Re: [smime] [Technical Errata Reported] RFC5084 (4774)

[Quan Nguyen <[e-mail]>]

On Thu, Aug 18, 2016 at 1:13 PM, Jim Schaad <[e-mail]> wrote:

> I did a brief look at the code you pointed to and I would run screaming
> from it for several reasons.
>
>
>
> I do not have a problem if the tag is defaulted to a length of 12 bytes,
>

Note that I were talking about Java Cipher which is used in different
scenarios where some requires high-level of security. It's the library
responsibility to choose a default safe option. Note that in general, all
BouncyCastle, Conscrypt, OpenJDK use 16-bytes as default. The only place
where they use/used 12-byte is where they cite/cited the RFC.

> but I do not see any way to set it to a different length if desired.
> There should be a version that allows for getting the tag length.  At a
> minimum the length of the tag should be parameterized.
>

There are different ways to initialize Java Cipher and some of them allow
user to choose the authentication tag size.

>
>
> This is making me want to fix the code, but not necessarily wanting to fix
> the spec.  In the context of the document the default is not unreasonable.
>

If it was designed specifically for your use-case and it uses key only once
for decryption then I agree it's fine. I believe the core problem is people
only looked at section 3.2 and then applied it for *general* usecase.

Honestly, I don't know what to do in this situation.

>
>
> Jim
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> *From:* smime [mailto:[e-mail]] *On Behalf Of *Quan Nguyen
> *Sent:* Thursday, August 18, 2016 12:07 PM
> *To:* Russ Housley <[e-mail]>
> *Cc:* Kathleen Moriarty <[e-mail]>; IETF SMIME <
> [e-mail]>; David McGrew <[e-mail]>; Stephen Farrell <
> [e-mail]>
> *Subject:* Re: [smime] [Technical Errata Reported] RFC5084 (4774)
>
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> On Thu, Aug 18, 2016 at 11:39 AM, Russ Housley <[e-mail]>
> wrote:
>
> Quan:
>
>
>
> I just read the cited paper from Niels Ferguson <
> http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/commen
> ts/CWC-GCM/Ferguson2.pdf>.  Niels says:
>
>
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>    After 2^16 forgery attempts we can expect a successful forgery.
>
>
>
> And, then Niels talks about an encrypted voice environment where this
> attack might lead to disastrous consequences.
>
>
>
> Also, Niels points out that the AES-GCM security proof is unbroken by this
> attack.  He is staying within the bounds of the proven security.
>
>
>
> Yeah, but the proven security wasn't clear about the weakness/fragility of
> GCM. The attack explicitly showed how to exploit the weakness.
>
>
>
> It is hard to imagine a protocol environment that uses CMS where 2^16
> extra messages would be undetected.
>
>
>
> In addition RFC 5084 requires automated key management.  This means that a
> fresh AES-GCM key ought to be used for each of the messages.
>
>
>
> Oh, this is interesting. The problem I saw with OpenJDK, BouncyCastle,
> Conscrypt is when Java Cipher (https://docs.oracle.com/javas
> e/7/docs/api/javax/crypto/Cipher.html) is initialized to use GCM mode:
>
>
>
>    1. In one case, if the parameter is ASN.1 encoded and aes-ICVlen is
> missing then it's interpreted as 12-byte.
>
>    2. In other case, they use 12-byte tag, citing RFC 5084 recommendation.
> For instance, see : https://github.com/bcgit/bc-
> java/blob/master/prov/src/main/java/org/bouncycastle/jcajce/
> provider/symmetric/AES.java#L497
>
>
>
> Note that Cipher is a *general *crypto primitive which may be used to
> encrypt *multiple* messages. So there may be misunderstanding every now
> and then. It's worth to note that I had a hard time convincing developers
> to change because they cited RFC :(
>
>
>
>  I recognize that attackers do not follow the specification, but it means
> that they cannot just use an existing implementation.
>
>
>
> These two observations make me wonder whether the is enough of a problem
> to bother with an update to RFC 5084.  What do you think?
>
>
>
> Russ
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>
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> On Mon, Aug 15, 2016 at 1:55 PM, Russ Housley <[e-mail]>
> wrote:
>
> Quan:
>
>
>
> I do not think that we can change the DEFAULT value associated with these
> OIDs.  Changing the meaning of an absent aes-ICVlen will result in too many
> interoperability problems.
>
>
>
> Yeah, I'm aware of it and I understand your concern.
>
>
>
>  However, we could put out a very short RFC that updates RFC 5084 to
> recommend the use of 16 octet authentication tags in all situations.
>
>
>
> Thanks for doing this :) It's SGTM.
>
>
>
> Russ
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>
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> On Aug 11, 2016, at 2:49 PM, Quan Nguyen <[e-mail]> wrote:
>
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> On Thu, Aug 11, 2016 at 11:47 AM, RFC Errata System <
> [e-mail]> wrote:
>
> The following errata report has been submitted for RFC5084,
> "Using AES-CCM and AES-GCM Authenticated Encryption in the Cryptographic
> Message Syntax (CMS)".
>
> --------------------------------------
> You may review the report below and at:
> http://www.rfc-editor.org/errata_search.php?rfc=5084&eid=4774
>
> --------------------------------------
> Type: Technical
> Reported by: QUAN NGUYEN <[e-mail]>
>
> Section: 3.2
>
> Original Text
> -------------
> aes-ICVlen       AES-GCM-ICVlen DEFAULT 12
>
> A length of 12 octets is RECOMMENDED.
>
> Corrected Text
> --------------
> aes-ICVlen       AES-GCM-ICVlen DEFAULT 16
>
> A length of 16 octets is RECOMMENDED.
>
> Notes
> -----
> Many JCE providers including OpenJDK, BouncyCastle, Conscrypt have a bug
> to use 12 bytes authentication tag (aes-ICVlen) as default if the code path
> [1] uses CMS. According to Ferguson's attack (
> http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/commen
> ts/CWC-GCM/Ferguson2.pdf), if a user encrypts 2^32 block length message,
> then 12 bytes authentication tag length has only 96 - 32 = 64 bits security
> which is not good enough nowadays. Furthermore, once a forgery happens then
> authentication is leaked.
>
>
>
> Sorry, I meant "authentication *key*" is leaked.
>
>
> [1] In other code paths, all providers use 16 bytes authentication tag as
> default.
>
> Instructions:
> -------------
> This erratum is currently posted as "Reported". If necessary, please
> use "Reply All" to discuss whether it should be verified or
> rejected. When a decision is reached, the verifying party (IESG)
> can log in to change the status and edit the report, if necessary.
>
> --------------------------------------
> RFC5084 (draft-ietf-smime-cms-aes-ccm-and-gcm-03)
> --------------------------------------
> Title               : Using AES-CCM and AES-GCM Authenticated Encryption
> in the Cryptographic Message Syntax (CMS)
> Publication Date    : November 2007
> Author(s)           : R. Housley
> Category            : PROPOSED STANDARD
> Source              : S/MIME Mail Security
> Area                : Security
> Stream              : IETF
> Verifying Party     : IESG
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