22
23 int RAND_egd(const char *path);
24
25 void RAND_set_rand_method(RAND_METHOD *meth);
26 RAND_METHOD *RAND_get_rand_method(void);
27 RAND_METHOD *RAND_SSLeay(void);
28
29 void RAND_cleanup(void);
30
31=head1 DESCRIPTION
32
33These functions implement a cryptographically secure pseudo-random
34number generator (PRNG). It is used by other library functions for
35example to generate random keys, and applications can use it when they
36need randomness.
37
38A cryptographic PRNG must be seeded with unpredictable data such as
39mouse movements or keys pressed at random by the user. This is
40described in L<RAND_add(3)|RAND_add(3)>. Its state can be saved in a seed file
41(see L<RAND_load_file(3)|RAND_load_file(3)>) to avoid having to go through the
42seeding process whenever the application is started.
43
44L<RAND_bytes(3)|RAND_bytes(3)> describes how to obtain random data from the
45PRNG.
46
47=head1 INTERNALS
48
49The RAND_SSLeay() method implements a PRNG based on a cryptographic
50hash function.
51
52The following description of its design is based on the SSLeay
53documentation:
54
55First up I will state the things I believe I need for a good RNG.
56
57=over 4
58
59=item 1
60
61A good hashing algorithm to mix things up and to convert the RNG 'state'
62to random numbers.
63
64=item 2
65
66An initial source of random 'state'.
67
68=item 3
69
70The state should be very large. If the RNG is being used to generate
714096 bit RSA keys, 2 2048 bit random strings are required (at a minimum).
72If your RNG state only has 128 bits, you are obviously limiting the
73search space to 128 bits, not 2048. I'm probably getting a little
74carried away on this last point but it does indicate that it may not be
75a bad idea to keep quite a lot of RNG state. It should be easier to
76break a cipher than guess the RNG seed data.
77
78=item 4
79
80Any RNG seed data should influence all subsequent random numbers
81generated. This implies that any random seed data entered will have
82an influence on all subsequent random numbers generated.
83
84=item 5
85
86When using data to seed the RNG state, the data used should not be
87extractable from the RNG state. I believe this should be a
88requirement because one possible source of 'secret' semi random
89data would be a private key or a password. This data must
90not be disclosed by either subsequent random numbers or a
91'core' dump left by a program crash.
92
93=item 6
94
95Given the same initial 'state', 2 systems should deviate in their RNG state
96(and hence the random numbers generated) over time if at all possible.
97
98=item 7
99
100Given the random number output stream, it should not be possible to determine
101the RNG state or the next random number.
102
103=back
104
105The algorithm is as follows.
106
107There is global state made up of a 1023 byte buffer (the 'state'), a
108working hash value ('md'), and a counter ('count').
109
110Whenever seed data is added, it is inserted into the 'state' as
111follows.
112
113The input is chopped up into units of 20 bytes (or less for
114the last block). Each of these blocks is run through the hash
115function as follows: The data passed to the hash function
116is the current 'md', the same number of bytes from the 'state'
117(the location determined by in incremented looping index) as
118the current 'block', the new key data 'block', and 'count'
119(which is incremented after each use).
120The result of this is kept in 'md' and also xored into the
121'state' at the same locations that were used as input into the
122hash function. I
123believe this system addresses points 1 (hash function; currently
124SHA-1), 3 (the 'state'), 4 (via the 'md'), 5 (by the use of a hash
125function and xor).
126
127When bytes are extracted from the RNG, the following process is used.
128For each group of 10 bytes (or less), we do the following:
129
130Input into the hash function the top 10 bytes from the local 'md'
131(which is initialized from the global 'md' before any bytes are
132generated), the bytes that are to be overwritten by the random bytes,
133and bytes from the 'state' (incrementing looping index). From this
134digest output (which is kept in 'md'), the top (up to) 10 bytes are
135returned to the caller and the bottom (up to) 10 bytes are xored into
136the 'state'.
137
138Finally, after we have finished 'num' random bytes for the caller,
139'count' (which is incremented) and the local and global 'md' are fed
140into the hash function and the results are kept in the global 'md'.
141
142I believe the above addressed points 1 (use of SHA-1), 6 (by hashing
143into the 'state' the 'old' data from the caller that is about to be
144overwritten) and 7 (by not using the 10 bytes given to the caller to
145update the 'state', but they are used to update 'md').
146
147So of the points raised, only 2 is not addressed (but see
148L<RAND_add(3)|RAND_add(3)>).
149
150=head1 SEE ALSO
151
152L<BN_rand(3)|BN_rand(3)>, L<RAND_add(3)|RAND_add(3)>,
153L<RAND_load_file(3)|RAND_load_file(3)>, L<RAND_egd(3)|RAND_egd(3)>,
154L<RAND_bytes(3)|RAND_bytes(3)>,
155L<RAND_set_rand_method(3)|RAND_set_rand_method(3)>,
156L<RAND_cleanup(3)|RAND_cleanup(3)>
157
158=cut
| 22
23 int RAND_egd(const char *path);
24
25 void RAND_set_rand_method(RAND_METHOD *meth);
26 RAND_METHOD *RAND_get_rand_method(void);
27 RAND_METHOD *RAND_SSLeay(void);
28
29 void RAND_cleanup(void);
30
31=head1 DESCRIPTION
32
33These functions implement a cryptographically secure pseudo-random
34number generator (PRNG). It is used by other library functions for
35example to generate random keys, and applications can use it when they
36need randomness.
