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authorStephan Mueller <smueller@chronox.de>2015-03-06 21:34:22 +0100
committerHerbert Xu <herbert@gondor.apana.org.au>2015-03-09 21:06:18 +1100
commitdbe5fe7e1b3b3632bef2c09964a5f5505de4d744 (patch)
tree2479f39dfca4a23dc2a569597551949866198e8d /Documentation/DocBook
parentcde001e4c3c3625c60b68a83eb1f1c2572dee07a (diff)
downloadkernel-dbe5fe7e1b3b3632bef2c09964a5f5505de4d744.tar.gz
crypto: doc - AEAD / RNG AF_ALG interface
The patch moves the information provided in Documentation/crypto/crypto-API-userspace.txt into a separate chapter in the kernel crypto API DocBook. Some corrections are applied (such as removing a reference to Netlink when the AF_ALG socket is referred to). In addition, the AEAD and RNG interface description is now added. Also, a brief description of the zero-copy interface with an example code snippet is provided. Signed-off-by: Stephan Mueller <smueller@chronox.de> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
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diff --git a/Documentation/DocBook/crypto-API.tmpl b/Documentation/DocBook/crypto-API.tmpl
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+++ b/Documentation/DocBook/crypto-API.tmpl
@@ -1072,6 +1072,602 @@ kernel crypto API | Caller
</sect1>
</chapter>
+ <chapter id="User"><title>User Space Interface</title>
+ <sect1><title>Introduction</title>
+ <para>
+ The concepts of the kernel crypto API visible to kernel space is fully
+ applicable to the user space interface as well. Therefore, the kernel
+ crypto API high level discussion for the in-kernel use cases applies
+ here as well.
+ </para>
+
+ <para>
+ The major difference, however, is that user space can only act as a
+ consumer and never as a provider of a transformation or cipher algorithm.
+ </para>
+
+ <para>
+ The following covers the user space interface exported by the kernel
+ crypto API. A working example of this description is libkcapi that
+ can be obtained from [1]. That library can be used by user space
+ applications that require cryptographic services from the kernel.
+ </para>
+
+ <para>
+ Some details of the in-kernel kernel crypto API aspects do not
+ apply to user space, however. This includes the difference between
+ synchronous and asynchronous invocations. The user space API call
+ is fully synchronous.
+ </para>
+
+ <para>
+ [1] http://www.chronox.de/libkcapi.html
+ </para>
+
+ </sect1>
+
+ <sect1><title>User Space API General Remarks</title>
+ <para>
+ The kernel crypto API is accessible from user space. Currently,
+ the following ciphers are accessible:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>Message digest including keyed message digest (HMAC, CMAC)</para>
+ </listitem>
+
+ <listitem>
+ <para>Symmetric ciphers</para>
+ </listitem>
+
+ <listitem>
+ <para>AEAD ciphers</para>
+ </listitem>
+
+ <listitem>
+ <para>Random Number Generators</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ The interface is provided via socket type using the type AF_ALG.
+ In addition, the setsockopt option type is SOL_ALG. In case the
+ user space header files do not export these flags yet, use the
+ following macros:
+ </para>
+
+ <programlisting>
+#ifndef AF_ALG
+#define AF_ALG 38
+#endif
+#ifndef SOL_ALG
+#define SOL_ALG 279
+#endif
+ </programlisting>
+
+ <para>
+ A cipher is accessed with the same name as done for the in-kernel
+ API calls. This includes the generic vs. unique naming schema for
+ ciphers as well as the enforcement of priorities for generic names.
+ </para>
+
+ <para>
+ To interact with the kernel crypto API, a socket must be
+ created by the user space application. User space invokes the cipher
+ operation with the send()/write() system call family. The result of the
+ cipher operation is obtained with the read()/recv() system call family.
+ </para>
+
+ <para>
+ The following API calls assume that the socket descriptor
+ is already opened by the user space application and discusses only
+ the kernel crypto API specific invocations.
+ </para>
+
+ <para>
+ To initialize the socket interface, the following sequence has to
+ be performed by the consumer:
+ </para>
+
+ <orderedlist>
+ <listitem>
+ <para>
+ Create a socket of type AF_ALG with the struct sockaddr_alg
+ parameter specified below for the different cipher types.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Invoke bind with the socket descriptor
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Invoke accept with the socket descriptor. The accept system call
+ returns a new file descriptor that is to be used to interact with
+ the particular cipher instance. When invoking send/write or recv/read
+ system calls to send data to the kernel or obtain data from the
+ kernel, the file descriptor returned by accept must be used.
