Mailing List Archive

On Wed, Jul 30, 2008 at 1:25 PM, Jabka Atu <mashrom.head@gmail.com> wrote:
> The next comment should be added in encrypt / decrypt :
>
>
> "When using ARCFOUR you should use two different handlers (one for
> encryption and other for decryption with the same key ) ."
>
>
> Special thank <Adam Langley> that gave me that solutione ( i wasted 36
> coding hours on that ) .

Well... that's true of any handle which keeps state, right? What about
CTR mode block ciphers, or CBC, or ... Or the K(block) style stream
ciphers like Salsa20.. or probably anything that isn't ECB. It's been
a while since I used the gcrypt API, but I believe that's true.



AGL

--
Adam Langley agl@imperialviolet.org http://www.imperialviolet.org

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On Wed, 30 Jul 2008 22:52, agl@imperialviolet.org said:

> Well... that's true of any handle which keeps state, right? What about
> CTR mode block ciphers, or CBC, or ... Or the K(block) style stream
> ciphers like Salsa20.. or probably anything that isn't ECB. It's been
> a while since I used the gcrypt API, but I believe that's true.

You are right. A context/handle is there to convey state - if we would
not need state there would be no need for a context.

The problem at hand is an improper use of the API. If you want to start
a operation you alocate a context, set the key and the IV and start. Do
the same if you are going to decrypt. Actually you don't need to get a
allocate new context but call gcry_cipher_reset to put it back into a
vanilla state.

I also advice against the use of arcfour becuase there are so many
things you can get wrong in using this particular stream cipher. It is
pretty easy to use arcfour in an unsafe way and have the community fun
decrypting it. Use AES and never use ECB mode with any cipher.


Shalom-Salam,

Werner


--
Die Gedanken sind frei. Auschnahme regelt ein Bundeschgesetz.


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