--- /dev/null
+=pod
+
+=head1 NAME
+
+rand - pseudo-random number generator
+
+=head1 SYNOPSIS
+
+ #include <openssl/rand.h>
+
+ int RAND_set_rand_engine(ENGINE *engine);
+
+ int RAND_bytes(unsigned char *buf, int num);
+ int RAND_pseudo_bytes(unsigned char *buf, int num);
+
+ void RAND_seed(const void *buf, int num);
+ void RAND_add(const void *buf, int num, int entropy);
+ int RAND_status(void);
+
+ int RAND_load_file(const char *file, long max_bytes);
+ int RAND_write_file(const char *file);
+ const char *RAND_file_name(char *file, size_t num);
+
+ int RAND_egd(const char *path);
+
+ void RAND_set_rand_method(const RAND_METHOD *meth);
+ const RAND_METHOD *RAND_get_rand_method(void);
+ RAND_METHOD *RAND_SSLeay(void);
+
+ void RAND_cleanup(void);
+
+ /* For Win32 only */
+ void RAND_screen(void);
+ int RAND_event(UINT, WPARAM, LPARAM);
+
+=head1 DESCRIPTION
+
+Since the introduction of the ENGINE API, the recommended way of controlling
+default implementations is by using the ENGINE API functions. The default
+B<RAND_METHOD>, as set by RAND_set_rand_method() and returned by
+RAND_get_rand_method(), is only used if no ENGINE has been set as the default
+"rand" implementation. Hence, these two functions are no longer the recommened
+way to control defaults.
+
+If an alternative B<RAND_METHOD> implementation is being used (either set
+directly or as provided by an ENGINE module), then it is entirely responsible
+for the generation and management of a cryptographically secure PRNG stream. The
+mechanisms described below relate solely to the software PRNG implementation
+built in to OpenSSL and used by default.
+
+These functions implement a cryptographically secure pseudo-random
+number generator (PRNG). It is used by other library functions for
+example to generate random keys, and applications can use it when they
+need randomness.
+
+A cryptographic PRNG must be seeded with unpredictable data such as
+mouse movements or keys pressed at random by the user. This is
+described in L<RAND_add(3)|RAND_add(3)>. Its state can be saved in a seed file
+(see L<RAND_load_file(3)|RAND_load_file(3)>) to avoid having to go through the
+seeding process whenever the application is started.
+
+L<RAND_bytes(3)|RAND_bytes(3)> describes how to obtain random data from the
+PRNG.
+
+=head1 INTERNALS
+
+The RAND_SSLeay() method implements a PRNG based on a cryptographic
+hash function.
+
+The following description of its design is based on the SSLeay
+documentation:
+
+First up I will state the things I believe I need for a good RNG.
+
+=over 4
+
+=item 1
+
+A good hashing algorithm to mix things up and to convert the RNG 'state'
+to random numbers.
+
+=item 2
+
+An initial source of random 'state'.
+
+=item 3
+
+The state should be very large. If the RNG is being used to generate
+4096 bit RSA keys, 2 2048 bit random strings are required (at a minimum).
+If your RNG state only has 128 bits, you are obviously limiting the
+search space to 128 bits, not 2048. I'm probably getting a little
+carried away on this last point but it does indicate that it may not be
+a bad idea to keep quite a lot of RNG state. It should be easier to
+break a cipher than guess the RNG seed data.
+
+=item 4
+
+Any RNG seed data should influence all subsequent random numbers
+generated. This implies that any random seed data entered will have
+an influence on all subsequent random numbers generated.
+
+=item 5
+
+When using data to seed the RNG state, the data used should not be
+extractable from the RNG state. I believe this should be a
+requirement because one possible source of 'secret' semi random
+data would be a private key or a password. This data must
+not be disclosed by either subsequent random numbers or a
+'core' dump left by a program crash.
+
+=item 6
+
+Given the same initial 'state', 2 systems should deviate in their RNG state
+(and hence the random numbers generated) over time if at all possible.
+
+=item 7
+
+Given the random number output stream, it should not be possible to determine
+the RNG state or the next random number.
+
+=back
+
+The algorithm is as follows.
+
+There is global state made up of a 1023 byte buffer (the 'state'), a
+working hash value ('md'), and a counter ('count').
+
+Whenever seed data is added, it is inserted into the 'state' as
+follows.
+
+The input is chopped up into units of 20 bytes (or less for
+the last block). Each of these blocks is run through the hash
+function as follows: The data passed to the hash function
+is the current 'md', the same number of bytes from the 'state'
+(the location determined by in incremented looping index) as
+the current 'block', the new key data 'block', and 'count'
+(which is incremented after each use).
+The result of this is kept in 'md' and also xored into the
+'state' at the same locations that were used as input into the
+hash function. I
+believe this system addresses points 1 (hash function; currently
+SHA-1), 3 (the 'state'), 4 (via the 'md'), 5 (by the use of a hash
+function and xor).
+
+When bytes are extracted from the RNG, the following process is used.
+For each group of 10 bytes (or less), we do the following:
+
+Input into the hash function the local 'md' (which is initialized from
+the global 'md' before any bytes are generated), the bytes that are to
+be overwritten by the random bytes, and bytes from the 'state'
+(incrementing looping index). From this digest output (which is kept
+in 'md'), the top (up to) 10 bytes are returned to the caller and the
+bottom 10 bytes are xored into the 'state'.
+
+Finally, after we have finished 'num' random bytes for the caller,
+'count' (which is incremented) and the local and global 'md' are fed
+into the hash function and the results are kept in the global 'md'.
+
+I believe the above addressed points 1 (use of SHA-1), 6 (by hashing
+into the 'state' the 'old' data from the caller that is about to be
+overwritten) and 7 (by not using the 10 bytes given to the caller to
+update the 'state', but they are used to update 'md').
+
+So of the points raised, only 2 is not addressed (but see
+L<RAND_add(3)|RAND_add(3)>).
+
+=head1 SEE ALSO
+
+L<BN_rand(3)|BN_rand(3)>, L<RAND_add(3)|RAND_add(3)>,
+L<RAND_load_file(3)|RAND_load_file(3)>, L<RAND_egd(3)|RAND_egd(3)>,
+L<RAND_bytes(3)|RAND_bytes(3)>,
+L<RAND_set_rand_method(3)|RAND_set_rand_method(3)>,
+L<RAND_cleanup(3)|RAND_cleanup(3)>
+
+=cut
projects / cassiopeia.git / blobdiff