1. rand 查看man帮助文档
#man sslrand
RAND(1) OpenSSL RAND(1)用户指令
NAME
rand - generate pseudo-random bytes 生成‘伪随机数’字节
SYNOPSIS
openssl rand [-out file] [-rand file(s)] [-base64] [-hex] num
DESCRIPTION
The rand command outputs num pseudo-random bytes after seeding the random number generator once. As in other openssl command line tools, PRNG seeding uses the file $HOME/.rnd or .rnd inaddition to the files given in the -rand option. A new $HOME/.rnd or .rnd file will be written back if enough seeding was obtained from these sources.
rand命令在一次播种“随机数生成器（seed）”之后输出num个伪随机字节。与其他openssl命令行工具中一样，PRNG ((Pseudo Random Number Generator))seeding使用文件$ HOME / .rnd或附加到-rand选项中给出的.rnd文件。新的$ HOME / .rnd或.rnd文件将从这些源获得足够的播种（seeding）被写回。
随机数字产生器需要一个seed，在没有/dev/random或/dev/urandom linux系统下的解决方法是自己做一个~/.rnd文件 Openssl rand命令（这句参考这里）
OPTIONS
-out file
Write to file instead of standard output.
-rand file(s)
Use specified file or files or EGD socket (see RAND_egd(3)) for seeding the random number generator. 使用指定的file 或files或EGD套接字（请参阅RAND_egd（3））为“随机数生成器”设定seek。 Multiple files can be specified separated by a OS-dependent character. The separator is ; for MS-Windows, , for OpenVMS, and : for all others.
-base64
Perform base64 encoding on the output.对输出执行base64编码。
-hex
Show the output as a hex string. 将输出显示为十六进制字符串。
SEE ALSO
RAND_bytes(3)
1.0.1e 2013-02-11 RAND(1)
2.随机数生成器设备 man帮助文档
#man 4 random
RANDOM(4) Linux Programmer’s Manual RANDOM(4) 设备和特殊文件
NAME
random, urandom - kernel random number source devices 内核随机数源设备
DESCRIPTION
The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) provide an interface to the kernel’s random number generator 这两个特殊字符设备文件提供了一个内核随机数生成器的接口. File /dev/random has major device number 1 and minor device number 8. File /dev/urandom has major device number 1 and minor device num-ber 9.
crw-rw-rw- 1 root root 1, 8 Sep 10 2014 /dev/random 主设备号1，次设备号8 crw-rw-rw- 1 root root 1, 9 Sep 10 2014 /dev/urandom 主设备号1，次设备号9
The random number generator gathers environmental noise from device drivers and other sources into an entropy pool. The generator also keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool random numbers are created.
When read, the /dev/random device will only return random bytes within the estimated number of bits of noise in the entropy pool. /dev/random should be suitable for uses that need very high quality randomness such as one-time pad or key generation. When the entropy pool is empty, reads from /dev/random will block until additional environmental noise is gathered.
A read from the /dev/urandom device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vul-nerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current non-classified literature, but it is theoretically possi-ble that such an attack may exist. If this is a concern in your application, use /dev/random
instead.
Usage
If you are unsure about whether you should use /dev/random or /dev/urandom, then probably you want to use the latter. As a general rule, /dev/urandom should be used for everything except long-lived GPG/SSL/SSH keys.
If a seed file is saved across reboots as recommended above (all major Linux distributions have done this since 2000 at least), the output is cryptographically secure against attackers with-out local root access as soon as it is reloaded in the boot sequence, and perfectly adequate for network encryption session keys. Since reads from /dev/random may block, users will usu-ally want to open it in non-blocking mode (or perform a read with timeout), and provide some
sort of user notification if the desired entropy is not immediately available.
The kernel random-number generator is designed to produce a small amount of high-quality seed material to seed a cryptographic pseudo-random number generator (CPRNG). It is designed for security, not speed, and is poorly suited to generating large amounts of random data. Users should be very economical in the amount of seed material that they read from /dev/urandom (and /dev/random); unnecessarily reading large quantities of data from this device will have a nega-tive impact on other users of the device.
The amount of seed material required to generate a cryptographic key equals the effective key
size of the key. For example, a 3072-bit RSA or Diffie-Hellman private key has an effective
key size of 128 bits (it requires about 2^128 operations to break) so a key generator only
needs 128 bits (16 bytes) of seed material from /dev/random.
While some safety margin above that minimum is reasonable, as a guard against flaws in the CPRNG algorithm, no cryptographic primitive available today can hope to promise more than 256 bits of security, so if any program reads more than 256 bits (32 bytes) from the kernel random pool per invocation, or per reasonable re-seed interval (not less than one minute), that should be taken as a sign that its cryptography is not skilfully implemented.
Configuration
If your system does not have /dev/random and /dev/urandom created already, they can be created with the following commands: 当你的系统事先没有这两个设备文件，可以使用下列命令进行创建:
mknod -m 644 /dev/random c 1 8
mknod -m 644 /dev/urandom c 1 9
chown root:root /dev/random /dev/urandom
When a Linux system starts up without much operator interaction, the entropy pool may be in a fairly predictable state. This reduces the actual amount of noise in the entropy pool below the estimate. In order to counteract this effect, it helps to carry entropy pool information across shut-downs and start-ups. To do this, add the following lines to an appropriate script which is run during the Linux system start-up sequence:
echo "Initializing random number generator..."
random_seed=/var/run/random-seed
# Carry a random seed from start-up to start-up
# Load and then save the whole entropy pool
if [ -f $random_seed ]; then
cat $random_seed >/dev/urandom
else
touch $random_seed
fi
chmod 600 $random_seed
poolfile=/proc/sys/kernel/random/poolsize
[ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
Also, add the following lines in an appropriate script which is run during the Linux system shutdown:
# Carry a random seed from shut-down to start-up
# Save the whole entropy pool
echo "Saving random seed..."
random_seed=/var/run/random-seed
touch $random_seed
chmod 600 $random_seed
poolfile=/proc/sys/kernel/random/poolsize
[ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
/proc Interface
The files in the directory /proc/sys/kernel/random (present since 2.3.16) provide an additional interface to the /dev/random device.
The read-only file entropy_avail gives the available entropy. Normally, this will be 4096 (bits), a full entropy pool.
The file poolsize gives the size of the entropy pool. The semantics of this file vary across kernel versions:
Linux 2.4: This file gives the size of the entropy pool in bytes. Normally, this file
will have the value 512, but it is writable, and can be changed to any value
for which an algorithm is available. The choices are 32, 64, 128, 256, 512,
1024, or 2048.
Linux 2.6: This file is read-only, and gives the size of the entropy pool in bits. It contains the value 4096.
The file read_wakeup_threshold contains the number of bits of entropy required for waking up processes that sleep waiting for entropy from /dev/random. The default is 64. The file write_wakeup_threshold contains the number of bits of entropy below which we wake up processes that do a select(2) or poll(2) for write access to /dev/random. These values can be changed by writing to the files.
The read-only files uuid and boot_id contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9. The former is generated afresh for each read, the latter was generated once.
FILES
/dev/random
/dev/urandom
SEE ALSO
mknod (1)
mknod [OPTION]... NAME TYPE [MAJOR MINOR]
RFC 1750, "Randomness Recommendations for Security"
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-06-20 RANDOM(4)
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