Support ECDSA based AK's on Windows (#415)

attest/activation.go

@@ -3,6 +3,7 @@ package attest
 import (
 	"bytes"
 	"crypto"
+	"crypto/ecdsa"
 	"crypto/rand"
 	"crypto/rsa"
 	"errors"
@@ -20,6 +21,8 @@ import (
 const (
 	// minRSABits is the minimum accepted bit size of an RSA key.
 	minRSABits = 2048
+	// minECCBits is the minimum accepted bit size of an ECC key.
+	minECCBits = 256
 	// activationSecretLen is the size in bytes of the generated secret
 	// which is generated for credential activation.
 	activationSecretLen = 32
@@ -115,6 +118,12 @@ func (p *ActivationParameters) checkTPM20AKParameters() error {
 		if pub.RSAParameters.KeyBits < minRSABits {
 			return fmt.Errorf("attestation key too small: must be at least %d bits but was %d bits", minRSABits, pub.RSAParameters.KeyBits)
 		}
+	case tpm2.AlgECC:
+		if len(pub.ECCParameters.Point.XRaw)*8 < minECCBits {
+			return fmt.Errorf("attestation key too small: must be at least %d bits but was %d bits", minECCBits, len(pub.ECCParameters.Point.XRaw)*8)
+		} else if len(pub.ECCParameters.Point.YRaw)*8 < minECCBits {
+			return fmt.Errorf("attestation key too small: must be at least %d bits but was %d bits", minECCBits, len(pub.ECCParameters.Point.YRaw)*8)
+		}
 	default:
 		return fmt.Errorf("public key of alg 0x%x not supported", pub.Type)
 	}
@@ -160,6 +169,17 @@ func (p *ActivationParameters) checkTPM20AKParameters() error {
 	}
 
 	// Check the signature over the attestation data verifies correctly.
+	switch pub.Type {
+	case tpm2.AlgRSA:
+		return verifyRSASignature(pub, p)
+	case tpm2.AlgECC:
+		return verifyECDSASignature(pub, p)
+	default:
+		return fmt.Errorf("public key of alg 0x%x not supported", pub.Type)
+	}
+}
+
+func verifyRSASignature(pub tpm2.Public, p *ActivationParameters) error {
 	pk := rsa.PublicKey{E: int(pub.RSAParameters.Exponent()), N: pub.RSAParameters.Modulus()}
 	signHash, err := pub.RSAParameters.Sign.Hash.Hash()
 	if err != nil {
@@ -188,6 +208,40 @@ func (p *ActivationParameters) checkTPM20AKParameters() error {
 	return nil
 }
 
+func verifyECDSASignature(pub tpm2.Public, p *ActivationParameters) error {
+	key, err := pub.Key()
+	if err != nil {
+		return nil
+	}
+	pk, ok := key.(*ecdsa.PublicKey)
+	if !ok {
+		return fmt.Errorf("expected *ecdsa.PublicKey, got %T", key)
+	}
+	signHash, err := pub.ECCParameters.Sign.Hash.Hash()
+	if err != nil {
+		return err
+	}
+	hsh := signHash.New()
+	_, err = hsh.Write(p.AK.CreateAttestation)
+	if err != nil {
+		return err
+	}
+
+	if len(p.AK.CreateSignature) < 8 {
+		return fmt.Errorf("signature invalid: length of %d is shorter than 8", len(p.AK.CreateSignature))
+	}
+
+	sig, err := tpm2.DecodeSignature(bytes.NewBuffer(p.AK.CreateSignature))
+	if err != nil {
+		return fmt.Errorf("DecodeSignature() failed: %v", err)
+	}
+
+	if !ecdsa.Verify(pk, hsh.Sum(nil), sig.ECC.R, sig.ECC.S) {
+		return fmt.Errorf("unable to verify attestation for ecdsa credential")
+	}
+	return nil
+}
+
 // Generate returns a credential activation challenge, which can be provided
 // to the TPM to verify the AK parameters given are authentic & the AK
 // is present on the same TPM as the EK.

