// AddEndorsements reads the SerializedEndorsements in an attestation and adds
// the ones that are predicates signed by a guard's policy key.
func AddEndorsements(guard Guard, a *Attestation, v *Verifier) error {
// Before validating against the guard, check to see if there are any
// predicates endorsed by the policy key. This allows truncated principals
// to get the Tao CA to sign a statement of the form
// TrustedHash(ext.Program(...)).
for _, e := range a.SerializedEndorsements {
var ea Attestation
if err := proto.Unmarshal(e, &ea); err != nil {
return err
}
f, err := auth.UnmarshalForm(ea.SerializedStatement)
if err != nil {
return err
}
says, ok := f.(auth.Says)
if !ok {
return fmt.Errorf("a serialized endorsement must be an auth.Says")
}
// TODO(tmroeder): check that this endorsement hasn't expired.
pred, ok := says.Message.(auth.Pred)
if !ok {
return fmt.Errorf("the message in an endorsement must be a predicate")
}
signerPrin := auth.NewPrin(*ea.SignerType, ea.SignerKey)
if !signerPrin.Identical(says.Speaker) {
return fmt.Errorf("the speaker of an endorsement must be the signer: %v vs %v", signerPrin, says.Speaker)
}
if !v.ToPrincipal().Identical(signerPrin) {
return fmt.Errorf("the signer of an endorsement must be the guard's policy key")
}
if ok, err := v.Verify(ea.SerializedStatement, AttestationSigningContext, ea.Signature); (err != nil) || !ok {
return fmt.Errorf("the signature on an endorsement didn't pass verification")
}
return guard.AddRule(pred.String())
}
return nil
}
// Checks the following:
// (1) the endorsement attestation is valid
// (2) the key being endorsed is kPrin
// (3) the subject being endorsed is a trusted host
// Finally the function returns the key principal signing the endorsement
func validateEndorsementAttestation(attestation *tao.Attestation, guard tao.Guard,
kPrin *auth.Prin) (*auth.Prin, error) {
saysStatement, err := attestation.Validate()
if err != nil {
return nil, err
}
_, key, host, err := parseSaysStatement(&saysStatement)
if err != nil {
return nil, err
}
if !key.Identical(kPrin) {
return nil, errors.New("endorsement does not endorse signer of (previous) attestaton")
}
if !guard.IsAuthorized(*host, "Host", []string{}) {
return nil, errors.New("endorsement host not authorized to run in this domain")
}
signerType := attestation.SignerType
if signerType == nil {
return nil, errors.New("endorsement chain has attestation with missing SignerType")
}
signerPrin := auth.NewPrin(*signerType, attestation.SignerKey)
return &signerPrin, nil
}
// ValidSigner checks the signature on an attestation and, if so, returns the
// principal name for the signer.
func (a *Attestation) ValidSigner() (auth.Prin, error) {
signer := auth.NewPrin(*a.SignerType, a.SignerKey)
switch *a.SignerType {
case "tpm":
// The PCRs are contained in the Speaker of an auth.Says statement that
// makes up the a.SerializedStatement.
f, err := auth.UnmarshalForm(a.SerializedStatement)
if err != nil {
return auth.Prin{}, newError("tao: couldn't unmarshal the statement: %s", err)
}
// A TPM attestation must be an auth.Says.
says, ok := f.(auth.Says)
if !ok {
return auth.Prin{}, newError("tao: the attestation statement was not an auth.Says statement")
}
// Signer is tpm; use tpm-specific signature verification. Extract the
// PCRs from the issuer name, unmarshal the key as an RSA key, and call
// tpm.VerifyQuote().
speaker, ok := says.Speaker.(auth.Prin)
if !ok {
return auth.Prin{}, newError("tao: the speaker of an attestation must be an auth.Prin")
}
pcrNums, pcrVals, err := extractPCRs(speaker)
if err != nil {
return auth.Prin{}, newError("tao: couldn't extract TPM PCRs from attestation: %s", err)
}
pk, err := extractTPMKey(a.SignerKey)
if err != nil {
return auth.Prin{}, newError("tao: couldn't extract TPM key from attestation: %s", err)
}
if err := tpm.VerifyQuote(pk, a.SerializedStatement, a.Signature, pcrNums, pcrVals); err != nil {
return auth.Prin{}, newError("tao: TPM quote failed verification: %s", err)
}
return signer, nil
case "tpm2":
// TODO -- tpm2
// The PCRs are contained in the Speaker of an auth.Says statement that
// makes up the a.SerializedStatement.
f, err := auth.UnmarshalForm(a.SerializedStatement)
if err != nil {
return auth.Prin{}, newError("tao: couldn't unmarshal the statement: %s", err)
}
// put this back in
// A TPM attestation must be an auth.Says.
says, ok := f.(auth.Says)
if !ok {
return auth.Prin{}, newError("tao: the attestation statement was not an auth.Says statement")
}
// Signer is tpm; use tpm-specific signature verification. Extract the
// PCRs from the issuer name, unmarshal the key as an RSA key, and call
// tpm2.VerifyQuote().
