crypto: doc - AEAD / RNG AF_ALG interface

   </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