Date: Fri, 5 May 2000 11:17:30 +1000 From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: Mark Murray <mark@grondar.za> Cc: "Andrew J. Korty" <ajk@iu.edu>, security@FreeBSD.ORG Subject: Re: Cryptographic dump(8) Message-ID: <00May5.111951est.116340@border.alcanet.com.au> In-Reply-To: <200005042015.WAA07617@grimreaper.grondar.za>; from mark@grondar.za on Fri, May 05, 2000 at 06:16:30AM %2B1000 References: <Pine.BSF.4.21.0005041318270.26492-100000@verbal.uits.iupui.edu> <ajk@iu.edu> <200005042015.WAA07617@grimreaper.grondar.za>
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On 2000-May-05 06:16:30 +1000, Mark Murray <mark@grondar.za> wrote:
>> I was under the impression that the CBC mode would also propagate this
>> entry throughout the block. Must I use one of the feedback modes?
>
>Yes.
Not exactly.
> I have no hard references in front of me, but I'll put ${bodypart}
>on a block that CBC is 8-bytes-at-a-time-with-same-key-each-time.
Bzzzt. You lose (I hope you didn't set ${bodypart} to anything important).
You are thinking of ECB (Electronic Code Book): This mode takes each
block and individually encrypts it.
CBC (Cipher Block Chaining) is Cn = Ek(C(n-1) XOR Pn) An initialisation
vector (IV) is XOR'd with the first block.
CFB (Cipher feedback) is Cn = Pn XOR Ek(C(n-1))
OFB (Output feedback) is Cn = Pn XOR (Rn = Ek(R(n-1)))
As for propagation:
ECB: the output block depends on the key only.
OFB: the output block depends on the key and IV only.
CBC and CFB: the output block depends on key, IV and all preceeding plaintext
(They all have different behaviours regarding corruption and
synchronisation - which is important if your backup tape loses a few
bits).
Peter
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