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I needed to generate a random password from a shell script, I figured that this was solved long ago, so I turned to teh interwebz to quickly copy/paste a working solution.
Inspecting the first few links that turned up, I noticed many of the proposed solutions are dubious at best.

The date ain’t random, buddy

The most obviously wrong are:

$ date +%s | sha256sum | base64 | head -c32
$ date | md5sum
$ ping -c 1 yahoo.com | md5 | head -c8

To paraphrase a quote from holy scripture The Hitchhikers guide to the Galaxy: “This is obviously some strange usage of the word ‘random’ that I hadn’t previously been aware of”.

Both SHA256 & MD5 also output in hex, so that would limit the total amount of characters to just 16, instead of 92.

tr means translate characters

Most of the other commands suffer from a dubious usage of tr(1).

tr(1) works on characters, not byte streams, /dev/urandom outputs a byte stream, not characters.
If your locale is set to (extended) ASCII or a variant thereof (ISO-8859-1, Windows-1252) this is more or less okay, since every byte is a character or escape code.

However, with UTF-8 or another multibyte character sets, it gets more complicated. Not every random byte stream is a valid set of UTF-8 characters, the chances of a random byte stream also being a valid UTF-8 character stream are quite small.

Yet, it seems to work on Linux with GNU tr. Why? Here’s a clue:

$ echo 'I løv€ π' | tr '[:lower:]' '[:upper:]' 
I LøV€ π

$ echo 'I løv€ π' | tr øπ€ X
I lXXvXXX XX

We would expect the ø and π to be uppercased, but they’re not, and the ø, π, and € getting replaced by 2 or 3 X’s?

The astute reader will have recognized what this means, GNU tr doesn’t handle multibyte characters, and always assumes an ASCII character set, which is somewhat disappointing, since it’s 2014, not 1974.

FreeBSD, for example, does this correctly, it also gives an error message on invalid UTF-8 sequences:

$ echo 'I løv€ π' | tr '[:lower:]' '[:upper:]'
I LØV€ Π

$ echo 'I løv€ π' | tr øπ€ X
I lXvX X

$ head -c5 /dev/urandom | tr X Y
tr: Illegal byte sequence

$ setenv LC_CTYPE C  

$ head -c5 /dev/urandom | tr X Y
f��!�

The moral here is: byte streams are not character streams, don’t use ’em as such. It may work for now, but whenever someone adds multibyte support to GNU tr, your command will fail. It’s 2014, always assume multibyte.

Other problems

While I’m whining anyway…

$ openssl rand -base64 8 | md5 | head -c8

Using openssl rand is a good idea, but piping it to md5 isn’t. base64 gives me 64 characters, md5 gives me 16, making the password a lot easier to brute force. Also, 8 characters is too short, use at least 15.

$ curl -s http://sensiblepassword.com/?harder=1

Getting a random password from the internet is spectacularly stupid & naive. Someone now knows:

I can now cross-reference the info with other data collected about you. For example, you once posted to a mailing list, your IP address is in the mail’s header, so we now have a password, name, and an email address.
I hope you can finish the scenario from here…

Just don’t do this. Ever. Randomly banging on the keyboard is a lot better.

Good solutions

$ strings -n 1 < /dev/urandom | tr -d '[:space:]' | head -c15
$ openssl rand -base64 15
$ gpg2 --armor --gen-random 1 15

The first solution could be considered slightly better, since it includes more characters (92 instead of 64). It also doesn’t require external tools (although openssl is almost always available these days).

Lessons

Feedback

You can mail me at martin@arp242.net for feedback, questions, etc.

Copyright © 2010-2017 Martin Tournoij <martin@arp242.net>
This document is licensed under a cc-by 4.0 license.