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8d23da22ac
Add the KPP API documentation to the kernel crypto API Sphinx documentation. This addition includes the documentation of the ECDH and DH helpers which are needed to create the approrpiate input data for the crypto_kpp_set_secret function. Signed-off-by: Stephan Mueller <smueller@chronox.de> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
442 lines
15 KiB
ReStructuredText
442 lines
15 KiB
ReStructuredText
Kernel Crypto API Architecture
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==============================
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Cipher algorithm types
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----------------------
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The kernel crypto API provides different API calls for the following
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cipher types:
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- Symmetric ciphers
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- AEAD ciphers
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- Message digest, including keyed message digest
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- Random number generation
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- User space interface
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Ciphers And Templates
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---------------------
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The kernel crypto API provides implementations of single block ciphers
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and message digests. In addition, the kernel crypto API provides
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numerous "templates" that can be used in conjunction with the single
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block ciphers and message digests. Templates include all types of block
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chaining mode, the HMAC mechanism, etc.
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Single block ciphers and message digests can either be directly used by
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a caller or invoked together with a template to form multi-block ciphers
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or keyed message digests.
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A single block cipher may even be called with multiple templates.
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However, templates cannot be used without a single cipher.
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See /proc/crypto and search for "name". For example:
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- aes
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- ecb(aes)
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- cmac(aes)
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- ccm(aes)
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- rfc4106(gcm(aes))
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- sha1
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- hmac(sha1)
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- authenc(hmac(sha1),cbc(aes))
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In these examples, "aes" and "sha1" are the ciphers and all others are
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the templates.
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Synchronous And Asynchronous Operation
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--------------------------------------
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The kernel crypto API provides synchronous and asynchronous API
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operations.
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When using the synchronous API operation, the caller invokes a cipher
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operation which is performed synchronously by the kernel crypto API.
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That means, the caller waits until the cipher operation completes.
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Therefore, the kernel crypto API calls work like regular function calls.
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For synchronous operation, the set of API calls is small and
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conceptually similar to any other crypto library.
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Asynchronous operation is provided by the kernel crypto API which
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implies that the invocation of a cipher operation will complete almost
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instantly. That invocation triggers the cipher operation but it does not
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signal its completion. Before invoking a cipher operation, the caller
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must provide a callback function the kernel crypto API can invoke to
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signal the completion of the cipher operation. Furthermore, the caller
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must ensure it can handle such asynchronous events by applying
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appropriate locking around its data. The kernel crypto API does not
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perform any special serialization operation to protect the caller's data
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integrity.
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Crypto API Cipher References And Priority
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-----------------------------------------
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A cipher is referenced by the caller with a string. That string has the
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following semantics:
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::
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template(single block cipher)
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where "template" and "single block cipher" is the aforementioned
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template and single block cipher, respectively. If applicable,
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additional templates may enclose other templates, such as
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::
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template1(template2(single block cipher)))
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The kernel crypto API may provide multiple implementations of a template
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or a single block cipher. For example, AES on newer Intel hardware has
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the following implementations: AES-NI, assembler implementation, or
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straight C. Now, when using the string "aes" with the kernel crypto API,
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which cipher implementation is used? The answer to that question is the
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priority number assigned to each cipher implementation by the kernel
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crypto API. When a caller uses the string to refer to a cipher during
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initialization of a cipher handle, the kernel crypto API looks up all
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implementations providing an implementation with that name and selects
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the implementation with the highest priority.
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Now, a caller may have the need to refer to a specific cipher
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implementation and thus does not want to rely on the priority-based
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selection. To accommodate this scenario, the kernel crypto API allows
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the cipher implementation to register a unique name in addition to
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common names. When using that unique name, a caller is therefore always
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sure to refer to the intended cipher implementation.
