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Attestation

Ensuring system integrity and security is critical in modern computing environments. Confidential computing protects sensitive data during processing, ensuring it remains encrypted and secure. A key component of this security model is remote attestation, which allows one party to verify the integrity of another system before sharing sensitive information.

This document provides an overview of remote attestation, TPM, Platform Configuration Registers (PCRs), and runtime measurements, explaining how they work together to ensure a trusted computing environment.

Remote Attestation​

Attestation is a process in which one system (the attester) gathers information about itself and sends it to a relying party (or client) for verification. Successful verification ensures that the Confidential Virtual Machine (CVM) is running the expected code on trusted hardware with the correct configuration. If deemed trustworthy, the relying party can securely send confidential code or data to the attester.

Several models of remote attestation exist, with two main models: Passport Model and Backgroud-Check Model. Attested TLS

In Passport Model (left side of the figure above) the attester sends evidence to the verifier and receives an attestation result that it can use to prove its trustworthiness to relying parties.

In the Background-Check Model (right side of the figure above) the attester sends evidence to the relying party, who forwards it to the verifier for appraisal. The verifier then sends the attestation results to the relying party, who makes the final decision about trusting the attester based on the comparison of its own appraisal policy against the attestation result.

Attestation in Cocos​

Cocos implements the Background-Check model of remote attestation using two key software components:

  1. Agent – A software application running inside the CVM. It is responsible for fetching the attestation report and executing secure computations.
  2. Cocos CLI – A command-line tool used by Secure Multiparty Computation (SMPC) members. It verifies the attestation report and securely transmits confidential code and data to the Agent.

The attestation report consists of several components which will be described in the remainder of the document.

SEV-SNP attestation​

AMD SEV-SNP (Secure Encrypted Virtualization - Secure Nested Paging) provides hardware-based memory encryption and integrity protection for virtual machines (VMs), allowing them to run securely even on untrusted hypervisors. A core feature of SEV-SNP is its attestation capability, which enables a VM to cryptographically prove its integrity and configuration to a remote verifier.

Attestation Workflow​

The SEV-SNP attestation process involves several key components:

  • Guest VM (Attester): Runs inside a secure VM with SEV-SNP enabled.
  • VMM/Hypervisor: Facilitates access to attestation interfaces but is considered untrusted.
  • AMD Secure Processor (ASP): A trusted firmware component that handles attestation logic and signs attestation reports.
  • Remote Verifier: External party that verifies the integrity and identity of the attested VM.
  • AMD Attestation Signing Service (ASK/ARK): AMD-operated certificate infrastructure that signs attestation keys and allows verifiers to validate signatures.

The typical workflow includes:

  • The guest VM requests an attestation report by issuing an SNP_GUEST_REQUEST_REPORT instruction.
  • The request includes a user-supplied nonce to guarantee freshness and may include additional runtime data (e.g., measurements).
  • The Secure Processor returns a binary attestation report, signed with AMD’s device-specific key.
  • The VM sends the report to the remote verifier.
  • The verifier uses AMD’s ARK (AMD Root Key) and ASK (AMD Signing Key) certificate chain to validate the report signature and verify platform integrity and configuration.

Attestation Report Structure​

The SEV-SNP attestation report is a binary structure (typically ~4 KB) that contains the following fields:

