# Linux Kernel Crypto API

Stephan
Mueller

          smueller@chronox.de

Marek
Vasut

          marek@denx.de

2014
Stephan Mueller

This documentation is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

For more details see the file COPYING in the source distribution of Linux.

## Kernel Crypto API Interface Specification

### Introduction

The kernel crypto API offers a rich set of cryptographic ciphers as well as other data transformation mechanisms and methods to invoke these. This document contains a description of the API and provides example code.

To understand and properly use the kernel crypto API a brief explanation of its structure is given. Based on the architecture, the API can be separated into different components. Following the architecture specification, hints to developers of ciphers are provided. Pointers to the API function call documentation are given at the end.

The kernel crypto API refers to all algorithms as "transformations". Therefore, a cipher handle variable usually has the name "tfm". Besides cryptographic operations, the kernel crypto API also knows compression transformations and handles them the same way as ciphers.

The kernel crypto API serves the following entity types:

- consumers requesting cryptographic services

- data transformation implementations (typically ciphers) that can be called by consumers using the kernel crypto API

This specification is intended for consumers of the kernel crypto API as well as for developers implementing ciphers. This API specification, however, does not discusses all API calls available to data transformation implementations (i.e. implementations of ciphers and other transformations (such as CRC or even compression algorithms) that can register with the kernel crypto API).

Note: The terms "transformation" and cipher algorithm are used interchangably.

### Terminology

The transformation implementation is an actual code or interface to hardware which implements a certain transformation with precisely defined behavior.

The transformation object (TFM) is an instance of a transformation implementation. There can be multiple transformation objects associated with a single transformation implementation. Each of those transformation objects is held by a crypto API consumer or another transformation. Transformation object is allocated when a crypto API consumer requests a transformation implementation. The consumer is then provided with a structure, which contains a transformation object (TFM).

The structure that contains transformation objects may also be referred to as a "cipher handle". Such a cipher handle is always subject to the following phases that are reflected in the API calls applicable to such a cipher handle:

1. Initialization of a cipher handle.

2. Execution of all intended cipher operations applicable for the handle where the cipher handle must be furnished to every API call.

3. Destruction of a cipher handle.

When using the initialization API calls, a cipher handle is created and returned to the consumer. Therefore, please refer to all initialization API calls that refer to the data structure type a consumer is expected to receive and subsequently to use. The initialization API calls have all the same naming conventions of crypto_alloc_*.

The transformation context is private data associated with the transformation object.

## Kernel Crypto API Architecture

### Cipher algorithm types

The kernel crypto API provides different API calls for the following cipher types:

- Symmetric ciphers

- AEAD ciphers

- Message digest, including keyed message digest

- Random number generation

- User space interface

### Ciphers And Templates

The kernel crypto API provides implementations of single block ciphers and message digests. In addition, the kernel crypto API provides numerous "templates" that can be used in conjunction with the single block ciphers and message digests. Templates include all types of block chaining mode, the HMAC mechanism, etc.

Single block ciphers and message digests can either be directly used by a caller or invoked together with a template to form multi-block ciphers or keyed message digests.

A single block cipher may even be called with multiple templates. However, templates cannot be used without a single cipher.

See /proc/crypto and search for "name". For example:

- aes

- ecb(aes)

- cmac(aes)

- ccm(aes)

- rfc4106(gcm(aes))

- sha1

- hmac(sha1)

- authenc(hmac(sha1),cbc(aes))

In these examples, "aes" and "sha1" are the ciphers and all others are the templates.

### Synchronous And Asynchronous Operation

The kernel crypto API provides synchronous and asynchronous API operations.

When using the synchronous API operation, the caller invokes a cipher operation which is performed synchronously by the kernel crypto API. That means, the caller waits until the cipher operation completes. Therefore, the kernel crypto API calls work like regular function calls. For synchronous operation, the set of API calls is small and conceptually similar to any other crypto library.

Asynchronous operation is provided by the kernel crypto API which implies that the invocation of a cipher operation will complete almost instantly. That invocation triggers the cipher operation but it does not signal its completion. Before invoking a cipher operation, the caller must provide a callback function the kernel crypto API can invoke to signal the completion of the cipher operation. Furthermore, the caller must ensure it can handle such asynchronous events by applying appropriate locking around its data. The kernel crypto API does not perform any special serialization operation to protect the caller's data integrity.

