[Openid-specs-ab] Encryption

Nat Sakimura sakimura at gmail.com
Fri Oct 28 20:43:57 UTC 2011


P.S., It does not have to be HMAC, but any MAC or signature.
For OpenID Connect, applying MAC over the JWE is not hard, but in the
generic JWE, as we may not have established the symmetric key, it may be
difficult. We could require it to be asymmetrically over signed with keys
from the encrypting party if the symmetric key has not been established.

P.S 2: For Connect, making sure that error message does not contain enough
information that the attacker can use should actually suffice as John points
out. For generic JWE, we probably need to do something, I guess. It may just
end up in the security consideration that applications that use CBC mode of
operation should make sure that it will not give out unnecessary
information, and strongly RECOMMEND that GCM to be used. This may be the
most realistic solution.

On Sat, Oct 29, 2011 at 5:29 AM, Nat Sakimura <sakimura at gmail.com> wrote:

> As I have pointed out in the earlier mail, being a variant of padding
> oracle attack, not giving out information when error occurs is one key part
> of mitigation, as it seems. Integrity checking / authenticating the cipher
> text is one way to do it. Fortunately, we do not have flaws like XML
> Signature Wrapping attack. So applying HMAC seems to be adequate to mitigate
> this attack. This actually has to be a part of JWE, I suppose.
>
> While checking these, I found bugs in Messages:
>
> Messages - 3.1.2.1 iss and aud
>
> It states "If signed, the OpenID Request Object SHOULD contain the
> standard JWT "iss" and "aud" claims." It MUST contain them to thwart
> surrepetitious forwarding etc.
>
> Messages - 3.3.2 Must require iss and aud for signed response
>
> I will check if they are already ticketed, and if not, will ticket them.
>
> Nat
>
>
>
> On Sat, Oct 29, 2011 at 1:52 AM, John Bradley <ve7jtb at ve7jtb.com> wrote:
>
>> It is a complicated attack.  One of the main things it relies on is an
>> ability to differentiate between decryption (padding) errors and XML
>> validation errors.
>>
>> Part of it is also based on XML syntax.  Though I suspect that you could
>> do the same thing for JSON.
>>
>> Having an integrity check so that the attacker only gets back deception
>> errors is one way to stop the attack.
>>
>> Updating PHP and other things will take time no matter what we want.
>>
>> We should better understand the problem and other potential mitigations
>> before completely throwing out AES-CBC.
>>
>> John B.
>> On 2011-10-28, at 1:18 PM, Mike Jones wrote:
>>
>> > We pretty much reached the same conclusion during the encryption working
>> group session at IIW.  The only problem, as Nat pointed out, is that PHP
>> libraries, as currently distributed, do not support GCM (although the
>> underlying OpenSSL libraries that PHP uses do).  Of course, maybe we can use
>> this as a forcing function to get PHP to support GCM by default (without
>> requiring recompilation, which is possible now).
>> >
>> >                               -- Mike
>> >
>> > -----Original Message-----
>> > From: openid-specs-ab-bounces at lists.openid.net [mailto:
>> openid-specs-ab-bounces at lists.openid.net] On Behalf Of Anthony Nadalin
>> > Sent: Friday, October 28, 2011 9:13 AM
>> > To: Axel.Nennker at telekom.de; jbradley at mac.com
>> > Cc: openid-specs-ab at lists.openid.net
>> > Subject: Re: [Openid-specs-ab] Encryption
>> >
>> > As I see it we need to require the GCM mode of operation (an
>> authenticated encryption mode) for AES (moving AES-GCM from option to
>> mandatory).
>> >
>> > -----Original Message-----
>> > From: openid-specs-ab-bounces at lists.openid.net [mailto:
>> openid-specs-ab-bounces at lists.openid.net] On Behalf Of
>> Axel.Nennker at telekom.de
>> > Sent: Friday, October 28, 2011 8:55 AM
>> > To: jbradley at mac.com
>> > Cc: openid-specs-ab at lists.openid.net
>> > Subject: Re: [Openid-specs-ab] Encryption
>> >
>> > Here is the link to the paper:
>> >
>> http://www.nds.rub.de/media/nds/veroeffentlichungen/2011/10/22/HowToBreakXMLenc.pdf
>> >
