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authorStephan Mueller <smueller@chronox.de>2015-02-27 20:00:00 +0100
committerHerbert Xu <herbert@gondor.apana.org.au>2015-03-04 22:12:39 +1300
commit7b24d97f16f561cc90eab1658100598d54a414fd (patch)
tree2fa00f1b217a27099f6c00c62c4a0fc0fda9f245 /Documentation/DocBook
parentd9850fc529efc0c1b3b938aab6916698dcf31f01 (diff)
downloadkernel-7b24d97f16f561cc90eab1658100598d54a414fd.tar.gz
crypto: doc - describe internal structure
The kernel crypto API has many indirections which warrant a description as otherwise one can get easily lost. The description explains the layers of the kernel crypto API based on examples. Signed-off-by: Stephan Mueller <smueller@chronox.de> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
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select it due to the used type and mask field.
</para>
</sect1>
+
+ <sect1><title>Internal Structure of Kernel Crypto API</title>
+
+ <para>
+ The kernel crypto API has an internal structure where a cipher
+ implementation may use many layers and indirections. This section
+ shall help to clarify how the kernel crypto API uses
+ various components to implement the complete cipher.
+ </para>
+
+ <para>
+ The following subsections explain the internal structure based
+ on existing cipher implementations. The first section addresses
+ the most complex scenario where all other scenarios form a logical
+ subset.
+ </para>
+
+ <sect2><title>Generic AEAD Cipher Structure</title>
+
+ <para>
+ The following ASCII art decomposes the kernel crypto API layers
+ when using the AEAD cipher with the automated IV generation. The
+ shown example is used by the IPSEC layer.
+ </para>
+
+ <para>
+ For other use cases of AEAD ciphers, the ASCII art applies as
+ well, but the caller may not use the GIVCIPHER interface. In
+ this case, the caller must generate the IV.
+ </para>
+
+ <para>
+ The depicted example decomposes the AEAD cipher of GCM(AES) based
+ on the generic C implementations (gcm.c, aes-generic.c, ctr.c,
+ ghash-generic.c, seqiv.c). The generic implementation serves as an
+ example showing the complete logic of the kernel crypto API.
+ </para>
+
+ <para>
+ It is possible that some streamlined cipher implementations (like
+ AES-NI) provide implementations merging aspects which in the view
+ of the kernel crypto API cannot be decomposed into layers any more.
+ In case of the AES-NI implementation, the CTR mode, the GHASH
+ implementation and the AES cipher are all merged into one cipher
+ implementation registered with the kernel crypto API. In this case,
+ the concept described by the following ASCII art applies too. However,
+ the decomposition of GCM into the individual sub-components
+ by the kernel crypto API is not done any more.
+ </para>
+
+ <para>
+ Each block in the following ASCII art is an independent cipher
+ instance obtained from the kernel crypto API. Each block
+ is accessed by the caller or by other blocks using the API functions
+ defined by the kernel crypto API for the cipher implementation type.
+ </para>
+
+ <para>
+ The blocks below indicate the cipher type as well as the specific
+ logic implemented in the cipher.
+ </para>
+
+ <para>
+ The ASCII art picture also indicates the call structure, i.e. who
+ calls which component. The arrows point to the invoked block
+ where the caller uses the API applicable to the cipher type
+ specified for the block.
+ </para>
+
+ <programlisting>
+<![CDATA[
+kernel crypto API | IPSEC Layer
+ |
++-----------+ |
+| | (1)
+| givcipher | <----------------------------------- esp_output
+| (seqiv) | ---+
++-----------+ |
+ | (2)
++-----------+ |
+| | <--+ (2)
+| aead | <----------------------------------- esp_input
+| (gcm) | ------------+
++-----------+ |
+ | (3) | (5)
+ v v
++-----------+ +-----------+
+| | | |
+| ablkcipher| | ahash |
+| (ctr) | ---+ | (ghash) |
++-----------+ | +-----------+
+ |
++-----------+ | (4)
+| | <--+
+| cipher |
+| (aes) |
++-----------+
+]]>
+ </programlisting>
+
+ <para>
+ The following call sequence is applicable when the IPSEC layer
+ triggers an encryption operation with the esp_output function. During
+ configuration, the administrator set up the use of rfc4106(gcm(aes)) as
+ the cipher for ESP. The following call sequence is now depicted in the
+ ASCII art above:
+ </para>
+
+ <orderedlist>
+ <listitem>
+ <para>
+ esp_output() invokes crypto_aead_givencrypt() to trigger an encryption
+ operation of the GIVCIPHER implementation.
