zmq4: evolve Security interface to support PLAIN and CURVE

Updates #25.
Updates #26.

conn.go

@@ -80,7 +80,7 @@ func (conn *Conn) init(sec Security, md map[string]string) error {
 		return errors.Wrapf(err, "zmq4: could not exchange greetings")
 	}
 
-	err = conn.sec.Handshake()
+	err = conn.sec.Handshake(conn, conn.server)
 	if err != nil {
 		return errors.Wrapf(err, "zmq4: could not perform security handshake")
 	}

security.go

@@ -10,14 +10,16 @@ import (
 
 // Security is an interface for ZMTP security mechanisms
 type Security interface {
+	// Type returns the security mechanism type.
 	Type() SecurityType
+
 	// Handshake implements the ZMTP security handshake according to
 	// this security mechanism.
 	// see:
 	//  https://rfc.zeromq.org/spec:23/ZMTP/
 	//  https://rfc.zeromq.org/spec:24/ZMTP-PLAIN/
 	//  https://rfc.zeromq.org/spec:25/ZMTP-CURVE/
-	Handshake() error
+	Handshake(conn *Conn, server bool) error
 
 	// Encrypt writes the encrypted form of data to w.
 	Encrypt(w io.Writer, data []byte) (int, error)
@@ -58,7 +60,7 @@ func (nullSecurity) Type() SecurityType {
 //  https://rfc.zeromq.org/spec:23/ZMTP/
 //  https://rfc.zeromq.org/spec:24/ZMTP-PLAIN/
 //  https://rfc.zeromq.org/spec:25/ZMTP-CURVE/
-func (nullSecurity) Handshake() error {
+func (nullSecurity) Handshake(conn *Conn, server bool) error {
 	return nil
 }
 

security/null/null.go

@@ -30,7 +30,7 @@ func (security) Type() zmq4.SecurityType {
 //  https://rfc.zeromq.org/spec:23/ZMTP/
 //  https://rfc.zeromq.org/spec:24/ZMTP-PLAIN/
 //  https://rfc.zeromq.org/spec:25/ZMTP-CURVE/
-func (security) Handshake() error {
+func (security) Handshake(conn *zmq4.Conn, server bool) error {
 	return nil
 }
 

security/null/null_test.go

@@ -12,13 +12,13 @@ import (
 	"github.com/go-zeromq/zmq4/security/null"
 )
 
-func TestNullSecurity(t *testing.T) {
+func TestSecurity(t *testing.T) {
 	sec := null.Security()
 	if got, want := sec.Type(), zmq4.NullSecurity; got != want {
 		t.Fatalf("got=%v, want=%v", got, want)
 	}
 
-	err := sec.Handshake()
+	err := sec.Handshake(nil, false)
 	if err != nil {
 		t.Fatalf("error doing handshake: %v", err)
 	}

security_test.go

@@ -15,7 +15,7 @@ func TestNullSecurity(t *testing.T) {
 		t.Fatalf("got=%v, want=%v", got, want)
 	}
 
-	err := sec.Handshake()
+	err := sec.Handshake(nil, false)
 	if err != nil {
 		t.Fatalf("error doing handshake: %v", err)
 	}