Careful with subkeys - Re: Hi again, and an invitation to kibitz

[Peter M Allan]

Amanda Walker wrote:

>  I'm considering DES-EDE (the easy option), Blowfish (also pretty easy),
>  or the DES variant Bruce Schneier describes in Applied Cryptography, 2nd ed.
>  (the one with independent subkeys).


This might be a bad idea.  Rumor has it that independent subkeys
are eaten alive by related-key attacks (not very practical usually).
I think I saw this in a post by Matt Blaze about last November on coderpunks.

That post suggested 2 key schedule strategies:

1)  planned, like DES, by people who know how a particular schedule
    affects related-key attacks

2)  scrambled, like Turtle & Blowfish, so that key bits all depend on
    each other in a messy way

I mark the margin of my AC book with snippets like this.
I don't seem to have kept the post in question.


 -- Peter Allan    peter.allan at aeat.co.uk