Last Call Review of draft-ietf-marf-redaction-
review-ietf-marf-redaction-genart-lc-black-2012-02-04-00

The -08 version is a significant improvement that aligns the draft's
recommendations on mechanisms for redaction and anonymization with the
situation-dependent levels of security that are appropriate for those
purposes.

idnits 2.12.13 didn't find anything.

The -08 version is ready for publication as a Standards Track RFC.

Thanks,
--David


> -----Original Message-----
> From: Black, David
> Sent: Thursday, January 19, 2012 7:10 PM
> To: ietf at cybernothing.org; Murray S. Kucherawy; gen-art at ietf.org; ietf at ietf.org
> Cc: marf at ietf.org; presnick at qualcomm.com; Black, David
> Subject: Gen-ART review of draft-ietf-marf-redaction-05
> 
> Based on discussion with the authors, the -05 version of this draft resolves the
> issues raised in the Gen-ART review of the -04 version.  An important element of
> the approach taken to issue [1] has been to explain why the security requirements
> for redaction are significantly weaker than the strength of the secure hashes
> that are suggested by the draft.
> 
> Thanks,
> --David
> 
> > -----Original Message-----
> > From: Black, David
> > Sent: Tuesday, January 10, 2012 9:44 PM
> > To: ietf at cybernothing.org; Murray S. Kucherawy; gen-art at ietf.org; ietf at ietf.org
> > Cc: Black, David; marf at ietf.org; presnick at qualcomm.com
> > Subject: Gen-ART review of draft-ietf-marf-redaction-04
> >
> > I am the assigned Gen-ART reviewer for this draft. For background on Gen-ART, please
> > see the FAQ at <http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.
> >
> > Please resolve these comments along with any other Last Call comments you may receive.
> >
> > Document: draft-ietf-marf-redaction-04
> > Reviewer: David L. Black
> > Review Date: January 10, 2012
> > IETF LC End Date: January 18, 2011
> > IESG Telechat Date: January 19, 2011
> >
> > Summary: This draft is on the right track but has open issues, described in the review.
> >
> > This draft specifies a method for redacting information from email abuse reports
> > (e.g., hiding the local part [user] of an email address), while still allowing
> > correlation of the redacted information across related abuse reports from the same
> > source. The draft is short, clear, and well written.
> >
> > There are two open issues:
> >
> > [1] The first open issue is the absence of security guidance to ensure that this
> > redaction technique effectively hides the redacted information.  The redaction
> > technique is to concatenate a secret string (called the "redaction key") to the
> > information to be redacted, apply "any hashing/digest algorithm", convert the output
> > to base64 and use that base64 string to replace the redacted information.
> >
> > There are two important ways in which this technique could fail to effectively hide
> > the redacted information:
> > 	- The secret string may inject insufficient entropy.
> > 	- The hashing/digest algorithm may be weak.
> >
> > To take an extreme example, if the secret string ("redaction key") consists of a
> > single ASCII character, and a short email local part is being redacted, then the
> > output is highly vulnerable to dictionary and brute force attacks because only 6 bits
> > of entropy are added (the result may look secure, but it's not).  Beyond this extreme
> > example, this is a potentially real concern - e.g., applying the rule of thumb that
> > ASCII text contains 4-5 bits of entropy per character, the example in Appendix A
> > uses a "redaction key" of "potatoes" that injects at most 40 bits of entropy -
> > is that sufficient for email redaction purposes?
> >
> > To take a silly example, if a CRC is used as the hash with that sort of short input,
> > the result is not particularly difficult to invert.
> >
> > I suggest a couple of changes:
> > 1) Change "any hashing/digest algorithm" to require use of a secure hash, and
> > 	explain what is meant by "secure hash" in the security considerations section.
> > 2) Require a minimum length of the "redaction key" string, and strongly suggest
> > 	(SHOULD) that it be randomly generated (e.g., by running sufficient output
> > 	of an entropy-rich random number generator through a base64 converter).
> >
> > For the latter change, figure out the amount of entropy that should be used
> > for redaction - the recommended string length will be larger because printable
> > ASCII is not entropy-dense (at best it's good for 6 bits of entropy in each
> > 8-bit character, and human-written text such as this message has significantly
> > less).
> >
> > From a pure security perspective, use of HMAC with specified secure hashes
> > (SHA2-family) and an approach of hashing the "redaction key" down to a binary
> > key for HMAC would be a stronger approach. I suggest that authors consider
> > approach, but  there may be practical usage concerns that suggest not adopting it.
> >
> > [2] The second open issue is absence of security considerations for the redaction
> > key.  The security considerations section needs to caution that the redaction key
> > is a secret key that must be managed and protected as a secret key.  Disclosure
> > of a redaction key removes the redaction from all reports that used that key.
> > As part of this, guidance should be provided on when and how to change the
> > redaction key in order to limit the effects of loss of secrecy for a single
> > redaction key.
> >
> > Editorial Nit: I believe that "anonymization" is a better description of what
> > this draft is doing (as opposed to "redaction"), particularly as the result is
> > intended to be correlatable via string match across reports from the same source.
> >
> > idnits 2.12.13 didn't find any nits.
> >
> > Thanks,
> > --David
> > ----------------------------------------------------
> > David L. Black, Distinguished Engineer
> > EMC Corporation, 176 South St., Hopkinton, MA  01748
> > +1 (508) 293-7953             FAX: +1 (508) 293-7786
> > david.black at emc.com        Mobile: +1 (978) 394-7754
> > ----------------------------------------------------