mirror of
https://github.com/edgelesssys/constellation.git
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233 lines
7.9 KiB
Go
233 lines
7.9 KiB
Go
package vtpm
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import (
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"bytes"
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"crypto"
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"crypto/rsa"
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"crypto/sha256"
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"encoding/json"
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"errors"
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"fmt"
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"io"
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tpmClient "github.com/google/go-tpm-tools/client"
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"github.com/google/go-tpm-tools/proto/attest"
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tpmProto "github.com/google/go-tpm-tools/proto/tpm"
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tpmServer "github.com/google/go-tpm-tools/server"
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"github.com/google/go-tpm/tpm2"
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)
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var (
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// AzurePCRSelection are the PCR values verified for Azure Constellations.
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// PCR[0] is excluded due to changing rarely, but unpredictably.
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// PCR[6] is excluded due to being different for any 2 VMs. See: https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClient_PFP_r1p05_v23_pub.pdf#%5B%7B%22num%22%3A157%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C33%2C400%2C0%5D
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// PCR[10] is excluded since its value is derived from a digest of PCR[0-7]. See: https://sourceforge.net/p/linux-ima/wiki/Home/#ima-measurement-list
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AzurePCRSelection = tpm2.PCRSelection{
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Hash: tpm2.AlgSHA256,
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PCRs: []int{1, 2, 3, 4, 5, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23},
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}
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// GCPPCRSelection are the PCR values verified for GCP Constellations.
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// On GCP firmware and other host controlled systems are static. This results in the same PCRs for any 2 VMs using the same image.
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GCPPCRSelection = tpmClient.FullPcrSel(tpm2.AlgSHA256)
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// QEMUPCRSelection are the PCR values verified for QEMU based Contellations.
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// PCR[1] is excluded. See: https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClient_PFP_r1p05_v23_pub.pdf#%5B%7B%22num%22:157,%22gen%22:0%7D,%7B%22name%22:%22XYZ%22%7D,33,400,0%5D
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// PCR[10] is excluded since its value is derived from a digest of PCR[0-7]. See: https://sourceforge.net/p/linux-ima/wiki/Home/#ima-measurement-list
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QEMUPCRSelection = tpm2.PCRSelection{
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Hash: tpm2.AlgSHA256,
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PCRs: []int{0, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23},
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}
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)
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type (
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// GetTPMAttestationKey loads a TPM key to perform attestation.
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GetTPMAttestationKey func(tpm io.ReadWriter) (*tpmClient.Key, error)
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// GetTPMTrustedAttestationPublicKey verifies and returns the attestation public key.
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GetTPMTrustedAttestationPublicKey func(akPub []byte, instanceInfo []byte) (crypto.PublicKey, error)
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// GetInstanceInfo returns VM metdata.
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GetInstanceInfo func(tpm io.ReadWriteCloser) ([]byte, error)
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// ValidateCVM validates confidential computing capabilities of the instance issuing the attestation.
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ValidateCVM func(attestation AttestationDocument) error
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// VerifyUserData verifies signed user data.
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VerifyUserData func(pub crypto.PublicKey, hash crypto.Hash, hashed, sig []byte) error
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)
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// AttestationDocument contains the TPM attestation with signed user data.
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type AttestationDocument struct {
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// Attestation contains the TPM event log, PCR values and quotes, and public key of the key used to sign the attestation.
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Attestation *attest.Attestation
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// InstanceInfo is used to verify the provided public key.
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InstanceInfo []byte
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// arbitrary data, signed by the TPM.
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UserData []byte
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UserDataSignature []byte
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}
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// Issuer handles issuing of TPM based attestation documents.
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type Issuer struct {
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openTPM TPMOpenFunc
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getAttestationKey GetTPMAttestationKey
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getInstanceInfo GetInstanceInfo
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}
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// NewIssuer returns a new Issuer.
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func NewIssuer(openTPM TPMOpenFunc, getAttestationKey GetTPMAttestationKey, getInstanceInfo GetInstanceInfo) *Issuer {
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return &Issuer{
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openTPM: openTPM,
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getAttestationKey: getAttestationKey,
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getInstanceInfo: getInstanceInfo,
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}
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}
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// Issue generates an attestation document using a TPM.
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func (i *Issuer) Issue(userData []byte, nonce []byte) ([]byte, error) {
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tpm, err := i.openTPM()
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if err != nil {
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return nil, fmt.Errorf("opening TPM: %w", err)
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}
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defer tpm.Close()
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// Load the TPM's attestation key
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aK, err := i.getAttestationKey(tpm)
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if err != nil {
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return nil, fmt.Errorf("loading attestation key: %w", err)
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}
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defer aK.Close()
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// Create an attestation using the loaded key
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attestation, err := aK.Attest(tpmClient.AttestOpts{Nonce: nonce})
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if err != nil {
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return nil, fmt.Errorf("creating attestation: %w", err)
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}
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// Fetch instance info of the VM
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instanceInfo, err := i.getInstanceInfo(tpm)
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if err != nil {
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return nil, fmt.Errorf("fetching instance info: %w", err)
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}
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// Sign user provided data using the loaded key
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userDataSigned, err := aK.SignData(userData)
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if err != nil {
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return nil, fmt.Errorf("signing user data: %w", err)
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}
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attDoc := AttestationDocument{
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Attestation: attestation,
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InstanceInfo: instanceInfo,
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UserData: userData,
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UserDataSignature: userDataSigned,
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}
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return json.Marshal(attDoc)
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}
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// Validator handles validation of TPM based attestation.
