293 lines
9.3 KiB
Groff
293 lines
9.3 KiB
Groff
.\" $NetBSD: openssl_rand.3,v 1.1 2001/04/12 10:45:49 itojun Exp $
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.\" Thu Apr 12 19:27:14 2001
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.\" ======================================================================
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.\"
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.IX Title "rand 3"
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.TH rand 3 "0.9.6a" "2001-04-12" "OpenSSL"
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.UC
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.SH "NAME"
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rand \- pseudo-random number generator
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.SH "LIBRARY"
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libcrypto, -lcrypto
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/rand.h>
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.Ve
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.Vb 2
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\& int RAND_bytes(unsigned char *buf, int num);
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\& int RAND_pseudo_bytes(unsigned char *buf, int num);
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.Ve
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.Vb 4
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\& void RAND_seed(const void *buf, int num);
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\& void RAND_add(const void *buf, int num, int entropy);
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\& int RAND_status(void);
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\& void RAND_screen(void);
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.Ve
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.Vb 3
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\& int RAND_load_file(const char *file, long max_bytes);
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\& int RAND_write_file(const char *file);
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\& const char *RAND_file_name(char *file, size_t num);
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.Ve
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.Vb 1
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\& int RAND_egd(const char *path);
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.Ve
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.Vb 3
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\& void RAND_set_rand_method(RAND_METHOD *meth);
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\& RAND_METHOD *RAND_get_rand_method(void);
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\& RAND_METHOD *RAND_SSLeay(void);
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.Ve
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.Vb 1
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\& void RAND_cleanup(void);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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These functions implement a cryptographically secure pseudo-random
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number generator (\s-1PRNG\s0). It is used by other library functions for
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example to generate random keys, and applications can use it when they
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need randomness.
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.PP
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A cryptographic \s-1PRNG\s0 must be seeded with unpredictable data such as
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mouse movements or keys pressed at random by the user. This is
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described in RAND_add(3). Its state can be saved in a seed file
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(see RAND_load_file(3)) to avoid having to go through the
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seeding process whenever the application is started.
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.PP
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RAND_bytes(3) describes how to obtain random data from the
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\&\s-1PRNG\s0.
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.SH "INTERNALS"
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.IX Header "INTERNALS"
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The \fIRAND_SSLeay()\fR method implements a \s-1PRNG\s0 based on a cryptographic
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hash function.
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.PP
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The following description of its design is based on the SSLeay
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documentation:
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.PP
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First up I will state the things I believe I need for a good \s-1RNG\s0.
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.Ip "1" 4
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.IX Item "1"
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A good hashing algorithm to mix things up and to convert the \s-1RNG\s0 'state'
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to random numbers.
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.Ip "2" 4
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.IX Item "2"
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An initial source of random 'state'.
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.Ip "3" 4
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.IX Item "3"
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The state should be very large. If the \s-1RNG\s0 is being used to generate
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4096 bit \s-1RSA\s0 keys, 2 2048 bit random strings are required (at a minimum).
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If your \s-1RNG\s0 state only has 128 bits, you are obviously limiting the
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search space to 128 bits, not 2048. I'm probably getting a little
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carried away on this last point but it does indicate that it may not be
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a bad idea to keep quite a lot of \s-1RNG\s0 state. It should be easier to
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break a cipher than guess the \s-1RNG\s0 seed data.
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.Ip "4" 4
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.IX Item "4"
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Any \s-1RNG\s0 seed data should influence all subsequent random numbers
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generated. This implies that any random seed data entered will have
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an influence on all subsequent random numbers generated.
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.Ip "5" 4
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.IX Item "5"
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When using data to seed the \s-1RNG\s0 state, the data used should not be
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extractable from the \s-1RNG\s0 state. I believe this should be a
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requirement because one possible source of 'secret' semi random
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data would be a private key or a password. This data must
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not be disclosed by either subsequent random numbers or a
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\&'core' dump left by a program crash.
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.Ip "6" 4
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.IX Item "6"
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Given the same initial 'state', 2 systems should deviate in their \s-1RNG\s0 state
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(and hence the random numbers generated) over time if at all possible.
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.Ip "7" 4
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.IX Item "7"
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Given the random number output stream, it should not be possible to determine
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the \s-1RNG\s0 state or the next random number.
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.PP
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The algorithm is as follows.
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.PP
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There is global state made up of a 1023 byte buffer (the 'state'), a
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working hash value ('md'), and a counter ('count').
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.PP
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Whenever seed data is added, it is inserted into the 'state' as
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follows.
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.PP
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The input is chopped up into units of 20 bytes (or less for
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the last block). Each of these blocks is run through the hash
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function as follows: The data passed to the hash function
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is the current 'md', the same number of bytes from the 'state'
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(the location determined by in incremented looping index) as
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the current 'block', the new key data 'block', and 'count'
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(which is incremented after each use).
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The result of this is kept in 'md' and also xored into the
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\&'state' at the same locations that were used as input into the
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hash function. I
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believe this system addresses points 1 (hash function; currently
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\&\s-1SHA-1\s0), 3 (the 'state'), 4 (via the 'md'), 5 (by the use of a hash
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function and xor).
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.PP
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When bytes are extracted from the \s-1RNG\s0, the following process is used.
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For each group of 10 bytes (or less), we do the following:
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.PP
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Input into the hash function the top 10 bytes from the local 'md'
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(which is initialized from the global 'md' before any bytes are
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generated), the bytes that are to be overwritten by the random bytes,
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and bytes from the 'state' (incrementing looping index). From this
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digest output (which is kept in 'md'), the top (up to) 10 bytes are
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returned to the caller and the bottom (up to) 10 bytes are xored into
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the 'state'.
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.PP
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Finally, after we have finished 'num' random bytes for the caller,
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\&'count' (which is incremented) and the local and global 'md' are fed
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into the hash function and the results are kept in the global 'md'.
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.PP
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I believe the above addressed points 1 (use of \s-1SHA-1\s0), 6 (by hashing
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into the 'state' the 'old' data from the caller that is about to be
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overwritten) and 7 (by not using the 10 bytes given to the caller to
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update the 'state', but they are used to update 'md').
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.PP
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So of the points raised, only 2 is not addressed (but see
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RAND_add(3)).
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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BN_rand(3), RAND_add(3),
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RAND_load_file(3), RAND_egd(3),
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RAND_bytes(3),
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RAND_set_rand_method(3),
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RAND_cleanup(3)
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