### Crypto API Cipher References And Priority

A cipher is referenced by the caller with a string. That string has the following semantics:

    	template(single block cipher)

where "template" and "single block cipher" is the aforementioned template and single block cipher, respectively. If applicable, additional templates may enclose other templates, such as

    	template1(template2(single block cipher)))

The kernel crypto API may provide multiple implementations of a template or a single block cipher. For example, AES on newer Intel hardware has the following implementations: AES-NI, assembler implementation, or straight C. Now, when using the string "aes" with the kernel crypto API, which cipher implementation is used? The answer to that question is the priority number assigned to each cipher implementation by the kernel crypto API. When a caller uses the string to refer to a cipher during initialization of a cipher handle, the kernel crypto API looks up all implementations providing an implementation with that name and selects the implementation with the highest priority.

Now, a caller may have the need to refer to a specific cipher implementation and thus does not want to rely on the priority-based selection. To accommodate this scenario, the kernel crypto API allows the cipher implementation to register a unique name in addition to common names. When using that unique name, a caller is therefore always sure to refer to the intended cipher implementation.

The list of available ciphers is given in /proc/crypto. However, that list does not specify all possible permutations of templates and ciphers. Each block listed in /proc/crypto may contain the following information -- if one of the components listed as follows are not applicable to a cipher, it is not displayed:

- name: the generic name of the cipher that is subject to the priority-based selection -- this name can be used by the cipher allocation API calls (all names listed above are examples for such generic names)

- driver: the unique name of the cipher -- this name can be used by the cipher allocation API calls

- module: the kernel module providing the cipher implementation (or "kernel" for statically linked ciphers)

- priority: the priority value of the cipher implementation

- refcnt: the reference count of the respective cipher (i.e. the number of current consumers of this cipher)

- selftest: specification whether the self test for the cipher passed

- type:

  - blkcipher for synchronous block ciphers

  - ablkcipher for asynchronous block ciphers

  - cipher for single block ciphers that may be used with an additional template

  - shash for synchronous message digest

  - ahash for asynchronous message digest

  - aead for AEAD cipher type

  - compression for compression type transformations

  - rng for random number generator

  - givcipher for cipher with associated IV generator (see the geniv entry below for the specification of the IV generator type used by the cipher implementation)

- blocksize: blocksize of cipher in bytes

- keysize: key size in bytes

- ivsize: IV size in bytes

- seedsize: required size of seed data for random number generator

- digestsize: output size of the message digest

- geniv: IV generation type:

  - eseqiv for encrypted sequence number based IV generation

  - seqiv for sequence number based IV generation

  - chainiv for chain iv generation

  - <builtin> is a marker that the cipher implements IV generation and handling as it is specific to the given cipher

### Key Sizes

When allocating a cipher handle, the caller only specifies the cipher type. Symmetric ciphers, however, typically support multiple key sizes (e.g. AES-128 vs. AES-192 vs. AES-256). These key sizes are determined with the length of the provided key. Thus, the kernel crypto API does not provide a separate way to select the particular symmetric cipher key size.

### Cipher Allocation Type And Masks

The different cipher handle allocation functions allow the specification of a type and mask flag. Both parameters have the following meaning (and are therefore not covered in the subsequent sections).

The type flag specifies the type of the cipher algorithm. The caller usually provides a 0 when the caller wants the default handling. Otherwise, the caller may provide the following selections which match the the aforementioned cipher types:

- CRYPTO_ALG_TYPE_CIPHER Single block cipher

- CRYPTO_ALG_TYPE_COMPRESS Compression

- CRYPTO_ALG_TYPE_AEAD Authenticated Encryption with Associated Data (MAC)

- CRYPTO_ALG_TYPE_BLKCIPHER Synchronous multi-block cipher

- CRYPTO_ALG_TYPE_ABLKCIPHER Asynchronous multi-block cipher

- CRYPTO_ALG_TYPE_GIVCIPHER Asynchronous multi-block cipher packed together with an IV generator (see geniv field in the /proc/crypto listing for the known IV generators)

- CRYPTO_ALG_TYPE_DIGEST Raw message digest

- CRYPTO_ALG_TYPE_HASH Alias for CRYPTO_ALG_TYPE_DIGEST

- CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash

- CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash

- CRYPTO_ALG_TYPE_RNG Random Number Generation

- CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression / decompression instead of performing the operation on one segment only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted.

