Libgcrypt makes it possible to load additional `cipher modules'; these ciphers can be used just like the cipher algorithms that are built into the library directly. For an introduction into extension modules, see See Modules.

— Data type: **gcry_cipher_spec_t**

This is the `module specification structure' needed for registering cipher modules, which has to be filled in by the user before it can be used to register a module. It contains the following members:

`const char *name`

- The primary name of the algorithm.
`const char **aliases`

- A list of strings that are `aliases' for the algorithm. The list must be terminated with a NULL element.
`gcry_cipher_oid_spec_t *oids`

- A list of OIDs that are to be associated with the algorithm. The list's last element must have it's `oid' member set to NULL. See below for an explanation of this type.
`size_t blocksize`

- The block size of the algorithm, in bytes.
`size_t keylen`

- The length of the key, in bits.
`size_t contextsize`

- The size of the algorithm-specific `context', that should be allocated for each handle.
`gcry_cipher_setkey_t setkey`

- The function responsible for initializing a handle with a provided key. See below for a description of this type.
`gcry_cipher_encrypt_t encrypt`

- The function responsible for encrypting a single block. See below for a description of this type.
`gcry_cipher_decrypt_t decrypt`

- The function responsible for decrypting a single block. See below for a description of this type.
`gcry_cipher_stencrypt_t stencrypt`

- Like `encrypt', for stream ciphers. See below for a description of this type.
`gcry_cipher_stdecrypt_t stdecrypt`

- Like `decrypt', for stream ciphers. See below for a description of this type.

— Data type: **gcry_cipher_oid_spec_t**

This type is used for associating a user-provided algorithm implementation with certain OIDs. It contains the following members:

`const char *oid`

- Textual representation of the OID.
`int mode`

- Cipher mode for which this OID is valid.

— Data type: **gcry_cipher_setkey_t**

Type for the `setkey' function, defined as: gcry_err_code_t (*gcry_cipher_setkey_t) (void *c, const unsigned char *key, unsigned keylen)

— Data type: **gcry_cipher_encrypt_t**

Type for the `encrypt' function, defined as: gcry_err_code_t (*gcry_cipher_encrypt_t) (void *c, const unsigned char *outbuf, const unsigned char *inbuf)

— Data type: **gcry_cipher_decrypt_t**

Type for the `decrypt' function, defined as: gcry_err_code_t (*gcry_cipher_decrypt_t) (void *c, const unsigned char *outbuf, const unsigned char *inbuf)

— Data type: **gcry_cipher_stencrypt_t**

Type for the `stencrypt' function, defined as: gcry_err_code_t (*gcry_cipher_stencrypt_t) (void *c, const unsigned char *outbuf, const unsigned char *, unsigned int n)

— Data type: **gcry_cipher_stdecrypt_t**

Type for the `stdecrypt' function, defined as: gcry_err_code_t (*gcry_cipher_stdecrypt_t) (void *c, const unsigned char *outbuf, const unsigned char *, unsigned int n)

— Function: gcry_error_t **gcry_cipher_register** (`gcry_cipher_spec_t *cipher, unsigned int *algorithm_id, gcry_module_t *module`)

Register a new cipher module whose specification can be found in

cipher. On success, a new algorithm ID is stored inalgorithm_idand a pointer representing this module is stored inmodule.

— Function: void **gcry_cipher_unregister** (`gcry_module_t module`)

Unregister the cipher identified by

module, which must have been registered with gcry_cipher_register.

— Function: gcry_error_t **gcry_cipher_list** (`int *list, int *list_length`)

Get a list consisting of the IDs of the loaded cipher modules. If

listis zero, write the number of loaded cipher modules tolist_lengthand return. Iflistis non-zero, the first *list_lengthalgorithm IDs are stored inlist, which must be of according size. In case there are less cipher modules than *list_length, *list_lengthis updated to the correct number.