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: [email protected]; Murray S. Kucherawy; [email protected]; > [email protected] > Cc: Black, David; [email protected]; [email protected] > 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 > [email protected] Mobile: +1 (978) 394-7754 > ---------------------------------------------------- _______________________________________________ marf mailing list [email protected] https://www.ietf.org/mailman/listinfo/marf