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The list of available ciphers is given in /proc/crypto. However, that
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list does not specify all possible permutations of templates and
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ciphers. Each block listed in /proc/crypto may contain the following
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information -- if one of the components listed as follows are not
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applicable to a cipher, it is not displayed:
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- name: the generic name of the cipher that is subject to the
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priority-based selection -- this name can be used by the cipher
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allocation API calls (all names listed above are examples for such
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generic names)
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- driver: the unique name of the cipher -- this name can be used by the
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cipher allocation API calls
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- module: the kernel module providing the cipher implementation (or
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"kernel" for statically linked ciphers)
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- priority: the priority value of the cipher implementation
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- refcnt: the reference count of the respective cipher (i.e. the number
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of current consumers of this cipher)
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- selftest: specification whether the self test for the cipher passed
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- type:
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- skcipher for symmetric key ciphers
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- cipher for single block ciphers that may be used with an
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additional template
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- shash for synchronous message digest
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- ahash for asynchronous message digest
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- aead for AEAD cipher type
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- compression for compression type transformations
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- rng for random number generator
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- givcipher for cipher with associated IV generator (see the geniv
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entry below for the specification of the IV generator type used by
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the cipher implementation)
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- kpp for a Key-agreement Protocol Primitive (KPP) cipher such as
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an ECDH or DH implementation
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- blocksize: blocksize of cipher in bytes
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- keysize: key size in bytes
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- ivsize: IV size in bytes
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- seedsize: required size of seed data for random number generator
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- digestsize: output size of the message digest
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- geniv: IV generation type:
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- eseqiv for encrypted sequence number based IV generation
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- seqiv for sequence number based IV generation
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- chainiv for chain iv generation
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- <builtin> is a marker that the cipher implements IV generation and
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handling as it is specific to the given cipher
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Key Sizes
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---------
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When allocating a cipher handle, the caller only specifies the cipher
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type. Symmetric ciphers, however, typically support multiple key sizes
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(e.g. AES-128 vs. AES-192 vs. AES-256). These key sizes are determined
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with the length of the provided key. Thus, the kernel crypto API does
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not provide a separate way to select the particular symmetric cipher key
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size.
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Cipher Allocation Type And Masks
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--------------------------------
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The different cipher handle allocation functions allow the specification
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of a type and mask flag. Both parameters have the following meaning (and
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are therefore not covered in the subsequent sections).
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The type flag specifies the type of the cipher algorithm. The caller
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usually provides a 0 when the caller wants the default handling.
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Otherwise, the caller may provide the following selections which match
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the aforementioned cipher types:
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- CRYPTO_ALG_TYPE_CIPHER Single block cipher
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- CRYPTO_ALG_TYPE_COMPRESS Compression
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- CRYPTO_ALG_TYPE_AEAD Authenticated Encryption with Associated Data
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(MAC)
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- CRYPTO_ALG_TYPE_BLKCIPHER Synchronous multi-block cipher
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- CRYPTO_ALG_TYPE_ABLKCIPHER Asynchronous multi-block cipher
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- CRYPTO_ALG_TYPE_GIVCIPHER Asynchronous multi-block cipher packed
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together with an IV generator (see geniv field in the /proc/crypto
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listing for the known IV generators)
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- CRYPTO_ALG_TYPE_KPP Key-agreement Protocol Primitive (KPP) such as
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an ECDH or DH implementation
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- CRYPTO_ALG_TYPE_DIGEST Raw message digest
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- CRYPTO_ALG_TYPE_HASH Alias for CRYPTO_ALG_TYPE_DIGEST
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- CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash
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- CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash
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- CRYPTO_ALG_TYPE_RNG Random Number Generation
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- CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher
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- CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of
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CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression /
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decompression instead of performing the operation on one segment
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only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace
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CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted.
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The mask flag restricts the type of cipher. The only allowed flag is
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CRYPTO_ALG_ASYNC to restrict the cipher lookup function to
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asynchronous ciphers. Usually, a caller provides a 0 for the mask flag.
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When the caller provides a mask and type specification, the caller
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limits the search the kernel crypto API can perform for a suitable
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cipher implementation for the given cipher name. That means, even when a
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caller uses a cipher name that exists during its initialization call,
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the kernel crypto API may not select it due to the used type and mask
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field.
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Internal Structure of Kernel Crypto API
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---------------------------------------
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The kernel crypto API has an internal structure where a cipher
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implementation may use many layers and indirections. This section shall
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help to clarify how the kernel crypto API uses various components to
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implement the complete cipher.
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The following subsections explain the internal structure based on
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existing cipher implementations. The first section addresses the most
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complex scenario where all other scenarios form a logical subset.
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Generic AEAD Cipher Structure
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The following ASCII art decomposes the kernel crypto API layers when
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using the AEAD cipher with the automated IV generation. The shown
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example is used by the IPSEC layer.
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For other use cases of AEAD ciphers, the ASCII art applies as well, but
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the caller may not use the AEAD cipher with a separate IV generator. In
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this case, the caller must generate the IV.
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The depicted example decomposes the AEAD cipher of GCM(AES) based on the
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generic C implementations (gcm.c, aes-generic.c, ctr.c, ghash-generic.c,
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seqiv.c). The generic implementation serves as an example showing the
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complete logic of the kernel crypto API.
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It is possible that some streamlined cipher implementations (like
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AES-NI) provide implementations merging aspects which in the view of the
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kernel crypto API cannot be decomposed into layers any more. In case of
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the AES-NI implementation, the CTR mode, the GHASH implementation and
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the AES cipher are all merged into one cipher implementation registered
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with the kernel crypto API. In this case, the concept described by the
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following ASCII art applies too. However, the decomposition of GCM into
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the individual sub-components by the kernel crypto API is not done any
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more.
