Predefined Constants
The constants below are defined by this extension, and
will only be available when the extension has either
been compiled into PHP or dynamically loaded at runtime.
Mcrypt can operate in four block cipher modes (CBC, OFB, CFB, and
ECB). If linked against libmcrypt-2.4.x or higher the functions can also operate
in the block cipher mode nOFB and in STREAM mode. Below you find a list
with all supported encryption modes together with the constants that are
defines for the encryption mode. For a more complete reference and
discussion see Applied Cryptography by Schneier (ISBN 0-471-11709-9).
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MCRYPT_MODE_ECB (electronic codebook) is suitable for random data,
such as encrypting other keys. Since data there is short and random,
the disadvantages of ECB have a favorable negative effect.
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MCRYPT_MODE_CBC (cipher block chaining) is especially suitable for
encrypting files where the security is increased over ECB
significantly.
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MCRYPT_MODE_CFB (cipher feedback) is the best mode for encrypting byte
streams where single bytes must be encrypted.
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MCRYPT_MODE_OFB (output feedback, in 8bit) is comparable to CFB, but
can be used in applications where error propagation cannot
be tolerated. It's insecure (because it operates in 8bit
mode) so it is not recommended to use it.
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MCRYPT_MODE_NOFB (output feedback, in nbit) is comparable to OFB, but
more secure because it operates on the block size of the algorithm.
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MCRYPT_MODE_STREAM is an extra mode to include some stream algorithms
like WAKE or RC4.
Some other mode and random device constants:
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MCRYPT_ENCRYPT
(integer)
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MCRYPT_DECRYPT
(integer)
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MCRYPT_DEV_RANDOM
(integer)
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MCRYPT_DEV_URANDOM
(integer)
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MCRYPT_RAND
(integer)
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