Offset [Byte]BitsNameDescription
00h31:0VersionThe current value is 2 or 3, but we will focus on version 2.
04h31:0Guest SVNThe Guest SVN. Can be set when launching the CVM.
08h63:0PolicyThe policy of the CVM. The launch will fail if the platform does not match the launch policy.
10h127:0Family IDArbitrary number that can be set when launching the CVM.
20h127:0Image IDArbitrary number that can be set when launching the CVM.
30h31:0VMPLVirtual Machine Privilege Level (VMPL) from which the attestation report was fetched.
34h31:0Signature algoThe signature algorithm that was used to sign the report.
38h63:0Current TCBThe current TCB value.
40h63:0Platform infoVarious information about the platform.
48h31:5-Reserved field. Must be 0.
48h4:2Signing keyEncoding of the keys that are used to sign the report. If the value is 0, VCEK is used.
48h1Mask chip key.The attestation report is not signed if the mask chip key is 0.
48h0Author key en.Indicates if the digest of the guest owner key is part of the report.
4Ch31:0-Reserved field. Must be 0.
50h511:0Report dataCVM-provided data at the moment the attestation report is requested.
90h383:0MeasurementThe hash of initial guest memory.
C0h255:0Host dataData provided by the hypervisor to the guest at the time of CVM launch.
E0h383:0ID key digestSHA-384 digest of the ID public key that signed the ID block.
110h383:0Author key digestSHA-384 digest of the Author public key that certified the ID key.
140h255:0Report IDReport ID of the guest.
160h255:0Report ID of MAReport ID of the Migration Agent (MA).
180h63:0Reported TCBThe reported TCB value.
188h-19Fh--Reserved field.
1A0h-1DFh511:0Chip IDUnique identifier of the platform chip.
1E0h63:0Committed TCBThe committed TCB value.
1E8h7:0Current BuildBuild number of the current firmware.
1E9h7:0Current MinorMinor number of the current firmware.
1EAh7:0Current MajorMajor number of the current firmware.
1EBh7:0-Reserved field.
1ECh7:0Committed BuildBuild number of the committed firmware.
1EDh7:0Committed MinorMinor number of the committed firmware.
1EEh7:0Committed MajorMajor number of the committed firmware.
1EFh7:0-Reserved field.
1F0h63:0Launch TCBThe TCB valued at the time the CVM was launched.
1F8h-29Fh--Reserved field.
2A0h-49Fh-SignatureSignature of the report. Bytes from 0h to 29Fh.

Security Guarantees​

The report allows a verifier to validate:

  • Platform authenticity, via the AMD certificate chain.
  • Guest integrity, via the measurement field (comparable to a PCR hash).
  • Freshness, via the report_data which should contain a hash of the verifier’s nonce.
  • Configuration policy, ensuring the guest runs with secure settings (e.g., no debug access).
  • Key binding, where the attestation report binds the measurement to a public key used later for secure communication (e.g., TLS or key wrapping).

Trusted Platform Module (TPM)​

A TPM is a dedicated security chip designed to perform tamper-resistant cryptographic functions. It securely manages sensitive artifacts such as encryption keys, certificates, and integrity measurements. In scenarios where TPM functionality is implemented via software instead of hardware, it is referred to as a virtual TPM (vTPM).

The Trusted Platform Module (TPM) plays a fundamental role in the process of attestation by providing a tamper-resistant foundation for cryptographic operations, securing sensitive artifacts, measuring system state, and enabling attestation mechanisms.

Remote attestation verifies the integrity of a client platform by retrieving a quote from the TPM, which includes signed Platform Configuration Register (PCR) values. This process involves Event Log Verification, which ensures that recorded system events align with the expected platform state.

To enable vTPM follow instructions in the table below.

Private cloud should include AMD SEV-SNP CPUs. To enable vTPM on these CPUs use the firmware from COCONUT Secure VM Service Module.

Platform Configuration Register (PCR)​

A Platform Configuration Register (PCR) is a secure memory region within the TPM that records system integrity measurements. Instead of being overwritten, PCR values are extended through a cryptographic hash function:

PCR[N]=HASHalg(PCR[N] ∣∣ NewMeasurement)PCR[N] = \text{HASH}_{\text{alg}}( PCR[N] \, || \, \text{NewMeasurement} )

This ensures that all recorded values remain linked and verifiable, making PCRs essential for attestation and system security.

Trusted Boot and Integrity Measurement​

System integrity is ensured by measuring and recording each boot component into the Trusted Platform Module (TPM). The firmware acts as the Static Root of Trust for Measurement (SRTM), measuring critical components and storing their hashes in the TPM's Platform Configuration Registers (PCRs). These values are extended immutably, creating a tamper-proof record.

This process enables secure attestation, ensuring that only verified software executes and detecting unauthorized modifications. The TPM also functions as a Root of Trust for Storage (RTS) and Root of Trust for Reporting (RTR), allowing for remote verification of system integrity.

What is Verified in Each PCR​

A typical TPM has 24 PCRs. What is verified in each PCR depends on the platform and the specific implementation of the TPM. The table below shows the PCRs used in CocosAI and what is verified in each of them for a different platform.