The mask flag restricts the type of cipher. The only allowed flag is CRYPTO_ALG_ASYNC to restrict the cipher lookup function to asynchronous ciphers. Usually, a caller provides a 0 for the mask flag.

When the caller provides a mask and type specification, the caller limits the search the kernel crypto API can perform for a suitable cipher implementation for the given cipher name. That means, even when a caller uses a cipher name that exists during its initialization call, the kernel crypto API may not select it due to the used type and mask field.

## Developing Cipher Algorithms

### Registering And Unregistering Transformation

There are three distinct types of registration functions in the Crypto API. One is used to register a generic cryptographic transformation, while the other two are specific to HASH transformations and COMPRESSion. We will discuss the latter two in a separate chapter, here we will only look at the generic ones.

Before discussing the register functions, the data structure to be filled with each, struct crypto_alg, must be considered -- see below for a description of this data structure.

The generic registration functions can be found in include/linux/crypto.h and their definition can be seen below. The former function registers a single transformation, while the latter works on an array of transformation descriptions. The latter is useful when registering transformations in bulk.

       int crypto_register_alg(struct crypto_alg *alg);
       int crypto_register_algs(struct crypto_alg *algs, int count);

The counterparts to those functions are listed below.

       int crypto_unregister_alg(struct crypto_alg *alg);
       int crypto_unregister_algs(struct crypto_alg *algs, int count);

Notice that both registration and unregistration functions do return a value, so make sure to handle errors. A return code of zero implies success. Any return code < 0 implies an error.

The bulk registration / unregistration functions require that struct crypto_alg is an array of count size. These functions simply loop over that array and register / unregister each individual algorithm. If an error occurs, the loop is terminated at the offending algorithm definition. That means, the algorithms prior to the offending algorithm are successfully registered. Note, the caller has no way of knowing which cipher implementations have successfully registered. If this is important to know, the caller should loop through the different implementations using the single instance *_alg functions for each individual implementation.

### Single-Block Symmetric Ciphers [CIPHER]

Example of transformations: aes, arc4, ...

This section describes the simplest of all transformation implementations, that being the CIPHER type used for symmetric ciphers. The CIPHER type is used for transformations which operate on exactly one block at a time and there are no dependencies between blocks at all.

#### Registration specifics

The registration of [CIPHER] algorithm is specific in that struct crypto_alg field .cra_type is empty. The .cra_u.cipher has to be filled in with proper callbacks to implement this transformation.

See struct cipher_alg below.

#### Cipher Definition With struct cipher_alg

Struct cipher_alg defines a single block cipher.

Here are schematics of how these functions are called when operated from other part of the kernel. Note that the .cia_setkey() call might happen before or after any of these schematics happen, but must not happen during any of these are in-flight.

             KEY ---.    PLAINTEXT ---.
                    v                 v
              .cia_setkey() -> .cia_encrypt()
                                      |
                                      '-----> CIPHERTEXT

Please note that a pattern where .cia_setkey() is called multiple times is also valid:

      KEY1 --.    PLAINTEXT1 --.         KEY2 --.    PLAINTEXT2 --.
             v                 v                v                 v
       .cia_setkey() -> .cia_encrypt() -> .cia_setkey() -> .cia_encrypt()
                               |                                  |
                               '---> CIPHERTEXT1                  '---> CIPHERTEXT2

### Multi-Block Ciphers [BLKCIPHER] [ABLKCIPHER]

Example of transformations: cbc(aes), ecb(arc4), ...

This section describes the multi-block cipher transformation implementations for both synchronous [BLKCIPHER] and asynchronous [ABLKCIPHER] case. The multi-block ciphers are used for transformations which operate on scatterlists of data supplied to the transformation functions. They output the result into a scatterlist of data as well.

#### Registration Specifics

The registration of [BLKCIPHER] or [ABLKCIPHER] algorithms is one of the most standard procedures throughout the crypto API.

