Explicitly set scheme for ECDSA signing

My system is returning RCScheme if TPM_ALG_NULL is passed here. This should
be causing the key's default scheme to be used, but for some reason it
seems unhappy. Just explicitly set the scheme for now to avoid that.

attest/key_windows.go

@@ -270,7 +270,7 @@ func (k *windowsAK20) sign(tb tpmBase, digest []byte, pub crypto.PublicKey, opts
 
 	switch p := pub.(type) {
 	case *ecdsa.PublicKey:
-		return signECDSA(rw, hnd, digest, p.Curve)
+		return signECDSA(rw, hnd, digest, p.Curve, opts)
 	case *rsa.PublicKey:
 		return signRSA(rw, hnd, digest, opts)
 	}

attest/wrapped_tpm20.go

@@ -503,14 +503,24 @@ func (k *wrappedKey20) sign(tb tpmBase, digest []byte, pub crypto.PublicKey, opt
 	}
 	switch p := pub.(type) {
 	case *ecdsa.PublicKey:
-		return signECDSA(t.rwc, k.hnd, digest, p.Curve)
+		return signECDSA(t.rwc, k.hnd, digest, p.Curve, opts)
 	case *rsa.PublicKey:
 		return signRSA(t.rwc, k.hnd, digest, opts)
 	}
 	return nil, fmt.Errorf("unsupported signing key type: %T", pub)
 }
 
-func signECDSA(rw io.ReadWriter, key tpmutil.Handle, digest []byte, curve elliptic.Curve) ([]byte, error) {
+func signECDSA(rw io.ReadWriter, key tpmutil.Handle, digest []byte, curve elliptic.Curve, opts crypto.SignerOpts) ([]byte, error) {
+	h, err := tpm2.HashToAlgorithm(opts.HashFunc())
+	if err != nil {
+		return nil, fmt.Errorf("incorrect hash algorithm: %v", err)
+	}
+
+	scheme := &tpm2.SigScheme{
+		Alg:  tpm2.AlgECDSA,
+		Hash: h,
+	}
+
 	// https://cs.opensource.google/go/go/+/refs/tags/go1.19.2:src/crypto/ecdsa/ecdsa.go;l=181
 	orderBits := curve.Params().N.BitLen()
 	orderBytes := (orderBits + 7) / 8
@@ -524,7 +534,7 @@ func signECDSA(rw io.ReadWriter, key tpmutil.Handle, digest []byte, curve ellipt
 	}
 	digest = ret.Bytes()
 
-	sig, err := tpm2.Sign(rw, key, "", digest, nil, nil)
+	sig, err := tpm2.Sign(rw, key, "", digest, nil, scheme)
 	if err != nil {
 		return nil, fmt.Errorf("cannot sign: %v", err)
 	}