Peter Trei envisions data recovery in a TCPA world:

HoM: I want to recover my data. Me: OK: We'll pull the HD, and get the data off it. HoM: Good - mount it as a secondary HD in my new system. Me: That isn't going to work now we have TCPA and Palladium. HoM: Well, what do you have to do? Me: Oh, it's simple. We encrypt the data under Intel's TPME key, and send it off to Intel. Since Intel has all the keys, they can unseal all your data to plaintext, copy it, and then re-seal it for your new system. It only costs $1/Mb. HoM: Let me get this straight - the only way to recover this data is to let Intel have a copy, AND pay them for it? Me: Um... Yes. I think MS might be involved as well, if your were using Word. HoM: You are *so* dead.

It's not quite as bad as all this, but it is still pretty bad. You don't have to send your data to Intel, just a master storage key. This key encrypts the other keys which encrypt your data. Normally this master key never leaves your TPM, but there is this optional feature where it can be backed up, encrypted to the manufacturer's public key, for recovery purposes. I think it is also in blinded form. Obviously you'd need to do this backup step before the TPM crashed; afterwards is too late. So maybe when you first get your system it generates the on-chip storage key (called the SRK, storage root key), and then exports the recovery blob. You'd put that on a floppy or some other removable medium and store it somewhere safe. Then when your system dies you pull out the disk and get the recovery blob. You communicate with the manufacturer, give him this recovery blob, along with the old TPM key and the key to your new TPM in the new machine. The manufacturer decrypts the blob and re-encrypts it to the TPM in the new machine. It also issues and distributes a CRL revoking the cert on the old TPM key so that the old machine can't be used to access remote TCPA data any more. (Note, the CRL is not used by the TPM itself, it is just used by remote servers to decide whether to believe client requests.) The manufacturer sends the data back to you and you load it into the TPM in your new machine, which decrypts it and stores the master storage key. Now it can read your old data. Someone asked if you'd have to go through all this if you just upgraded your OS. I'm not sure. There are several secure registers on the TPM, called PCRs, which can hash different elements of the BIOS, OS, and other software. You can lock a blob to any one of these registers. So in some circumstances it might be that upgrading the OS would keep the secure data still available. In other cases you might have to go through some kind of recovery procedure. I think this recovery business is a real Achilles heel of the TCPA and Palladium proposals. They are paranoid about leaking sealed data, because the whole point is to protect it. So they can't let you freely copy it to new machines, or decrypt it from an insecure OS. This anal protectiveness is inconsistent with the flexibility needed in an imperfect world where stuff breaks. My conclusion is that the sealed storage of TCPA will be used sparingly. Ross Anderson and others suggest that Microsoft Word will seal all of its documents so that people can't switch to StarOffice. I think that approach would be far too costly and risky, given the realities I have explained above. Instead, I would expect that only highly secure data would be sealed, and that there would often be some mechanism to recover it from elsewhere. For example, in a DRM environment, maybe the central server has a record of all the songs you have downloaded. Then if your system crashes, rather than go through a complicated crypto protocol to recover, you just buy a new machine, go to the server, and re-download all the songs you were entitled to. Or in a closed environment, like a business which seals sensitive documents, the data could be backed up redundantly to multiple central file servers, each of which seal it. Then if one machine crashes, the data is available from others and there is no need to go through the recovery protocol. So there are solutions, but they will add complexity and cost. At the same time they do add genuine security and value. Each application and market will have to find its own balance of the costs and benefits. --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majordomo@wasabisystems.com