speaker, ok := says.Speaker.(auth.Prin)
if !ok {
return auth.Prin{}, newError("tao: the speaker of an attestation must be an auth.Prin")
}
key, err := extractTPM2Key(a.SignerKey)
if err != nil {
return auth.Prin{}, newError("tao: couldn't extract TPM key from attestation: %s", err)
}
pcrNums, pcrVal, err := extractTpm2PCRs(speaker)
if err != nil {
return auth.Prin{}, newError("tao: couldn't extract TPM PCRs from attestation: %s", err)
}
ok, err = tpm2.VerifyTpm2Quote(a.SerializedStatement,
pcrNums, pcrVal, a.Tpm2QuoteStructure, a.Signature,
key)
if err != nil {
return auth.Prin{}, newError("tao: TPM quote verification error")
}
if !ok {
return auth.Prin{}, newError("tao: TPM quote failed verification")
}
return signer, nil
case "key":
// Signer is ECDSA key, use Tao signature verification.
v, err := UnmarshalKey(a.SignerKey)
if err != nil {
return auth.Prin{}, err
}
ok, err := v.Verify(a.SerializedStatement, AttestationSigningContext, a.Signature)
if err != nil {
return auth.Prin{}, err
}
if !ok {
return auth.Prin{}, newError("tao: attestation signature invalid")
}
return signer, nil
default:
return auth.Prin{}, newError("tao: attestation signer principal unrecognized: %s", signer.String())
}
}
// This function makes the following checks
// (1) Checks if the attestation signature is valid and the statement is of the form
// 'Speaker says Key speaks for Program'.
// (2) Checks that 'Program' in the above statement is allowed to Execute in the domain policy.
// In particular, the policy should allow the predicate:
// Authorized(ProgramTaoName, "Execute")
// (3) Checks that 'Speaker' in the above statement is a key principal endorsed by the policy key,
// or rootCerts, via an endorsement chain. Each endorsement in this chain endorses the key
// signing the previous endorsement (starting with the 'Speaker' key).
//
// An endorsement endorses either a host key, in which case it is an attestation,
// or the root hardware key, in which case it is certificate.
// This function also checks that each host or root hardware encoutered along this endorsement
// chain is allowed as per domain policy. In particular the policy should allow the predicates
// Authorized(HostTaoName, "Host") and Authorized(EncodedMachineInformation, "Root")
//
// A valid attestation chain must either end in a attestation signed by the policy key
// or a certificate signed by one of the rootCerts.
//
// If all above checks go through, the function returns the principals: Speaker, Key, Program.
func VerifyHostAttestation(serializedHostAttestation []byte, domain *tao.Domain,
rootCerts *x509.CertPool) (*auth.Prin, *auth.Prin, *auth.Prin, error) {
var hostAttestation tao.Attestation
err := proto.Unmarshal(serializedHostAttestation, &hostAttestation)
if err != nil {
return nil, nil, nil, errors.New(
"domain_service: error deserialiaizng host attestation: " + err.Error())
}
// First check if attestation is valid.
statement, err := hostAttestation.Validate()
if err != nil {
return nil, nil, nil, errors.New(
"host attestation fails validation check: " + err.Error())
}
// Next, check if SpeaksFor delegator is authorized to execute (i.e. the program is allowed to
// run as per policy).
speaker, key, prog, err := parseSaysStatement(&statement)
if err != nil {
return nil, nil, nil, err
}
if !domain.Guard.IsAuthorized(*prog, "Execute", []string{}) {
return nil, nil, nil, errors.New(
"program not authorized to run in this domain")
}
// Look for endorsement cert(s), rooted in the policy key, that ultimately certify the
// key of the signer.
signingKey := hostAttestation.GetSignerKey()
if hostAttestation.SignerType == nil {
return nil, nil, nil, errors.New("host attestation missing SignerType field")
}
signingPrin := auth.NewPrin(*hostAttestation.SignerType, signingKey)
if !speaker.Identical(signingPrin) {
// TODO: endorsement endorses speaker or signer?
}
// Look for endorsement(s) of signer, rooted in policy key.
serializedEndorsements := hostAttestation.GetSerializedEndorsements()
var realErr error
var kPrin *auth.Prin
kPrin = &signingPrin
for _, serializedEndorsement := range serializedEndorsements {
// serializedEndorsement could be X.509 certificate or Tao attestation.
var attestation tao.Attestation
err := proto.Unmarshal(serializedEndorsement, &attestation)
if err == nil {
kPrin, realErr = validateEndorsementAttestation(&attestation, domain.Guard, kPrin)
if realErr != nil {
return nil, nil, nil, realErr
}
} else if cert, err1 := x509.ParseCertificate(serializedEndorsement); err1 == nil {
realErr = validateEndorsementCertificate(cert, domain.Guard, kPrin, rootCerts)
if realErr != nil {
return nil, nil, nil, realErr
} else {
// Endorsement certs are the root of the endorsement chain.
// If they are valid, then no more checking is required.
return speaker, key, prog, nil
}
} else {
return nil, nil, nil, errors.New("error parsing host endorsement")
}
}
if domain.Keys.SigningKey.ToPrincipal().Identical(*kPrin) {
return speaker, key, prog, nil
}
return nil, nil, nil, errors.New("endorsement chain does not terminate in policy key")
}
