Block Cipher Algorithm Definitions¶

These data structures define modular crypto algorithm implementations, managed via crypto_register_alg() and crypto_unregister_alg().

struct cipher_alg¶: single-block symmetric ciphers definition

Definition:

struct cipher_alg {
    unsigned int cia_min_keysize;
    unsigned int cia_max_keysize;
    int (*cia_setkey)(struct crypto_tfm *tfm, const u8 *key, unsigned int keylen);
    void (*cia_encrypt)(struct crypto_tfm *tfm, u8 *dst, const u8 *src);
    void (*cia_decrypt)(struct crypto_tfm *tfm, u8 *dst, const u8 *src);
};

Members

cia_min_keysize: Minimum key size supported by the transformation. This is the smallest key length supported by this transformation algorithm. This must be set to one of the pre-defined values as this is not hardware specific. Possible values for this field can be found via git grep "_MIN_KEY_SIZE" include/crypto/
cia_max_keysize: Maximum key size supported by the transformation. This is the largest key length supported by this transformation algorithm. This must be set to one of the pre-defined values as this is not hardware specific. Possible values for this field can be found via git grep "_MAX_KEY_SIZE" include/crypto/
cia_setkey: Set key for the transformation. This function is used to either program a supplied key into the hardware or store the key in the transformation context for programming it later. Note that this function does modify the transformation context. This function can be called multiple times during the existence of the transformation object, so one must make sure the key is properly reprogrammed into the hardware. This function is also responsible for checking the key length for validity.
cia_encrypt: Encrypt a single block. This function is used to encrypt a single block of data, which must be cra_blocksize big. This always operates on a full cra_blocksize and it is not possible to encrypt a block of smaller size. The supplied buffers must therefore also be at least of cra_blocksize size. Both the input and output buffers are always aligned to cra_alignmask. In case either of the input or output buffer supplied by user of the crypto API is not aligned to cra_alignmask, the crypto API will re-align the buffers. The re-alignment means that a new buffer will be allocated, the data will be copied into the new buffer, then the processing will happen on the new buffer, then the data will be copied back into the original buffer and finally the new buffer will be freed. In case a software fallback was put in place in the cra_init call, this function might need to use the fallback if the algorithm doesn't support all of the key sizes. In case the key was stored in transformation context, the key might need to be re-programmed into the hardware in this function. This function shall not modify the transformation context, as this function may be called in parallel with the same transformation object.
cia_decrypt: Decrypt a single block. This is a reverse counterpart to cia_encrypt, and the conditions are exactly the same.

Description

All fields are mandatory and must be filled.

struct compress_alg¶: compression/decompression algorithm

Definition:

struct compress_alg {
    int (*coa_compress)(struct crypto_tfm *tfm, const u8 *src, unsigned int slen, u8 *dst, unsigned int *dlen);
    int (*coa_decompress)(struct crypto_tfm *tfm, const u8 *src, unsigned int slen, u8 *dst, unsigned int *dlen);
};

Members

coa_compress: Compress a buffer of specified length, storing the resulting data in the specified buffer. Return the length of the compressed data in dlen.
coa_decompress: Decompress the source buffer, storing the uncompressed data in the specified buffer. The length of the data is returned in dlen.

Description

All fields are mandatory.

struct crypto_alg¶: definition of a cryptograpic cipher algorithm

Definition:

struct crypto_alg {
    struct list_head cra_list;
    struct list_head cra_users;
    u32 cra_flags;
    unsigned int cra_blocksize;
    unsigned int cra_ctxsize;
    unsigned int cra_alignmask;
    int cra_priority;
    refcount_t cra_refcnt;
    char cra_name[CRYPTO_MAX_ALG_NAME];
    char cra_driver_name[CRYPTO_MAX_ALG_NAME];
    const struct crypto_type *cra_type;
    union {
        struct cipher_alg cipher;
        struct compress_alg compress;
    } cra_u;
    int (*cra_init)(struct crypto_tfm *tfm);
    void (*cra_exit)(struct crypto_tfm *tfm);
    void (*cra_destroy)(struct crypto_alg *alg);
    struct module *cra_module;
};