attest/activation_test.go

@@ -67,3 +67,33 @@ func TestActivationTPM20(t *testing.T) {
 		t.Fatalf("secret = %v, want %v", got, want)
 	}
 }
+
+func TestECCActivationTPM20(t *testing.T) {
+	priv := ekCertSigner(t)
+	rand := rand.New(rand.NewSource(123456))
+
+	// These parameters represent an ECC P256 AK generated on a real-world,
+	// Google vTPM.
+	params := ActivationParameters{
+		TPMVersion: TPMVersion20,
+		AK: AttestationParameters{
+			Public:            decodeBase64("ACMACwAFBHIAIJ3/y/NsODrmmfuYaNxty4nXFTiEvigDkiwSQVi/rSKuABAAGAAEAAMAEAAgydcfFWW6O7PjW9vnOE4IDx3545TxylD1iVHP8MIFI78AIJuD/QM9EbqM+3SEl7PgiXlWV1NhmvnmE2AHiEfI/hUn", t),
+			CreateData:        decodeBase64("AAAAAAAg47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFUBAAsAIgALLEyPGewwEIKqPNw9Lx7QXsfp0MsOZFt4EzHFT4tSXekAIgALf7O+OxqTNzTuIhi1YGQoulPZoyxsNKHpBgT4dHqT3+UAAA==", t),
+			CreateAttestation: decodeBase64("/1RDR4AaACIAC8Uqr5OAkfcyLJ2gLU2oSvIoPi8XdoLHGdpS5h/JJz8aAAAAAAAEXezQqarG49k44bBnAUna81DZr1xhACIAC3AoqIzDrusUtdH3uAwqbqUrybtu35H1XPQDyLBHVGF+ACCKEKpwL1LFbw/IG+vtJ6CtXHIYBhVWyrkAqYLkHleYMw==", t),
+			CreateSignature:   decodeBase64("ABgABAAgbTEBcZvjb9uEEYZCiSqPh/XO0BZQS+egnJ8WKtpSbmkAIDNgvF9iyiOCvd5480hOCzjRj7GP3YZ3XqjEVvN3Q3Ca", t),
+		},
+		EK: &rsa.PublicKey{
+			E: priv.E,
+			N: priv.N,
+		},
+		Rand: rand,
+	}
+
+	secret, _, err := params.Generate()
+	if err != nil {
+		t.Fatalf("Generate() returned err: %v", err)
+	}
+	if got, want := secret, decodeBase64("0vhS7HtORX9uf/iyQ8Sf9WkpJuoJ1olCfTjSZuyNNxY=", t); !bytes.Equal(got, want) {
+		t.Fatalf("secret = %v, want %v", got, want)
+	}
+}

attest/application_key.go

@@ -63,6 +63,13 @@ type Algorithm string
 const (
 	ECDSA Algorithm = "ECDSA"
 	RSA   Algorithm = "RSA"
+
+	// Windows specific ECDSA CNG algorithm identifiers.
+	// NOTE: Using ECDSA will default to ECDSA_P256.
+	// Ref: https://learn.microsoft.com/en-us/windows/win32/SecCNG/cng-algorithm-identifiers
+	ECDSA_P256 Algorithm = "ECDSA_P256"
+	ECDSA_P384 Algorithm = "ECDSA_P384"
+	ECDSA_P521 Algorithm = "ECDSA_P521"
 )
 
 // KeyConfig encapsulates parameters for minting keys.
@@ -88,6 +95,24 @@ var defaultConfig = &KeyConfig{
 	Size:      256,
 }
 
+// Size returns the bit size associated with an algorithm.
+func (a Algorithm) Size() int {
+	switch a {
+	case RSA:
+		return 2048
+	case ECDSA:
+		return 256
+	case ECDSA_P256:
+		return 256
+	case ECDSA_P384:
+		return 384
+	case ECDSA_P521:
+		return 521
+	default:
+		return 0
+	}
+}
+
 // Public returns the public key corresponding to the private key.
 func (k *Key) Public() crypto.PublicKey {
 	return k.pub

attest/attest.go

@@ -190,6 +190,8 @@ func (k *AK) Certify(tpm *TPM, handle interface{}) (*CertificationParameters, er
 
 // AKConfig encapsulates parameters for minting keys.
 type AKConfig struct {
+	// Optionally set unique name for AK on Windows.
+	Name string
 	// Parent describes the Storage Root Key that will be used as a parent.
 	// If nil, the default SRK (i.e. RSA with handle 0x81000001) is assumed.
 	// Supported only by TPM 2.0 on Linux.