+ </para>
+ </listitem>
+ </orderedlist>
+ </sect1>
+
+ <sect1><title>In-place Cipher operation</title>
+ <para>
+ Just like the in-kernel operation of the kernel crypto API, the user
+ space interface allows the cipher operation in-place. That means that
+ the input buffer used for the send/write system call and the output
+ buffer used by the read/recv system call may be one and the same.
+ This is of particular interest for symmetric cipher operations where a
+ copying of the output data to its final destination can be avoided.
+ </para>
+
+ <para>
+ If a consumer on the other hand wants to maintain the plaintext and
+ the ciphertext in different memory locations, all a consumer needs
+ to do is to provide different memory pointers for the encryption and
+ decryption operation.
+ </para>
+ </sect1>
+
+ <sect1><title>Message Digest API</title>
+ <para>
+ The message digest type to be used for the cipher operation is
+ selected when invoking the bind syscall. bind requires the caller
+ to provide a filled struct sockaddr data structure. This data
+ structure must be filled as follows:
+ </para>
+
+ <programlisting>
+struct sockaddr_alg sa = {
+ .salg_family = AF_ALG,
+ .salg_type = "hash", /* this selects the hash logic in the kernel */
+ .salg_name = "sha1" /* this is the cipher name */
+};
+ </programlisting>
+
+ <para>
+ The salg_type value "hash" applies to message digests and keyed
+ message digests. Though, a keyed message digest is referenced by
+ the appropriate salg_name. Please see below for the setsockopt
+ interface that explains how the key can be set for a keyed message
+ digest.
+ </para>
+
+ <para>
+ Using the send() system call, the application provides the data that
+ should be processed with the message digest. The send system call
+ allows the following flags to be specified:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ MSG_MORE: If this flag is set, the send system call acts like a
+ message digest update function where the final hash is not
+ yet calculated. If the flag is not set, the send system call
+ calculates the final message digest immediately.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ With the recv() system call, the application can read the message
+ digest from the kernel crypto API. If the buffer is too small for the
+ message digest, the flag MSG_TRUNC is set by the kernel.
+ </para>
+
+ <para>
+ In order to set a message digest key, the calling application must use
+ the setsockopt() option of ALG_SET_KEY. If the key is not set the HMAC
+ operation is performed without the initial HMAC state change caused by
+ the key.
+ </para>
+ </sect1>
+
+ <sect1><title>Symmetric Cipher API</title>
+ <para>
+ The operation is very similar to the message digest discussion.
+ During initialization, the struct sockaddr data structure must be
+ filled as follows:
+ </para>
+
+ <programlisting>
+struct sockaddr_alg sa = {
+ .salg_family = AF_ALG,
+ .salg_type = "skcipher", /* this selects the symmetric cipher */
+ .salg_name = "cbc(aes)" /* this is the cipher name */
+};
+ </programlisting>
+
+ <para>
+ Before data can be sent to the kernel using the write/send system
+ call family, the consumer must set the key. The key setting is
+ described with the setsockopt invocation below.
+ </para>
+
+ <para>
+ Using the sendmsg() system call, the application provides the data that should be processed for encryption or decryption. In addition, the IV is
+ specified with the data structure provided by the sendmsg() system call.
+ </para>
+
+ <para>
+ The sendmsg system call parameter of struct msghdr is embedded into the
+ struct cmsghdr data structure. See recv(2) and cmsg(3) for more
+ information on how the cmsghdr data structure is used together with the
+ send/recv system call family. That cmsghdr data structure holds the
+ following information specified with a separate header instances:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ specification of the cipher operation type with one of these flags:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>ALG_OP_ENCRYPT - encryption of data</para>
+ </listitem>
+ <listitem>
+ <para>ALG_OP_DECRYPT - decryption of data</para>
+ </listitem>
+ </itemizedlist>
+ </listitem>
+
+ <listitem>
+ <para>
+ specification of the IV information marked with the flag ALG_SET_IV
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ The send system call family allows the following flag to be specified:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ MSG_MORE: If this flag is set, the send system call acts like a
+ cipher update function where more input data is expected
+ with a subsequent invocation of the send system call.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ Note: The kernel reports -EINVAL for any unexpected data. The caller
+ must make sure that all data matches the constraints given in
+ /proc/crypto for the selected cipher.
