# 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 defined 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 a block cipher mode that is generally unsuitable for most purposes. The use of this mode is not recommended.
- `MCRYPT_MODE_CBC` (`cipher block chaining`) is a block cipher mode that is significantly more secure than `ECB` mode.
- `MCRYPT_MODE_CFB` (`cipher feedback, in 8-bit mode`) is a stream cipher mode. It is recommended to use `NCFB` mode rather than `CFB` mode.
- `MCRYPT_MODE_OFB` (`output feedback, in 8-bit mode`) is a stream cipher mode comparable to `CFB`, but can be used in applications where error propagation cannot be tolerated. It is recommended to use `NOFB` mode rather than `OFB` mode.
- `MCRYPT_MODE_NOFB` (`output feedback, in n-bit mode`) is comparable to `OFB` mode, but operates on the full block size of the algorithm.
- `MCRYPT_MODE_STREAM` is an extra mode to include some stream algorithms like `"WAKE"` or `"RC4"`.

Mcrypt supports some other modes of operation for which there are no predefined constants. They can be utilised by passing a string in place of the missing constants.

- `"ctr"` (`counter mode`) is a stream cipher mode.
- `"ncfb"` (`cipher feedback, in n-bit mode`) is comparable to `CFB` mode, but operates on the full block size of the algorithm.

Some other mode and random device constants:

`MCRYPT_ENCRYPT` (int)
`MCRYPT_DECRYPT` (int)
`MCRYPT_DEV_RANDOM` (int)
`MCRYPT_DEV_URANDOM` (int)
`MCRYPT_RAND` (int)