randombytes(3)

  NAME

randombytes - fill a buffer with random data

  SYNOPSIS

    #include <randombytes.h>
    unsigned char x[xbytes];
    randombytes(x,xbytes);

Link with -lrandombytes.

  DESCRIPTION

randombytes sets x[0], x[1], ..., x[xbytes-1] to random bytes of data.

Randomness APIs vary in three major ways. randombytes is designed in each way to simplify callers:

•   Like RAND_bytes, randombytes automatically generates separate randomness for any number of bytes in any number of calls in any number of threads in any number of programs. For comparison, some randomness APIs (e.g., random) recycle randomness from one program to another unless the program does extra work to set a separate “seed”, and can recycle randomness across multiple threads unless the program does further work.

•   Like getrandom and getentropy and RAND_bytes, randombytes aims for the stringent goal of ensuring that no feasible computation will ever be able to tell the difference between the output bytes and true randomness. The caller can treat each returned byte as the result of 8 fresh coin flips. For comparison, some randomness APIs (e.g., random) do not aim for this goal and do not view detectable patterns as a bug, as long as most applications do not notice the patterns.

•   Like random, randombytes always succeeds. Any necessary resources (e.g., opening a file descriptor for /dev/urandom, on systems that need this) are allocated at program startup, rather than being deferred until the first randombytes call; also, dynamic failure cases such as EINTR are handled inside randombytes. For comparison, some randomness APIs (e.g., getrandom and getentropy and RAND_bytes) can return error codes to be handled by the caller.

There are some programs that try to close all file descriptors. These programs must limit their library use to libraries that promise not to keep file descriptors open. In particular, these programs must not use randombytes (which keeps a file descriptor open on some systems) or other libraries calling randombytes.

  LINK DETAILS

Currently -lrandombytes is a frontend symbolic link to either -lrandombytes-kernel or -lrandombytes-openssl as a backend library. To simplify system-wide replacement of the backend library, typical applications should dynamically link to -lrandombytes rather than to -lrandombytes-kernel or -lrandombytes-openssl.

Applications that link statically to -lrandombytes also need -lcrypto if -lrandombytes is -lrandombytes-openssl.

Currently randombytes is a macro, where the function actually linked is randombytes_internal_void_voidstar_longlong.

  HISTORY

The randombytes API was introduced in 2008 as part of the SUPERCOP (https://bench.cr.yp.to) benchmarking framework for cryptographic software.

Similar previous APIs include RAND_bytes and arc4random_buf, but RAND_bytes was allowed to return failures and arc4random_buf was using the broken RC4 stream cipher.

  SEE ALSO

getrandom(2), getentropy(2), rand(3), random(3), arc4random(3), urandom(4)