[553] in cryptography@c2.net mail archive
RC4 rollback/rollforward question
daemon@ATHENA.MIT.EDU (Rodney Thayer)
Wed Apr 16 11:03:11 1997
Date: Tue, 15 Apr 1997 20:03:37 -0400
To: cryptography@c2.net
From: Rodney Thayer <rodney@sabletech.com>
On the IPsec list we've been talking about using streaming ciphers and
dealing with out of order packets.
The comment has been made that you can deal with out of order input to a
streaming cipher, like RC4. One thing I didn't quite catch was what you do
to "roll forward" the RC4 state so you can handle this case:
stream offset xxx data aaa bbb ccc ddd
stream offset xxx+yyy data eee fff ggg hhh
In other words, I feed aaa bbb ccc ddd to the RC4 engine, then I have to
skip forwards by 'yyy' bytes to feed the engine the 'eee fff ggg hhh' bytes.
Is this discussed somewhere I could go read? It sounds to me a bit more
than trivial and of course it doesn't sound like a vanilla operation for
(my favorite licensed) RC4 implementation.
[Rodney, who hasn't really posted here before and hopes he's respectfully
following the guidelines for this list.]
Rodney Thayer <rodney@sabletech.com> +1 617 332 7292
Sable Technology Corp, 246 Walnut St., Newton MA 02160 USA
Fax: +1 617 332 7970 http://www.shore.net/~sable
"Developers of communications software"