+ </para>
+
+ <para>
+ With the recv() system call, the application can read the result of
+ the cipher operation from the kernel crypto API. The output buffer
+ must be at least as large as to hold all blocks of the encrypted or
+ decrypted data. If the output data size is smaller, only as many
+ blocks are returned that fit into that output buffer size.
+ </para>
+ </sect1>
+
+ <sect1><title>AEAD Cipher API</title>
+ <para>
+ The operation is very similar to the symmetric cipher discussion.
+ During initialization, the struct sockaddr data structure must be
+ filled as follows:
+ </para>
+
+ <programlisting>
+struct sockaddr_alg sa = {
+ .salg_family = AF_ALG,
+ .salg_type = "aead", /* this selects the symmetric cipher */
+ .salg_name = "gcm(aes)" /* this is the cipher name */
+};
+ </programlisting>
+
+ <para>
+ Before data can be sent to the kernel using the write/send system
+ call family, the consumer must set the key. The key setting is
+ described with the setsockopt invocation below.
+ </para>
+
+ <para>
+ In addition, before data can be sent to the kernel using the
+ write/send system call family, the consumer must set the authentication
+ tag size. To set the authentication tag size, the caller must use the
+ setsockopt invocation described below.
+ </para>
+
+ <para>
+ Using the sendmsg() system call, the application provides the data that should be processed for encryption or decryption. In addition, the IV is
+ specified with the data structure provided by the sendmsg() system call.
+ </para>
+
+ <para>
+ The sendmsg system call parameter of struct msghdr is embedded into the
+ struct cmsghdr data structure. See recv(2) and cmsg(3) for more
+ information on how the cmsghdr data structure is used together with the
+ send/recv system call family. That cmsghdr data structure holds the
+ following information specified with a separate header instances:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ specification of the cipher operation type with one of these flags:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>ALG_OP_ENCRYPT - encryption of data</para>
+ </listitem>
+ <listitem>
+ <para>ALG_OP_DECRYPT - decryption of data</para>
+ </listitem>
+ </itemizedlist>
+ </listitem>
+
+ <listitem>
+ <para>
+ specification of the IV information marked with the flag ALG_SET_IV
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ specification of the associated authentication data (AAD) with the
+ flag ALG_SET_AEAD_ASSOCLEN. The AAD is sent to the kernel together
+ with the plaintext / ciphertext. See below for the memory structure.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ The send system call family allows the following flag to be specified:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ MSG_MORE: If this flag is set, the send system call acts like a
+ cipher update function where more input data is expected
+ with a subsequent invocation of the send system call.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ Note: The kernel reports -EINVAL for any unexpected data. The caller
+ must make sure that all data matches the constraints given in
+ /proc/crypto for the selected cipher.
+ </para>
+
+ <para>
+ With the recv() system call, the application can read the result of
+ the cipher operation from the kernel crypto API. The output buffer
+ must be at least as large as defined with the memory structure below.
+ If the output data size is smaller, the cipher operation is not performed.
+ </para>
+
+ <para>
+ The authenticated decryption operation may indicate an integrity error.
+ Such breach in integrity is marked with the -EBADMSG error code.
+ </para>
+
+ <sect2><title>AEAD Memory Structure</title>
+ <para>
+ The AEAD cipher operates with the following information that
+ is communicated between user and kernel space as one data stream:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>plaintext or ciphertext</para>
+ </listitem>
+
+ <listitem>
+ <para>associated authentication data (AAD)</para>
+ </listitem>
+
+ <listitem>
+ <para>authentication tag</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ The sizes of the AAD and the authentication tag are provided with
+ the sendmsg and setsockopt calls (see there). As the kernel knows
+ the size of the entire data stream, the kernel is now able to
+ calculate the right offsets of the data components in the data
+ stream.
+ </para>
+
+ <para>
+ The user space caller must arrange the aforementioned information
+ in the following order:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ AEAD encryption input: AAD || plaintext
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ AEAD decryption input: AAD || ciphertext || authentication tag
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ The output buffer the user space caller provides must be at least as
+ large to hold the following data:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ AEAD encryption output: ciphertext || authentication tag
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ AEAD decryption output: plaintext
+ </para>
+ </listitem>
+ </itemizedlist>
+ </sect2>
+ </sect1>
+
+ <sect1><title>Random Number Generator API</title>
+ <para>
+ Again, the operation is very similar to the other APIs.
