Add credential authenticator interface

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

registry/auth/auth.go

@@ -33,6 +33,7 @@
 package auth
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -49,6 +50,14 @@ const (
 	UserNameKey = "auth.user.name"
 )
 
+var (
+	// ErrInvalidCredential is returned when the auth token does not authenticate correctly.
+	ErrInvalidCredential = errors.New("invalid authorization credential")
+
+	// ErrAuthenticationFailure returned when authentication failure to be presented to agent.
+	ErrAuthenticationFailure = errors.New("authentication failure")
+)
+
 // UserInfo carries information about
 // an autenticated/authorized client.
 type UserInfo struct {
@@ -97,6 +106,11 @@ type AccessController interface {
 	Authorized(ctx context.Context, access ...Access) (context.Context, error)
 }
 
+// CredentialAuthenticator is an object which is able to validate credentials
+type CredentialAuthenticator interface {
+	AuthenticateUser(username, password string) error
+}
+
 // WithUser returns a context with the authorized user info.
 func WithUser(ctx context.Context, user UserInfo) context.Context {
 	return userInfoContext{