vendor/upspin.io/rpc/server.go - gcp - Git at Google

 // Copyright 2016 The Upspin Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package rpc

 import (
 	"crypto/rand"
 	"crypto/sha256"
 	"encoding/binary"
 	"fmt"
 	"io/ioutil"
 	"math/big"
 	"net/http"
 	"strings"
 	"time"

 	pb "github.com/golang/protobuf/proto"

 	"upspin.io/errors"
 	"upspin.io/factotum"
 	"upspin.io/log"
 	"upspin.io/upspin"
 	"upspin.io/valid"
 )

 // Errors returned in case of various authentication failure scenarios.
 var (
 	errUnauthenticated  = errors.Str("user not authenticated")
 	errExpired          = errors.Str("auth token expired")
 	errMissingSignature = errors.Str("missing or invalid signature")

 	authTokenDuration = 20 * time.Hour // Max duration an auth token lasts.
 )

 const (
 	// authTokenHeader is the key in the context's metadata for the auth token.
 	authTokenHeader = "Upspin-Auth-Token"

 	// authRequestHeader is the key for inline user authentication.
 	authRequestHeader = "Upspin-Auth-Request"

 	// authErrorHeader is the key for inline user authentication errors.
 	authErrorHeader = "Upspin-Auth-Error"

 	// proxyRequestHeader key is for inline proxy configuration requests.
 	proxyRequestHeader = "Upspin-Proxy-Request"

 	// authTokenEntropyLen is the size of random bytes in an auth token.
 	authTokenEntropyLen = 16

 	// clientAuthMagic is a string used in validating the client's user name.
 	clientAuthMagic = " Authenticate "

 	// serverAuthMagic is a string used in validating the server's user name.
 	serverAuthMagic = " AuthenticateServer "
 )

 // Service describes an RPC service.
 type Service struct {
 	// The name of the service, which forms the first path component of any
 	// HTTP request.
 	Name string

 	// The RPC methods to serve.
 	Methods map[string]Method

 	// The RPC methods to serve without validating sessions.
 	UnauthenticatedMethods map[string]UnauthenticatedMethod

 	// The streaming RPC methods to serve.
 	Streams map[string]Stream

 	// Lookup is KeyServer.Lookup function that should be used for key
 	// lookups during authentication.
 	// If nil, PublicUserKeyService will be used.
 	Lookup func(userName upspin.UserName) (upspin.PublicKey, error)
 }

 // Method describes an authenticated RPC method.
 type Method func(s Session, reqBytes []byte) (pb.Message, error)

 // UnauthenticatedMethod describes an RPC method that does not require
 // server-side authentication.
 type UnauthenticatedMethod func(reqBytes []byte) (pb.Message, error)

 // Stream describes an authenticated streaming RPC method.
 type Stream func(s Session, reqBytes []byte, done <-chan struct{}) (<-chan pb.Message, error)

 // NewServer returns a new Server that uses the given ServerConfig.
 func NewServer(cfg upspin.Config, svc Service) http.Handler {
 	// Validate Service.
 	if svc.Name == "" {
 		panic("ServerConfig provided with empty Name")
 	}
 	for name := range svc.Methods {
 		if _, ok := svc.UnauthenticatedMethods[name]; ok {
 			panic(fmt.Sprintf("Method %q also specified as UnauthenticatedMethod", name))
 		}
 		if _, ok := svc.Streams[name]; ok {
 			panic(fmt.Sprintf("Method %q also specified as Stream", name))
 		}
 	}
 	for name := range svc.Streams {
 		if _, ok := svc.UnauthenticatedMethods[name]; ok {
 			panic(fmt.Sprintf("Stream %q also specified as UnauthenticatedMethod", name))
 		}
 	}

 	return &serverImpl{
 		config:  cfg,
 		service: svc,
 	}
 }

 type serverImpl struct {
 	config  upspin.Config
 	service Service
 }

 func (s *serverImpl) lookup(u upspin.UserName) (upspin.PublicKey, error) {
 	if s.service.Lookup == nil {
 		return PublicUserKeyService(s.config)(u)
 	}
 	return s.service.Lookup(u)
 }

 func generateRandomToken() (string, error) {
 	var buf [authTokenEntropyLen]byte
 	n, err := rand.Read(buf[:])
 	if err != nil {
 		return "", err
 	}
 	if n != len(buf) {
 		return "", errors.Str("random bytes too short")
 	}
 	return fmt.Sprintf("%X", buf), nil
 }

