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Re: EncryptionMethod in XMLEnc and SignatureMethod in XMLDSig

From: Christian Geuer-Pollmann <geuer-pollmann@nue.et-inf.uni-siegen.de>
Date: Tue, 02 Apr 2002 15:39:21 +0200
To: Karel Wouters <Karel.Wouters@esat.kuleuven.ac.be>, Aleksey Sanin <aleksey@aleksey.com>
Cc: xml-encryption@w3.org
Message-id: <20432069.1017761961@pinkpanther>
Hi Karel,

but this attack is always possible, isn't it? If the verifying application 
allows (accepts) weak signature methods, an attacker can change the 
ds:SignatureMethod/@Algorithm value and the ds:SignatureValue . This attack 
does not depend on whether the SignatureMethod is optional or not.

Or am I completely on the wrong track.


--On Dienstag, 2. April 2002 16:00 +0200 Karel Wouters 
<Karel.Wouters@esat.kuleuven.ac.be> wrote:

> Hi,
> I think that SignatureMethod in ds:SignedInfo should be present in
> each signature, because it restricts the attacker:
> If I leave out SignatureMethod, the attacker might be able to come up with
> a weaker SignatureMethod, tamper with the references and claim that the
> signature was produced with this method.
> RSA with a weak hash algorithm would suffice.
> (actually, he might produce 'any' signature if the hash function is weak
> enough)
> So specifying the SignatureMethod ensures the verifier that this signature
> is generated with a solid technique.
> (mailinglist, correct me if I'm wrong)
> Karel.
> On Mon, 1 Apr 2002, Aleksey Sanin wrote:
>> Sorry for mistype, actually Imeant SignatureMethod in XMLDSig:
>> A minor issue but probably it's worth to fix it: the EncryptionMethod
>> in XMLEncryption and SignatureMethod in XMLDSig both have the same
>> meaning: algorithm selection. However, EncryptionMethod is *optional*
>> element and SignatureMethod is *required*. From my point of view it is
>> inconsistent. Either both should be required or both should be optional.
>> I would prefer the second (both optional) since application can have this
>> information from the context.
>> Aleksey Sanin <aleksey@aleksey.com>
>> http://www.aleksey.com/xmlsec
Received on Tuesday, 2 April 2002 08:34:57 UTC

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