Let's hope this doesn't get picked up by the (corporate) masses... the last thing I want is my browser offering personal TLS certificates to every server I visit as some kind of identity verification or fingerprint/tracking.
Currently yes, but there's not much stopping Chrome etc. from adding a new feature that has a way of presenting a client certificate to a website in a backwards-compatible manner.
Of course the website itself would need to support that, but it's all possible in time.
Yes that's what I'm saying this could be used for. Browser generates a client-side key (or any kind of identifier, possibly derived from the site's TLS cert/domain or some other info) for the current browser/user/device/whatever and just offer that to every website as an extra header so that whoever wants to track you, can.
Wouldn't this in practice be a lot like Passkeys? But it might be more difficult to integrate this kind of approach to the stacks we use, whereas Passkeys fits in relatively easily.
I suppose client cert would protect against from a MitM attack, if the client failed to notice it, or if the MitMer has the website keys to make a perfect attack.
That's right, I'm learning in public here. In RATS we prove a machine is sound by measuring it and appraising the evidence. But after attestation the usual thing is to hand the machine a short-lived identity saying it is attested, and when that machine then authenticates over mTLS to something like an HSM, the thing that gives that machine its identity is a private key in a file. That bothered me, and there is surprisingly little written about closing that gap in RATS specifically. What I want is to tie that key in the TPM to the evidence of the confidential VM itself, and let that be the identity the machine carries after attestation. They're just working notes, feel free to correct me if my thinking is wrong here.
Never mind, I had no idea who you where, looked you up, please take a bow, apologies if the comment came out rude, more power to your work and agree learning in public and publishing more will what will make this idea better.
Exactly, you could do this also with the Microsoft Cryptographic Provider long time ago, which is the basic Provider called by the go-tpm library, when running under Windows
Attested TLS has had some rough patches lately which can be attributed to making big changes to a complex protocol.
It really better to separate the attestation, the check against policy and then the TLS stuff. Solve one problem at a time, sign that progress and move on.
I wish the author provided some latency numbers for this. One issue with tpms is that they are slow relative to performing the same operation on a modern CPU.
Thats the “what it costs” section? Im a bit impressed if they are down to ~3ms per handshake. When I last looked at TPM signing (many years ago) it was more like single digit transactions per second.
That said, even 3ms TPM signatures are going to be for special cases or novelty. Plain old CPU tls will do about 1ms cpu time per request which will scale by cpu core count. One or two orders of magnitude more throughput per host.
For a company I work for I needed to ship a machine through unknown channels and have some confidence that it wasn't fiddled with.
my threat model was reasonably technical engineer swapping drives for some reason, or someone claiming that the machine is "different". (no nation state shit)
after the machine was imaged, it would connect to our central config server, get its hostname and exchange keys which would be embedded in the TPM.
once the machine is shipped and booted, it'll check in and sign a challenge. any kind of action on the central API could have a challenge. Each machine is attested at least once an hour.
I'm not sure how "secure" it all is, but it seems to work.
I helped design the attestation framework for https://docs.cloud.google.com/transfer-appliance/docs/4.0/re... - the goal was to ensure that the device you're about to copy a bunch of sensitive information onto is actually the device you were shipped and is running the expected software. This is definitely used in the real world.
The benchmarks are from GCP, where the vTPM is implemented in the hypervisor rather than on something that's plausibly an 8051[1]. Doing this on actual client hardware is going to be a bunch slower.
[1] Typically ARM these days, but most system vendors aren't picking TPM vendors based on performance
What mjg59 says. The benchmarks are against a vTPM, that was what I had access to, and it's the environment I'm implementing the RATS side in.
Worth adding that not every outbound connection needs to go through the TPM (IMO). It's for the handful of services where the machine-identity actually matters, a secret store, or an HSM releasing key material onto an attested confidential VM, in my case.
You didn't really go into actually verifying the machine identity - obviously if you have a trusted mechanism to do that in advance then that's easy enough, but otherwise you'd want something like https://github.com/google/go-attestation and then to use control plane APIs to identify the vTPM EK to tie the TPM to the VM.
Isn't this a well-discussed issue already, and not specific to TPM?
We faced a similar issue (we use OpenSSL). OpenSSL does have OPENSSL_secure_malloc() which prevents sensitive memory from being dumped. However, the problem is that not all paths use the secure allocator. For example, this issue: https://github.com/openssl/openssl/issues/27603
Not sure if this has changed in OpenSSL 4.x, but it is certainly something desirable.
The link between attestation and the key is nicely made with TAS. TAS gives you a cert and Spiffe then requires a cert like that to give a SVID that you use as a certificate for mTLS.
This means that the root of trust threads through software (TAS) that verified that your attestation evidence matches the live policy. This works with no changes to Spiffe.
This doesn't really meet your requirements to keep the key out of memory since the resulting SVID lasts for several minutes in memory, but it does meet most people's needs.
This feels like a somewhat odd design choice - you have a TEE, most TEEs (outside TPMs) are fast so there's little overhead in pushing your signing through there, why bother with short-lived credentials instead of just attesting to private key material ownership and having that be what the SPIFFE cert is issued to? Bearer token SVIDs are an awful thing that we should be getting as far away from as possible.
I suppose the value of this depends on your threat model.
The TPM will give you stronger assurance that a machine owns a key, but it's likely that a dedicated HSM would be much harder to extract the key material from.
TPM being inside the machine is a double edged sword. On one hand it makes attestation feasible, but on the other you now have the security black box inside the same physical domain as the machine that uses it. Risk of side channel extraction goes up dramatically when these systems coexist. It's a lot harder to instrument an HSM across the network.
