// 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")
}