Note, if a cipher implementation requires a proper alignment of data, the caller should use the functions of crypto_blkcipher_alignmask() or crypto_ablkcipher_alignmask() respectively to identify a memory alignment mask. The kernel crypto API is able to process requests that are unaligned. This implies, however, additional overhead as the kernel crypto API needs to perform the realignment of the data which may imply moving of data.

#### Cipher Definition With struct blkcipher_alg and ablkcipher_alg

Struct blkcipher_alg defines a synchronous block cipher whereas struct ablkcipher_alg defines an asynchronous block cipher.

Please refer to the single block cipher description for schematics of the block cipher usage. The usage patterns are exactly the same for [ABLKCIPHER] and [BLKCIPHER] as they are for plain [CIPHER].

#### Specifics Of Asynchronous Multi-Block Cipher

There are a couple of specifics to the [ABLKCIPHER] interface.

First of all, some of the drivers will want to use the Generic ScatterWalk in case the hardware needs to be fed separate chunks of the scatterlist which contains the plaintext and will contain the ciphertext. Please refer to the ScatterWalk interface offered by the Linux kernel scatter / gather list implementation.

### Hashing [HASH]

Example of transformations: crc32, md5, sha1, sha256,...

#### Registering And Unregistering The Transformation

There are multiple ways to register a HASH transformation, depending on whether the transformation is synchronous [SHASH] or asynchronous [AHASH] and the amount of HASH transformations we are registering. You can find the prototypes defined in include/crypto/internal/hash.h:

       int crypto_register_ahash(struct ahash_alg *alg);

       int crypto_register_shash(struct shash_alg *alg);
       int crypto_register_shashes(struct shash_alg *algs, int count);

The respective counterparts for unregistering the HASH transformation are as follows:

       int crypto_unregister_ahash(struct ahash_alg *alg);

       int crypto_unregister_shash(struct shash_alg *alg);
       int crypto_unregister_shashes(struct shash_alg *algs, int count);

#### Cipher Definition With struct shash_alg and ahash_alg

Here are schematics of how these functions are called when operated from other part of the kernel. Note that the .setkey() call might happen before or after any of these schematics happen, but must not happen during any of these are in-flight. Please note that calling .init() followed immediately by .finish() is also a perfectly valid transformation.

       I)   DATA -----------.
                            v
             .init() -> .update() -> .final()      ! .update() might not be called
                         ^    |         |            at all in this scenario.
                         '----'         '---> HASH

       II)  DATA -----------.-----------.
                            v           v
             .init() -> .update() -> .finup()      ! .update() may not be called
                         ^    |         |            at all in this scenario.
                         '----'         '---> HASH

       III) DATA -----------.
                            v
                        .digest()                  ! The entire process is handled
                            |                        by the .digest() call.
                            '---------------> HASH

Here is a schematic of how the .export()/.import() functions are called when used from another part of the kernel.

       KEY--.                 DATA--.
            v                       v                  ! .update() may not be called
        .setkey() -> .init() -> .update() -> .export()   at all in this scenario.
                                 ^     |         |
                                 '-----'         '--> PARTIAL_HASH

       ----------- other transformations happen here -----------

       PARTIAL_HASH--.   DATA1--.
                     v          v
                 .import -> .update() -> .final()     ! .update() may not be called
                             ^    |         |           at all in this scenario.
                             '----'         '--> HASH1

       PARTIAL_HASH--.   DATA2-.
                     v         v
                 .import -> .finup()
                               |
                               '---------------> HASH2

#### Specifics Of Asynchronous HASH Transformation

Some of the drivers will want to use the Generic ScatterWalk in case the implementation needs to be fed separate chunks of the scatterlist which contains the input data. The buffer containing the resulting hash will always be properly aligned to .cra_alignmask so there is no need to worry about this.

