### Tip: Creating a String of Random Hex Digits

Every once in a while a string of random hex digits comes in handy*, perhaps when you need a random alphanumeric name for something. The `unpack`

operator seems like it should come in handy for this purpose, given that it can translate character values into hex strings, and so it does. Here's a snippet that creates a string of 32 random hex digits:

my $rand_hex = join "", map { unpack "H*", chr(rand(256)) } 1..16;

`rand(256)`

produces floating-point numbers `0.0 <= n < 256.0`

. `unpack "H2"`

turns a character (a single-character string to be precise) into a 2-character MSB-first hex string - the `chr`

is needed to turn the numeric value from `rand`

into a string. The `map { ... } 1..16`

creates a list of 16 different 2-character strings, and `join`

gloms them all together.

Here's another - simpler - way to generate 32 random hex digits:

use Digest::MD5 qw(md5_hex);my $rand_hex2 = md5_hex(rand);

*Or maybe you think that Perl looks like random hex digits all on its own. Most people, however, think it looks like modem line noise $$817 d3#@ ss NO CARRIER

## 4 Comments:

md5_hex(rand) will not give you a string of random hex digits. In fact, it's considerably less than random, since rand() gives 32, 48, or 64 bits of randomness, but a md5 hash is 128 bits. So there are 2^128 possible md5 hashes but your md5_hex(rand) will choose from only at most 2^64 of them. So it only looks random -- be careful!

Of course it's a random string. However, it doesn't have 128 bits of entropy.

It's also unlikely that glomming together hex digits resulting from repeated calls to

randwill really produce 128 bits of randomness - because the pseudorandom number generator on your favorite platform is probably not cryptographically strong.my $0,2

perl -e 'print ((0..9,a..f)[rand(16)]) for 1..32'

Thanks.You can always run your Perl code under Perl's w flag.

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