Implementing-cryptography-using-python

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CHAPTER5Stream Ciphers and Block CiphersIn this chapter, you’ll learn the means of support of the entire cryptographicworld. You will learn how to encrypt and decrypt messages with a shared secret,which is how the vast majority of today’s encryption on the Internet functions. Infact, encryption is used every day using techniques you’ll learn in this chapter.This chapter introduces you to the development of stream ciphers, block ciphers,and cryptographically secure pseudorandom number generators. You’ll gainan understanding of some of the analyses of the underlying algorithms and thebest attacks that exist in the real world. You’ll also learn the amount of securityyou must build in, based on the client you are serving, and how long it needs tostay secure. You’ll walk away with being able to randomize every message yousend, which will help you build a solid career in cryptography. Through thischapter, you’ll gain cryptographic knowledge as you do the following:■■Learn how to convert between hexdigest and plaintext■■Gain an understanding of stream ciphers and CSPRNGs■■Learn about block ciphers■■Explore various modes of encryption and their weaknesses■■Gain an understanding of blocks as streamsImplementing Cryptography Using Python®, First Edition. Shannon W. Bray.© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.139
140 Chapter 5 ■ Stream Ciphers and Block CiphersConvert between Hexdigest and PlaintextBefore we dive into stream and block ciphers, I will show you some of the harderbits that involve the conversion between hexdigest and plaintext. To perform theconversions in Python, you’ll use the binascii library. The binascii module providesseveral methods to convert between ASCII-ended binary expressions andbinary. It comprises low-level functions written in C for superior performance.In this section, you will learn about hexlify and unhexlify, modules in thebinascii library. Hexlify returns the hexadecimal representation; each byteof data converts into a two-digit hex representation. The output results in thereturned hexadecimal being twice the length of the data passed into the hexlifyfunction. The following code converts the string “apples are red” to itshexadecimal representation. You will find that the length of the value referencedby passhex is twice the size of the value referenced by the password variable.import binasciipassword = b"apples are red"print(password)apples are red>>> passhex = binascii.hexlify(password)>>> print(passhex)b'6170706c65732061726520726564'>>> print(len(password))14>>> print(len(passhex))28You probably noticed that the first two digits of the passhex variable equal61; you can look up the code, hex, and characters in an ASCII table. To demonstratehow to convert from characters, code, and hex, examine the followingPython code:>>> passhexb'6170706c65732061726520726564'>>> print(ord('a'))97>>> print(hex(97))0x61>>> print(format(ord('a'), 'x'))61The binascii function unhexlify, which takes a hexadecimal string as aparameter, is the inverse. As you might imagine, the hexadecimal string must
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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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Chapter 6 ■ Using Cryptography wi
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200 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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