## Programming Interface

### Block Cipher Context Data Structures

!Pinclude/linux/crypto.h Block Cipher Context Data Structures !Finclude/linux/crypto.h aead_request

### Block Cipher Algorithm Definitions

!Pinclude/linux/crypto.h Block Cipher Algorithm Definitions !Finclude/linux/crypto.h crypto_alg !Finclude/linux/crypto.h ablkcipher_alg !Finclude/linux/crypto.h aead_alg !Finclude/linux/crypto.h blkcipher_alg !Finclude/linux/crypto.h cipher_alg !Finclude/linux/crypto.h rng_alg

### Asynchronous Block Cipher API

!Pinclude/linux/crypto.h Asynchronous Block Cipher API !Finclude/linux/crypto.h crypto_alloc_ablkcipher !Finclude/linux/crypto.h crypto_free_ablkcipher !Finclude/linux/crypto.h crypto_has_ablkcipher !Finclude/linux/crypto.h crypto_ablkcipher_ivsize !Finclude/linux/crypto.h crypto_ablkcipher_blocksize !Finclude/linux/crypto.h crypto_ablkcipher_setkey !Finclude/linux/crypto.h crypto_ablkcipher_reqtfm !Finclude/linux/crypto.h crypto_ablkcipher_encrypt !Finclude/linux/crypto.h crypto_ablkcipher_decrypt

### Asynchronous Cipher Request Handle

!Pinclude/linux/crypto.h Asynchronous Cipher Request Handle !Finclude/linux/crypto.h crypto_ablkcipher_reqsize !Finclude/linux/crypto.h ablkcipher_request_set_tfm !Finclude/linux/crypto.h ablkcipher_request_alloc !Finclude/linux/crypto.h ablkcipher_request_free !Finclude/linux/crypto.h ablkcipher_request_set_callback !Finclude/linux/crypto.h ablkcipher_request_set_crypt

### Authenticated Encryption With Associated Data (AEAD) Cipher API

!Pinclude/linux/crypto.h Authenticated Encryption With Associated Data (AEAD) Cipher API !Finclude/linux/crypto.h crypto_alloc_aead !Finclude/linux/crypto.h crypto_free_aead !Finclude/linux/crypto.h crypto_aead_ivsize !Finclude/linux/crypto.h crypto_aead_authsize !Finclude/linux/crypto.h crypto_aead_blocksize !Finclude/linux/crypto.h crypto_aead_setkey !Finclude/linux/crypto.h crypto_aead_setauthsize !Finclude/linux/crypto.h crypto_aead_encrypt !Finclude/linux/crypto.h crypto_aead_decrypt

### Asynchronous AEAD Request Handle

!Pinclude/linux/crypto.h Asynchronous AEAD Request Handle !Finclude/linux/crypto.h crypto_aead_reqsize !Finclude/linux/crypto.h aead_request_set_tfm !Finclude/linux/crypto.h aead_request_alloc !Finclude/linux/crypto.h aead_request_free !Finclude/linux/crypto.h aead_request_set_callback !Finclude/linux/crypto.h aead_request_set_crypt !Finclude/linux/crypto.h aead_request_set_assoc

### Synchronous Block Cipher API

!Pinclude/linux/crypto.h Synchronous Block Cipher API !Finclude/linux/crypto.h crypto_alloc_blkcipher !Finclude/linux/crypto.h crypto_free_blkcipher !Finclude/linux/crypto.h crypto_has_blkcipher !Finclude/linux/crypto.h crypto_blkcipher_name !Finclude/linux/crypto.h crypto_blkcipher_ivsize !Finclude/linux/crypto.h crypto_blkcipher_blocksize !Finclude/linux/crypto.h crypto_blkcipher_setkey !Finclude/linux/crypto.h crypto_blkcipher_encrypt !Finclude/linux/crypto.h crypto_blkcipher_encrypt_iv !Finclude/linux/crypto.h crypto_blkcipher_decrypt !Finclude/linux/crypto.h crypto_blkcipher_decrypt_iv !Finclude/linux/crypto.h crypto_blkcipher_set_iv !Finclude/linux/crypto.h crypto_blkcipher_get_iv

### Single Block Cipher API

!Pinclude/linux/crypto.h Single Block Cipher API !Finclude/linux/crypto.h crypto_alloc_cipher !Finclude/linux/crypto.h crypto_free_cipher !Finclude/linux/crypto.h crypto_has_cipher !Finclude/linux/crypto.h crypto_cipher_blocksize !Finclude/linux/crypto.h crypto_cipher_setkey !Finclude/linux/crypto.h crypto_cipher_encrypt_one !Finclude/linux/crypto.h crypto_cipher_decrypt_one

