Hash Functions and Checksums
=============================

Hash functions are one-way functions, which map data of arbitrary size to a
fixed output length. Most of the hash functions in Botan are designed to be
cryptographically secure, which means that it is computationally infeasible to
create a collision (finding two inputs with the same hash) or preimages (given a
hash output, generating an arbitrary input with the same hash). But note that
not all such hash functions meet their goals, in particular MD4 and MD5 are
trivially broken. However they are still included due to their wide adoption in
various protocols.

The class :cpp:class:`HashFunction` is defined in `botan/hash.h`.

Using a hash function is typically split into three stages: initialization,
update, and finalization (often referred to as a IUF interface). The
initialization stage is implicit: after creating a hash function object, it is
ready to process data. Then update is called one or more times. Calling update
several times is equivalent to calling it once with all of the arguments
concatenated. After completing a hash computation (eg using ``final``), the
internal state is reset to begin hashing a new message.

.. cpp:class:: HashFunction

  .. cpp:function:: size_t output_length()

    Return the size (in *bytes*) of the output of this function.

  .. cpp:function:: void update(const uint8_t* input, size_t length)

    Updates the computation with *input*.

  .. cpp:function:: void update(uint8_t input)

    Updates the computation with *input*.

  .. cpp:function:: void update(const std::vector<uint8_t>& input)

    Updates the computation with *input*.

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

    Updates the computation with *input*.

  .. cpp:function:: void final(uint8_t* out)

    Finalize the 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 algorithm is reset to
    its initial state, so it may be reused immediately.

  .. cpp:function:: secure_vector<uint8_t> final()

    Similar to the other function of the same name, except it returns
    the result in a newly allocated vector.

  .. cpp:function:: secure_vector<uint8_t> process(const uint8_t in[], size_t length)

     Equivalent to calling ``update`` followed by ``final``.

  .. cpp:function:: secure_vector<uint8_t> process(const std::string& in)

     Equivalent to calling ``update`` followed by ``final``.

Code Example
------------

Assume we want to calculate the SHA-256, SHA-384, and SHA-3 hash digests of the STDIN stream using the Botan library.

.. code-block:: cpp

    #include <botan/hash.h>
    #include <botan/hex.h>
    #include <iostream>
    int main ()
       {
       std::unique_ptr<Botan::HashFunction> hash1(Botan::HashFunction::create("SHA-256"));
       std::unique_ptr<Botan::HashFunction> hash2(Botan::HashFunction::create("SHA-384"));
       std::unique_ptr<Botan::HashFunction> hash3(Botan::HashFunction::create("SHA-3"));
       std::vector<uint8_t> buf(2048);

       while(std::cin.good())
          {
          //read STDIN to buffer
          std::cin.read(reinterpret_cast<char*>(buf.data()), buf.size());
          size_t readcount = std::cin.gcount();
          //update hash computations with read data
          hash1->update(buf.data(),readcount);
          hash2->update(buf.data(),readcount);
          hash3->update(buf.data(),readcount);
          }
       std::cout << "SHA-256: " << Botan::hex_encode(hash1->final()) << std::endl;
       std::cout << "SHA-384: " << Botan::hex_encode(hash2->final()) << std::endl;
       std::cout << "SHA-3: " << Botan::hex_encode(hash3->final()) << std::endl;
       return 0;
       }

Available Hash Functions
------------------------------

The following cryptographic hash functions are implemented. If in doubt,
any of SHA-384, SHA-3, BLAKE2b, or Skein-512 are fine choices.

BLAKE2b
^^^^^^^^^

Available if ``BOTAN_HAS_BLAKE2B`` is defined.

A recently designed hash function. Very fast on 64-bit processors. Can output a
hash of any length between 1 and 64 bytes, this is specified by passing a value
to the constructor with the desired length.

GOST-34.11
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_GOST_34_11`` is defined.

Russian national standard hash. It is old, slow, and has some weaknesses. Avoid
it unless you must.

Keccak-1600
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_KECCAK`` is defined.

An older (and incompatible) variant of SHA-3, but sometimes used. Prefer SHA-3 in
new code.