A dedicated HSM will give you stronger trust that the private key material can't be extracted, but there's no real way to bind an HSM to a specific client and that's a very easy thing to do in the vTPM case.
I wonder, at what point will it be cheaper to kidnap and ransom those remote attestation engineers' families for key material than to work around those schemes with technical measures. Keeping in mind that people set up bot farms with physical phones just for attestation keys, it seems like tightening it all too much will just shift the balance towards the $5 wrench approach...
I might be missing something: is this any conceptually different from using PKCS11 provider for TPM in OpenSSL?
Also, with real TPM, the key could be locked to a specific configuration register value, which makes less sense for VMs. “Quote” is mentioned and I guess author means that, but did not elaborate further.
> PKCS11 doesn't allow you to attest that the key is resident in the PKCS11 provider, which as you say, the author alludes to, but doesn't cover.
You don’t need that if you include quote in CSR and then CA validates the quote and writes the validation result in the certificate. Certificate then contains proof that private key is in TPM.
I think your central thesis is that at the time of TLS establishment, why not use PKCS11 (with a cert signed by a CA that has validated a TPM certification), and I agree: services should probably integrate via PKCS11.
I just read your original comment "is this any conceptually different from using PKCS11 provider for TPM in OpenSSL?" and yes, I agree it's not conceptually different.
42 comments
[ 0.26 ms ] story [ 23.1 ms ] threadOf course the website itself would need to support that, but it's all possible in time.
I suppose client cert would protect against from a MitM attack, if the client failed to notice it, or if the MitMer has the website keys to make a perfect attack.
https://datatracker.ietf.org/doc/draft-fossati-tls-attestati... https://www.youtube.com/watch?v=MF9AwkMJOlw
Hat Tip!
It really better to separate the attestation, the check against policy and then the TLS stuff. Solve one problem at a time, sign that progress and move on.
That said, even 3ms TPM signatures are going to be for special cases or novelty. Plain old CPU tls will do about 1ms cpu time per request which will scale by cpu core count. One or two orders of magnitude more throughput per host.
my threat model was reasonably technical engineer swapping drives for some reason, or someone claiming that the machine is "different". (no nation state shit)
after the machine was imaged, it would connect to our central config server, get its hostname and exchange keys which would be embedded in the TPM.
once the machine is shipped and booted, it'll check in and sign a challenge. any kind of action on the central API could have a challenge. Each machine is attested at least once an hour.
I'm not sure how "secure" it all is, but it seems to work.
https://blog.habets.se/2012/02/Benchmarking-TPM-backed-SSL.h...
https://blog.habets.se/2012/02/TPM-backed-SSL.html
Well, it's been over 14 years so I should hope so.
[1] Typically ARM these days, but most system vendors aren't picking TPM vendors based on performance
Worth adding that not every outbound connection needs to go through the TPM (IMO). It's for the handful of services where the machine-identity actually matters, a secret store, or an HSM releasing key material onto an attested confidential VM, in my case.
Not suitable for servers, since it's such an easy DoS vector.
The War Against General Purpose Computing ticks on.
We faced a similar issue (we use OpenSSL). OpenSSL does have OPENSSL_secure_malloc() which prevents sensitive memory from being dumped. However, the problem is that not all paths use the secure allocator. For example, this issue: https://github.com/openssl/openssl/issues/27603
Not sure if this has changed in OpenSSL 4.x, but it is certainly something desirable.
This means that the root of trust threads through software (TAS) that verified that your attestation evidence matches the live policy. This works with no changes to Spiffe.
This doesn't really meet your requirements to keep the key out of memory since the resulting SVID lasts for several minutes in memory, but it does meet most people's needs.
https://github.com/TEE-Attestation/tas
The TPM will give you stronger assurance that a machine owns a key, but it's likely that a dedicated HSM would be much harder to extract the key material from.
TPM being inside the machine is a double edged sword. On one hand it makes attestation feasible, but on the other you now have the security black box inside the same physical domain as the machine that uses it. Risk of side channel extraction goes up dramatically when these systems coexist. It's a lot harder to instrument an HSM across the network.
ARM TrustZone, for example, can run this OSS TPM: https://github.com/OP-TEE/optee_ftpm
I expect there are equivalents for Intel/AMD.
Error: Forbidden Your client does not have permission to get URL /posts/go-tpm-tls/ from this server.
Also, with real TPM, the key could be locked to a specific configuration register value, which makes less sense for VMs. “Quote” is mentioned and I guess author means that, but did not elaborate further.
PKCS11 doesn't allow you to attest that the key is resident in the PKCS11 provider, which as you say, the author alludes to, but doesn't cover.
> with real TPM, the key could be locked to a specific configuration register value, which makes less sense for VMs.
A vTPM is as real as a physical TPM chip.
The question is which TPM endorsement certificate CAs you are willing to trust.
For some that might the manufacturer of TPM chips, for others it might be their VM provider. (For some, none: for some both!)
Trusting their VM provider isn't so crazy if the VM provider is able to influence the guest code anyway.
You don’t need that if you include quote in CSR and then CA validates the quote and writes the validation result in the certificate. Certificate then contains proof that private key is in TPM.
That said, TPM quotes are attesting the value of TPM PCRs which are just registers of hashes (representing the state of the machine).
When making a CSR, you can use https://tpm2-tools.readthedocs.io/en/latest/man/tpm2_certify... to attest a key is TPM-resident.
I think your central thesis is that at the time of TLS establishment, why not use PKCS11 (with a cert signed by a CA that has validated a TPM certification), and I agree: services should probably integrate via PKCS11.
Sorry to have missed that!