Members

cra_list: internally used
cra_users: internally used
cra_flags: Flags describing this transformation. See include/linux/crypto.h CRYPTO_ALG_* flags for the flags which go in here. Those are used for fine-tuning the description of the transformation algorithm.
cra_blocksize: Minimum block size of this transformation. The size in bytes of the smallest possible unit which can be transformed with this algorithm. The users must respect this value. In case of HASH transformation, it is possible for a smaller block than cra_blocksize to be passed to the crypto API for transformation, in case of any other transformation type, an error will be returned upon any attempt to transform smaller than cra_blocksize chunks.
cra_ctxsize: Size of the operational context of the transformation. This value informs the kernel crypto API about the memory size needed to be allocated for the transformation context.
cra_alignmask: Alignment mask for the input and output data buffer. The data buffer containing the input data for the algorithm must be aligned to this alignment mask. The data buffer for the output data must be aligned to this alignment mask. Note that the Crypto API will do the re-alignment in software, but only under special conditions and there is a performance hit. The re-alignment happens at these occasions for different cra_u types: cipher -- For both input data and output data buffer; ahash -- For output hash destination buf; shash -- For output hash destination buf. This is needed on hardware which is flawed by design and cannot pick data from arbitrary addresses.
cra_priority: Priority of this transformation implementation. In case multiple transformations with same cra_name are available to the Crypto API, the kernel will use the one with highest cra_priority.
cra_refcnt: internally used
cra_name: Generic name (usable by multiple implementations) of the transformation algorithm. This is the name of the transformation itself. This field is used by the kernel when looking up the providers of particular transformation.
cra_driver_name: Unique name of the transformation provider. This is the name of the provider of the transformation. This can be any arbitrary value, but in the usual case, this contains the name of the chip or provider and the name of the transformation algorithm.
cra_type: Type of the cryptographic transformation. This is a pointer to struct crypto_type, which implements callbacks common for all transformation types. There are multiple options, such as crypto_skcipher_type, crypto_ahash_type, crypto_rng_type. This field might be empty. In that case, there are no common callbacks. This is the case for: cipher, compress, shash.
cra_u: Callbacks implementing the transformation. This is a union of multiple structures. Depending on the type of transformation selected by cra_type and cra_flags above, the associated structure must be filled with callbacks. This field might be empty. This is the case for ahash, shash.
cra_u.cipher: Union member which contains a single-block symmetric cipher definition. See struct cipher_alg.
cra_u.compress: Union member which contains a (de)compression algorithm. See struct compress_alg.
cra_init: Initialize the cryptographic transformation object. This function is used to initialize the cryptographic transformation object. This function is called only once at the instantiation time, right after the transformation context was allocated. In case the cryptographic hardware has some special requirements which need to be handled by software, this function shall check for the precise requirement of the transformation and put any software fallbacks in place.
cra_exit: Deinitialize the cryptographic transformation object. This is a counterpart to cra_init, used to remove various changes set in cra_init.
cra_destroy: internally used
cra_module: Owner of this transformation implementation. Set to THIS_MODULE

Description

The struct crypto_alg describes a generic Crypto API algorithm and is common for all of the transformations. Any variable not documented here shall not be used by a cipher implementation as it is internal to the Crypto API.

Symmetric Key Cipher API¶

Symmetric key cipher API is used with the ciphers of type CRYPTO_ALG_TYPE_SKCIPHER (listed as type "skcipher" in /proc/crypto).

Asynchronous cipher operations imply that the function invocation for a cipher request returns immediately before the completion of the operation. The cipher request is scheduled as a separate kernel thread and therefore load-balanced on the different CPUs via the process scheduler. To allow the kernel crypto API to inform the caller about the completion of a cipher request, the caller must provide a callback function. That function is invoked with the cipher handle when the request completes.