### Synchronous Message Digest API

!Pinclude/linux/crypto.h Synchronous Message Digest API !Finclude/linux/crypto.h crypto_alloc_hash !Finclude/linux/crypto.h crypto_free_hash !Finclude/linux/crypto.h crypto_has_hash !Finclude/linux/crypto.h crypto_hash_blocksize !Finclude/linux/crypto.h crypto_hash_digestsize !Finclude/linux/crypto.h crypto_hash_init !Finclude/linux/crypto.h crypto_hash_update !Finclude/linux/crypto.h crypto_hash_final !Finclude/linux/crypto.h crypto_hash_digest !Finclude/linux/crypto.h crypto_hash_setkey

### Message Digest Algorithm Definitions

!Pinclude/crypto/hash.h Message Digest Algorithm Definitions !Finclude/crypto/hash.h hash_alg_common !Finclude/crypto/hash.h ahash_alg !Finclude/crypto/hash.h shash_alg

### Asynchronous Message Digest API

!Pinclude/crypto/hash.h Asynchronous Message Digest API !Finclude/crypto/hash.h crypto_alloc_ahash !Finclude/crypto/hash.h crypto_free_ahash !Finclude/crypto/hash.h crypto_ahash_init !Finclude/crypto/hash.h crypto_ahash_digestsize !Finclude/crypto/hash.h crypto_ahash_reqtfm !Finclude/crypto/hash.h crypto_ahash_reqsize !Finclude/crypto/hash.h crypto_ahash_setkey !Finclude/crypto/hash.h crypto_ahash_finup !Finclude/crypto/hash.h crypto_ahash_final !Finclude/crypto/hash.h crypto_ahash_digest !Finclude/crypto/hash.h crypto_ahash_export !Finclude/crypto/hash.h crypto_ahash_import

### Asynchronous Hash Request Handle

!Pinclude/crypto/hash.h Asynchronous Hash Request Handle !Finclude/crypto/hash.h ahash_request_set_tfm !Finclude/crypto/hash.h ahash_request_alloc !Finclude/crypto/hash.h ahash_request_free !Finclude/crypto/hash.h ahash_request_set_callback !Finclude/crypto/hash.h ahash_request_set_crypt

### Synchronous Message Digest API

!Pinclude/crypto/hash.h Synchronous Message Digest API !Finclude/crypto/hash.h crypto_alloc_shash !Finclude/crypto/hash.h crypto_free_shash !Finclude/crypto/hash.h crypto_shash_blocksize !Finclude/crypto/hash.h crypto_shash_digestsize !Finclude/crypto/hash.h crypto_shash_descsize !Finclude/crypto/hash.h crypto_shash_setkey !Finclude/crypto/hash.h crypto_shash_digest !Finclude/crypto/hash.h crypto_shash_export !Finclude/crypto/hash.h crypto_shash_import !Finclude/crypto/hash.h crypto_shash_init !Finclude/crypto/hash.h crypto_shash_update !Finclude/crypto/hash.h crypto_shash_final !Finclude/crypto/hash.h crypto_shash_finup

### Crypto API Random Number API

!Pinclude/crypto/rng.h Random number generator API !Finclude/crypto/rng.h crypto_alloc_rng !Finclude/crypto/rng.h crypto_rng_alg !Finclude/crypto/rng.h crypto_free_rng !Finclude/crypto/rng.h crypto_rng_get_bytes !Finclude/crypto/rng.h crypto_rng_reset !Finclude/crypto/rng.h crypto_rng_seedsize !Cinclude/crypto/rng.h

## Code Examples

### Code Example For Asynchronous Block Cipher Operation

    struct tcrypt_result {
    	struct completion completion;
    	int err;
    };

    /* tie all data structures together */
    struct ablkcipher_def {
    	struct scatterlist sg;
    	struct crypto_ablkcipher *tfm;
    	struct ablkcipher_request *req;
    	struct tcrypt_result result;
    };

    /* Callback function */
    static void test_ablkcipher_cb(struct crypto_async_request *req, int error)
    {
    	struct tcrypt_result *result = req->data;

    	if (error == -EINPROGRESS)
    		return;
    	result->err = error;
    	complete(&result->completion);
    	pr_info("Encryption finished successfully\n");
    }

