phase 3

ui/core/canon/signature.go

@@ -0,0 +1,51 @@
+package canon
+
+import (
+	"crypto/ed25519"
+	"encoding/base64"
+	"errors"
+)
+
+var (
+	// ErrInvalidClientPublicKey reports that the provided client public key is
+	// not a base64-encoded raw Ed25519 public key.
+	ErrInvalidClientPublicKey = errors.New("client_public_key is not a valid base64-encoded Ed25519 public key")
+
+	// ErrInvalidRequestSignature reports that a request signature is not a raw
+	// Ed25519 signature for the canonical request signing input.
+	ErrInvalidRequestSignature = errors.New("invalid request signature")
+)
+
+// VerifyRequestSignature decodes the base64-encoded raw Ed25519 client public
+// key, builds the canonical v1 signing input from fields, and verifies that
+// signature authenticates the request.
+//
+// The base64 string form matches how a backend hands the client public key
+// back to the gateway after device-session resolution
+// (see docs/ARCHITECTURE.md §15).
+func VerifyRequestSignature(clientPublicKey string, signature []byte, fields RequestSigningFields) error {
+	publicKey, err := decodeClientPublicKey(clientPublicKey)
+	if err != nil {
+		return err
+	}
+	if len(signature) != ed25519.SignatureSize {
+		return ErrInvalidRequestSignature
+	}
+	if !ed25519.Verify(publicKey, BuildRequestSigningInput(fields), signature) {
+		return ErrInvalidRequestSignature
+	}
+
+	return nil
+}
+
+func decodeClientPublicKey(value string) (ed25519.PublicKey, error) {
+	decoded, err := base64.StdEncoding.Strict().DecodeString(value)
+	if err != nil {
+		return nil, ErrInvalidClientPublicKey
+	}
+	if len(decoded) != ed25519.PublicKeySize {
+		return nil, ErrInvalidClientPublicKey
+	}
+
+	return ed25519.PublicKey(decoded), nil
+}