415 lines
14 KiB
ReStructuredText
415 lines
14 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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- 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 generator (obsolete)
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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_KPP Key-agreement Protocol Primitive (KPP) such as
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an ECDH or DH implementation
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- CRYPTO_ALG_TYPE_HASH Raw message 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 seqiv(rfc4106(gcm(aes)))
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as the cipher for ESP. The following call sequence is now depicted in
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the 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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The SEQIV generates the IV.
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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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