func (a *BasicAuth) RequireAuth(w http.ResponseWriter, r *http.Request) {
	w.Header().Set(contentType, a.Headers.V().UnauthContentType)
	w.Header().Set(a.Headers.V().Authenticate, `Basic realm="`+a.Realm+`"`)
	w.WriteHeader(a.Headers.V().UnauthCode)
	w.Write([]byte(a.Headers.V().UnauthResponse))
}
/*
http.Handler for BasicAuth which initiates the authentication process
(or requires reauthentication).
package auth
import (
	"bytes"
	"crypto/sha1"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"net/http"
	"strings"
	"golang.org/x/crypto/bcrypt"
	"golang.org/x/net/context"
)
type compareFunc func(hashedPassword, password []byte) error
var (
	errMismatchedHashAndPassword = errors.New("mismatched hash and password")
	compareFuncs = []struct {
		prefix  string
		compare compareFunc
	}{
		{"", compareMD5HashAndPassword},  
		{"{SHA}", compareShaHashAndPassword},
		{"$2a$", bcrypt.CompareHashAndPassword},
		{"$2b$", bcrypt.CompareHashAndPassword},
		{"$2x$", bcrypt.CompareHashAndPassword},
		{"$2y$", bcrypt.CompareHashAndPassword},
	}
)
type BasicAuth struct {
	Realm   string
	Secrets SecretProvider
	Headers *Headers
}
var _ = (AuthenticatorInterface)((*BasicAuth)(nil))
func (a *BasicAuth) CheckAuth(r *http.Request) string {
	s := strings.SplitN(r.Header.Get(a.Headers.V().Authorization), " ", 2)
	if len(s) != 2 || s[0] != "Basic" {
		return ""
	}
	b, err := base64.StdEncoding.DecodeString(s[1])
	if err != nil {
		return ""
	}
	pair := strings.SplitN(string(b), ":", 2)
	if len(pair) != 2 {
		return ""
	}
	user, password := pair[0], pair[1]
	secret := a.Secrets(user, a.Realm)
	if secret == "" {
		return ""
	}
	compare := compareFuncs[0].compare
	for _, cmp := range compareFuncs[1:] {
		if strings.HasPrefix(secret, cmp.prefix) {
			compare = cmp.compare
			break
		}
	}
	if compare([]byte(secret), []byte(password)) != nil {
		return ""
	}
	return pair[0]
}
func compareShaHashAndPassword(hashedPassword, password []byte) error {
	d := sha1.New()
	d.Write(password)
	if subtle.ConstantTimeCompare(hashedPassword[5:], []byte(base64.StdEncoding.EncodeToString(d.Sum(nil)))) != 1 {
		return errMismatchedHashAndPassword
	}
	return nil
}
func compareMD5HashAndPassword(hashedPassword, password []byte) error {
	parts := bytes.SplitN(hashedPassword, []byte("$"), 4)
	if len(parts) != 4 {
		return errMismatchedHashAndPassword
	}
	magic := []byte("$" + string(parts[1]) + "$")
	salt := parts[2]
	if subtle.ConstantTimeCompare(hashedPassword, MD5Crypt(password, salt, magic)) != 1 {
		return errMismatchedHashAndPassword
	}
	return nil
}