PCR IndexWhat is Stored?What We Verify?Why It Is Important for Integrity?
PCR[0]Firmware measurements: CRTM version and EFI platform firmware blobsEnsures the measured firmware components (CRTM, UEFI firmware) match expected hashes.Prevents unauthorized or tampered firmware from running, which could compromise the entire boot chain.
PCR[1]Platform configuration flags and boot variables (e.g., BootOrder, Boot#### entries)Ensures that critical UEFI variables and boot settings haven’t been altered.Stops attackers from changing boot parameters or order to load malicious components.
PCR[4]EFI Boot Services Application image (e.g., bootloader or kernel image loaded via UEFI)Verifies that the correct bootloader/kernel image is measured before execution.Prevents unsigned or altered boot applications from being launched, ensuring a trusted OS load.
PCR[5]EFI Actions around Exit Boot Services (e.g., “Exit BootServices Invocation/Return”)Ensures that the transition from firmware to OS (via Exit Boot Services) is unmodified.Guards the handoff phase so that no unauthorized hooks or code injections occur just before OS runtime.
PCR[7]EFI variable configurations related to Secure Boot: SecureBoot flag, PK, KEK, db, dbx, and authority dataVerifies that Secure Boot keys and policy databases are exactly as expected (no rogue keys or removed revocations).Crucial for ensuring only signed, authorized binaries can load; prevents boot-time attacks via unauthorized keys or certificates.
PCR[9]Event tags for loaded images and initramfs options (e.g., “LOADED_IMAGE::loadOptions”, “Linux initrd”)Ensures that the load options or initrd identifiers provided to the OS are as intended.Protects early user-space setup by detecting tampering in initramfs or kernel command-line inputs.

By leveraging PCR measurements, systems maintain trusted execution environments, ensuring the integrity of Confidential Virtual Machines (CVMs) and other secure workloads.

Event Log Verification​

In addition to the PCR values, the vTPM quote also includes a log of all significant events that happened during the boot process. This ensures the platform’s integrity by checking if the recorded events match the expected state.

  1. The CLI requests a quote from the client.
  2. The Agent asks the TPM to sign the current PCR values using the AK private key and sends the signed quote to the CLI.
  3. The CLI verifies the quote’s signature using the AK public key, ensuring the PCR values are legitimate.
  4. The CLI replays the event log to reconstruct the PCR values. If the recalculated values match the received PCRs, the event log is verified.
  5. The verified PCR values are then compared against predefined golden(good) values for the TPM. If they match the expected values in the vTPM quote, the platform's integrity is confirmed.

A secure communication channel is required between the attester and the relying party, which is established using attested TLS.

Attestation Report Verification​

Regardless of the provider, the attestation report always consists of a vTPM quote and SEV-SNP quote. SEV-SNP quote can always be verfied. However, verifying the vTPM quote differs across platforms.

Since all components of the CVM are known in advance, all the values present in the attestation report can be verified against the values calculated in advance.

How is the attestation report fetched?​

The Agent is responsible for fetching the attestation report from the CVM. This procedure is safe because the Kernel and the ASP can exchange encrypted messages that can only be decrypted by the Kernel and the ASP. The keys used for the encryption/decryption are inserted by the ASP into the memory of the CVM during boot, thus ensuring that only the ASP and the CVM have the keys for safe communication.

Additionally, the Agent interacts with the vTPM inside the CVM using go-tpm-tools to retrieve attestation data, which includes cryptographic measurements of the enclave’s boot and runtime state. These measurements are used to generate the attestation report, which the Manager or an external verifier can validate to ensure the enclave’s integrity.

Linux IMA (Integrity Measurement Architecture)​

Linux Integrity Measurement Architecture (IMA) is a Linux kernel feature that provides a mechanism to record and verify the integrity of files and other system objects at load or access time. At a high level, IMA computes cryptographic hashes of files (or other data) when they are used, maintains a log of these measurements, and can optionally enforce appraisal policies (e.g., signature checks) before allowing execution or access. This helps detect unauthorized modifications and supports building trust in the system state.

Overview​

Primary objectives of Linux IMA are:

  • Detection of Unauthorized Modifications: By computing hashes of files and comparing them to known-good values or verifying signatures, IMA can detect tampering.
  • Support for Remote Attestation: Measurements can be extended into TPM PCRs (typically PCR 10 on AMD SEV-SNP setups) to allow a remote party to verify the boot-time and runtime integrity state of the system.
  • Policy-Driven Measurement and Appraisal: Flexible policy allows administrators to specify which files or operations to measure or appraise, balancing performance and security needs.
  • Integration with Extended Attributes (EVM): Ensures integrity metadata (e.g., security labels, signatures stored as xattrs) cannot themselves be tampered with without detection.

