// ValidSigner checks the signature on an attestation and, if so, returns the signer.
func (a *Attestation) ValidSigner() (auth.Prin, error) {
signer, err := auth.UnmarshalPrin(a.Signer)
if err != nil {
return auth.Prin{}, err
}
if len(signer.Ext) != 0 {
return auth.Prin{}, newError("tao: attestation signer principal malformed: %s", signer)
}
switch signer.Type {
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 PCRs from the signer: %s", err)
}
pk, err := ExtractAIK(speaker)
if err != nil {
return auth.Prin{}, newError("tao: couldn't extract the AIK from the signer: %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 "key":
// Signer is ECDSA key, use Tao signature verification.
v, err := FromPrincipal(signer)
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())
}
}
// 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())
}
}