Privacy With a 4096 Bit RSA Key — Offline, On Paper

HavanaF writes "Online backup is practical, but can it offer any privacy? The Dutch security company Safeberg developed an Offline Private Key Protocol, with an asymmetric key scheme. The protocol demands that the private (decryption) key be stored away from the 'source' computer, which presumably is 'too vulnerable.' The catch is that the private key needs to be fairly large to be secure: a 4,096-bit RSA key should suffice for some years. But how to store an 800-character key offline? Safeberg introduces a machine readable paper key, with the 4k-bit key crammed in a giant 2D Datamatrix barcode. This video on key strength tells the story."

How much added security?

[SmilingBoy]

If the source computer is vulnerable, the private key will be vulnerable as well as soon as you use a device connected to the compromised computer to scan it.

Smartcard ?

[KermitTheFragger]

After surfing around a bit on the source site I can't find any compelling reasons why I should use a giant unwieldy printable 2D Barcode instead of a smartcard ? A smartcard reader costs 25 bucks now a days so that cant be much of an obstacle.

Re:Don't use datamatrix

[Kostya]

The wikipedia article on DataMatrix (http://en.wikipedia.org/wiki/Data_Matrix#Patent_issues) seems to imply it is unencumbered--perhaps I'm misunderstanding something?

Prior to the expiration of U.S. Patent 5,612,524, intellectual property company Acacia Technologies claimed that Data Matrix was partially covered by its contents. As the patent owner, Acacia allegedly contacted Data Matrix users demanding license fees related to the patent.

Cognex Corporation, a large manufacturer of 2D barcode devices, filed a declaratory judgment complaint on March 13, 2006 after receiving information that Acacia had contacted its customers demanding licensing fees. On May 19, 2008 Judge Joan N. Ericksen of the U.S. District Court in Minnesota ruled in favor of Cognex. The ruling held that the '524 patent, which claimed to cover a system for capturing and reading 2D symbology codes, is both invalid and unenforceable due to inequitable conduct by the defendants during the procurement of the patent.

Notably, since the '524 patent expired in November 2007, a ruling against Cognex wouldn't have affected current use of Data Matrix anyway. However, it would have established that use of Data Matrix prior to November 2007 could potentially be covered by the '524 patent.

Re:How is this any more secure

[Locke2005]

Simple: you print the key in a blank spot on a random page of War and Peace. Good luck to anybody trying to find it without knowing the page number! Whereas in a standalone computer, any disk analysis software should be able to find the key. The point is, as in The Purloined Letter, you put the key in a place no one would think to look for it. Searching your computer and computer media is the _first_ thing anyone looking for the key is going to do! When they come in with a warrant to confiscate your computer, do you think that warrant covers your book collection as well? No, it just covers computers, hard drives, USB drives, CDs/DVDs, etc.

Ummmm....

[jemenake]

I'm not sure I grok this notion of not storing the key with the source machine. I mean... if I can get to the machine you backed up... I don't really need to get to the backup, do I? I've got fresher data right there in front of me.

Now, if you're really trying to protect some kind of historical record of how your data has progressed over time, then that would be a reason why access to the source computer still didn't get the intruder access to what you're trying to protect... but that's a very special case.

Dunno. Maybe I'm just missing the point.

Re:no thanks my Hard drive is too big

[Anonymous Coward]

Why would you try to backup 1.5TB with USB? eSATA would really cut down on the time required.

I'll hold out

[egcagrac0]

... until there's a 640kbit key. 640k ought to be enough for anybody.

But seriously, it was just a few years back when we though 128bit keys were unbreakably long. Now 2048bit is standard, and about to get broken. 4096bit isn't enough right now. 16kbit is just about right, but that will get broken in early 2015.

Re:hide the key in a book: great idea!

[mysidia]

You convert it into a Base-52 or Base-26 representation, so all keybits are represented by ordinary letters such as A-Z (you might expand it a little to include common punctuation marks), and underline different characters on the page that correspond to digits of your key..

Also, you can XOR your 4096 key, by a truly random 4096 bit value.

Destroy the original key, and print the random number and the XOR result in two different books at completely different places.

You can XOR it a few more times, and divide the key into 5 or 6 pieces.

Then go to the library with a pencil one night, and (covertly) store a copy of your key in various books.

Making plenty of redundant copies of course, and visiting multiple libraries (for fear that part of your key would be unavailable due to someone else having checked out one of your bits).

Re:Another plausible scenario I have to watch out

[Red Flayer]

I like a little alliteration in my catchphrases. How about:

Void thine vellum!
Oust thine onion skin!

And that's about all I can come up with.

Except maybe "Shit the sheet", but that doesn't sound as nice.