+ During initialization, the struct sockaddr data structure must be
+ filled as follows:
+ </para>
+
+ <programlisting>
+struct sockaddr_alg sa = {
+ .salg_family = AF_ALG,
+ .salg_type = "rng", /* this selects the symmetric cipher */
+ .salg_name = "drbg_nopr_sha256" /* this is the cipher name */
+};
+ </programlisting>
+
+ <para>
+ Depending on the RNG type, the RNG must be seeded. The seed is provided
+ using the setsockopt interface to set the key. For example, the
+ ansi_cprng requires a seed. The DRBGs do not require a seed, but
+ may be seeded.
+ </para>
+
+ <para>
+ Using the read()/recvmsg() system calls, random numbers can be obtained.
+ The kernel generates at most 128 bytes in one call. If user space
+ requires more data, multiple calls to read()/recvmsg() must be made.
+ </para>
+
+ <para>
+ WARNING: The user space caller may invoke the initially mentioned
+ accept system call multiple times. In this case, the returned file
+ descriptors have the same state.
+ </para>
+
+ </sect1>
+
+ <sect1><title>Zero-Copy Interface</title>
+ <para>
+ In addition to the send/write/read/recv system call familty, the AF_ALG
+ interface can be accessed with the zero-copy interface of splice/vmsplice.
+ As the name indicates, the kernel tries to avoid a copy operation into
+ kernel space.
+ </para>
+
+ <para>
+ The zero-copy operation requires data to be aligned at the page boundary.
+ Non-aligned data can be used as well, but may require more operations of
+ the kernel which would defeat the speed gains obtained from the zero-copy
+ interface.
+ </para>
+
+ <para>
+ The system-interent limit for the size of one zero-copy operation is
+ 16 pages. If more data is to be sent to AF_ALG, user space must slice
+ the input into segments with a maximum size of 16 pages.
+ </para>
+
+ <para>
+ Zero-copy can be used with the following code example (a complete working
+ example is provided with libkcapi):
+ </para>
+
+ <programlisting>
+int pipes[2];
+
+pipe(pipes);
+/* input data in iov */
+vmsplice(pipes[1], iov, iovlen, SPLICE_F_GIFT);
+/* opfd is the file descriptor returned from accept() system call */
+splice(pipes[0], NULL, opfd, NULL, ret, 0);
+read(opfd, out, outlen);
+ </programlisting>
+
+ </sect1>
+
+ <sect1><title>Setsockopt Interface</title>
+ <para>
+ In addition to the read/recv and send/write system call handling
+ to send and retrieve data subject to the cipher operation, a consumer
+ also needs to set the additional information for the cipher operation.
+ This additional information is set using the setsockopt system call
+ that must be invoked with the file descriptor of the open cipher
+ (i.e. the file descriptor returned by the accept system call).
+ </para>
+
+ <para>
+ Each setsockopt invocation must use the level SOL_ALG.
+ </para>
+
+ <para>
+ The setsockopt interface allows setting the following data using
+ the mentioned optname:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ ALG_SET_KEY -- Setting the key. Key setting is applicable to:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>the skcipher cipher type (symmetric ciphers)</para>
+ </listitem>
+ <listitem>
+ <para>the hash cipher type (keyed message digests)</para>
+ </listitem>
+ <listitem>
+ <para>the AEAD cipher type</para>
+ </listitem>
+ <listitem>
+ <para>the RNG cipher type to provide the seed</para>
+ </listitem>
+ </itemizedlist>
+ </listitem>
+
+ <listitem>
+ <para>
+ ALG_SET_AEAD_AUTHSIZE -- Setting the authentication tag size
+ for AEAD ciphers. For a encryption operation, the authentication
+ tag of the given size will be generated. For a decryption operation,
+ the provided ciphertext is assumed to contain an authentication tag
+ of the given size (see section about AEAD memory layout below).
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ </sect1>
+
+ <sect1><title>User space API example</title>
+ <para>
+ Please see [1] for libkcapi which provides an easy-to-use wrapper
+ around the aforementioned Netlink kernel interface. [1] also contains
+ a test application that invokes all libkcapi API calls.
+ </para>
+
+ <para>
+ [1] http://www.chronox.de/libkcapi.html
+ </para>
+
+ </sect1>
+
+ </chapter>
+
<chapter id="API"><title>Programming Interface</title>
<sect1><title>Block Cipher Context Data Structures</title>
!Pinclude/linux/crypto.h Block Cipher Context Data Structures