Message-ID: <1415226082.5083.24.camel@oc8377654763.ibm.com>
Date:	Wed, 05 Nov 2014 16:21:22 -0600
From:	"Joy M. Latten" <jmlatten@...ux.vnet.ibm.com>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Marek Vasut <marex@...x.de>,
	Jason Cooper <cryptography@...edaemon.net>,
	Grant Likely <grant.likely@...retlab.ca>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2 01/11] crypto: Documentation - crypto API high level
 spec

Hi Stephan,

Great docs! I think they will be very useful! Thanks!
I only have 2 comments below.

regards,
Joy

On Sun, 2014-11-02 at 21:35 +0100, Stephan Mueller wrote:
> The design of the kernel crypto API as well as hints to program with
> the kernel crypto API are given.
> 
> The documentation contains:
>  * design aspects of crypto API
>  * develper specific hints
>  * references to the API function description
> 
> Signed-off-by: Stephan Mueller <smueller@...onox.de>
> CC: Marek Vasut <marex@...x.de>
> ---
>  Documentation/crypto/crypto-API-spec.txt | 721 +++++++++++++++++++++++++++++++
>  1 file changed, 721 insertions(+)
>  create mode 100644 Documentation/crypto/crypto-API-spec.txt
> 
> diff --git a/Documentation/crypto/crypto-API-spec.txt b/Documentation/crypto/crypto-API-spec.txt
> new file mode 100644
> index 0000000..8a24c98
> --- /dev/null
> +++ b/Documentation/crypto/crypto-API-spec.txt
> @@ -0,0 +1,721 @@
> +Kernel Crypto API Interface Specification
> +=========================================
> +
> +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 "transformation". Therefore, a
> +cipher handle variable usually has the name "tfm". Besides cryptographic
> +operations, the kernel crypto API also knows compression transforms 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 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.

Although you go into further explanation shortly, I think an example or
two here might help reader to immediately grab concept. i.e. aes,
cbc(aes), ...
> +
> +
> +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.
> +
> +
> +Kernel 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. When reading that file,
> +all available ciphers are listed. However, that list does not specify all
> +possible permutations of templates and ciphers. Each block listed in
> +/proc/crypto contains the following information:
> +
> +	* 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
> +
> +	* 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 symmetric block ciphers
> +		- ablkcipher for asymmetric block ciphers
> +		- cipher for single block ciphers that may be used with an
> +		  additional template
> +		- shash for symmetric message digest
> +		- ahash for asymmetric message digest
> +		- aead for AEAD cipher type
> +		- compression for compression type transforms
> +		- rng for random number generator
> +		- givcipher for cipher with associated IV generator
> +
> +	* 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	Raw 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
> + * 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
> +
> +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
> +kernel crypto API´s search 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.
> +
> +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.
> +
> +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.
> +
> +Single-block ciphers [CIPHER]
> +-----------------------------
> +Example of transformations: aes, arc4, ...
> +
> +This section describes the simplest of all transformation implementations,
> +that being the CIPHER type. 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.
> +
Picky, but I think maybe transitioning reader to the symmetric ciphers
might be a bit helpful here... because I was still interpreting "cipher"
as meaning everything. But after scanning below I think we are only
talking about symmetric ciphers now...

> +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.
> +
> +Fields in struct cipher_alg explained
> +.....................................
> +
> +This section explains the .cra_u.cipher fields and how they are called.
> +All of the fields are mandatory and must be filled:
> +
> +   .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.
> +                        - 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.
> +   .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.
> +
> +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. There are no specifics for
> +this case other that re-aligning of input and output buffers does not happen
> +automatically within the crypto API, but is the responsibility of the crypto
> +API consumer. The crypto API consumer shall use crypto_blkcipher_alignmask()
> +or crypto_ablkcipher_alignmask() respectively to determine the needs of the
> +transformation object and prepare the scatterlist with data accordingly.
> +
> +Fields in struct blkcipher_alg and struct ablkcipher_alg explained
> +..................................................................
> +
> +This section explains the .cra_u.blkcipher and .cra_u.cra_ablkcipher fields
> +and how they are called. Please note that this is very similar to the basic
> +CIPHER case for all but minor details. All of the fields but .geniv are
> +mandatory and must be filled:
> +
> +   .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/
> +   .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/
> +   .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.
> +                    - 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.
> +   .encrypt() ..... Encrypt a scatterlist of blocks.
> +                    - This function is used to encrypt the supplied
> +                      scatterlist containing the blocks of data. The crypto
> +                      API consumer is responsible for aligning the entries
> +                      of the scatterlist properly and making sure the
> +                      chunks are correctly sized.
> +                    - 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.
> +   .decrypt() ..... Decrypt a single block.
> +                    - This is a reverse counterpart to .encrypt(), and the
> +                      conditions are exactly the same.
> +
> +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
> +below for a description and usage of the Generic ScatterWalk interface.
> +
> +It is recommended to enqueue cryptographic transformation requests into
> +generic crypto queues. This allows for these requests to be processed in
> +sequence as the cryptographic hardware becomes free.
> +
> +
> +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);
> +
> +Common fields of struct shash_alg and ahash_alg explained
> +.........................................................
> +
> +For definition of these structures, please refer to include/crypto/hash.h .
> +We will now explain the meaning of each field:
> +
> +   .init() ......... Initialize the transformation context.
> +                     - Intended only to initialize the state of the HASH
> +                       transformation at the begining. This shall fill in
> +                       the internal structures used during the entire duration
> +                       of the whole transformation.
> +                     - No data processing happens at this point.
> +   .update() ....... Push chunk of data into the driver for transformation.
> +                     - This function actually pushes blocks of data from upper
> +                       layers into the driver, which then passes those to the
> +                       hardware as seen fit.
> +                     - This function must not finalize the HASH transformation,
> +                       this only adds more data into the transformation.
> +                     - This function shall not modify the transformation
> +                       context, as this function may be called in parallel
> +                       with the same transformation object.
> +                     - Data processing can happen synchronously [SHASH] or
> +                       asynchronously [AHASH] at this point.
> +   .final() ....... Retrieve result from the driver.
> +                     - This function finalizes the transformation and retrieves
> +                       the resulting hash from the driver and pushes it back to
> +                       upper layers.
> +                     - No data processing happens at this point.
> +   .finup() ........ Combination of update()+final() .
> +                     - This function is effectively a combination of update()
> +                       and final() calls issued in sequence.
> +                     - As some hardware cannot do update() and final()
> +                       separately, this callback was added to allow such
> +                       hardware to be used at least by IPsec.
> +                     - Data processing can happen synchronously [SHASH] or
> +                       asynchronously [AHASH] at this point.
> +   .digest() ....... Combination of init()+update()+final() .
> +                     - This function effectively behaves as the entire chain
> +                       of operations, init(), update() and final() issued in
> +                       sequence.
> +                     - Just like .finup(), this was added for hardware which
> +                       cannot do even the .finup(), but can only do the whole
> +                       transformation in one run.
> +                     - Data processing can happen synchronously [SHASH] or
> +                       asynchronously [AHASH] at this point.
> +
> +   .setkey() ....... Set optional key used by the hashing algorithm .
> +                     - Intended to push optional key used by the hashing
> +                       algorithm from upper layers into the driver.
> +                     - This function can store the key in the transformation
> +                       context or can outright program it into the hardware.
> +                       In the former case, one must be careful to program
> +                       the key into the hardware at appropriate time and one
> +                       must be careful that .setkey() can be called multiple
> +                       times during the existence of the transformation
> +                       object.
> +                     - Not all hashing algorithms do implement this function.
> +                       -> SHAx/MDx/CRCx do NOT implement this function.
> +                       -> HMAC(MDx)/HMAC(SHAx) do implement this function.
> +                     - This function must be called before any other of the
> +                       init()/update()/final()/finup()/digest() is called.
> +
Maybe it would be good to mention that setkey is only required for
"keyed message digests" such as the hmacs. Although you do point this
out above by naming them.

