path: root/doc/lowlevel.txt

The Low-Level Interface
=================================

Botan has two different interfaces. The one documented in this section is meant
more for implementing higher-level types (see the section on filters, earlier in
this manual) than for use by applications. Using it safely requires a solid
knowledge of encryption techniques and best practices, so unless you know, for
example, what CBC mode and nonces are, and why PKCS #1 padding is important,
you should avoid this interface in favor of something working at a higher level
(such as the CMS interface).

Basic Algorithm Abilities
---------------------------------

There are a small handful of functions implemented by most of Botan's
algorithm objects. Among these are:

.. cpp:function:: std::string Algorithm::name()

Returns a human-readable string of the name of this
algorithm. Examples of names returned are "AES-128" and
"HMAC(SHA-512)". You can turn names back into algorithm objects using
the functions in ``lookup.h``.

.. cpp:function:: void Algorithm::clear()

Clear out the algorithm's internal state. A block cipher object will
"forget" its key, a hash function will "forget" any data put into it,
etc. The object will look and behave as it did when you initially
allocated it.

.. cpp:function:: T* Algorithm::clone()

This function is central to Botan's name-based interface. The
``clone`` has many different return types, such as ``BlockCipher``\*
and ``HashFunction``\*, depending on what kind of object it is called
on. Note that unlike Java's clone, this returns a new object in a
"pristine" state; that is, operations done on the initial object
before calling ``clone`` do not affect the initial state of the new
clone.

Cloned objects can (and should) be deallocated with the C++ ``delete``
operator.

Keys and IVs
---------------------------------

Both symmetric keys and initialization values can be considered byte
(or octet) strings. These are represented by the classes
``SymmetricKey`` and ``InitializationVector``, which are
subclasses of ``OctetString``.

Since often it's hard to distinguish between a key and IV, many things
(such as key derivation mechanisms) return ``OctetString`` instead of
``SymmetricKey`` to allow its use as a key or an IV.

.. cpp:function:: OctetString::OctetString(RandomNumberGenerator& rng, size_t length)

  This constructor creates a new random key *length* bytes long using
  the random number generator.

.. cpp:function:: OctetString::OctetString(std::string str)

  The argument *str* is assumed to be a hex string; it is converted to
  binary and stored. Whitespace is ignored.

.. cpp:function:: OctetString::OctetString(const byte* input, size_t length)

  This constructor copies its input.

Symmetrically Keyed Algorithms
---------------------------------

Block ciphers, stream ciphers, and MACs are all keyed operations; to
be useful, they have to be set to use a particular key, which is a
randomly chosen string of bits of a specified length.  The length
required by any particular algorithm may vary, depending on both the
algorithm specification and the implementation. You can query any
botan object to find out what key length(s) it supports.

To make this similarity in terms of keying explicit, all algorithms of
those types are derived from the ``SymmetricAlgorithm`` base
class. This type provides functions for setting the key, and querying
restrictions on the size of the key:

.. cpp:function:: void SymmetricAlgorithm::set_key(const byte* key, size_t length)

  This sets the key to the value specified. Most algorithms only
  accept keys of certain lengths. If you attempt to call ``set_key``
  with a key length that is not supported, the exception
  ``Invalid_Key_Length`` will be thrown. There is also another version
  of ``set_key`` that takes a ``SymmetricKey`` as an argument.

  In all cases, ``set_key`` must be called on an object before any
  data processing (encryption, decryption, etc) is done by that
  object. If this is not done, the results are undefined, but probably
  will not be good.

.. cpp:function:: bool SymmetricAlgorithm::valid_keylength(size_t length) const

  This function returns true if and only if *length* is a valid
  keylength for the algorithm.

.. cpp:function:: size_t SymmetricAlgorithm::minimum_keylength() const

  Return the smallest key length (in bytes) that is acceptible for the
  algorithm.

.. cpp:function:: size_t SymmetricAlgorithm::maximum_keylength() const

  Return the largest key length (in bytes) that is acceptible for the
  algorithm

Block Ciphers
---------------------------------

Block ciphers implement the interface ``BlockCipher``, found in
``block_cipher.h``, as well as the ``SymmetricAlgorithm`` interface.

.. cpp:function:: size_t BlockCipher::block_size() const

  Returns the block size of the cipher in bytes

.. cpp:function:: void BlockCipher::encrypt_n(const byte* in, byte* out, size_t n) const

  Encrypt *n* blocks of data, taking the input from the array *in* and
  placing the ciphertext into *out*. The two pointers may be
  identical, but should not overlap ranges.

.. cpp:function:: void BlockCipher::encrypt(const byte* in, byte* out) const

  Encrypt a single block, taking the input from *in* and placing it
  in *out*. Acts like :cpp:func:`encrypt_n`\ (in, out, 1).

.. cpp:function:: void BlockCipher::encrypt(byte* block) const

  Identical to :cpp:func:`encrypt`\ (block, block)

.. cpp:function:: void BlockCipher::decrypt_n(const byte* in, byte out, size_t n) const

  Decrypt *n* blocks of data, taking the input from *in* and placing
  the plaintext in *out*. The two pointers may be identical, but
  should not overlap ranges.

.. cpp:function:: void BlockCipher::decrypt(const byte* in, byte* out) const

  Decrypt a single block, taking the input from *in* and placing it
  in *out*. Acts like :cpp:func:`decrypt_n`\ (in, out, 1).

.. cpp:function:: void BlockCipher::decrypt(byte* block) const

  Identical to :cpp:func:`decrypt`\ (block, block)


Stream Ciphers
---------------------------------

Stream ciphers are somewhat different from block ciphers, in that
encrypting data results in changing the internal state of the
cipher. Also, you may encrypt any length of data in one go (in byte
amounts).

