attest: ActivateCredentialWithEK() method that can be used with non-default EKs. (#340)

attest/attest.go

@@ -131,13 +131,24 @@ func (k *AK) Marshal() ([]byte, error) {
 }
 
 // ActivateCredential decrypts the secret using the key to prove that the AK
-// was generated on the same TPM as the EK.
+// was generated on the same TPM as the EK. This method can be used with TPMs
+// that have the default EK, i.e. RSA EK with handle 0x81010001.
 //
 // This operation is synonymous with TPM2_ActivateCredential.
 func (k *AK) ActivateCredential(tpm *TPM, in EncryptedCredential) (secret []byte, err error) {
 	return k.ak.activateCredential(tpm.tpm, in, k.ek)
 }
 
+// ActivateCredential decrypts the secret using the key to prove that the AK
+// was generated on the same TPM as the EK. This method can be used with TPMs
+// that have an ECC EK. The 'ek' argument must be one of EKs returned from
+// TPM.EKs() or TPM.EKCertificates().
+//
+// This operation is synonymous with TPM2_ActivateCredential.
+func (k *AK) ActivateCredentialWithEK(tpm *TPM, in EncryptedCredential, ek EK) (secret []byte, err error) {
+	return k.ak.activateCredential(tpm.tpm, in, &ek)
+}
+
 // Quote returns a quote over the platform state, signed by the AK.
 //
 // This is a low-level API. Consumers seeking to attest the state of the

attest/attest_simulated_tpm20_test.go

@@ -115,11 +115,7 @@ func testActivateCredential(t *testing.T, useEK bool) {
 	}
 	ek := chooseEK(t, EKs)
 
-	var akConfig *AKConfig
-	if useEK {
-		akConfig = &AKConfig{EK: &ek}
-	}
-	ak, err := tpm.NewAK(akConfig)
+	ak, err := tpm.NewAK(nil)
 	if err != nil {
 		t.Fatalf("NewAK() failed: %v", err)
 	}
@@ -135,7 +131,12 @@ func testActivateCredential(t *testing.T, useEK bool) {
 		t.Fatalf("Generate() failed: %v", err)
 	}
 
-	decryptedSecret, err := ak.ActivateCredential(tpm, *challenge)
+	var decryptedSecret []byte
+	if useEK {
+		decryptedSecret, err = ak.ActivateCredentialWithEK(tpm, *challenge, ek)
+	} else {
+		decryptedSecret, err = ak.ActivateCredential(tpm, *challenge)
+	}
 	if err != nil {
 		t.Errorf("ak.ActivateCredential() failed: %v", err)
 	}

attest/example_test.go

@@ -91,12 +91,62 @@ func ExampleAK_credentialActivation() {
 	}
 }
 
+func ExampleAK_credentialActivationWithEK() {
+	tpm, err := attest.OpenTPM(nil)
+	if err != nil {
+		log.Fatalf("Failed to open TPM: %v", err)
+	}
+	defer tpm.Close()
+
+	// Create a new AK.
+	ak, err := tpm.NewAK(nil)
+	if err != nil {
+		log.Fatalf("Failed to create AK: %v", err)
+	}
+	defer ak.Close(tpm)
+
+	// Read the EK certificates.
+	ekCerts, err := tpm.EKCertificates()
+	if err != nil {
+		log.Fatalf("Failed to enumerate EKs: %v", err)
+	}
+
+	// Read parameters necessary to generate a challenge.
+	ap := ak.AttestationParameters()
+
+	// Try activating with each EK certificate.
+	for _, ek := range ekCerts {
+		// Generate a credential activation challenge (usually done on the server).
+		activation := attest.ActivationParameters{
+			TPMVersion: tpm.Version(),
+			EK:         ek.Public,
+			AK:         ap,
+		}
+		secret, challenge, err := activation.Generate()
+		if err != nil {
+			log.Fatalf("Failed to generate activation challenge: %v", err)
+		}
+
+		// Challenge the AK & EK properties to recieve the decrypted secret.
+		decrypted, err := ak.ActivateCredentialWithEK(tpm, *challenge, ek)
+		if err != nil {
+			log.Fatalf("Failed to activate credential: %v", err)
+		}
+
+		// Check that the AK completed the challenge (usually done on the server).
+		if subtle.ConstantTimeCompare(secret, decrypted) == 0 {
+			log.Fatal("Activation response did not match secret")
+		}
+	}
+}
+
 func TestExampleAK(t *testing.T) {
 	if !*testExamples {
 		t.SkipNow()
 	}
 	ExampleAK()
 	ExampleAK_credentialActivation()
+	ExampleAK_credentialActivationWithEK()
 }
 
 func TestExampleTPM(t *testing.T) {