    /* Perform cipher operation */
    static unsigned int test_ablkcipher_encdec(struct ablkcipher_def *ablk,
    					   int enc)
    {
    	int rc = 0;

    	if (enc)
    		rc = crypto_ablkcipher_encrypt(ablk->req);
    	else
    		rc = crypto_ablkcipher_decrypt(ablk->req);

    	switch (rc) {
    	case 0:
    		break;
    	case -EINPROGRESS:
    	case -EBUSY:
    		rc = wait_for_completion_interruptible(
    			&ablk->result.completion);
    		if (!rc && !ablk->result.err) {
    			reinit_completion(&ablk->result.completion);
    			break;
    		}
    	default:
    		pr_info("ablkcipher encrypt returned with %d result %d\n",
    		       rc, ablk->result.err);
    		break;
    	}
    	init_completion(&ablk->result.completion);

    	return rc;
    }

    /* Initialize and trigger cipher operation */
    static int test_ablkcipher(void)
    {
    	struct ablkcipher_def ablk;
    	struct crypto_ablkcipher *ablkcipher = NULL;
    	struct ablkcipher_request *req = NULL;
    	char *scratchpad = NULL;
    	char *ivdata = NULL;
    	unsigned char key[32];
    	int ret = -EFAULT;

    	ablkcipher = crypto_alloc_ablkcipher("cbc-aes-aesni", 0, 0);
    	if (IS_ERR(ablkcipher)) {
    		pr_info("could not allocate ablkcipher handle\n");
    		return PTR_ERR(ablkcipher);
    	}

    	req = ablkcipher_request_alloc(ablkcipher, GFP_KERNEL);
    	if (IS_ERR(req)) {
    		pr_info("could not allocate request queue\n");
    		ret = PTR_ERR(req);
    		goto out;
    	}

    	ablkcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG,
    					test_ablkcipher_cb,
    					&ablk.result);

    	/* AES 256 with random key */
    	get_random_bytes(&key, 32);
    	if (crypto_ablkcipher_setkey(ablkcipher, key, 32)) {
    		pr_info("key could not be set\n");
    		ret = -EAGAIN;
    		goto out;
    	}

    	/* IV will be random */
    	ivdata = kmalloc(16, GFP_KERNEL);
    	if (!ivdata) {
    		pr_info("could not allocate ivdata\n");
    		goto out;
    	}
    	get_random_bytes(ivdata, 16);

    	/* Input data will be random */
    	scratchpad = kmalloc(16, GFP_KERNEL);
    	if (!scratchpad) {
    		pr_info("could not allocate scratchpad\n");
    		goto out;
    	}
    	get_random_bytes(scratchpad, 16);

    	ablk.tfm = ablkcipher;
    	ablk.req = req;

    	/* We encrypt one block */
    	sg_init_one(&ablk.sg, scratchpad, 16);
    	ablkcipher_request_set_crypt(req, &ablk.sg, &ablk.sg, 16, ivdata);
    	init_completion(&ablk.result.completion);

    	/* encrypt data */
    	ret = test_ablkcipher_encdec(&ablk, 1);
    	if (ret)
    		goto out;

    	pr_info("Encryption triggered successfully\n");

    out:
    	if (ablkcipher)
    		crypto_free_ablkcipher(ablkcipher);
    	if (req)
    		ablkcipher_request_free(req);
    	if (ivdata)
    		kfree(ivdata);
    	if (scratchpad)
    		kfree(scratchpad);
    	return ret;
    }

