feat: JWT bearer assertions for client authentication (#566)

Co-authored-by: Kyle Mendell <ksm@ofkm.us>
Co-authored-by: Kyle Mendell <kmendell@ofkm.us>
Co-authored-by: Elias Schneider <login@eliasschneider.com>

backend/internal/utils/jwk_util.go

@@ -0,0 +1,69 @@
+package utils
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"io"
+
+	"github.com/lestrrat-go/jwx/v3/jwa"
+	"github.com/lestrrat-go/jwx/v3/jwk"
+)
+
+const (
+	// KeyUsageSigning is the usage for the private keys, for the "use" property
+	KeyUsageSigning = "sig"
+)
+
+// ImportRawKey imports a crypto key in "raw" format (e.g. crypto.PrivateKey) into a jwk.Key.
+// It also populates additional fields such as the key ID, usage, and alg.
+func ImportRawKey(rawKey any) (jwk.Key, error) {
+	key, err := jwk.Import(rawKey)
+	if err != nil {
+		return nil, fmt.Errorf("failed to import generated private key: %w", err)
+	}
+
+	// Generate the key ID
+	kid, err := generateRandomKeyID()
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate key ID: %w", err)
+	}
+	_ = key.Set(jwk.KeyIDKey, kid)
+
+	// Set other required fields
+	_ = key.Set(jwk.KeyUsageKey, KeyUsageSigning)
+	EnsureAlgInKey(key)
+
+	return key, nil
+}
+
+// generateRandomKeyID generates a random key ID.
+func generateRandomKeyID() (string, error) {
+	buf := make([]byte, 8)
+	_, err := io.ReadFull(rand.Reader, buf)
+	if err != nil {
+		return "", fmt.Errorf("failed to read random bytes: %w", err)
+	}
+	return base64.RawURLEncoding.EncodeToString(buf), nil
+}
+
+// EnsureAlgInKey ensures that the key contains an "alg" parameter, set depending on the key type
+func EnsureAlgInKey(key jwk.Key) {
+	_, ok := key.Algorithm()
+	if ok {
+		// Algorithm is already set
+		return
+	}
+
+	switch key.KeyType() {
+	case jwa.RSA():
+		// Default to RS256 for RSA keys
+		_ = key.Set(jwk.AlgorithmKey, jwa.RS256())
+	case jwa.EC():
+		// Default to ES256 for ECDSA keys
+		_ = key.Set(jwk.AlgorithmKey, jwa.ES256())
+	case jwa.OKP():
+		// Default to EdDSA for OKP keys
+		_ = key.Set(jwk.AlgorithmKey, jwa.EdDSA())
+	}
+}