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CHAPTER 5: Cryptography
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What would it take to protect the data How hard will it be to protect the data How long will it take Will it inconvenience the user
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If, after a thorough analysis, you decide that the app needs to protect its data, you can proceed to a consideration of the best method to do so.
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When most people hear the word cryptography, they may first think of cipher encryption that is, transforming a plaintext, such as hello, into seeming gibberish called a ciphertext, such as ifmmp. Ciphers have existed for millennia, and modern ones have grown to be incredibly sophisticated. All but the most trivial ciphers rely on use of a key. The key is a secret piece of data that is used to encrypt a message. For example, consider a cipher that adds the key value to the plaintext value. We might have a plain text of hello and a key of world. We can convert those letters into numeric values, starting with 1 for A and 26 for Z . Then, for each letter, we add the value of the plain text to the value of the key. If the total is greater than 26, we subtract 26 so we end up with a value between 1 and 26. Finally, we convert the number back to the numeric value. In this example, h has a value of 8 since H is the 8th letter in the English alphabet; w has a value of 23 since it is the 23rd letter. 8 + 23 = 31. We subtract 26 to get a value of 5, which corresponds to the letter e . Table 5-1 shows how to apply these steps to the entire words.
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Table 5-1. Applying a Simple Cipher
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Key Letter
w o r l d
Plaintext Number
8 5 12 12 15
Key Number
23 15 18 12 4
5 20 4 24 19
Cipher Letter
e t d x n
Without any other context, the word etdxn doesn t seem to mean anything, and any would-be attackers are left frustrated. Even if they know how the cipher works, without the extra information supplied by the key, they will be stuck. What happens if someone figured out your key, though When this happens, the cipher becomes useless, and attackers will be able to decrypt any ciphertext that they come across later. It is imperative to keep your keys secret. Some modern systems generate keys on the fly based on secret processes, such as the time of day or the motion of a lava lamp. Apart from the keys, modern ciphers are, of course, far more complex than
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CHAPTER 5: Cryptography
the example shown above, and often involve scrambling the order of letters in addition to performing various permutations on each byte of data. There are many types of ciphers available for use in software applications. Most fall into one of two major categories. A block-based cipher encrypts data in certain chunk sizes. In our previous example, since our key is 5 letters long, we would want to process plaintext in sets of 5 letters. If the incoming plaintext has only 13 letters, then we can substitute random characters for the last 2 letters and discard them when we decode the message. Alternately, a stream-based cipher can process plaintexts of arbitrary length. Neither type is inherently more secure than the other.
Data Decryption
If a server sends your app encrypted data, you will need to write code to decrypt that data so you can process it. Similarly, if you encrypt your user s data and store it on the local filesystem, you ll decrypt that data when you need to access it. Decryption is the reverse of encryption, and transforms a ciphertext back into a plaintext, as shown in Figure 5-1.
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