attest/attest_test.go

@@ -131,12 +131,13 @@ func TestAKCreateAndLoad(t *testing.T) {
 			}
 			defer loaded.Close(tpm)
 
-			k1, k2 := ak.ak.(*wrappedKey20), loaded.ak.(*wrappedKey20)
+			k1 := ak.ak.attestationParameters()
+			k2 := loaded.ak.attestationParameters()
 
-			if !bytes.Equal(k1.public, k2.public) {
+			if !bytes.Equal(k1.Public, k2.Public) {
 				t.Error("Original & loaded AK public blobs did not match.")
-				t.Logf("Original = %v", k1.public)
-				t.Logf("Loaded   = %v", k2.public)
+				t.Logf("Original = %v", k1.Public)
+				t.Logf("Loaded   = %v", k2.Public)
 			}
 		})
 	}

attest/pcp_windows.go

@@ -453,19 +453,19 @@ func getPCPCerts(hProv uintptr, propertyName string) ([][]byte, error) {
 }
 
 // NewAK creates a persistent attestation key of the specified name.
-func (h *winPCP) NewAK(name string) (uintptr, error) {
+func (h *winPCP) NewAK(name string, alg Algorithm) (uintptr, error) {
 	var kh uintptr
 	utf16Name, err := windows.UTF16FromString(name)
 	if err != nil {
 		return 0, err
 	}
-	utf16RSA, err := windows.UTF16FromString("RSA")
+	utf16Alg, err := windows.UTF16FromString(string(alg))
 	if err != nil {
 		return 0, err
 	}
 
 	// Create a persistent RSA key of the specified name.
-	r, _, msg := nCryptCreatePersistedKey.Call(h.hProv, uintptr(unsafe.Pointer(&kh)), uintptr(unsafe.Pointer(&utf16RSA[0])), uintptr(unsafe.Pointer(&utf16Name[0])), 0, 0)
+	r, _, msg := nCryptCreatePersistedKey.Call(h.hProv, uintptr(unsafe.Pointer(&kh)), uintptr(unsafe.Pointer(&utf16Alg[0])), uintptr(unsafe.Pointer(&utf16Name[0])), 0, 0)
 	if r != 0 {
 		if tpmErr := maybeWinErr(r); tpmErr != nil {
 			msg = tpmErr
@@ -477,7 +477,7 @@ func (h *winPCP) NewAK(name string) (uintptr, error) {
 	if err != nil {
 		return 0, err
 	}
-	var length uint32 = 2048
+	var length uint32 = uint32(alg.Size())
 	r, _, msg = nCryptSetProperty.Call(kh, uintptr(unsafe.Pointer(&utf16Length[0])), uintptr(unsafe.Pointer(&length)), unsafe.Sizeof(length), 0)
 	if r != 0 {
 		if tpmErr := maybeWinErr(r); tpmErr != nil {

attest/tpm_windows.go

@@ -287,13 +287,26 @@ func decryptCredential(secretKey, blob []byte) ([]byte, error) {
 }
 
 func (t *windowsTPM) newAK(opts *AKConfig) (*AK, error) {
-	nameHex := make([]byte, 5)
-	if n, err := rand.Read(nameHex); err != nil || n != len(nameHex) {
-		return nil, fmt.Errorf("rand.Read() failed with %d/%d bytes read and error: %v", n, len(nameHex), err)
-	}
-	name := fmt.Sprintf("ak-%x", nameHex)
+	var name string
+	var alg Algorithm
 
-	kh, err := t.pcp.NewAK(name)
+	if opts != nil && opts.Name != "" {
+		name = opts.Name
+	} else {
+		nameHex := make([]byte, 5)
+		if n, err := rand.Read(nameHex); err != nil || n != len(nameHex) {
+			return nil, fmt.Errorf("rand.Read() failed with %d/%d bytes read and error: %v", n, len(nameHex), err)
+		}
+		name = fmt.Sprintf("ak-%x", nameHex)
+	}
+	if opts != nil && opts.Algorithm != "" {
+		alg = opts.Algorithm
+	} else {
+		// Default to RSA based AK.
+		alg = RSA
+	}
+
+	kh, err := t.pcp.NewAK(name, alg)
 	if err != nil {
 		return nil, fmt.Errorf("pcp failed to mint attestation key: %v", err)
 	}