diff --git a/dkim.go b/dkim.go
index 8eb14f0..d98f347 100644
--- a/dkim.go
+++ b/dkim.go
@@ -1 +1,39 @@
 package dkim
+
+import (
+	"bytes"
+	"time"
+)
+
+// sigOptions represents signing options
+type sigOptions struct {
+	PrivateKey       string
+	Domain           string
+	Selector         string
+	Auid             string
+	Canonicalization string
+	Algo             string
+	Headers          []string
+	Timestamp        time.Time
+	Expiration       time.Time
+}
+
+// NewSigOption returns new sigoption with some defaults value
+func NewSigOptions() sigOptions {
+	return sigOptions{
+		Canonicalization: "simple/simple",
+		Algo:             "rsa-sha256",
+	}
+}
+
+// Sign signs an email
+func Sign(email *bytes.Reader, options sigOptions) (*bytes.Reader, error) {
+	// check config
+
+	// private key (not empty & TODO: valid)
+	if options.PrivateKey == "" {
+		return nil, ErrConfigNoPrivateKey
+	}
+
+	return nil, nil
+}