Implement PCR Extend

[lukehinds]

PCR Extend performs a hash extend operation into the TPM.

A PCR is a platform configuration register

Let’s explore PCRs a bit more.

A Platform Configuration Register is a value that can only be set by the TPM and is a recorded measurement of a system object (by object we could mean file, firmware, bootloader, kernel, etc).

One of the main functions of a PCR is the extend operation.

tpm2_pcrextend 10:sha256=b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c

The above command asks the TPM to extend the hash b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c into PCR 10

Each time an extend is made the following mathematical operation occurs:

PCR New Value = Digest of (PCR old value || data to extend)

Here is an example in python

import hashlib

hash_one = hashlib.sha1(b'hello')
hash_two = hashlib.sha1(b'world')
hex_dig_one = hash_one.hexdigest()
hex_dig_two = hash_two.hexdigest()

# concatenate hashes
extend_hash = hex_dig_one.encode('utf-8') + hex_dig_two.encode('utf-8')

# recompute concatenated hash
extend = hashlib.sha1(extend_hash)
extend_dig = extend.hexdigest()

print("hello hash: ", hex_dig_one)
print("world hash: ", hex_dig_two)
print("extended hash: ", extend_dig)

$ python ext.py                                                                                                                                                                                                                                              
hello hash:  aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d
world hash:  7c211433f02071597741e6ff5a8ea34789abbf43
extended hash:  39955b37b910e57748368b0922fd44fbff72bda5

This is a one way hash. A one-way hash function is designed in such a way that it is hard to reverse the process, that is, to find a string that hashes to a given value (hence the name one-way.) A hash function also makes it hard to find two strings that would produce the same hash value.

To play with this using the tools, first install the tools:

dnf install tpm2-tools

you can then use tpm2_pcrread to look at the values

tpm2_pcrread| grep 10:                                                                                                                                                         ✔  ⚡ 
  10: 0x44F12027AB81DFB6E096018F5A9F19645F988D45529CDED3427159DC0032D921

and extend it:

tpm2_pcrextend 10:sha256=b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae49abc

tpm2_pcrread| grep 10:                                                                                                                                                         ✔  ⚡ 
  10: 0x1CEA6BDAD4B6F40B2A69F916FA81692AB263329E92B069C7B9C5ADDE6405DC3C

We will need to implement this as a function as can be seen in the python code

https://github.com/keylime/keylime/blob/1ef444e2c1f18815dde9826accc4e36da7ba3fd6/keylime/tpm/tpm2.py#L1151-L1155

To do this you can use the pcrextend function in the ESAPI wrapper here:
https://github.com/parallaxsecond/rust-tss-esapi/blob/b74837ceb229a0010c05f6149efa65c4f41f9426/src/context.rs#L774

Check out the existing code to see how you can get a context in my PR #113

[lukehinds]

as mentioned, you could set up a sandbox and just do this in your own main.rs.

we will need to think about captureing the return output. Previously we were marshalling this into YAML, but I don't know how we should approach this yet with the esapi create. We might need a helper function here to serialise output of calls to the tss, @puiterwijk will have some ideas here.

[lkatalin]

Thanks for this clear write-up, @lukehinds . tpm2_pcrextend works perfectly on the command line and I'm creating an example with the ESAPI function.

Edit: I made this small example and will implement the Python function next.

[lkatalin]

@lukehinds The ESAPI pcr_extend function already seems to have most of the functionality of the Python version extendPCR, minus setting a default algorithm and dealing with locking. Would it make more sense to directly call tss_esapi::Context::pcr_extend where we need it, rather than implementing a wrapper for it in tpm.rs? Otherwise I do have a tiny PR for the wrapper.

[lkatalin]

Closing as this is implemented by the ESAPI and already used here.