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The cipher functions are used for symmetrical cryptography, i.e. cryptography using a shared key. The programming model follows an open/process/close paradigm and is in that similar to other building blocks provided by Libgcrypt.
5.1 Available ciphers | List of ciphers supported by the library. | |
5.2 Cipher modules | How to work with cipher modules. | |
5.3 Available cipher modes | List of cipher modes supported by the library. | |
5.4 Working with cipher handles | How to perform operations related to cipher handles. | |
5.5 General cipher functions | General cipher functions independent of cipher handles. |
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GCRY_CIPHER_NONE
This is not a real algorithm but used by some functions as error return. The value always evaluates to false.
GCRY_CIPHER_IDEA
This is the IDEA algorithm. The constant is provided but there is currently no implementation for it because the algorithm is patented.
GCRY_CIPHER_3DES
Triple-DES with 3 Keys as EDE. The key size of this algorithm is 168 but you have to pass 192 bits because the most significant bits of each byte are ignored.
GCRY_CIPHER_CAST5
CAST128-5 block cipher algorithm. The key size is 128 bits.
GCRY_CIPHER_BLOWFISH
The blowfish algorithm. The current implementation allows only for a key size of 128 bits.
GCRY_CIPHER_SAFER_SK128
Reserved and not currently implemented.
GCRY_CIPHER_DES_SK
Reserved and not currently implemented.
GCRY_CIPHER_AES
GCRY_CIPHER_AES128
GCRY_CIPHER_RIJNDAEL
GCRY_CIPHER_RIJNDAEL128
AES (Rijndael) with a 128 bit key.
GCRY_CIPHER_AES192
GCRY_CIPHER_RIJNDAEL192
AES (Rijndael) with a 192 bit key.
GCRY_CIPHER_AES256
GCRY_CIPHER_RIJNDAEL256
AES (Rijndael) with a 256 bit key.
GCRY_CIPHER_TWOFISH
The Twofish algorithm with a 256 bit key.
GCRY_CIPHER_TWOFISH128
The Twofish algorithm with a 128 bit key.
GCRY_CIPHER_ARCFOUR
An algorithm which is 100% compatible with RSA Inc.'s RC4 algorithm. Note that this is a stream cipher and must be used very carefully to avoid a couple of weaknesses.
GCRY_CIPHER_DES
Standard DES with a 56 bit key. You need to pass 64 bit but the high bits of each byte are ignored. Note, that this is a weak algorithm which can be broken in reasonable time using a brute force approach.
GCRY_CIPHER_SERPENT128
GCRY_CIPHER_SERPENT192
GCRY_CIPHER_SERPENT256
The Serpent cipher from the AES contest.
GCRY_CIPHER_RFC2268_40
GCRY_CIPHER_RFC2268_128
Ron's Cipher 2 in the 40 and 128 bit variants. Note, that we currently only support the 40 bit variant. The identifier for 128 is reserved for future use.
GCRY_CIPHER_SEED
A 128 bit cipher as described by RFC4269.
GCRY_CIPHER_CAMELLIA128
GCRY_CIPHER_CAMELLIA192
GCRY_CIPHER_CAMELLIA256
The Camellia cipher by NTT. See http://info.isl.ntt.co.jp/crypt/eng/camellia/specifications.html.
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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 section Modules.
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.
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.
Type for the `setkey' function, defined as: gcry_err_code_t (*gcry_cipher_setkey_t) (void *c, const unsigned char *key, unsigned keylen)
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)
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)
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)
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)
Register a new cipher module whose specification can be found in cipher. On success, a new algorithm ID is stored in algorithm_id and a pointer representing this module is stored in module.
Unregister the cipher identified by module, which must have been registered with gcry_cipher_register.
Get a list consisting of the IDs of the loaded cipher modules. If list is zero, write the number of loaded cipher modules to list_length and return. If list is non-zero, the first *list_length algorithm IDs are stored in list, which must be of according size. In case there are less cipher modules than *list_length, *list_length is updated to the correct number.
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GCRY_CIPHER_MODE_NONE
No mode specified, may be set later using other functions. The value of this constant is always 0.
GCRY_CIPHER_MODE_ECB
Electronic Codebook mode.
GCRY_CIPHER_MODE_CFB
Cipher Feedback mode.
GCRY_CIPHER_MODE_CBC
Cipher Block Chaining mode.
GCRY_CIPHER_MODE_STREAM
Stream mode, only to be used with stream cipher algorithms.
GCRY_CIPHER_MODE_OFB
Output Feedback mode.
GCRY_CIPHER_MODE_CTR
Counter mode.
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To use a cipher algorithm, you must first allocate an according handle. This is to be done using the open function:
This function creates the context handle required for most of the other cipher functions and returns a handle to it in `hd'. In case of an error, an according error code is returned.
The ID of algorithm to use must be specified via algo. See See section Available ciphers, for a list of supported ciphers and the according constants.
Besides using the constants directly, the function
gcry_cipher_map_name
may be used to convert the textual name of
an algorithm into the according numeric ID.
The cipher mode to use must be specified via mode. See See section Available cipher modes, for a list of supported cipher modes and the according constants. Note that some modes are incompatible with some algorithms - in particular, stream mode (GCRY_CIPHER_MODE_STREAM) only works with stream ciphers. Any block cipher mode (GCRY_CIPHER_MODE_ECB, GCRY_CIPHER_MODE_CBC, GCRY_CIPHER_MODE_CFB, GCRY_CIPHER_MODE_OFB or GCRY_CIPHER_MODE_CTR) will work with any block cipher algorithm.
The third argument flags can either be passed as 0
or as
the bit-wise OR of the following constants.
