/*
- silcSilcRng.h
-
+ silcrng.h
+
+ COPYRIGHT
+
Author: Pekka Riikonen <priikone@poseidon.pspt.fi>
-
- Copyright (C) 1997 - 2000 Pekka Riikonen
-
+
+ Copyright (C) 1997 - 2001 Pekka Riikonen
+
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
-
+
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
-
+
*/
+/****h* silccrypt/SilcRNGAPI
+ *
+ * DESCRIPTION
+ *
+ * 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.
+ *
+ * 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.
+ *
+ *
+ * Random Pool Initialization
+ *
+ * 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.
+ *
+ * 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.
+ *
+ *
+ * Stirring the Random Pool
+ *
+ * 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.
+ *
+ * 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.
+ *
+ * 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.
+ *
+ *
+ * Stirring Threshholds
+ *
+ * The random pool has two threshholds 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.
+ *
+ * The first threshhold gets soft noise from system and stirs the random pool.
+ * The threshhold 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
+ * threshhold is reached.
+ *
+ * The second threshhold gets hard noise from system and stirs the random
+ * pool. The threshhold is reached after 160 bits of random output. After the
+ * noise is acquired (from /dev/urandom) the random pool is stirred and the
+ * threshholds are set to zero. The process is repeated again after 64 bits of
+ * output for first threshhold and after 160 bits of output for the second
+ * threshhold.
+ *
+ *
+ * Internal State of the Random Pool
+ *
+ * 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.
+ *
+ *
+ * Security Considerations
+ *
+ * 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.
+ *
+ * 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.
+ *
+ * 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.
+ *
+ * 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
+ * threshholds 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 threshholds are
+ * reached. This should mean that the RNG is never too long in an compromised
+ * state and recovers as fast as possible.
+ *
+ * Currently the SILC's RNG does not use random seed files to store some
+ * random data for future initializing. This is important and must be
+ * implemented in the future.
+ *
+ * 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.
+ *
+ ***/
+
#ifndef SILCRNG_H
#define SILCRNG_H
/* Forward declaration. Actual object is in source file. */
typedef struct SilcRngObjectStruct *SilcRng;
-/* Number of states to fetch data from pool. */
-#define SILC_RNG_STATE_NUM 4
-
-/* Byte size of the random data pool. */
-#define SILC_RNG_POOLSIZE 1024
-
/* Prototypes */
SilcRng silc_rng_alloc();
void silc_rng_free(SilcRng rng);
projects / silc.git / blobdiff