 // ServeHTTP exposes the configured Service as an HTTP API.
 func (s *serverImpl) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	d := &s.service
 	prefix := "/api/" + d.Name + "/"
 	if !strings.HasPrefix(r.URL.Path, prefix) {
 		http.NotFound(w, r)
 		return
 	}
 	name := strings.TrimPrefix(r.URL.Path, prefix)

 	method := d.Methods[name]
 	umethod := d.UnauthenticatedMethods[name]
 	stream := d.Streams[name]
 	if method == nil && umethod == nil && stream == nil {
 		http.NotFound(w, r)
 		return
 	}

 	var session Session
 	if umethod == nil {
 		var err error
 		session, err = s.SessionForRequest(w, r)
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusUnauthorized)
 			return
 		}
 	}

 	body, err := ioutil.ReadAll(r.Body)
 	r.Body.Close()
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}

 	switch {
 	case method != nil:
 		resp, err := method(session, body)
 		sendResponse(w, resp, err)
 	case umethod != nil:
 		resp, err := umethod(body)
 		sendResponse(w, resp, err)
 	case stream != nil:
 		serveStream(stream, session, w, body)
 	default:
 		panic("this should never happen")
 	}
 }

 func sendResponse(w http.ResponseWriter, resp pb.Message, err error) {
 	if err != nil {
 		sendError(w, err)
 		return
 	}
 	payload, err := pb.Marshal(resp)
 	if err != nil {
 		log.Error.Printf("error encoding response: %v", err)
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 	w.Write(payload)
 }

 func sendError(w http.ResponseWriter, err error) {
 	h := w.Header()
 	h.Set("Content-type", "application/octet-stream")
 	w.WriteHeader(http.StatusInternalServerError)
 	w.Write(errors.MarshalError(err))
 }

 func serveStream(s Stream, sess Session, w http.ResponseWriter, body []byte) {
 	done := make(chan struct{})
 	msgs, err := s(sess, body, done)
 	if err != nil {
 		sendError(w, err)
 		return
 	}

 	connClosed := w.(http.CloseNotifier).CloseNotify()
 	go func() {
 		<-connClosed
 		close(done)
 	}()

 	// Write the headers, beginning the stream.
 	w.Write([]byte("OK"))
 	w.(http.Flusher).Flush()

 	var lenBytes [4]byte // stores a uint32, the length of each output message
 	for {
 		select {
 		case msg, ok := <-msgs:
 			if !ok {
 				return
 			}
 			if done == nil {
 				// Drop this message as there's nobody to deliver to.
 				continue
 			}

 			b, err := pb.Marshal(msg)
 			if err != nil {
 				log.Error.Printf("rpc/auth: error encoding proto in stream: %v", err)
 				return
 			}

 			binary.BigEndian.PutUint32(lenBytes[:], uint32(len(b)))
 			if _, err := w.Write(lenBytes[:]); err != nil {
 				return
 			}
 			if _, err := w.Write(b); err != nil {
 				return
 			}
 			w.(http.Flusher).Flush()

 		case <-done:
 			done = nil
 		}
 	}
 }

 func (s *serverImpl) SessionForRequest(w http.ResponseWriter, r *http.Request) (session Session, err error) {
 	const op errors.Op = "rpc.SessionForRequest"

 	defer func() {
 		if err == nil {
 			return
 		}
 		// Capture session setup errors and
 		// send them to the client in the HTTP response Header.
 		w.Header().Set(authErrorHeader, err.Error())
 		// Attach the op to the error here, because the client doesn't
 		// care that this error originated in this function.
 		err = errors.E(op, err)
 	}()

 	if tok, ok := r.Header[authTokenHeader]; ok && len(tok) == 1 {
 		return s.validateToken(tok[0])
 	}

 	proxyRequest, ok := r.Header[proxyRequestHeader]
 	if ok && len(proxyRequest) != 1 {
 		return nil, errors.E(errors.Invalid, "invalid proxy request in header")
 	}

 	// Clients send a single header line with comma-separated values.
 	authRequest, ok := r.Header[authRequestHeader]
 	if !ok {
 		return nil, errors.E(errors.Invalid, "missing auth request header")
 	} else if len(authRequest) == 5 {
 		// Old-style authentication tokens should now fail,
 		// but provide an informative error message when they do.
 		// TODO(adg): Remove this if/else block on April 15.
 		return nil, errors.E(errors.Invalid, "invalid auth request header (please update your Upspin clients and servers)")
 	} else if len(authRequest) != 1 {
 		return nil, errors.E(errors.Invalid, "invalid auth request header")
 	}
 	authRequest = strings.Split(authRequest[0], ",")

 	return s.handleSessionRequest(w, authRequest, proxyRequest, r.Host)
 }

 func (s *serverImpl) validateToken(authToken string) (Session, error) {
 	if len(authToken) < authTokenEntropyLen {
 		return nil, errors.E(errors.Invalid, "invalid auth token")
 	}

 	// Get the session for this authToken
 	session := GetSession(authToken)
 	if session == nil {
 		// We don't know this client or have forgotten about it. We must authenticate.
 		// Log it so we can track how often this happens. Maybe we need to increase the session cache size.
 		log.Debug.Printf("Got token from user but there's no session for it.")
 		return nil, errors.E(errors.Permission, errUnauthenticated)
 	}