### Code Example For Synchronous Block Cipher Operation

    static int test_blkcipher(void)
    {
    	struct crypto_blkcipher *blkcipher = NULL;
    	char *cipher = "cbc(aes)";
    	// AES 128
    	charkey =
    "\x12\x34\x56\x78\x90\xab\xcd\xef\x12\x34\x56\x78\x90\xab\xcd\xef";
    	chariv =
    "\x12\x34\x56\x78\x90\xab\xcd\xef\x12\x34\x56\x78\x90\xab\xcd\xef";
    	unsigned int ivsize = 0;
    	char *scratchpad = NULL; // holds plaintext and ciphertext
    	struct scatterlist sg;
    	struct blkcipher_desc desc;
    	int ret = -EFAULT;

    	blkcipher = crypto_alloc_blkcipher(cipher, 0, 0);
    	if (IS_ERR(blkcipher)) {
    		printk("could not allocate blkcipher handle for %s\n", cipher);
    		return -PTR_ERR(blkcipher);
    	}

    	if (crypto_blkcipher_setkey(blkcipher, key, strlen(key))) {
    		printk("key could not be set\n");
    		ret = -EAGAIN;
    		goto out;
    	}

    	ivsize = crypto_blkcipher_ivsize(blkcipher);
    	if (ivsize) {
    		if (ivsize != strlen(iv))
    			printk("IV length differs from expected length\n");
    		crypto_blkcipher_set_iv(blkcipher, iv, ivsize);
    	}

    	scratchpad = kmalloc(crypto_blkcipher_blocksize(blkcipher), GFP_KERNEL);
    	if (!scratchpad) {
    		printk("could not allocate scratchpad for %s\n", cipher);
    		goto out;
    	}
    	/* get some random data that we want to encrypt */
    	get_random_bytes(scratchpad, crypto_blkcipher_blocksize(blkcipher));

    	desc.flags = 0;
    	desc.tfm = blkcipher;
    	sg_init_one(&sg, scratchpad, crypto_blkcipher_blocksize(blkcipher));

    	/* encrypt data in place */
    	crypto_blkcipher_encrypt(&desc, &sg, &sg,
    				 crypto_blkcipher_blocksize(blkcipher));

    	/* decrypt data in place
    	 * crypto_blkcipher_decrypt(&desc, &sg, &sg,
    	 */			 crypto_blkcipher_blocksize(blkcipher));

    	printk("Cipher operation completed\n");
    	return 0;

    out:
    	if (blkcipher)
    		crypto_free_blkcipher(blkcipher);
    	if (scratchpad)
    		kzfree(scratchpad);
    	return ret;
    }

### Code Example For Use of Operational State Memory With SHASH

    struct sdesc {
    	struct shash_desc shash;
    	char ctx[];
    };

    static struct sdescinit_sdesc(struct crypto_shash *alg)
    {
    	struct sdescsdesc;
    	int size;

    	size = sizeof(struct shash_desc) + crypto_shash_descsize(alg);
    	sdesc = kmalloc(size, GFP_KERNEL);
    	if (!sdesc)
    		return ERR_PTR(-ENOMEM);
    	sdesc->shash.tfm = alg;
    	sdesc->shash.flags = 0x0;
    	return sdesc;
    }

    static int calc_hash(struct crypto_shashalg,
    		     const unsigned chardata, unsigned int datalen,
    		     unsigned chardigest) {
    	struct sdescsdesc;
    	int ret;

    	sdesc = init_sdesc(alg);
    	if (IS_ERR(sdesc)) {
    		pr_info("trusted_key: can't alloc %s\n", hash_alg);
    		return PTR_ERR(sdesc);
    	}

    	ret = crypto_shash_digest(&sdesc->shash, data, datalen, digest);
    	kfree(sdesc);
    	return ret;
    }

### Code Example For Random Number Generator Usage

    static int get_random_numbers(u8 *buf, unsigned int len)
    {
    	struct crypto_rngrng = NULL;
    	chardrbg = "drbg_nopr_sha256"; /* Hash DRBG with SHA-256, no PR */
    	int ret;

    	if (!buf || !len) {
    		pr_debug("No output buffer provided\n");
    		return -EINVAL;
    	}

    	rng = crypto_alloc_rng(drbg, 0, 0);
    	if (IS_ERR(rng)) {
    		pr_debug("could not allocate RNG handle for %s\n", drbg);
    		return -PTR_ERR(rng);
    	}

    	ret = crypto_rng_get_bytes(rng, buf, len);
    	if (ret < 0)
    		pr_debug("generation of random numbers failed\n");
    	else if (ret == 0)
    		pr_debug("RNG returned no data");
    	else
    		pr_debug("RNG returned %d bytes of data\n", ret);

    out:
    	crypto_free_rng(rng);
    	return ret;
    }