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type Validator struct {
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trustedPcrs map[uint32][]byte
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getTrustedKey GetTPMTrustedAttestationPublicKey
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validateCVM ValidateCVM
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verifyUserData VerifyUserData
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}
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// NewValidator returns a new Validator.
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func NewValidator(trustedPcrs map[uint32][]byte, getTrustedKey GetTPMTrustedAttestationPublicKey, validateCVM ValidateCVM, verifyUserData VerifyUserData) *Validator {
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return &Validator{
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trustedPcrs: trustedPcrs,
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getTrustedKey: getTrustedKey,
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validateCVM: validateCVM,
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verifyUserData: verifyUserData,
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}
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}
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// Validate a TPM based attestation.
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func (v *Validator) Validate(attDocRaw []byte, nonce []byte) ([]byte, error) {
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var attDoc AttestationDocument
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if err := json.Unmarshal(attDocRaw, &attDoc); err != nil {
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return nil, fmt.Errorf("unmarshaling TPM attestation document: %w", err)
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}
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// Verify and retrieve the trusted attestation public key using the provided instance info
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aKP, err := v.getTrustedKey(attDoc.Attestation.AkPub, attDoc.InstanceInfo)
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if err != nil {
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return nil, fmt.Errorf("validating attestation public key: %w", err)
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}
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// Validate confidential computing capabilities of the VM
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if err := v.validateCVM(attDoc); err != nil {
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return nil, fmt.Errorf("verifying VM confidential computing capabilities: %w", err)
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}
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// Verify the TPM attestation
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if _, err := tpmServer.VerifyAttestation(
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attDoc.Attestation,
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tpmServer.VerifyOpts{
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Nonce: nonce,
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TrustedAKs: []crypto.PublicKey{aKP},
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AllowSHA1: false,
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},
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); err != nil {
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return nil, fmt.Errorf("verifying attestation document: %w", err)
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}
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// Verify PCRs
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quoteIdx, err := GetSHA256QuoteIndex(attDoc.Attestation.Quotes)
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if err != nil {
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return nil, err
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}
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for idx, pcr := range v.trustedPcrs {
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if !bytes.Equal(pcr, attDoc.Attestation.Quotes[quoteIdx].Pcrs.Pcrs[idx]) {
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return nil, fmt.Errorf("untrusted PCR value at PCR index %d", idx)
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}
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}
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// Verify signed user data
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digest := sha256.Sum256(attDoc.UserData)
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if err = v.verifyUserData(aKP, crypto.SHA256, digest[:], attDoc.UserDataSignature); err != nil {
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return nil, fmt.Errorf("verifying signed user data: %w", err)
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}
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return attDoc.UserData, nil
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}
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// GetSHA256QuoteIndex performs safety checks and returns the index for SHA256 PCR quotes.
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func GetSHA256QuoteIndex(quotes []*tpmProto.Quote) (int, error) {
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if len(quotes) == 0 {
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return 0, fmt.Errorf("attestation is missing quotes")
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}
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for idx, quote := range quotes {
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if quote == nil {
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return 0, fmt.Errorf("quote is nil")
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}
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if quote.Pcrs == nil {
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return 0, fmt.Errorf("no PCR data in attestation")
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}
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if quote.Pcrs.Hash == tpmProto.HashAlgo_SHA256 {
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return idx, nil
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}
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}
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return 0, fmt.Errorf("attestation did not include SHA256 hashed PCRs")
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}
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// VerifyPKCS1v15 is a convenience function to call rsa.VerifyPKCS1v15.
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func VerifyPKCS1v15(pub crypto.PublicKey, hash crypto.Hash, hashed, sig []byte) error {
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key, ok := pub.(*rsa.PublicKey)
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if !ok {
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return errors.New("key is not an RSA public key")
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}
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return rsa.VerifyPKCS1v15(key, hash, hashed, sig)
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}
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// GetSelectedPCRs returns a map of the selected PCR hashes.
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func GetSelectedPCRs(open TPMOpenFunc, selection tpm2.PCRSelection) (map[uint32][]byte, error) {
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tpm, err := open()
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if err != nil {
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return nil, err
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}
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defer tpm.Close()
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pcrList, err := tpmClient.ReadPCRs(tpm, selection)
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if err != nil {
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return nil, err
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}
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return pcrList.Pcrs, nil
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}
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