>> > The authors recommend "One possibility to avoid our attack is to use a
>> symmetric cryptographic primitive that does not only provide
>> confidentiality, but also integrity. This can for instance be achieved by
>> replacing the CBC mode of operation with a mode that provides message
>> integrity. Adequate choices have for instance been standardized in ISO/IEC
>> 19772:2009. We consider this solution as very recommendable for future
>> versions of the XML Encryption standard. Unfortunately, this may bring
>> deployment and backwards compatibility issues."
>> >
>> > http://www.iso.org/iso/catalogue_detail?csnumber=46345
>> >
>> > -Axel
>> >
>> > -----Original Message-----
>> > From: John Bradley [mailto:jbradley at mac.com]
>> > Sent: Freitag, 28. Oktober 2011 16:22
>> > To: Nennker, Axel
>> > Cc: Nat Sakimura; Michael Jones; openid-specs-ab at lists.openid.net
>> > Subject: Re: [Openid-specs-ab] Encryption
>> >
>> > We don't encryption it, but we do support it.
>> >
>> > I haven't seen the original paper only analysis of it.
>> >
>> > Mike should be able to get it.
>> >
>> > I don't think we should panic.   I have known about this for a week or
>> so.
>> >
>> > While the problem involves CBC it is not necessarily a CBC algorithm
>> vulnerability in itself.
>> >
>> > The problem is likely the xmlenc API error messages and having them
>> reported back over SOAP.
>> >
>> > As long as we are careful about not communicating too much in the error
>> message and implementers protect against side channel timing attacks, JWE
>> probably is OK as is with appropriate security considerations.
>> >
>> > I would be surprised if the attack works agains AES-CBC + RSA.
>> >
>> > It also probably is ineffective agains AES-CBC+keywrap.
>> >
>> > Yes GWC is better that is why it was created.
>> >
>> > We need the paper before trying to fix things that may not need fixing.
>> >
>> > John B.
>> >
>> >
>> >
>> >
>> > On 2011-10-28, at 10:13 AM, Axel.Nennker at telekom.de wrote:
>> >
>> >> Do we actually require encryption in the openid connect standards? I
>> thought we refer to JWS and JWS and that's it?
>> >> Axel
>> >>
>> >>
>> >>
>> >>
>> >> -----Original Message-----
>> >> From: openid-specs-ab-bounces at lists.openid.net
>> >> [mailto:openid-specs-ab-bounces at lists.openid.net] On Behalf Of
>> >> sakimura
>> >> Sent: Freitag, 28. Oktober 2011 13:36
>> >> To: Mike Jones; John Bradley; Anthony Nadalin; Openid specs ab
>> >> Subject: [Openid-specs-ab] Encryption
>> >>
>> >> So I was going over the recent XML Encryption vulnerability.
>> >> http://www.informationweek.com/news/security/vulnerabilities/231901532
>> >>
>> >> The flaw is that of CBC mode of operation combined with
>> >> unauthenticated encryption.
>> >> It is a kind of padding oracle attack.
>> >>
>> >> We have two choices here:
>> >>
>> >> 1) Require authenticated encryption mode such as GCM
>> >> 2) Require message authentication to be applied to the cipher text.
>> >>
>> >> Ideally 1) should be taken as operational efficiency is much greater
>> >> than 2), but in reality we do not have support for GCM in many
>> >> languages.
>> >>
>> >> Thus, while RECOMMENDing 1), we should REQUIRE HMAC to be applied on
>> >> the encrypted text (cipher text) in CBC mode.
>> >>
>> >> Thus, we should make it REQUIRED to sig+enc+mac, instead of sig+enc,
>> >> and REQUIRE the verifier to first verify the mac and if the mac is not
>> >> correct the process should abend returning mac error.
>> >>
>> >> Also, although same public-private keypair can be used for encryption
>> >> and signature in case of RSA, we should probably use two separate
>> >> keypair. That is safer.
>> >> Perhaps we would not REQUIRE it, but we should RECOMMEND it.
>> >>
>> >> =nat
>> >>
>> >> _______________________________________________
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>
>
> --
> Nat Sakimura (=nat)
> Chairman, OpenID Foundation
> http://nat.sakimura.org/
> @_nat_en
>
>


-- 
Nat Sakimura (=nat)
Chairman, OpenID Foundation
http://nat.sakimura.org/
@_nat_en
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