GCRY_CIPHER_SECURE
Make sure that all operations are allocated in secure memory. This is useful when the key material is highly confidential.
GCRY_CIPHER_ENABLE_SYNC
This flag enables the CFB sync mode, which is a special feature of
Libgcrypt's CFB mode implementation to allow for OpenPGP's CFB variant.
See gcry_cipher_sync
.
GCRY_CIPHER_CBC_CTS
Enable cipher text stealing (CTS) for the CBC mode. Cannot be used simultaneous as GCRY_CIPHER_CBC_MAC. CTS mode makes it possible to transform data of almost arbitrary size (only limitation is that it must be greater than the algorithm's block size).
GCRY_CIPHER_CBC_MAC
Compute CBC-MAC keyed checksums. This is the same as CBC mode, but only output the last block. Cannot be used simultaneous as GCRY_CIPHER_CBC_CTS.
Use the following function to release an existing handle:
This function releases the context created by gcry_cipher_open
.
In order to use a handle for performing cryptographic operations, a `key' has to be set first:
Set the key k used for encryption or decryption in the context denoted by the handle h. The length l of the key k must match the required length of the algorithm set for this context or be in the allowed range for algorithms with variable key size. The function checks this and returns an error if there is a problem. A caller should always check for an error.
Note that this is currently implemented as a macro but may be changed to a function in the future.
Most crypto modes requires an initialization vector (IV), which usually is a non-secret random string acting as a kind of salt value. The CTR mode requires a counter, which is also similar to a salt value. To set the IV or CTR, use these functions:
Set the initialization vector used for encryption or decryption. The vector is passed as the buffer K of length l and copied to internal data structures. The function checks that the IV matches the requirement of the selected algorithm and mode. Note that this is implemented as a macro.
Set the counter vector used for encryption or decryption. The counter is passed as the buffer c of length l and copied to internal data structures. The function checks that the counter matches the requirement of the selected algorithm (i.e., it must be the same size as the block size). Note that this is implemented as a macro.
Set the given handle's context back to the state it had after the last call to gcry_cipher_setkey and clear the initialization vector.
Note that gcry_cipher_reset is implemented as a macro.
The actual encryption and decryption is done by using one of the following functions. They may be used as often as required to process all the data.
gcry_cipher_encrypt
is used to encrypt the data. This function
can either work in place or with two buffers. It uses the cipher
context already setup and described by the handle h. There are 2
ways to use the function: If in is passed as NULL
and
inlen is 0
, in-place encryption of the data in out or
length outsize takes place. With in being not NULL
,
inlen bytes are encrypted to the buffer out which must have
at least a size of inlen. outsize must be set to the
allocated size of out, so that the function can check that there
is sufficient space. Note that overlapping buffers are not allowed.
Depending on the selected algorithms and encryption mode, the length of the buffers must be a multiple of the block size.
The function returns 0
on success or an error code.
gcry_cipher_decrypt
is used to decrypt the data. This function
can either work in place or with two buffers. It uses the cipher
context already setup and described by the handle h. There are 2
ways to use the function: If in is passed as NULL
and
inlen is 0
, in-place decryption of the data in out or
length outsize takes place. With in being not NULL
,
inlen bytes are decrypted to the buffer out which must have
at least a size of inlen. outsize must be set to the
allocated size of out, so that the function can check that there
is sufficient space. Note that overlapping buffers are not allowed.
Depending on the selected algorithms and encryption mode, the length of the buffers must be a multiple of the block size.
The function returns 0
on success or an error code.
OpenPGP (as defined in RFC-2440) requires a special sync operation in some places. The following function is used for this:
Perform the OpenPGP sync operation on context h. Note that this
is a no-op unless the context was created with the flag
GCRY_CIPHER_ENABLE_SYNC
Some of the described functions are implemented as macros utilizing a catch-all control function. This control function is rarely used directly but there is nothing which would inhibit it:
gcry_cipher_ctl
controls various aspects of the cipher module and
specific cipher contexts. Usually some more specialized functions or
macros are used for this purpose. The semantics of the function and its
parameters depends on the the command cmd and the passed context
handle h. Please see the comments in the source code
(src/global.c
) for details.
gcry_cipher_info
is used to retrieve various
information about a cipher context or the cipher module in general.
Currently no information is available.
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To work with the algorithms, several functions are available to map algorithm names to the internal identifiers, as well as ways to retrieve information about an algorithm or the current cipher context.
This function is used to retrieve information on a specific algorithm. You pass the cipher algorithm ID as algo and the type of information requested as what. The result is either returned as the return code of the function or copied to the provided buffer whose allocated length must be available in an integer variable with the address passed in nbytes. This variable will also receive the actual used length of the buffer.
Here is a list of supported codes for what:
GCRYCTL_GET_KEYLEN:
Return the length of the key. If the algorithm supports multiple key lengths, the maximum supported value is returned. The length is returned as number of octets (bytes) and not as number of bits in nbytes; buffer must be zero.
GCRYCTL_GET_BLKLEN:
Return the block length of the algorithm. The length is returned as a number of octets in nbytes; buffer must be zero.
GCRYCTL_TEST_ALGO:
Returns 0
when the specified algorithm is available for use.
buffer and nbytes must be zero.
gcry_cipher_algo_name
returns a string with the name of the
cipher algorithm algo. If the algorithm is not known or another
error occurred, the string "?"
is returned. This function should
not be used to test for the availability of an algorithm.
gcry_cipher_map_name
returns the algorithm identifier for the
cipher algorithm described by the string name. If this algorithm
is not available 0
is returned.
Return the cipher mode associated with an ASN.1 object
identifier. The object identifier is expected to be in the
IETF-style dotted decimal notation. The function returns
0
for an unknown object identifier or when no mode is associated
with it.
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