To support the asynchronous operation, additional information than just the cipher handle must be supplied to the kernel crypto API. That additional information is given by filling in the skcipher_request data structure.

For the symmetric key cipher API, the state is maintained with the tfm cipher handle. A single tfm can be used across multiple calls and in parallel. For asynchronous block cipher calls, context data supplied and only used by the caller can be referenced the request data structure in addition to the IV used for the cipher request. The maintenance of such state information would be important for a crypto driver implementer to have, because when calling the callback function upon completion of the cipher operation, that callback function may need some information about which operation just finished if it invoked multiple in parallel. This state information is unused by the kernel crypto API.

struct crypto_skcipher *crypto_alloc_skcipher(const char *alg_name, u32 type, u32 mask)¶: allocate symmetric key cipher handle

Parameters

const char *alg_name: is the cra_name / name or cra_driver_name / driver name of the skcipher cipher
u32 type: specifies the type of the cipher
u32 mask: specifies the mask for the cipher

Description

Allocate a cipher handle for an skcipher. The returned struct crypto_skcipher is the cipher handle that is required for any subsequent API invocation for that skcipher.

Return

allocated cipher handle in case of success; IS_ERR() is true in case: of an error, PTR_ERR() returns the error code.

void crypto_free_skcipher(struct crypto_skcipher *tfm)¶: zeroize and free cipher handle

Parameters

struct crypto_skcipher *tfm: cipher handle to be freed

Description

If tfm is a NULL or error pointer, this function does nothing.

int crypto_has_skcipher(const char *alg_name, u32 type, u32 mask)¶: Search for the availability of an skcipher.

Parameters

const char *alg_name: is the cra_name / name or cra_driver_name / driver name of the skcipher
u32 type: specifies the type of the skcipher
u32 mask: specifies the mask for the skcipher

Return

true when the skcipher is known to the kernel crypto API; false: otherwise

unsigned int crypto_skcipher_ivsize(struct crypto_skcipher *tfm)¶: obtain IV size

Parameters

struct crypto_skcipher *tfm: cipher handle

Description

The size of the IV for the skcipher referenced by the cipher handle is returned. This IV size may be zero if the cipher does not need an IV.

Return

IV size in bytes

unsigned int crypto_skcipher_blocksize(struct crypto_skcipher *tfm)¶: obtain block size of cipher

Parameters

struct crypto_skcipher *tfm: cipher handle

Description

The block size for the skcipher referenced with the cipher handle is returned. The caller may use that information to allocate appropriate memory for the data returned by the encryption or decryption operation

Return

block size of cipher

int crypto_skcipher_setkey(struct crypto_skcipher *tfm, const u8 *key, unsigned int keylen)¶: set key for cipher

Parameters

struct crypto_skcipher *tfm: cipher handle
const u8 *key: buffer holding the key
unsigned int keylen: length of the key in bytes

Description

The caller provided key is set for the skcipher referenced by the cipher handle.

Note, the key length determines the cipher type. Many block ciphers implement different cipher modes depending on the key size, such as AES-128 vs AES-192 vs. AES-256. When providing a 16 byte key for an AES cipher handle, AES-128 is performed.

Return

0 if the setting of the key was successful; < 0 if an error occurred

struct crypto_skcipher *crypto_skcipher_reqtfm(struct skcipher_request *req)¶: obtain cipher handle from request

Parameters

struct skcipher_request *req: skcipher_request out of which the cipher handle is to be obtained

Description

Return the crypto_skcipher handle when furnishing an skcipher_request data structure.

Return

crypto_skcipher handle

int crypto_skcipher_encrypt(struct skcipher_request *req)¶: encrypt plaintext

Parameters

struct skcipher_request *req: reference to the skcipher_request handle that holds all information needed to perform the cipher operation

Description

Encrypt plaintext data using the skcipher_request handle. That data structure and how it is filled with data is discussed with the skcipher_request_* functions.

