MacOS autologin password

If you need to set it programmatically

Originally reverse-engineered by Gavin Brock in 2007 (original website is down, but copy preserved in archive.org), and then repeated multiple times, most recently in tart MacOS sequoia image. Cheers to backwards-compatible XOR security!

Here's my attempt at making it into a perl oneliner:

PASS=newpassword
perl -e '$a=$ARGV[0]; use POSIX; $l=ceil(length($a)/12)*12; $s=unpack("u","+?8E2(]*\\W>JCN1\\`"); $k=substr($s x $l,0,$l); print $a ^. $k' "$PASS" | sudo tee -a /etc/kcpassword

In more readable format:

use POSIX
$a=$ARGV[0]
$l=ceil(length($a)/12)*12
$s=unpack("u","+?8E2(]*\\W>JCN1\\`")
$k=substr($s x $l,0,$l)
print $a ^. $k

• $a is your password, taken from first argument

• $l is its length, padded to nearest multiple of 12 (11->12, 12->12, 13->24, 14->24, etc)

• $s is Apple's secret string

• $k is encryption key - secret string, repeated as necessary and truncated to length $l

• Last line performs bitwise XOR between your password $a and encryption key $k

Note that according to perlop, ^. operator, when applied to strings, implicitly appends zero characters to it. If it's not good enough for you, you can always append zero bytes to the end of $a, like this: $a=pack("Z" . $l, $a).

Note that if in addition to having autologin, you want your user to have an empty password - then /etc/kcpassword file can contain a single } character followed by any 11 characters, for example like this:

echo '}-1-2-3-4-5' | sudo tee /etc/kcpassword

Note that echo adds an extra newline character, so you'll need to add only ten filler characters.