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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).
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.
MCRYPT_MODE_CBC (cipher block
chaining) is especially suitable for
encrypting files where the security is increased over ECB
significantly.
MCRYPT_MODE_CFB (cipher
feedback) is the best mode for encrypting byte
streams where single bytes must be encrypted.
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.
MCRYPT_MODE_NOFB (output feedback,
in nbit) is comparable to OFB, but
more secure because it operates on the block size of the algorithm.
MCRYPT_MODE_STREAM is an extra mode to include
some stream algorithms like "WAKE" or "RC4".
Some other mode and random device constants: