mirror of
https://github.com/google/go-attestation.git
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401 lines
11 KiB
Go
401 lines
11 KiB
Go
// Copyright 2019 Google Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License"); you may not
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// use this file except in compliance with the License. You may obtain a copy of
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// the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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// License for the specific language governing permissions and limitations under
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// the License.
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package attest
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import (
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"bytes"
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"crypto"
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"encoding/binary"
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"fmt"
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"io"
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"sort"
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// Ensure hashes are available.
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"crypto/rsa"
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"crypto/sha1"
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_ "crypto/sha256"
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"github.com/google/go-tpm/tpm2"
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"github.com/google/go-tpm/tpmutil"
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)
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// TPM algorithms. See the TPM 2.0 specification section 6.3.
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//
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// https://trustedcomputinggroup.org/wp-content/uploads/TPM-Rev-2.0-Part-2-Structures-01.38.pdf#page=42
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const (
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algSHA1 uint16 = 0x0004
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algSHA256 uint16 = 0x000B
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)
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// EventType indicates what kind of data an event is reporting.
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type EventType uint32
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// Event is a single event from a TCG event log. This reports descrete items such
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// as BIOs measurements or EFI states.
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type Event struct {
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// PCR index of the event.
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Index int
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// Type of the event.
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Type EventType
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// Data of the event. For certain kinds of events, this must match the event
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// digest to be valid.
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Data []byte
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// Digest is the verified digest of the event data. While an event can have
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// multiple for different hash values, this is the one that was matched to the
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// PCR value.
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Digest []byte
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// TODO(ericchiang): Provide examples or links for which event types must
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// match their data to their digest.
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}
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// ParseEventLog parses an unverified measurement log.
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func ParseEventLog(measurementLog []byte) (*EventLog, error) {
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rawEvents, err := parseEventLog(measurementLog)
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if err != nil {
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return nil, fmt.Errorf("parsing measurement log: %v", err)
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}
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return &EventLog{rawEvents}, nil
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}
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// EventLog is a parsed measurement log. This contains unverified data representing
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// boot events that must be replayed against PCR values to determine authenticity.
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type EventLog struct {
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rawEvents []rawEvent
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}
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// Verify replays the event log against a TPM's PCR values, returning events
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// from the event log, or an error if the replayed PCR values did not match the
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// provided PCR values.
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func (e *EventLog) Verify(pcrs []PCR) ([]Event, error) {
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events, err := replayEvents(e.rawEvents, pcrs)
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if err != nil {
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return nil, fmt.Errorf("pcrs failed to replay: %v", err)
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}
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return events, nil
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}
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type rawAttestationData struct {
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Version [4]byte // This MUST be 1.1.0.0
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Fixed [4]byte // This SHALL always be the string ‘QUOT’
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Digest [20]byte // PCR Composite Hash
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Nonce [20]byte // Nonce Hash
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}
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var (
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fixedQuote = [4]byte{'Q', 'U', 'O', 'T'}
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)
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type rawPCRComposite struct {
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Size uint16 // always 3
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PCRMask [3]byte
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Values tpmutil.U32Bytes
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}
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func (a *AIKPublic) validate12Quote(quote Quote, pcrs []PCR, nonce []byte) error {
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pub, ok := a.Public.(*rsa.PublicKey)
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if !ok {
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return fmt.Errorf("unsupported public key type: %T", a.Public)
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}
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qHash := sha1.Sum(quote.Quote)
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if err := rsa.VerifyPKCS1v15(pub, crypto.SHA1, qHash[:], quote.Signature); err != nil {
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return fmt.Errorf("invalid quote signature: %v", err)
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}
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var att rawAttestationData
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if _, err := tpmutil.Unpack(quote.Quote, &att); err != nil {
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return fmt.Errorf("parsing quote: %v", err)
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}
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// TODO(ericchiang): validate Version field.
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if att.Nonce != sha1.Sum(nonce) {
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return fmt.Errorf("invalid nonce")
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}
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if att.Fixed != fixedQuote {
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return fmt.Errorf("quote wasn't a QUOT object: %x", att.Fixed)
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}
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// See 5.4.1 Creating a PCR composite hash
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sort.Slice(pcrs, func(i, j int) bool { return pcrs[i].Index < pcrs[j].Index })
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var (
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pcrMask [3]byte // bitmap indicating which PCRs are active
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values []byte // appended values of all PCRs
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)
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for _, pcr := range pcrs {
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if pcr.Index < 0 || pcr.Index >= 24 {
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return fmt.Errorf("invalid PCR index: %d", pcr.Index)
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}
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pcrMask[pcr.Index/8] |= 1 << uint(pcr.Index%8)
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values = append(values, pcr.Digest...)
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}
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composite, err := tpmutil.Pack(rawPCRComposite{3, pcrMask, values})
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if err != nil {
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return fmt.Errorf("marshaling PCRs: %v", err)
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}
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if att.Digest != sha1.Sum(composite) {
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return fmt.Errorf("PCRs passed didn't match quote: %v", err)
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}
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return nil
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}
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func (a *AIKPublic) validate20Quote(quote Quote, pcrs []PCR, nonce []byte) error {
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sig, err := tpm2.DecodeSignature(bytes.NewBuffer(quote.Signature))
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if err != nil {
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return fmt.Errorf("parse quote signature: %v", err)
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}
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sigHash := a.Hash.New()
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sigHash.Write(quote.Quote)
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switch pub := a.Public.(type) {
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case *rsa.PublicKey:
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if sig.RSA == nil {
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return fmt.Errorf("rsa public key provided for ec signature")
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}
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sigBytes := []byte(sig.RSA.Signature)
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if err := rsa.VerifyPKCS1v15(pub, a.Hash, sigHash.Sum(nil), sigBytes); err != nil {
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return fmt.Errorf("invalid quote signature: %v", err)
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}
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default:
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// TODO(ericchiang): support ecdsa
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return fmt.Errorf("unsupported public key type %T", pub)
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}
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att, err := tpm2.DecodeAttestationData(quote.Quote)
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if err != nil {
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return fmt.Errorf("parsing quote signature: %v", err)
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}
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if att.Type != tpm2.TagAttestQuote {
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return fmt.Errorf("attestation isn't a quote, tag of type 0x%x", att.Type)
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}
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if !bytes.Equal([]byte(att.ExtraData), nonce) {
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return fmt.Errorf("nonce didn't match: %v", err)
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}
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pcrByIndex := map[int][]byte{}
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for _, pcr := range pcrs {
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pcrByIndex[pcr.Index] = pcr.Digest
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}
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n := len(att.AttestedQuoteInfo.PCRDigest)
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hash, ok := hashBySize[n]
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if !ok {
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return fmt.Errorf("quote used unsupported hash algorithm length: %d", n)
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}
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h := hash.New()
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for _, index := range att.AttestedQuoteInfo.PCRSelection.PCRs {
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digest, ok := pcrByIndex[index]
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if !ok {
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return fmt.Errorf("quote was over PCR %d which wasn't provided", index)
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}
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if len(digest) != hash.Size() {
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return fmt.Errorf("mismatch pcr and quote hash, pcr hash length=%d, quote hash length=%d", len(digest), hash.Size())
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}
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h.Write(digest)
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}
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if !bytes.Equal(h.Sum(nil), att.AttestedQuoteInfo.PCRDigest) {
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return fmt.Errorf("quote digest didn't match pcrs provided")
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}
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return nil
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}
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var hashBySize = map[int]crypto.Hash{
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crypto.SHA1.Size(): crypto.SHA1,
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crypto.SHA256.Size(): crypto.SHA256,
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}
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func extend(pcr, replay []byte, e rawEvent) ([]byte, Event, error) {
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h, ok := hashBySize[len(pcr)]
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if !ok {
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return nil, Event{}, fmt.Errorf("pcr %d was not a known hash size: %d", e.index, len(pcr))
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}
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for _, digest := range e.digests {
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if len(digest) != len(pcr) {
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continue
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}
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hash := h.New()
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if len(replay) != 0 {
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hash.Write(replay)
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} else {
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b := make([]byte, h.Size())
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hash.Write(b)
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}
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hash.Write(digest)
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return hash.Sum(nil), Event{e.index, e.typ, e.data, digest}, nil
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}
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return nil, Event{}, fmt.Errorf("no event digest matches pcr length: %d", len(pcr))
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}
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func replayEvents(rawEvents []rawEvent, pcrs []PCR) ([]Event, error) {
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events := []Event{}
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replay := map[int][]byte{}
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pcrByIndex := map[int][]byte{}
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for _, pcr := range pcrs {
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pcrByIndex[pcr.Index] = pcr.Digest
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}
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for i, e := range rawEvents {
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pcrValue, ok := pcrByIndex[e.index]
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if !ok {
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// Ignore events for PCRs that weren't included in the quote.
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continue
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}
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replayValue, event, err := extend(pcrValue, replay[e.index], e)
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if err != nil {
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return nil, fmt.Errorf("replaying event %d: %v", i, err)
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}
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replay[e.index] = replayValue
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events = append(events, event)
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}
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var invalidReplays []int
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for i, value := range replay {
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if !bytes.Equal(value, pcrByIndex[i]) {
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invalidReplays = append(invalidReplays, i)
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}
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}
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if len(invalidReplays) > 0 {
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return nil, fmt.Errorf("the following registers failed to replay: %d", invalidReplays)
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}
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return events, nil
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}
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// EV_NO_ACTION is a special event type that indicates information to the parser
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// instead of holding a measurement. For TPM 2.0, this event type is used to signal
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// switching from SHA1 format to a variable length digest.
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//
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// https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClientSpecPlat_TPM_2p0_1p04_pub.pdf#page=110
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const eventTypeNoAction = 0x03
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func parseEventLog(b []byte) ([]rawEvent, error) {
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r := bytes.NewBuffer(b)
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parseFn := parseRawEvent
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e, err := parseFn(r)
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if err != nil {
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return nil, fmt.Errorf("parse first event: %v", err)
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}
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var events []rawEvent
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if e.typ == eventTypeNoAction {
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// Switch to parsing crypto agile events. Don't include this in the
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// replayed events since it's intentionally switching from SHA1 to
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// SHA256 and will fail to extend a SHA256 PCR value.
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//
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// NOTE(ericchiang): to be strict, we could parse the event data as a
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// TCG_EfiSpecIDEventStruct and validate the algorithms. But for now,
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// assume this indicates a switch from SHA1 format to SHA1/SHA256.
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//
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// https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf#page=18
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parseFn = parseRawEvent2
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} else {
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events = append(events, e)
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}
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for r.Len() != 0 {
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e, err := parseFn(r)
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if err != nil {
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return nil, err
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}
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events = append(events, e)
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}
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return events, nil
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}
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type rawEvent struct {
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index int
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typ EventType
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data []byte
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digests [][]byte
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}
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// TPM 1.2 event log format. See "5.1 SHA1 Event Log Entry Format"
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// https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf#page=15
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type rawEventHeader struct {
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PCRIndex uint32
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Type uint32
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Digest [20]byte
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EventSize uint32
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}
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func parseRawEvent(r io.Reader) (event rawEvent, err error) {
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var h rawEventHeader
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if err = binary.Read(r, binary.LittleEndian, &h); err != nil {
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return event, err
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}
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data := make([]byte, int(h.EventSize))
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if _, err := io.ReadFull(r, data); err != nil {
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return event, err
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}
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return rawEvent{
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typ: EventType(h.Type),
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data: data,
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index: int(h.PCRIndex),
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digests: [][]byte{h.Digest[:]},
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}, nil
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}
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// TPM 2.0 event log format. See "5.2 Crypto Agile Log Entry Format"
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// https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf#page=15
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type rawEvent2Header struct {
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PCRIndex uint32
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Type uint32
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}
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func parseRawEvent2(r io.Reader) (event rawEvent, err error) {
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var h rawEvent2Header
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if err = binary.Read(r, binary.LittleEndian, &h); err != nil {
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return event, err
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}
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event.typ = EventType(h.Type)
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event.index = int(h.PCRIndex)
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// parse the event digests
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var numDigests uint32
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if err := binary.Read(r, binary.LittleEndian, &numDigests); err != nil {
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return event, err
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}
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for i := 0; i < int(numDigests); i++ {
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var algID uint16
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if err := binary.Read(r, binary.LittleEndian, &algID); err != nil {
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return event, err
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}
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var digest []byte
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switch algID {
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case algSHA1:
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digest = make([]byte, crypto.SHA1.Size())
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case algSHA256:
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digest = make([]byte, crypto.SHA256.Size())
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default:
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// ignore signatures that aren't SHA1 or SHA256
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continue
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}
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if _, err := io.ReadFull(r, digest); err != nil {
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return event, err
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}
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event.digests = append(event.digests, digest)
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}
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// parse event data
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var eventSize uint32
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if err = binary.Read(r, binary.LittleEndian, &eventSize); err != nil {
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return event, err
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}
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event.data = make([]byte, int(eventSize))
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if _, err := io.ReadFull(r, event.data); err != nil {
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return event, err
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}
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return event, err
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}
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