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Interesting but vulnerable scheme for tokenless auth
From: Glenn Everhart <everhart () gce com>
Date: Wed, 26 Apr 2006 20:07:15 -0700

Consider the following attempt at el-cheapo (no hardware) authentication (which occurred to me recently while reading some ads):

It is possible to imagine an authentication scheme that wants to use something like a certificate with signing, encrypting random nonces etc., to verify that someone agrees to some transaction(s). If the certificate is on a PC, though, it gets exposed to theft.

In the cert is a private key. If the system were required to contact a "backend" server first, passing it perhaps a cipher containing its serial number encrypted with its private key and its identity, the server could send back a (hopefully unique to that cert) decryption key that would decrypt the private key, allowing its use; the code at the PC would need to erase the cleartext private key when done. The server could check the serial number matched the "identity" (it would have the public key) to prevent a simple search of the server for these encrypting keys.

This all seems reasonable and deals well with the environment perhaps of the 1990s. Problem today is that it is still utterly vulnerable to backdoor code on the PC which could be arranged to either listen for the decrypting key or just pluck it out of memory while the real cert was being used in cleartext. While code tricks can minimize time of exposure and obscure the use of the underlying key, they cannot block it, and once the private key is gone the use of such quickly becomes worse than useless.

This is another demo of the difficulty of building any kind of software token that can be connected to uncontrolled environments and which can keep secrets. It may resist OFFLINE attack, but that is not the primary attack threat today for such a beast.

Glenn Everhart
Everhart () gce com

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