Stream Ciphers
========================

In contrast to block ciphers, stream ciphers operate on a plaintext stream
instead of blocks. Thus encrypting data results in changing the internal state
of the cipher and encryption of plaintext with arbitrary length is possible in
one go (in byte amounts). All implemented stream ciphers derive from the base
class :cpp:class:`StreamCipher` (`botan/stream_cipher.h`).

Note that some of the implemented stream ciphers require a fresh initialisation
vector.

.. cpp:class:: StreamCipher

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

     Returns a human-readable string of the name of this algorithm.

  .. cpp:function:: void clear()

     Clear the key.

  .. cpp:function:: StreamCipher* clone() const

     Return a newly allocated object of the same type as this one.

  .. cpp:function:: void set_key(const uint8_t* key, size_t length)

     Set the stream cipher key. If the length is not accepted, an
     ``Invalid_Key_Length`` exception is thrown.

  .. cpp:function:: bool 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 minimum_keylength() const

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

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

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

  .. cpp:function:: bool valid_iv_length(size_t iv_len) const

     This function returns true if and only if *length* is a valid IV length for
     the stream cipher. Some ciphers do not support IVs at all, and will return
     false for any value except zero.

  .. cpp:function:: void set_iv(const uint8_t*, size_t len)

     Load IV into the stream cipher state. This should happen after the key is
     set and before any operation (encrypt/decrypt/seek) is called.

  .. cpp:function:: void seek(uint64_t offset)

     Sets the state of the stream cipher and keystream according to the passed
     *offset*.  Therefore the key and the IV (if required) have to be set
     beforehand.

  .. cpp:function:: void cipher(const uint8_t* in, uint8_t* out, size_t n)

     Processes *n* bytes plain/ciphertext from *in* and writes the result to *out*.

  .. cpp:function:: void cipher1(uint8_t* inout, size_t n)

     Processes *n* bytes plain/ciphertext in place. Acts like :cpp:func:`cipher`\ (inout, inout, n).

  .. cpp:function:: void encipher(std::vector<uint8_t> inout)
  .. cpp:function:: void encrypt(std::vector<uint8_t> inout)
  .. cpp:function:: void decrypt(std::vector<uint8_t> inout)

     Processes plain/ciphertext *inout* in place. Acts like :cpp:func:`cipher`\ (inout.data(), inout.data(), inout.size()).

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

The following code encrypts a provided plaintext using ChaCha20.

.. code-block:: cpp

    #include <botan/stream_cipher.h>
    #include <botan/auto_rng.h>
    #include <botan/hex.h>
    #include <iostream>

    int main()
       {
       std::string plaintext("This is a tasty burger!");
       std::vector<uint8_t> pt(plaintext.data(),plaintext.data()+plaintext.length());
       const std::vector<uint8_t> key = Botan::hex_decode("000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F");
       std::unique_ptr<Botan::StreamCipher> cipher(Botan::StreamCipher::create("ChaCha(20)"));

       //generate fresh nonce (IV)
       std::unique_ptr<Botan::RandomNumberGenerator> rng(new Botan::AutoSeeded_RNG);
       std::vector<uint8_t> iv(8);
       rng->randomize(iv.data(),iv.size());

       //set key and IV
       cipher->set_key(key);
       cipher->set_iv(iv.data(),iv.size());
       cipher->encipher(pt);

       std::cout << cipher->name() << " with iv " << Botan::hex_encode(iv) << ": "
                 << Botan::hex_encode(pt) << "\n";
       return 0;
       }

Available Stream Ciphers
----------------------------

Botan provides the following stream ciphers. If in doubt use CTR or ChaCha.

CTR-BE
~~~~~~~

A cipher mode that converts a block cipher into a stream cipher. It offers
parallel execution and can seek within the output stream.

(The ``-BE`` suffix refers to big-endian convention for the counter.
This is the most common case.)

OFB
~~~~~

Another stream cipher based on a block cipher. Unlike CTR mode, it does not
allow parallel execution or seeking within the output stream. Prefer CTR.

Available if ``BOTAN_HAS_OFB`` is defined.

ChaCha
~~~~~~~~

A very fast cipher, now widely deployed in TLS as part of the ChaCha20Poly1305
AEAD. Can be used with 8 (fast but dangerous), 12 (balance, or 20 rounds
(conservative). Even with 20 rounds, ChaCha is very fast. Use 20 rounds.

Available if ``BOTAN_HAS_CHACHA`` is defined.

Salsa20
~~~~~~~~~

An earlier iteration of the ChaCha design, this cipher is popular due to its use
in the libsodium library. Prefer ChaCha.

.. note::

   The 'XSalsa20' variant of Salsa20 is also supported by the same class; this
   is selected by using a 192-bit nonce instead of Salsa20's 64-bit nonce.

Available if ``BOTAN_HAS_SALSA20`` is defined.

SHAKE-128
~~~~~~~~~~~~

This is the SHAKE-128 XOF exposed as a stream cipher. It is slower than ChaCha
and somewhat obscure.

Available if ``BOTAN_HAS_SHAKE_CIPHER`` is defined.

RC4
~~~~

An old and very widely deployed stream cipher notable for its
simplicity. Now broken. **Avoid in new code**

Available if ``BOTAN_HAS_RC4`` is defined.