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>

1 files changed, 596 insertions, 0 deletions

diff --git a/Documentation/DocBook/crypto-API.tmpl b/Documentation/DocBook/crypto-API.tmpl
index 33f63cfc00ca..efc8d90a9a3f 100644
--- a/Documentation/DocBook/crypto-API.tmpl
+++ 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