MD4
^^^^^^^^^

Available if ``BOTAN_HAS_MD4`` is defined.

An old hash function that is now known to be trivially breakable. It is very
fast, and may still be suitable as a (non-cryptographic) checksum.

MD5
^^^^^^^^^

Available if ``BOTAN_HAS_MD5`` is defined.

Widely used, now known to be broken.

RIPEMD-160
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_RIPEMD160`` is defined.

A 160 bit hash function, quite old but still thought to be secure (up to the
limit of 2**80 computation required for a collision which is possible with any
160 bit hash function). Somewhat deprecated these days.

SHA-1
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SHA1`` is defined.

Widely adopted NSA designed hash function. Starting to show significant signs of
weakness, and collisions can now be generated. Avoid in new designs.

SHA-256
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SHA2_32`` is defined.

Relatively fast 256 bit hash function, thought to be secure.

Also includes the variant SHA-224. There is no real reason to use SHA-224.

SHA-512
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SHA2_64`` is defined.

SHA-512 is faster than SHA-256 on 64-bit processors. Also includes
the truncated variants SHA-384 and SHA-512/256.

SHA-3
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SHA3`` is defined.

The new NIST standard hash. Fairly slow.

SHAKE (SHAKE-128, SHAKE-256)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SHAKE`` is defined.

These are actually XOFs (extensible output functions) based on SHA-3, which can
output a value of any length.

SM3
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SM3`` is defined.

Chinese national hash function, 256 bit output. Widely used in industry there.
Fast and seemingly secure.

Skein-512
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_SKEIN_512`` is defined.

A contender for the NIST SHA-3 competition. Very fast on 64-bit systems.  Can
output a hash of any length between 1 and 64 bytes. It also accepts a
"personalization string" which can create variants of the hash. This is useful
for domain separation.

Streebog (Streebog-256, Streebog-512)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_STREEBOG`` is defined.

Newly designed Russian national hash function. Due to use of input-dependent
table lookups, it is vulnerable to side channels. There is no reason to use it
unless compatibility is needed.

Tiger
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_TIGER`` is defined.

An older 192-bit hash function, optimized for 64-bit systems. Possibly
vulnerable to side channels due to its use of table lookups. Prefer Skein-512 or
BLAKE2b in new code.

Whirlpool
^^^^^^^^^^^^^^^

Available if ``BOTAN_HAS_WHIRLPOOL`` is defined.

A 512-bit hash function standardized by ISO and NESSIE. Relatively slow, and due
to the table based implementation it is (unlike almost all other hashes)
potentially vulnerable to cache based side channels. Prefer Skein-512 or BLAKE2b
in new code.

Hash Function Combiners
---------------------------

These are functions which combine multiple hash functions to create a new hash
function. They are typically only used in specialized applications.

Parallel
^^^^^^^^^^^^^

Available if ``BOTAN_HAS_PARALLEL_HASH`` is defined.

Parallel simply concatenates multiple hash functions. For example
"Parallel(SHA-256,SHA-512)" outputs a 256+512 bit hash created by hashing the
input with both SHA-256 and SHA-512 and concatenating the outputs.

Note that due to the "multicollision attack" it turns out that generating a
collision for multiple parallel hash functions is no harder than generating a
collision for the strongest hash function.

Comp4P
^^^^^^^^^^^^^

Available if ``BOTAN_HAS_COMB4P`` is defined.

This combines two cryptographic hashes in such a way that preimage and collision
attacks are provably at least as hard as a preimage or collision attack on the
strongest hash.

Checksums
----------------

.. note:: Checksums are not suitable for cryptographic use, but can be used for
          error checking purposes.

Adler32
^^^^^^^^^^^

Available if ``BOTAN_HAS_ADLER32`` is defined.

The Adler32 checksum is used in the zlib format. 32 bit output.

CRC24
^^^^^^^^^^^

Available if ``BOTAN_HAS_CRC24`` is defined.

This is the CRC function used in OpenPGP. 24 bit output.

CRC32
^^^^^^^^^^^

Available if ``BOTAN_HAS_CRC32`` is defined.

This is the 32-bit CRC used in protocols such as Ethernet, gzip, PNG, etc.
