# Constantes predefinidas

Estas constantes están definidas por esta extensión y estarán disponibles sólo cuando la extensión haya sido compilada con PHP, o bien sea cargada dinámicamente en ejecución.

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)