From beedb2de174086f6772fe776322caa43b1147a69 Mon Sep 17 00:00:00 2001
From: Matthew Garrett <mgarrett@aurora.tech>
Date: Tue, 23 May 2023 13:20:59 -0700
Subject: [PATCH] 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   |  2 +-
 attest/wrapped_tpm20.go | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/attest/key_windows.go b/attest/key_windows.go
index d7b62c0a..3a3f6b2e 100644
--- a/attest/key_windows.go
+++ b/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)
 	}
diff --git a/attest/wrapped_tpm20.go b/attest/wrapped_tpm20.go
index 5d334f89..ceff6345 100644
--- a/attest/wrapped_tpm20.go
+++ b/attest/wrapped_tpm20.go
@@ -503,14 +503,28 @@ 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) {
+	var scheme *tpm2.SigScheme
+
+	if opts != nil {
+		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 +538,7 @@ 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)
 	}