GCP 中机密虚拟机的“sevLaunchAttestationReportEvent”日志条目中的“finalDigest”是如何计算的？

Question

I've experimented with launching some confidential VM instances. The simple scenario includes:

1. Launch an instance named 'Alice'.
2. Stop and relaunch instance 'Alice'.
3. Delete instance 'Alice', create a new VM instance named 'Alice'

I checked the 'sevLaunchAttestationReportEvent' log entry. As expected, in all three cases the 'guestMemoryRegion' digest was identical in all cases.

However, the 'finalDigest' was different in all three cases. My questions are: A. How is the 'finalDigest' calculated? B. What is the purpose of a 'finalDigest' that is different at each launch of an identical VM image? C. Can the 'finalDigest' be pre-calculate before instantiation?

Thanks.

Answer 1

First of all, a Confidential Virtual Machine runs on hosts based on the second generation of AMD Epyc processors, it is optimized for security workloads and includes inline memory encryption that ensures that data is encrypted while it's in RAM. You can consult the following documentation to get further information.

Regarding your questions:

A. How is the 'finalDigest' calculated? To calculate the digest value, a Digest Algorithm you can be use, those algorithms could be:

• SHA-1
• SHA-256
• SHA-384
• SHA-512
• MD5

They are functions to take a large document and compute a "digest" (also called "hash"), this is typically used in a digital signing process.

B. What is the purpose of a 'finalDigest' that is different at each launch of an identical VM image?
A message digest or hash function is used to turn input of arbitrary length into an output of fixed length and this output can then be used in place of the original input, and the digest can be changed every time that the VM instance is turned on because some changes were executed internally in the instance. I mean, the hash algorithm takes into consideration those changes, even though a single byte is changed the digest or hash will change completely.

C. Can the 'finalDigest' be pre-calculate before instantiation? In my opinion this is not feasible because the digest algorithm is a one-way function, that is, a function which is practically infeasible to invert.

You can get more information about the hash functions on this link .