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Mark R V Murray
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.Dd March 22, 2021 .Dt RANDOM 9 .Os .Sh NAME .Nm arc4rand , .Nm arc4random , .Nm arc4random_buf , .Nm is_random_seeded , .Nm random , .Nm read_random , .Nm read_random_uio .Nd supply pseudo-random numbers .Sh SYNOPSIS n sys/libkern.h .Ft uint32_t .Fn arc4random "void" .Ft void .Fn arc4random_buf "void *ptr" "size_t len" .Ft void .Fn arc4rand "void *ptr" "u_int length" "int reseed"
p n sys/random.h .Ft bool .Fn is_random_seeded "void" .Ft void .Fn read_random "void *buffer" "int count" .Ft int .Fn read_random_uio "struct uio *uio" "bool nonblock" .Ss LEGACY ROUTINES n sys/libkern.h .Ft u_long .Fn random "void" .Sh DESCRIPTION The .Fn arc4random and .Fn arc4random_buf functions will return very good quality random numbers, suited for security-related purposes. Both are wrappers around the underlying .Fn arc4rand interface. .Fn arc4random returns a 32-bit random value, while .Fn arc4random_buf fills .Fa ptr with .Fa len bytes of random data.
p The .Fn arc4rand CSPRNG is seeded from the .Xr random 4 kernel abstract entropy device. Automatic reseeding happens at unspecified time and bytes (of output) intervals. A reseed can be forced by passing a non-zero .Fa reseed value.
p The .Fn read_random function is used to read entropy directly from the kernel abstract entropy device. .Fn read_random blocks if and until the entropy device is seeded. The provided .Fa buffer is filled with no more than .Fa count bytes. It is strongly advised that .Fn read_random is not used directly; instead, use the .Fn arc4rand family of functions.
p The .Fn is_random_seeded function can be used to check in advance if .Fn read_random will block. (If random is seeded, it will not block.)
p The .Fn read_random_uio function behaves identically to .Xr read 2 on
a /dev/random . The .Fa uio argument points to a buffer where random data should be stored. If .Fa nonblock is true and the random device is not seeded, this function does not return any data. Otherwise, this function may block interruptibly until the random device is seeded. If the function is interrupted before the random device is seeded, no data is returned.
p The deprecated .Fn random function will return a 31-bit value. It is obsolete and scheduled to be removed in .Fx 14.0 . Consider .Xr prng 9 instead and see .Sx SECURITY CONSIDERATIONS . .Sh RETURN VALUES The .Fn arc4rand function uses the Chacha20 algorithm to generate a pseudo-random sequence of bytes. The .Fn arc4random function uses .Fn arc4rand to generate pseudo-random numbers in the range from 0 to
p The .Fn read_random function returns the number of bytes placed in .Fa buffer .
p .Fn read_random_uio returns zero when successful, otherwise an error code is returned.
p .Fn random returns numbers in the range from 0 to .Sh ERRORS .Fn read_random_uio may fail if: l -tag -width Er t Bq Er EFAULT .Fa uio points to an invalid memory region. t Bq Er EWOULDBLOCK The random device is unseeded and .Fa nonblock is true. .El .Sh AUTHORS .An Dan Moschuk wrote .Fn arc4random . .An Mark R V Murray wrote .Fn read_random . .Sh SECURITY CONSIDERATIONS Do not use .Fn random in new code.
p It is important to remember that the .Fn random function is entirely predictable. It is easy for attackers to predict future output of .Fn random by recording some generated values. We cannot emphasize strongly enough that .Fn random must not be used to generate values that are intended to be unpredictable.