Return

0 if the cipher operation was successful; < 0 if an error occurred

int crypto_skcipher_decrypt(struct skcipher_request *req)¶: decrypt ciphertext

Parameters

struct skcipher_request *req: reference to the skcipher_request handle that holds all information needed to perform the cipher operation

Description

Decrypt ciphertext data using the skcipher_request handle. That data structure and how it is filled with data is discussed with the skcipher_request_* functions.

Return

0 if the cipher operation was successful; < 0 if an error occurred

Symmetric Key Cipher Request Handle¶

The skcipher_request data structure contains all pointers to data required for the symmetric key cipher operation. This includes the cipher handle (which can be used by multiple skcipher_request instances), pointer to plaintext and ciphertext, asynchronous callback function, etc. It acts as a handle to the skcipher_request_* API calls in a similar way as skcipher handle to the crypto_skcipher_* API calls.

unsigned int crypto_skcipher_reqsize(struct crypto_skcipher *tfm)¶: obtain size of the request data structure

Parameters

struct crypto_skcipher *tfm: cipher handle

Return

number of bytes

void skcipher_request_set_tfm(struct skcipher_request *req, struct crypto_skcipher *tfm)¶: update cipher handle reference in request

Parameters

struct skcipher_request *req: request handle to be modified
struct crypto_skcipher *tfm: cipher handle that shall be added to the request handle

Description

Allow the caller to replace the existing skcipher handle in the request data structure with a different one.

struct skcipher_request *skcipher_request_alloc(struct crypto_skcipher *tfm, gfp_t gfp)¶: allocate request data structure

Parameters

struct crypto_skcipher *tfm: cipher handle to be registered with the request
gfp_t gfp: memory allocation flag that is handed to kmalloc by the API call.

Description

Allocate the request data structure that must be used with the skcipher encrypt and decrypt API calls. During the allocation, the provided skcipher handle is registered in the request data structure.

Return

allocated request handle in case of success, or NULL if out of memory

void skcipher_request_free(struct skcipher_request *req)¶: zeroize and free request data structure

Parameters

struct skcipher_request *req: request data structure cipher handle to be freed

void skcipher_request_set_callback(struct skcipher_request *req, u32 flags, crypto_completion_t compl, void *data)¶: set asynchronous callback function

Parameters

struct skcipher_request *req: request handle
u32 flags: specify zero or an ORing of the flags CRYPTO_TFM_REQ_MAY_BACKLOG the request queue may back log and increase the wait queue beyond the initial maximum size; CRYPTO_TFM_REQ_MAY_SLEEP the request processing may sleep
crypto_completion_t compl: callback function pointer to be registered with the request handle
void *data: The data pointer refers to memory that is not used by the kernel crypto API, but provided to the callback function for it to use. Here, the caller can provide a reference to memory the callback function can operate on. As the callback function is invoked asynchronously to the related functionality, it may need to access data structures of the related functionality which can be referenced using this pointer. The callback function can access the memory via the "data" field in the crypto_async_request data structure provided to the callback function.

Description

This function allows setting the callback function that is triggered once the cipher operation completes.

The callback function is registered with the skcipher_request handle and must comply with the following template:

void callback_function(struct crypto_async_request *req, int error)

void skcipher_request_set_crypt(struct skcipher_request *req, struct scatterlist *src, struct scatterlist *dst, unsigned int cryptlen, void *iv)¶: set data buffers

Parameters

struct skcipher_request *req: request handle
struct scatterlist *src: source scatter / gather list
struct scatterlist *dst: destination scatter / gather list
unsigned int cryptlen: number of bytes to process from src
void *iv: IV for the cipher operation which must comply with the IV size defined by crypto_skcipher_ivsize

Description

This function allows setting of the source data and destination data scatter / gather lists.