Key aspects of IMA in general terms:

  • Measurement: When a file (such as an executable, shared library, or configuration file) is opened, mapped, or otherwise used in a way covered by policy, the kernel computes a hash of its contents (and possibly metadata) and appends that measurement to an in-memory log. This log can be exposed to user space for auditing or further processing.

  • Policy-Driven: Which files or operations are measured (or appraised) is determined by a policy. Administrators define simple rules (e.g., “measure all ELF binaries” or “appraise scripts under /usr/local/bin”) so that only relevant objects incur overhead.

  • Appraisal (Optional Enforcement): Beyond just measuring, IMA can verify that a file carries a valid digital signature (with the public key provisioned in the kernel) before permitting its use. If appraisal fails, the kernel can refuse to open or execute the file, or log a warning depending on configuration.

  • Logging and Auditing: The sequence of measurements is kept in a log that can be read or exported. This enables administrators or automated systems to review which files were loaded and whether any unexpected changes occurred.

  • Integration with TPM/Attestation (Contextual Use): While IMA itself computes and logs hashes, in many deployments those measurements are extended into a TPM’s Platform Configuration Registers (PCRs). A remote verifier can then obtain a TPM quote including these PCR values, providing evidence that the system booted and ran only approved components. (Note: this integration is set up separately, but IMA provides the runtime measurements that feed into it.)

  • Performance Considerations: Measuring every access can introduce overhead, so policies usually target only critical binaries or first-time loads. Subsequent accesses of immutable files may skip rehashing. Appraisal (signature checks) also adds cost, so it’s applied selectively.

  • Extended Metadata Protection: IMA often works alongside EVM (Extended Verification Module), which protects integrity-related extended attributes (e.g., signatures stored in xattrs) so that metadata tampering is also detectable.

In essence, IMA provides a flexible, policy-driven way to observe and enforce file integrity at runtime in Linux. It does not by itself decide trustworthiness—that comes from how measurements are evaluated (e.g., comparing hashes to a whitelist or verifying signatures)—but it ensures that each file’s state when accessed is recorded and, if configured, checked before use. This mechanism underpins many integrity and attestation solutions by supplying reliable, tamper-evident records of what code and data have actually been loaded on the system.

Integration with CocosAI​

Eventhough IMA offers an appraise feature, CocosAI doesn't use it. The reason for this decision is the fact that many public cloud provider offer images that don't have file measurements. In these cases the VM doesn't boot since some of the critital binaries (system services etc.) are not executed because of the missing measurements. Rather than relying on kernel-enforced appraisal, CocosAI adopts a local verification approach:

  • An in-VM agent can fetch the current IMA measurement log.
  • A CLI tool can request that data and combine it with TPM PCR10 values (or quotes).
  • Verification is performed in user space (either locally or by forwarding to a remote attestation service). This design avoids boot failures on images without built-in measurements, while still leveraging IMA’s tamper-evident logs for post-boot integrity checking.

Linux IMA is enabled through grub command line arguments. The command line needs to include ima_policy=tcb for IMA to be enabled. On VM startup, CocosAI applies a configuration file (in YAML format) that changes the grub command line. The change is made by editing `/etc/grub/default/ file. Afterwards grub is update and the VM is rebooted in order to activate IMA kernel feature.

To ensure every file on the filesystem is measured (so that the log contains entries for all root-owned binaries and other critical files), CocosAI uses a command that accesses each file without reading its contents:

sudo find / -fstype ext4 -type f -uid 0 -exec dd if='{}' of=/dev/null count=0 status=none \;

What this does:

  • find /: Traverse from the root directory.
  • -fstype ext4: Restrict to filesystems of type ext4 (adjust if other filesystem types are used).
  • -type f -uid 0: Match regular files owned by root.
  • -exec dd if='{}' of=/dev/null count=0 status=none \;: For each path, open it via dd but copy zero blocks (count=0), which causes the kernel to open the file (triggering IMA measurement) without reading data, minimizing I/O overhead. Each matched file is opened exactly once, causing IMA to compute and record its hash in the runtime measurement list. After the reboot all files in the system are measured and their measurements are stored in the /sys/kernel/security/integrity/ima/ascii_runtime_measurements file.

When the VM boots and the Agent starts, the measurements can be fetche with CLI tool. Every IMA mesurement extends PCR 10 register value. This means that the value of that register can be used to verify that the content of the file with measurements is not tampered with. When fetched, the measurements can be verified locally.