Cryptography and SSL/TLS Toolkit



openssl-rand, rand - generate pseudo-random bytes


openssl rand [-help] [-out file] [-rand file...] [-writerand file] [-base64] [-hex] num


This command generates num random bytes using a cryptographically secure pseudo random number generator (CSPRNG).

The random bytes are generated using the RAND_bytes(3) function, which provides a security level of 256 bits, provided it managed to seed itself successfully from a trusted operating system entropy source. Otherwise, the command will fail with a nonzero error code. For more details, see RAND_bytes(3), RAND(7), and RAND_DRBG(7).



Print out a usage message.

-out file

Write to file instead of standard output.

-rand file...

A file or files containing random data used to seed the random number generator. Multiple files can be specified separated by an OS-dependent character. The separator is ; for MS-Windows, , for OpenVMS, and : for all others. Explicitly specifying a seed file is in general not necessary, see the "NOTES" section for more information.

[-writerand file]

Writes random data to the specified file upon exit. This can be used with a subsequent -rand flag.


Perform base64 encoding on the output.


Show the output as a hex string.


Prior to OpenSSL 1.1.1, it was common for applications to store information about the state of the random-number generator in a file that was loaded at startup and rewritten upon exit. On modern operating systems, this is generally no longer necessary as OpenSSL will seed itself from a trusted entropy source provided by the operating system. The -rand and -writerand flags are still supported for special platforms or circumstances that might require them.

It is generally an error to use the same seed file more than once and every use of -rand should be paired with -writerand.


RAND_bytes(3), RAND(7), RAND_DRBG(7)

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