.. cpp:function:: void StreamCipher::encrypt(const byte* in, byte* out, size_t length)

.. cpp:function:: void StreamCipher::encrypt(byte* data, size_t length)

Stream ciphers implement the ``SymmetricAlgorithm`` interface.

Hash Functions / Message Authentication Codes
----------------------------------------------

Hash functions take their input without producing any output, only
producing anything when all input has already taken place. MACs are
very similar, but are additionally keyed. Both of these are derived
from the base class ``BufferedComputation``, which has the following
functions.

.. cpp:function:: size_t BufferedComputation::output_length()

Return the size of the output of this function.

.. cpp:function:: void BufferedComputation::update(const byte* input, size_t length)

.. cpp:function:: void BufferedComputation::update(byte input)

.. cpp:function:: void BufferedComputation::update(const std::string& input)

Updates the hash/mac calculation with *input*.

.. cpp:function:: void BufferedComputation::final(byte* out)

.. cpp:function:: SecureVector<byte> BufferedComputation::final()

Complete the hash/MAC calculation and place the result into ``out``.
For the argument taking an array, exactly ``output_length`` bytes will
be written. After you call ``final``, the hash function is reset to
its initial state, so it may be reused immediately.

The second method of using final is to call it with no arguments at
all, as shown in the second prototype. It will return the hash/mac
value in a memory buffer.

There is also a pair of functions called ``process``. They are
a combination of a single ``update``, and ``final``.
Both versions return the final value, rather than placing it an
array. Calling ``process`` with a single byte value isn't
available, mostly because it would rarely be useful.

A MAC can be viewed (in most cases) as a keyed hash function, so
classes that are derived from ``MessageAuthenticationCode`` have
``update`` and ``final`` classes just like a
``HashFunction`` (and like a ``HashFunction``, after
``final`` is called, it can be used to make a new MAC right
away; the key is kept around).

A MAC has the ``SymmetricAlgorithm`` interface in addition to the
``BufferedComputation`` interface.

Checksums
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Checksums are very similar to hash functions, and in fact share the
same interface. But there are some significant differences, the major
ones being that the output size is very small (usually in the range of
2 to 4 bytes), and is not cryptographically secure. But for their
intended purpose (error checking), they perform very well. Some
examples of checksums included in Botan are the Adler32 and CRC32
checksums.

Secure Memory
---------------------------------

A major concern with mixing modern multiuser OSes and cryptographic
code is that at any time the code (including secret keys) could be
swapped to disk, where it can later be read by an attacker. Botan
stores almost everything (and especially anything sensitive) in memory
buffers that a) clear out their contents when their destructors are
called, and b) have easy plugins for various memory locking functions,
such as the ``mlock`` call on many Unix systems.

Two of the allocation method used ("malloc" and "mmap") don't
require any extra privileges on Unix, but locking memory does. At
startup, each allocator type will attempt to allocate a few blocks
(typically totaling 128k), so if you want, you can run your
application ``setuid`` ``root``, and then drop privileges
immediately after creating your ``LibraryInitializer``. If you end
up using more than what's been allocated, some of your sensitive data
might end up being swappable, but that beats running as ``root``
all the time.

These classes should also be used within your own code for storing
sensitive data. They are only meant for primitive data types (int,
long, etc): if you want a container of higher level Botan objects, you
can just use a ``std::vector``, since these objects know how to clear
themselves when they are destroyed. You cannot, however, have a
``std::vector`` (or any other container) of ``Pipe`` objects or
filters, because these types have pointers to other filters, and
implementing copy constructors for these types would be both hard and
quite expensive (vectors of pointers to such objects is fine, though).

These types are not described in any great detail: for more information,
consult the definitive sources~--~the header files ``secmem.h`` and
``allocate.h``.

``SecureBuffer`` is a simple array type, whose size is specified at
compile time. It will automatically convert to a pointer of the
appropriate type, and has a number of useful functions, including
``clear()``, and ``size_t`` ``size()``, which returns the length of
the array. It is a template that takes as parameters a type, and a
constant integer which is how long the array is (for example:
``SecureBuffer<byte, 8> key;``).

``SecureVector`` is a variable length array. Its size can be increased
or decreased as need be, and it has a wide variety of functions useful
for copying data into its buffer. Like ``SecureBuffer``, it implements
``clear`` and ``size``.

Allocators
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The containers described above get their memory from allocators. As a
user of the library, you can add new allocator methods at run time for
containers, including the ones used internally by the library, to
use. The interface to this is in ``allocate.h``. Code needing to
allocate or deallocate memory calls ``get_allocator``, which returns a
pointer to an allocator object. This pointer should not be freed: the
caller does not own the allocator (it is shared among multiple
allocatore users, and uses a mutex to serialize access internally if
necessary). It is possible to call ``get_allocator`` with a specific
name to request a particular type of allocator, otherwise, a default
allocator type is returned.

At start time, the only allocator known is a ``Default_Allocator``,
which just allocates memory using ``malloc``, and zeroizes it when the
memory is released. It is known by the name "malloc". If you ask for
another type of allocator ("locking" and "mmap" are currently used),
and it is not available, some other allocator will be returned.

You can add in a new allocator type using ``add_allocator_type``. This
function takes a string and a pointer to an allocator. The string gives this
allocator type a name to which it can be referred when one is requesting it
with ``get_allocator``. If an error occurs (such as the name being
already registered), this function returns false. It will return true if the
allocator was successfully registered. If you ask it to,
``LibraryInitializer`` will do this for you.

Finally, you can set the default allocator type that will be returned
using the policy setting "default_alloc" to the name of any previously
registered allocator.