>                      - No data processing happens at this point.
> +
> +   .export() ....... Export partial state of the transformation .
> +                     - This function dumps the entire state of the ongoing
> +                       transformation into a provided block of data so it
> +                       can be .import()ed back later on.
> +                     - This is useful in case you want to save partial result
> +                       of the transformation after processing certain amount
> +                       of data and reload this partial result multiple times
> +                       later on for multiple re-use.
> +                     - No data processing happens at this point.
> +   .import() ....... Import partial state of the transformation .
> +                     - This function loads the entire state of the ongoing
> +                       transformation from a provided block of data so the
> +                       transformation can continue from this point onward.
> +                     - No data processing happens at this point.
> +
> +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 immediatelly
> +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
> +
> +The struct hash_alg_common fields and it's mirror in struct shash_alg
> +.....................................................................
> +
> +This structure defines various size constraints and generic properties of
> +the hashing algorithm that is being implemented. Let us first inspect the
> +size properties:
> +
> +   digestsize .... Size of the result of the transformation.
> +                   - A buffer of this size must be available to the .final()
> +                     and .finup() calls, so they can store the resulting hash
> +                     into it.
> +                   - For various predefined sizes, search include/crypto/
> +                     using 'git grep _DIGEST_SIZE include/crypto' .
> +   statesize ..... Size of the block for partial state of the transformation.
> +                   - A buffer of this size must be passed to the .export()
> +                     function as it will save the partial state of the
> +                     transformation into it. On the other side, the .import()
> +                     function will load the state from a buffer of this size
> +                     as well.
> +
> +
> +Specifics of asynchronous HASH transformation
> +.............................................
> +
> +There are a couple of specifics to the [AHASH] 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 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. Please refer to the section 9.1) of this document of the
> +description and usage of the Generic ScatterWalk interface.
> +
> +It is recommended to enqueue cryptographic transformation requests into
> +generic crypto queues. This allows for these requests to be processed in
> +sequence as the cryptographic hardware becomes free.
> +
> +
> +Single block cipher API
> +=======================
> +
> +See source code comments for the *_cipher* calls in include/linux/crypto.h.
> +
> +
> +Synchronous block cipher API
> +============================
> +
> +See source code comments for the *_blkcipher* calls in include/linux/crypto.h.
> +
> +
> +Asynchronous block cipher API
> +=============================
> +
> +See source code comments for the *_ablkcipher* calls and "ablkcipher_request_*
> +calls in include/linux/crypto.h.
> +
> +
> +Synchronous message digest API
> +==============================
> +
> +See source code comments for the *_hash* calls in include/linux/crypto.h.
> +
> +
> +Synchronous message digest API with caller-accessible state space
> +=================================================================
> +
> +See source code comments for the *_shash* calls in include/crypto/hash.h.
> +
> +
> +Asynchronous message digest API
> +===============================
> +
> +See source code comments for the *_ahash* calls and "ahash_request_*
> +calls in include/crypto/hash.h.
> +
> +
> +Random number generation API
> +============================
> +
> +See source code comments for the *_rng* calls in include/crypto/hash.h.
> +
> +
> +AEAD asynchronous cipher API
> +============================
> +
> +See source code comments for the *_aead* calls and "aead_request_*
> +calls in include/linux/crypto.h.
> +
> +
> +Authors
> +=======
> +
> +Stephan Mueller <smueller@...onox.de>
> +Marek Vasut <marex@...x.de>


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