0 R 0F k1F k2F k3F k4R 4 L 4ciphertextFigure 5.15: Feistel Network encryptionciphertextR 4 L 4F k4F k3F k2F k1L 0 R 0plaintextFigure 5.16: Feistel Network decryptionTo build the encryption scheme, write a function that consumes a length 64string and a salt that now acts like a key. Have your scheme do four rounds ofa Feistel network and return a hex string of the 512-bit output. For now, you canuse 5,000 iterations per super-hash.
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ImplementingCryptography UsingPytho
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To Stephanie, Eden, Hayden, and Ken
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viAbout the Authorbook, Shannon is
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Contents at a GlanceIntroductionxvi
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xiiContentsUsing Conditionals 16Usi
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xivContentsPseudorandomness115Break
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xviContentsInstalling and Testing W
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xviiiIntroductionprivacy advocates.
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CHAPTER1Introduction to Cryptograph
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Chapter 1 ■ Introduction to Crypt
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CHAPTER2Cryptographic Protocolsand
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Chapter 2 ■ Cryptographic Protoco
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66 Chapter 3 ■ Classical Cryptogr
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CHAPTER4Cryptographic Mathand Frequ
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Chapter 4 ■ Cryptographic Math an
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216 Chapter 7 ■ Message Integrity
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224 Chapter 8 ■ Cryptographic App
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CHAPTER9Mastering CryptographyUsing
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Chapter 9 ■ Mastering Cryptograph
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IndexSYMBOLS\ (backslash), 10- oper
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Index ■ D-D 279CIA Triad, 35-36ci
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Index ■ I-M 281hashlib module, 28
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Index ■ Q-S 283Preneel, Bart, 145
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