Qrpff

qrpff is a Perl script created by Keith Winstein and Marc Horowitz of the MIT SIPB.^[1] It performs DeCSS in six or seven lines. The name itself is an encoding of "decss" in rot-13. The algorithm was rewritten 77 times to condense it down to six lines.^[2]

In fact, two versions of qrpff exist: a short version (6 lines) and a fast version (7 lines). Both appear below.

Short:

#!/usr/bin/perl
# 472-byte qrpff, Keith Winstein and Marc Horowitz <[email protected]>
# MPEG 2 PS VOB file -> descrambled output on stdout.
# usage: perl -I <k1>:<k2>:<k3>:<k4>:<k5> qrpff
# where k1..k5 are the title key bytes in least to most-significant order

s''$/=\2048;while(<>){G=29;R=142;if((@a=unqT="C*",_)[20]&48){D=89;_=unqb24,qT,@
b=map{ord qB8,unqb8,qT,_^$a[--D]}@INC;s/...$/1$&/;Q=unqV,qb25,_;H=73;O=$b[4]<<9
|256|$b[3];Q=Q>>8^(P=(E=255)&(Q>>12^Q>>4^Q/8^Q))<<17,O=O>>8^(E&(F=(S=O>>14&7^O)
^S*8^S<<6))<<9,_=(map{U=_%16orE^=R^=110&(S=(unqT,"\xb\ntd\xbz\x14d")[_/16%8]);E
^=(72,@z=(64,72,G^=12*(U-2?0:S&17)),H^=_%64?12:0,@z)[_%8]}(16..271))[_]^((D>>=8
)+=P+(~F&E))for@a[128..$#a]}print+qT,@a}';s/[D-HO-U_]/\$$&/g;s/q/pack+/g;eval

Fast:

#!/usr/bin/perl -w
# 531-byte qrpff-fast, Keith Winstein and Marc Horowitz <[email protected]>
# MPEG 2 PS VOB file on stdin -> descrambled output on stdout
# arguments: title key bytes in least to most-significant order

$_='while(read+STDIN,$_,2048){$a=29;$b=73;$c=142;$t=255;@t=map{$_%16or$t^=$c^=(
$m=(11,10,116,100,11,122,20,100)[$_/16%8])&110;$t^=(72,@z=(64,72,$a^=12*($_%16
-2?0:$m&17)),$b^=$_%64?12:0,@z)[$_%8]}(16..271);if((@a=unx"C*",$_)[20]&48){$h
=5;$_=unxb24,join"",@b=map{xB8,unxb8,chr($_^$a[--$h+84])}@ARGV;s/...$/1$&/;$
d=unxV,xb25,$_;$e=256|(ord$b[4])<<9|ord$b[3];$d=$d>>8^($f=$t&($d>>12^$d>>4^
$d^$d/8))<<17,$e=$e>>8^($t&($g=($q=$e>>14&7^$e)^$q*8^$q<<6))<<9,$_=$t[$_]^
(($h>>=8)+=$f+(~$g&$t))for@a[128..$#a]}print+x"C*",@a}';s/x/pack+/g;eval

The fast version is actually fast enough to decode a movie in real-time.

qrpff and related memorabilia was sold for $2,500 at The Algorithm Auction, the world’s first auction of computer algorithms.^[3]

References

1. McCullagh, Declan (8 March 2001). "Descramble That DVD in 7 Lines". Wired. Retrieved 26 April 2013.
2. Hotz, Robert (27 May 2015). "What's Hot in the Art World? Algorithms". Wall Street Journal. Retrieved 27 May 2015.
3. "Keith Winstein – qrpff". Artsy. Artsy. Retrieved 18 June 2015.

External links

• qrpff (fast) explained
• Gallery of CSS descramblers