547Chapter 29SecurityThe Unix version of PHP provides a (Web host 4 life)
547Chapter 29SecurityThe Unix version of PHP provides a set of functions that implement single-key encryption, using a publicly available library called mcrypt.To use these functions, you must downloadand install mcrypt(there is a link to the library s source available in the PHP manual) andrecompile PHP with the –enable-mcryptconfiguration option. When compiling this version of mcrypt, you must specify the configuration option –disable-posix-threadsduring the mcryptconfiguration. Missing this step causesApache to crash. mcryptoffers a choice between a number of ciphers different single-key algorithms. Eachhas its relative pros and cons in terms of speed and strength. In general, DES and Blowfish are fairly well-known algorithms with a good balance of speed and strength, but if you needextreme speed or great strength, you should research the algorithms available in your imple- mentation (listed in mcrypt.h) and choose the one most suited to your needs. mcryptalso allows you to choose among four cipher modes. These are summarized in Table 29-1. Table 29-1: Cipher Modes Provided by mcryptModeDescriptionInitialization vector (IV) ECB (electronic code book)Just translate the block of data given. noSuitable for small blocks of data that aren t very predictable, such as other keys. Do not use for text: The high frequency of letters and punctuation may be used to break the encryption. CBC (cipher block chaining)This stronger mode is far better optsuited for use with textual data. CFB (cipher feedback)Like ECB, CFB is well suited for short yesblocks of data. OFB (output feedback)OFB is very similar to CFB but designed yesto be better behaved when it encounters errors in its input. The last two modes require an initialization vector (abbreviated IV), which functions as astarting state for the encryption algorithm. The differences between these modes are relevantto interactive use, where individual keystrokes are encrypted one at a time. In that case, it iscrucial that the algorithm not encrypt athe same way each time. The PHP interface to mcryptonly allows us to encrypt strings, however, so any of the modes except ECB are perfectlyacceptable. Depending on the cipher mode you want to use, call mcrypt_ecb(), mcrypt_cbc(), mcrypt_cfb(), or mcrypt_ofb()like this: mcrypt_cbc(cipher, key, data, direction, [iv]) where cipheris MCRYPT_DES, MCRYPT_BLOWFISH, or whichever cipher you have chosen. (Seethe PHP documentation for an updated list of supported ciphers.) Pass your key and the dataCaution33
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