mirror of
https://github.com/moparisthebest/mailiverse
synced 2024-11-26 10:22:15 -05:00
130 lines
5.9 KiB
Plaintext
130 lines
5.9 KiB
Plaintext
.. _random_number_generators:
|
|
|
|
Random Number Generators
|
|
========================================
|
|
|
|
The random number generators provided in Botan are meant for creating
|
|
keys, IVs, padding, nonces, and anything else that requires 'random'
|
|
data. It is important to remember that the output of these classes
|
|
will vary, even if they are supplied with the same seed (ie, two
|
|
``Randpool`` objects with similar initial states will not produce the
|
|
same output, because the value of high resolution timers is added to
|
|
the state at various points).
|
|
|
|
To create a random number generator, instantiate a ``AutoSeeded_RNG``
|
|
object. This object will handle choosing the right algorithms from the
|
|
set of enabled ones and doing seeding using OS specific
|
|
routines. The main service a RandomNumberGenerator provides is, of
|
|
course, random numbers:
|
|
|
|
.. cpp:function:: byte RandomNumberGenerator::random()
|
|
|
|
Generates a single random byte and returns it
|
|
|
|
.. cpp:function:: void RandomNumberGenerator::randomize(byte* data, size_t length)
|
|
|
|
Places *length* bytes into the array pointed to by *data*
|
|
|
|
To ensure good quality output, a PRNG needs to be seeded with truly
|
|
random data. Normally this is done for you. However it may happen that
|
|
your application has access to data that is potentially unpredictable
|
|
to an attacker. If so, use
|
|
|
|
.. cpp:function:: void RandomNumberGenerator::add_entropy(const byte* data, \
|
|
size_t length)
|
|
|
|
which incorporates the data into the current randomness state. Don't
|
|
worry about filtering the data or doing any kind of cryptographic
|
|
preprocessing (such as hashing); the RNG objects in botan are designed
|
|
such that you can feed them any arbitrary non-random or even
|
|
maliciously chosen data - as long as at some point some of the seed
|
|
data was good the output will be secure.
|
|
|
|
|
|
Implementation Notes
|
|
----------------------------------------
|
|
|
|
Randpool
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
``Randpool`` is the primary PRNG within Botan. In recent versions all
|
|
uses of it have been wrapped by an implementation of the X9.31 PRNG
|
|
(see below). If for some reason you should have cause to create a PRNG
|
|
instead of using the "global" one owned by the library, it would be
|
|
wise to consider the same on the grounds of general caution; while
|
|
``Randpool`` is designed with known attacks and PRNG weaknesses in
|
|
mind, it is not an standard/official PRNG. The remainder of this
|
|
section is a (fairly technical, though high-level) description of the
|
|
algorithms used in this PRNG. Unless you have a specific interest in
|
|
this subject, the rest of this section might prove somewhat
|
|
uninteresting.
|
|
|
|
``Randpool`` has an internal state called pool, which is 512 bytes
|
|
long. This is where entropy is mixed into and extracted from. There is also a
|
|
small output buffer (called buffer), which holds the data which has already
|
|
been generated but has just not been output yet.
|
|
|
|
It is based around a MAC and a block cipher (which are currently
|
|
HMAC(SHA-256) and AES-256). Where a specific size is mentioned, it
|
|
should be taken as a multiple of the cipher's block size. For example,
|
|
if a 256-bit block cipher were used instead of AES, all the sizes
|
|
internally would double. Every time some new output is needed, we
|
|
compute the MAC of a counter and a high resolution timer. The
|
|
resulting MAC is XORed into the output buffer (wrapping as needed),
|
|
and the output buffer is then encrypted with AES, producing 16 bytes
|
|
of output.
|
|
|
|
After 8 blocks (or 128 bytes) have been produced, we mix the pool. To
|
|
do this, we first rekey both the MAC and the cipher; the new MAC key
|
|
is the MAC of the current pool under the old MAC key, while the new
|
|
cipher key is the MAC of the current pool under the just-chosen MAC
|
|
key. We then encrypt the entire pool in CBC mode, using the current
|
|
(unused) output buffer as the IV. We then generate a new output
|
|
buffer, using the mechanism described in the previous paragraph.
|
|
|
|
To add randomness to the PRNG, we compute the MAC of the input and XOR
|
|
the output into the start of the pool. Then we remix the pool and
|
|
produce a new output buffer. The initial MAC operation should make it
|
|
very hard for chosen inputs to harm the security of ``Randpool``, and
|
|
as HMAC should be able to hold roughly 256 bits of state, it is
|
|
unlikely that we are wasting much input entropy (or, if we are, it
|
|
doesn't matter, because we have a very abundant supply).
|
|
|
|
ANSI X9.31
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
``ANSI_X931_PRNG`` is the standard issue X9.31 Appendix A.2.4 PRNG,
|
|
though using AES-256 instead of 3DES as the block cipher. This PRNG
|
|
implementation has been checked against official X9.31 test vectors.
|
|
|
|
Internally, the PRNG holds a pointer to another PRNG (typically
|
|
Randpool). This internal PRNG generates the key and seed used by the
|
|
X9.31 algorithm, as well as the date/time vectors. Each time an X9.31
|
|
PRNG object receives entropy, it passes it along to the PRNG it is
|
|
holding, and then pulls out some random bits to generate a new key and
|
|
seed. This PRNG considers itself seeded as soon as the internal PRNG
|
|
is seeded.
|
|
|
|
|
|
Entropy Sources
|
|
---------------------------------
|
|
|
|
An ``EntropySource`` is an abstract representation of some method of
|
|
gather "real" entropy. This tends to be very system dependent. The
|
|
*only* way you should use an ``EntropySource`` is to pass it to a PRNG
|
|
that will extract entropy from it -- never use the output directly for
|
|
any kind of key or nonce generation!
|
|
|
|
``EntropySource`` has a pair of functions for getting entropy from
|
|
some external source, called ``fast_poll`` and ``slow_poll``. These
|
|
pass a buffer of bytes to be written; the functions then return how
|
|
many bytes of entropy were gathered.
|
|
|
|
Note for writers of ``EntropySource`` subclasses: it isn't necessary
|
|
to use any kind of cryptographic hash on your output. The data
|
|
produced by an EntropySource is only used by an application after it
|
|
has been hashed by the ``RandomNumberGenerator`` that asked for the
|
|
entropy, thus any hashing you do will be wasteful of both CPU cycles
|
|
and entropy.
|
|
|