add keychain and cert store support

client/internal/certproof/keystore.go

@@ -0,0 +1,77 @@
+package certproof
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/asn1"
+	"errors"
+	"fmt"
+	"math/big"
+	"slices"
+)
+
+var errUnsupportedScheme = errors.New("unsupported signature scheme for OS keystore")
+
+type sigScheme int
+
+const (
+	schemeECDSASHA256 sigScheme = iota + 1
+	schemeECDSASHA384
+	schemeRSAPSSSHA256
+)
+
+// schemeFor maps a crypto.Signer request onto the schemes the OS keystores perform.
+func schemeFor(pub crypto.PublicKey, opts crypto.SignerOpts) (sigScheme, error) {
+	switch pub.(type) {
+	case *ecdsa.PublicKey:
+		switch opts.HashFunc() {
+		case crypto.SHA256:
+			return schemeECDSASHA256, nil
+		case crypto.SHA384:
+			return schemeECDSASHA384, nil
+		}
+	case *rsa.PublicKey:
+		if pss, ok := opts.(*rsa.PSSOptions); ok && pss.Hash == crypto.SHA256 {
+			return schemeRSAPSSSHA256, nil
+		}
+	}
+	return 0, fmt.Errorf("%w: %T with %v", errUnsupportedScheme, pub, opts.HashFunc())
+}
+
+// buildChain extends leaf with the issuers found in pool up to a self-signed certificate.
+func buildChain(leaf *x509.Certificate, pool []*x509.Certificate) []*x509.Certificate {
+	chain := []*x509.Certificate{leaf}
+	current := leaf
+	for current.CheckSignatureFrom(current) != nil {
+		issuer := issuerIn(current, pool, chain)
+		if issuer == nil {
+			break
+		}
+		chain = append(chain, issuer)
+		current = issuer
+	}
+	return chain
+}
+
+func issuerIn(cert *x509.Certificate, pool, seen []*x509.Certificate) *x509.Certificate {
+	for _, candidate := range pool {
+		if slices.ContainsFunc(seen, candidate.Equal) {
+			continue
+		}
+		if cert.CheckSignatureFrom(candidate) == nil {
+			return candidate
+		}
+	}
+	return nil
+}
+
+// ecdsaSignatureASN1 converts the fixed-width r||s form emitted by CNG into the DER form Go verifies.
+func ecdsaSignatureASN1(raw []byte) ([]byte, error) {
+	if len(raw) == 0 || len(raw)%2 != 0 {
+		return nil, errors.New("malformed raw ECDSA signature")
+	}
+	half := len(raw) / 2
+	return asn1.Marshal(struct{ R, S *big.Int }{new(big.Int).SetBytes(raw[:half]), new(big.Int).SetBytes(raw[half:])})
+}