+ </para>
+
+ <para>
+ In case of GCM, the SEQIV implementation is registered as GIVCIPHER
+ in crypto_rfc4106_alloc().
+ </para>
+
+ <para>
+ The SEQIV performs its operation to generate an IV where the core
+ function is seqiv_geniv().
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Now, SEQIV uses the AEAD API function calls to invoke the associated
+ AEAD cipher. In our case, during the instantiation of SEQIV, the
+ cipher handle for GCM is provided to SEQIV. This means that SEQIV
+ invokes AEAD cipher operations with the GCM cipher handle.
+ </para>
+
+ <para>
+ During instantiation of the GCM handle, the CTR(AES) and GHASH
+ ciphers are instantiated. The cipher handles for CTR(AES) and GHASH
+ are retained for later use.
+ </para>
+
+ <para>
+ The GCM implementation is responsible to invoke the CTR mode AES and
+ the GHASH cipher in the right manner to implement the GCM
+ specification.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The GCM AEAD cipher type implementation now invokes the ABLKCIPHER API
+ with the instantiated CTR(AES) cipher handle.
+ </para>
+
+ <para>
+ During instantiation of the CTR(AES) cipher, the CIPHER type
+ implementation of AES is instantiated. The cipher handle for AES is
+ retained.
+ </para>
+
+ <para>
+ That means that the ABLKCIPHER implementation of CTR(AES) only
+ implements the CTR block chaining mode. After performing the block
+ chaining operation, the CIPHER implementation of AES is invoked.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The ABLKCIPHER of CTR(AES) now invokes the CIPHER API with the AES
+ cipher handle to encrypt one block.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The GCM AEAD implementation also invokes the GHASH cipher
+ implementation via the AHASH API.
+ </para>
+ </listitem>
+ </orderedlist>
+
+ <para>
+ When the IPSEC layer triggers the esp_input() function, the same call
+ sequence is followed with the only difference that the operation starts
+ with step (2).
+ </para>
+ </sect2>
+
+ <sect2><title>Generic Block Cipher Structure</title>
+ <para>
+ Generic block ciphers follow the same concept as depicted with the ASCII
+ art picture above.
+ </para>
+
+ <para>
+ For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The
+ ASCII art picture above applies as well with the difference that only
+ step (4) is used and the ABLKCIPHER block chaining mode is CBC.
+ </para>
+ </sect2>
+
+ <sect2><title>Generic Keyed Message Digest Structure</title>
+ <para>
+ Keyed message digest implementations again follow the same concept as
+ depicted in the ASCII art picture above.
+ </para>
+
+ <para>
+ For example, HMAC(SHA256) is implemented with hmac.c and
+ sha256_generic.c. The following ASCII art illustrates the
+ implementation:
+ </para>
+
+ <programlisting>
+<![CDATA[
+kernel crypto API | Caller
+ |
++-----------+ (1) |
+| | <------------------ some_function
+| ahash |
+| (hmac) | ---+
++-----------+ |
+ | (2)
++-----------+ |
+| | <--+
+| shash |
+| (sha256) |
++-----------+
+]]>
+ </programlisting>
+
+ <para>
+ The following call sequence is applicable when a caller triggers
+ an HMAC operation:
+ </para>
+
+ <orderedlist>
+ <listitem>
+ <para>
+ The AHASH API functions are invoked by the caller. The HMAC
+ implementation performs its operation as needed.
+ </para>
+
+ <para>
+ During initialization of the HMAC cipher, the SHASH cipher type of
+ SHA256 is instantiated. The cipher handle for the SHA256 instance is
+ retained.
+ </para>
+
+ <para>
+ At one time, the HMAC implementation requires a SHA256 operation
+ where the SHA256 cipher handle is used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The HMAC instance now invokes the SHASH API with the SHA256
+ cipher handle to calculate the message digest.
+ </para>
+ </listitem>
+ </orderedlist>
+ </sect2>
+ </sect1>
</chapter>
<chapter id="Development"><title>Developing Cipher Algorithms</title>