And now, smaller and faster than the PERL version...
/* efdtt.c Author: Charles M. Hannum <root@ihack.net> */
/* */
/* Usage is: cat title-key scrambled.vob | efdtt >clear.vob */
#define K(i)(x[i]^s[i+84])<<
unsigned char x[5],y,z,s[2048];main(n){for(read(0,x,5);read(0,s,n=2048);write(1
,s,n))if(s[y=s[13]%8+20]/16%4==1){int i=K(1)17^256+K(0)8,k=K(2)0,j=K(4)17^K(3)9
^k*2-k%8^8,a=0,b=0,c=26;for(s[y]-=16;--c;i/=2,j/=2)a=a*2^i&1,b=b*2^j&1;for(j=
127;++j<n;c=z+c>y)a^=a>>14,a=a>>8^(y=a^a*8^a<<6)<<9,b=b>>8^(z=b^b/8^b>>4^b>>12)
<<17,i=s[j],i="7Wo~'G_\216"[i&7]+2^"cr3sfw6v;*k+>/n."[i>>4]*2^i*257/8,s[j]=i^(i
&i*2&34)*6^z+c+~y;}}
Oh, and if you're worried about being arrested for carrying illegal source code, try this
Illegal prime number.