# Re: CFB mode problems

Garrison M. Venn (gvenn@vail.net)
Mon, 08 Mar 1999 20:13:07 -0700

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    Date: Mon, 08 Mar 1999 20:13:07 -0700

    From: "Garrison M. Venn" <gvenn@vail.net>

    To: Jan Luehe <Jan.Luehe@Eng>

    Subject: Re: CFB mode problems

Jan Luehe wrote:

> Garrison:
>
> In your program, you initialize the cipher object for encryption
> both when you write to the file and when you read from it.
>
> You have to initialize the cipher for DECRYPTION before you read
> from the file. When you do that, everything works fine.
>
> Jan
>
>

Thanks Jan!!

I realize now that I was confused between the concept of always running the block
cipher in encrypt "mode" for stream cipher behaviour and the concept of how the
mode fills its feedback register. This is where the semantics for OFB and CFB use
are not the same. The mechanism used to fill the feedback register in CFB mode is
not symmetric for encryption and decryption where as it is for OFB mode. Thanks
to your pointer I finally understand!

Garrison Venn
gvenn@vail.net

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