For encryption, the source is treated as the plaintext and the destination is the ciphertext. For a decryption operation, the use is reversed - the source is the ciphertext and the destination is the plaintext.

Single Block Cipher API¶

The single block cipher API is used with the ciphers of type CRYPTO_ALG_TYPE_CIPHER (listed as type "cipher" in /proc/crypto).

Using the single block cipher API calls, operations with the basic cipher primitive can be implemented. These cipher primitives exclude any block chaining operations including IV handling.

The purpose of this single block cipher API is to support the implementation of templates or other concepts that only need to perform the cipher operation on one block at a time. Templates invoke the underlying cipher primitive block-wise and process either the input or the output data of these cipher operations.

struct crypto_cipher *crypto_alloc_cipher(const char *alg_name, u32 type, u32 mask)¶: allocate single block cipher handle

Parameters

const char *alg_name: is the cra_name / name or cra_driver_name / driver name of the single block cipher
u32 type: specifies the type of the cipher
u32 mask: specifies the mask for the cipher

Description

Allocate a cipher handle for a single block cipher. The returned struct crypto_cipher is the cipher handle that is required for any subsequent API invocation for that single block cipher.

Return

allocated cipher handle in case of success; IS_ERR() is true in case: of an error, PTR_ERR() returns the error code.

void crypto_free_cipher(struct crypto_cipher *tfm)¶: zeroize and free the single block cipher handle

Parameters

struct crypto_cipher *tfm: cipher handle to be freed

int crypto_has_cipher(const char *alg_name, u32 type, u32 mask)¶: Search for the availability of a single block cipher

Parameters

const char *alg_name: is the cra_name / name or cra_driver_name / driver name of the single block cipher
u32 type: specifies the type of the cipher
u32 mask: specifies the mask for the cipher

Return

true when the single block cipher is known to the kernel crypto API;: false otherwise

unsigned int crypto_cipher_blocksize(struct crypto_cipher *tfm)¶: obtain block size for cipher

Parameters

struct crypto_cipher *tfm: cipher handle

Description

The block size for the single block cipher referenced with the cipher handle tfm is returned. The caller may use that information to allocate appropriate memory for the data returned by the encryption or decryption operation

Return

block size of cipher

int crypto_cipher_setkey(struct crypto_cipher *tfm, const u8 *key, unsigned int keylen)¶: set key for cipher

Parameters

struct crypto_cipher *tfm: cipher handle
const u8 *key: buffer holding the key
unsigned int keylen: length of the key in bytes

Description

The caller provided key is set for the single block cipher referenced by the cipher handle.

Note, the key length determines the cipher type. Many block ciphers implement different cipher modes depending on the key size, such as AES-128 vs AES-192 vs. AES-256. When providing a 16 byte key for an AES cipher handle, AES-128 is performed.

Return

0 if the setting of the key was successful; < 0 if an error occurred

void crypto_cipher_encrypt_one(struct crypto_cipher *tfm, u8 *dst, const u8 *src)¶: encrypt one block of plaintext

Parameters

struct crypto_cipher *tfm: cipher handle
u8 *dst: points to the buffer that will be filled with the ciphertext
const u8 *src: buffer holding the plaintext to be encrypted

Description

Invoke the encryption operation of one block. The caller must ensure that the plaintext and ciphertext buffers are at least one block in size.

void crypto_cipher_decrypt_one(struct crypto_cipher *tfm, u8 *dst, const u8 *src)¶: decrypt one block of ciphertext

Parameters

struct crypto_cipher *tfm: cipher handle
u8 *dst: points to the buffer that will be filled with the plaintext
const u8 *src: buffer holding the ciphertext to be decrypted

Description

Invoke the decryption operation of one block. The caller must ensure that the plaintext and ciphertext buffers are at least one block in size.

The Linux Kernel

Contents

This Page

Block Cipher Algorithm Definitions¶

Symmetric Key Cipher API¶

Symmetric Key Cipher Request Handle¶

Single Block Cipher API¶