+++ /dev/null
-<big><b>Introduction to Random Number Generator</b></big>
-
-<br /> <br /> <br />
-<b>Overview</b>
-
-<br /> <br />
-SILC Random Number Generator is cryptographically strong pseudo random
-number generator. It is used to generate all the random numbers needed
-in the SILC sessions. All key material and other sources needing random
-numbers use this generator.
-
-<br /> <br />
-The RNG has a random pool of 1024 bytes of size that provides the actual
-random numbers for the application. The pool is initialized when the
-RNG is allocated and initialized with silc_rng_alloc and silc_rng_init
-functions, respectively.
-
-
-<br /> <br /> <br />
-<b>Random Pool Initialization</b>
-
-<br /> <br />
-The RNG's random pool is the source of all random output data. The pool is
-initialized with silc_rng_init and application can reseed it at any time
-by calling the silc_rng_add_noise function.
-
-<br /> <br />
-The initializing phase attempts to set the random pool in a state that it
-is impossible to learn the input data to the RNG or any random output
-data. This is achieved by acquiring noise from various system sources. The
-first source is called to provide "soft noise". This noise is various
-data from system's processes. The second source is called to provide
-"medium noise". This noise is various output data from executed commands.
-Usually the commands are Unix `ps' and `ls' commands with various options.
-The last source is called to provide "hard noise" and is noise from
-system's /dev/random, if it exists.
-
-
-<br /> <br /> <br />
-<b>Stirring the Random Pool</b>
-
-<br /> <br />
-Every time data is acquired from any source, the pool is stirred. The
-stirring process performs an CFB (cipher feedback) encryption with SHA1
-algorithm to the entire random pool. First it acquires an IV (Initial
-Vector) from the constant (random) location of the pool and performs
-the first CFB pass. Then it acquires a new encryption key from variable
-location of the pool and performs the second CFB pass. The encryption
-key thus is always acquired from unguessable data.
-
-<br /> <br />
-The encryption process to the entire random pool assures that it is
-impossible to learn the input data to the random pool without breaking the
-encryption process. This would effectively mean breaking the SHA1 hash
-function. The encryption process also assures that each random output from
-the random pool is secured with cryptographically strong function, the
-SHA1 in this case.
-
-<br /> <br />
-The random pool can be restirred by the application at any point by
-calling the silc_rng_add_noise function. This function adds new noise to
-the pool and then stirs the entire pool.
-
-
-<br /> <br /> <br />
-<b>Stirring Thresholds</b>
-
-<br /> <br />
-The random pool has two thresholds that controls when the random pool
-needs more new noise and requires restirring. As previously mentioned, the
-application may do this by calling the silc_rng_add_noise. However, the
-RNG performs this also automatically.
-
-<br /> <br />
-The first threshold gets soft noise from system and stirs the random pool.
-The threshold is reached after 64 bits of random data has been fetched
-from the RNG. After the 64 bits, the soft noise acquiring and restirring
-process is performed every 8 bits of random output data until the second
-threshold is reached.
-
-<br /> <br />
-The second threshold gets hard noise from system and stirs the random
-pool. The threshold is reached after 160 bits of random output. After the
-noise is acquired (from /dev/urandom) the random pool is stirred and the
-thresholds are set to zero. The process is repeated again after 64 bits of
-output for first threshold and after 160 bits of output for the second
-threshold.
-
-
-<br /> <br /> <br />
-<b>Internal State of the Random Pool</b>
-
-<br /> <br />
-The random pool has also internal state that provides several variable
-distinct points to the random pool where the data is fetched. The state
-changes every 8 bits of output data and it is guaranteed that the fetched
-8 bits of data is from distinct location compared to the previous 8 bits.
-It is also guaranteed that the internal state never wraps before
-restirring the entire random pool. The internal state means that the data
-is not fetched linearly from the pool, eg. starting from zero and wrapping
-at the end of the pool. The internal state is not dependent of any random
-data in the pool. The internal states are initialized (by default the pool
-is splitted to four different sections (states)) at the RNG
-initialization phase. The state's current position is added linearly and
-wraps at the the start of the next state. The states provides the distinct
-locations.
-
-
-<br /> <br /> <br />
-<b>Security Considerations</b>
-
-<br /> <br />
-The security of this random number generator, like of any other RNG's,
-depends of the initial state of the RNG. The initial state of the random
-number generators must be unknown to an adversary. This means that after
-the RNG is initialized it is required that the input data to the RNG and
-the output data to the application has no correlation of any kind that
-could be used to compromise the acquired random numbers or any future
-random numbers.
-
-<br /> <br />
-It is, however, clear that the correlation exists but it needs to be
-hard to solve for an adversary. To accomplish this the input data to the
-random number generator needs to be secret. Usually this is impossible to
-achieve. That is why SILC's RNG acquires the noise from three different
-sources and provides for the application an interface to add more noise at
-any time. The first source ("soft noise") is known to the adversary but
-requires exact timing to get all of the input data. However, getting only
-partial data is easy. The second source ("medium noise") depends on the
-place of execution of the application. Getting at least partial data is
-easy but securing for example the user's home directory from outside access
-makes it harder. The last source ("hard noise") is considered to be the
-most secure source of data. An adversary is not considered to have any
-access on this data. This of course greatly depends on the operating system.
-
-<br /> <br />
-These three sources are considered to be adequate since the random pool is
-relatively large and the output of each bit of the random pool is secured
-by cryptographically secure function, the SHA1 in CFB mode encryption.
-Furthermore the application may provide other random data, such as random
-key strokes or mouse movement to the RNG. However, it is recommended that
-the application would not be the single point of source for the RNG, in
-either intializing or reseeding phases later in the session. Good solution
-is probably to use both, the application's seeds and the RNG's own
-sources, equally.
-
-<br /> <br />
-The RNG must also assure that any old or future random numbers are not
-compromised if an adversary would learn the initial input data (or any
-input data for that matter). The SILC's RNG provides good protection for
-this even if the some of the input bits would be compromised for old or
-future random numbers. The RNG reinitalizes (reseeds) itself using the
-thresholds after every 64 and 160 bits of output. This is considered to be
-adequate even if some of the bits would get compromised. Also, the
-applications that use the RNG usually fetches at least 256 bits from the
-RNG. This means that everytime RNG is accessed both of the thresholds are
-reached. This should mean that the RNG is never too long in an compromised
-state and recovers as fast as possible.
-
-
-<br /> <br /> <br />
-<b>Caveat Windows Programmer</b>
-
-<br /> <br />
-The caller must be cautios when using this RNG with native WIN32 system.
-The RNG most likely is impossible to set in unguessable state just by
-using the RNG's input data sources. On WIN32 it is stronly suggested
-that caller would add more random noise after the initialization of the
-RNG using the silc_rng_add_noise function. For example, random mouse
-movements may be used.
-
projects / crypto.git / blobdiff