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Each block in the following ASCII art is an independent cipher instance
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obtained from the kernel crypto API. Each block is accessed by the
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caller or by other blocks using the API functions defined by the kernel
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crypto API for the cipher implementation type.
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The blocks below indicate the cipher type as well as the specific logic
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implemented in the cipher.
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The ASCII art picture also indicates the call structure, i.e. who calls
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which component. The arrows point to the invoked block where the caller
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uses the API applicable to the cipher type specified for the block.
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::
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kernel crypto API | IPSEC Layer
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+-----------+ |
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| aead | <----------------------------------- esp_output
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| (seqiv) | ---+
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+-----------+ |
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| (2)
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+-----------+ |
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| | <--+ (2)
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| aead | <----------------------------------- esp_input
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| (gcm) | ------------+
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+-----------+ |
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| (3) | (5)
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v v
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+-----------+ +-----------+
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| skcipher | | ahash |
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| (ctr) | ---+ | (ghash) |
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+-----------+ | +-----------+
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+-----------+ | (4)
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| | <--+
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| cipher |
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| (aes) |
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+-----------+
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The following call sequence is applicable when the IPSEC layer triggers
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an encryption operation with the esp_output function. During
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configuration, the administrator set up the use of rfc4106(gcm(aes)) as
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the cipher for ESP. The following call sequence is now depicted in the
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ASCII art above:
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1. esp_output() invokes crypto_aead_encrypt() to trigger an
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encryption operation of the AEAD cipher with IV generator.
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In case of GCM, the SEQIV implementation is registered as GIVCIPHER
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in crypto_rfc4106_alloc().
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The SEQIV performs its operation to generate an IV where the core
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function is seqiv_geniv().
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2. Now, SEQIV uses the AEAD API function calls to invoke the associated
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AEAD cipher. In our case, during the instantiation of SEQIV, the
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cipher handle for GCM is provided to SEQIV. This means that SEQIV
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invokes AEAD cipher operations with the GCM cipher handle.
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During instantiation of the GCM handle, the CTR(AES) and GHASH
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ciphers are instantiated. The cipher handles for CTR(AES) and GHASH
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are retained for later use.
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The GCM implementation is responsible to invoke the CTR mode AES and
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the GHASH cipher in the right manner to implement the GCM
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specification.
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3. The GCM AEAD cipher type implementation now invokes the SKCIPHER API
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with the instantiated CTR(AES) cipher handle.
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During instantiation of the CTR(AES) cipher, the CIPHER type
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implementation of AES is instantiated. The cipher handle for AES is
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retained.
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That means that the SKCIPHER implementation of CTR(AES) only
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implements the CTR block chaining mode. After performing the block
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chaining operation, the CIPHER implementation of AES is invoked.
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4. The SKCIPHER of CTR(AES) now invokes the CIPHER API with the AES
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cipher handle to encrypt one block.
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5. The GCM AEAD implementation also invokes the GHASH cipher
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implementation via the AHASH API.
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When the IPSEC layer triggers the esp_input() function, the same call
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sequence is followed with the only difference that the operation starts
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with step (2).
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Generic Block Cipher Structure
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Generic block ciphers follow the same concept as depicted with the ASCII
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art picture above.
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For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The
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ASCII art picture above applies as well with the difference that only
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step (4) is used and the SKCIPHER block chaining mode is CBC.
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Generic Keyed Message Digest Structure
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Keyed message digest implementations again follow the same concept as
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depicted in the ASCII art picture above.
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For example, HMAC(SHA256) is implemented with hmac.c and
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sha256_generic.c. The following ASCII art illustrates the
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implementation:
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::
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kernel crypto API | Caller
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+-----------+ (1) |
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| | <------------------ some_function
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| ahash |
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| (hmac) | ---+
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+-----------+ |
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| (2)
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+-----------+ |
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| | <--+
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| shash |
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| (sha256) |
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+-----------+
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The following call sequence is applicable when a caller triggers an HMAC
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operation:
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1. The AHASH API functions are invoked by the caller. The HMAC
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implementation performs its operation as needed.
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During initialization of the HMAC cipher, the SHASH cipher type of
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SHA256 is instantiated. The cipher handle for the SHA256 instance is
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retained.
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At one time, the HMAC implementation requires a SHA256 operation
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where the SHA256 cipher handle is used.
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2. The HMAC instance now invokes the SHASH API with the SHA256 cipher
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handle to calculate the message digest.
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