37
38A cryptographic PRNG must be seeded with unpredictable data such as
39mouse movements or keys pressed at random by the user. This is
40described in L<RAND_add(3)|RAND_add(3)>. Its state can be saved in a seed file
41(see L<RAND_load_file(3)|RAND_load_file(3)>) to avoid having to go through the
42seeding process whenever the application is started.
43
44L<RAND_bytes(3)|RAND_bytes(3)> describes how to obtain random data from the
45PRNG.
46
47=head1 INTERNALS
48
49The RAND_SSLeay() method implements a PRNG based on a cryptographic
50hash function.
51
52The following description of its design is based on the SSLeay
53documentation:
54
55First up I will state the things I believe I need for a good RNG.
56
57=over 4
58
59=item 1
60
61A good hashing algorithm to mix things up and to convert the RNG 'state'
62to random numbers.
63
64=item 2
65
66An initial source of random 'state'.
67
68=item 3
69
70The state should be very large. If the RNG is being used to generate
714096 bit RSA keys, 2 2048 bit random strings are required (at a minimum).
72If your RNG state only has 128 bits, you are obviously limiting the
73search space to 128 bits, not 2048. I'm probably getting a little
74carried away on this last point but it does indicate that it may not be
75a bad idea to keep quite a lot of RNG state. It should be easier to
76break a cipher than guess the RNG seed data.
77
78=item 4
79
80Any RNG seed data should influence all subsequent random numbers
81generated. This implies that any random seed data entered will have
82an influence on all subsequent random numbers generated.
83
84=item 5
85
86When using data to seed the RNG state, the data used should not be
87extractable from the RNG state. I believe this should be a
88requirement because one possible source of 'secret' semi random
89data would be a private key or a password. This data must
90not be disclosed by either subsequent random numbers or a
91'core' dump left by a program crash.
92
93=item 6
94
95Given the same initial 'state', 2 systems should deviate in their RNG state
96(and hence the random numbers generated) over time if at all possible.
97
98=item 7
99
100Given the random number output stream, it should not be possible to determine
101the RNG state or the next random number.
102
103=back
104
105The algorithm is as follows.
106
107There is global state made up of a 1023 byte buffer (the 'state'), a
108working hash value ('md'), and a counter ('count').
109
110Whenever seed data is added, it is inserted into the 'state' as
111follows.
112
113The input is chopped up into units of 20 bytes (or less for
114the last block). Each of these blocks is run through the hash
115function as follows: The data passed to the hash function
116is the current 'md', the same number of bytes from the 'state'
117(the location determined by in incremented looping index) as
118the current 'block', the new key data 'block', and 'count'
119(which is incremented after each use).
120The result of this is kept in 'md' and also xored into the
121'state' at the same locations that were used as input into the
122hash function. I
123believe this system addresses points 1 (hash function; currently
124SHA-1), 3 (the 'state'), 4 (via the 'md'), 5 (by the use of a hash
125function and xor).
126
127When bytes are extracted from the RNG, the following process is used.
128For each group of 10 bytes (or less), we do the following:
129
130Input into the hash function the top 10 bytes from the local 'md'
131(which is initialized from the global 'md' before any bytes are
132generated), the bytes that are to be overwritten by the random bytes,
133and bytes from the 'state' (incrementing looping index). From this
134digest output (which is kept in 'md'), the top (up to) 10 bytes are
135returned to the caller and the bottom (up to) 10 bytes are xored into
136the 'state'.
137
138Finally, after we have finished 'num' random bytes for the caller,
139'count' (which is incremented) and the local and global 'md' are fed
140into the hash function and the results are kept in the global 'md'.
141
142I believe the above addressed points 1 (use of SHA-1), 6 (by hashing
143into the 'state' the 'old' data from the caller that is about to be
144overwritten) and 7 (by not using the 10 bytes given to the caller to
145update the 'state', but they are used to update 'md').
146
147So of the points raised, only 2 is not addressed (but see
148L<RAND_add(3)|RAND_add(3)>).
149
150=head1 SEE ALSO
151
152L<BN_rand(3)|BN_rand(3)>, L<RAND_add(3)|RAND_add(3)>,
153L<RAND_load_file(3)|RAND_load_file(3)>, L<RAND_egd(3)|RAND_egd(3)>,
154L<RAND_bytes(3)|RAND_bytes(3)>,
155L<RAND_set_rand_method(3)|RAND_set_rand_method(3)>,
156L<RAND_cleanup(3)|RAND_cleanup(3)>
157
158=cut
|