Re: RC4 implementation question

From: Gregory G Rose (ggr_at_qualcomm.com)
Date: 04/05/04 

Date: 5 Apr 2004 04:28:49 -0700

In article <c4rb5u$2gqfdv$1@ID-161723.news.uni-berlin.de>,
lallous <lallous@lgwm.org> wrote:
>Everytime I encrypt, 'k1' is getting modified. In order to decrypt, I have
>to have the key in the original initiated form and call rc4_crypt() in the
>same order as the buffers were called to encrypt.
>
>1)Is this a good RC4 implementation?

The array and variables associated with RC4 are
usually referred to as "state", rather than "key".
The key is the "abcd" you used to initialise it
with in the first place.

So, in the sense that this implementation calls it
"key", I would not call it a good implementation.

>2)Does the usual RC4 implementation change the 'key' ?

Yes, it changes the state as it encrypts. The
implementation is probably faithful (I didn't
check).

RC4 is a "synchronous stream cipher", which means
that the encryption and decryption ends must
remain correctly synchronised. That's what you're
seeing.

Greg. 

-- 
Greg Rose
232B EC8F 44C6 C853 D68F  E107 E6BF CD2F 1081 A37C
Qualcomm Australia: http://www.qualcomm.com.au

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