 	// If session has expired, forcibly remove it from the cache and return an error.
 	if session.Expires().Before(time.Now()) {
 		ClearSession(authToken)
 		return nil, errors.E(errors.Permission, errExpired)
 	}

 	return session, nil
 }

 func (s *serverImpl) handleSessionRequest(w http.ResponseWriter, authRequest []string, proxyRequest []string, host string) (Session, error) {
 	// Validate the username.
 	user := upspin.UserName(authRequest[0])
 	if err := valid.UserName(user); err != nil {
 		return nil, errors.E(user, err)
 	}

 	// Get user's public key.
 	key, err := s.lookup(user)
 	if err != nil {
 		return nil, errors.E(user, err)
 	}

 	// If this is a proxy request, extract the endpoint and
 	// set the signed host to that endpoint.
 	ep := &upspin.Endpoint{}
 	if len(proxyRequest) == 1 {
 		if pUser := s.config.UserName(); user != pUser {
 			return nil, errors.E(errors.Permission, errors.Errorf("client %q and proxy %q users mismatched", user, pUser))
 		}
 		ep, err = upspin.ParseEndpoint(proxyRequest[0])
 		if err != nil {
 			return nil, errors.E(errors.Invalid, errors.Errorf("invalid proxy endpoint: %v", err))
 		}
 		host = string(ep.NetAddr)
 	}

 	now := time.Now()

 	// Validate signature.
 	if err := verifyUser(key, authRequest, clientAuthMagic, host, now); err != nil {
 		return nil, errors.E(errors.Permission, user, errors.Errorf("invalid signature: %v", err))
 	}

 	// Generate an auth token and bind it to a session for the client.
 	expiration := now.Add(authTokenDuration)
 	authToken, err := generateRandomToken()
 	if err != nil {
 		return nil, err
 	}
 	w.Header().Set(authTokenHeader, authToken)

 	// If there is a proxy request, authenticate server to client.
 	if len(proxyRequest) == 1 {
 		// Authenticate the server to the user.
 		authMsg, err := signUser(s.config, serverAuthMagic, "[localproxy]")
 		if err != nil {
 			return nil, errors.E(errors.Permission, err)
 		}
 		w.Header()[authRequestHeader] = authMsg
 	}

 	return NewSession(user, expiration, authToken, ep, nil), nil
 }

 // verifyUser authenticates the remote user.
 // msg is a slice of strings: user, host, time, sig.R, sig.S
 func verifyUser(key upspin.PublicKey, msg []string, magic, host string, now time.Time) error {
 	if len(msg) != 5 {
 		return errors.Str("bad header")
 	}

 	if host != msg[1] || len(host) == 0 {
 		return errors.Errorf("signature was for host %q but this is %q", msg[1], host)
 	}

 	// Make sure the challenge time is sane.
 	msgNow, err := time.Parse(time.ANSIC, msg[2])
 	if err != nil {
 		return err
 	}
 	// Currently just print a message if the time is too far off.
 	// TODO(p): we have to do better than this.
 	if msgNow.After(now.Add(30*time.Second)) || msgNow.Before(now.Add(-45*time.Second)) {
 		log.Info.Printf("verifying %s: timestamp is far wrong (%v); proceeding anyway", msg[0], now.Sub(msgNow))
 	}

 	// Parse signature
 	var rs, ss big.Int
 	if _, ok := rs.SetString(msg[3], 10); !ok {
 		return errMissingSignature
 	}
 	if _, ok := ss.SetString(msg[4], 10); !ok {
 		return errMissingSignature
 	}

 	// Validate signature.
 	hash := hashUser(magic, msg[0], msg[1], msg[2])
 	err = factotum.Verify(hash, upspin.Signature{R: &rs, S: &ss}, key)
 	if err != nil {
 		shortKey := string(key)
 		if len(shortKey) > 16 {
 			shortKey = shortKey[:16] + "..."
 		}
 		user := msg[0]
 		log.Debug.Printf("rpc/server: signature fails to validate using key %q for %q: %s", shortKey, user, err)
 		return err
 	}
 	return nil
 }

 // signUser creates a header authenticating the local user.
 func signUser(cfg upspin.Config, magic, host string) ([]string, error) {
 	if cfg == nil {
 		return nil, errors.Str("nil config")
 	}
 	f := cfg.Factotum()
 	if f == nil {
 		return nil, errors.E(cfg.UserName(), "no factotum available")
 	}

 	user := string(cfg.UserName())
 	if len(host) == 0 {
 		return nil, errors.Str("unset host")
 	}
 	now := time.Now().UTC().Format(time.ANSIC)
 	sig, err := f.Sign(hashUser(magic, user, host, now))
 	if err != nil {
 		log.Error.Printf("proxyRequest signing server user: %v", err)
 		return nil, err
 	}
 	return []string{
 		user,
 		host,
 		now,
 		sig.R.String(),
 		sig.S.String(),
 	}, nil
 }

 func hashUser(magic, user, host, now string) []byte {
 	h := sha256.New()
 	h.Write([]byte(magic))
 	w := func(s string) {
 		var l [4]byte
 		binary.BigEndian.PutUint32(l[:], uint32(len(s)))
 		h.Write(l[:])
 		h.Write([]byte(s))
 	}
 	w(user)
 	w(host)
 	w(now)
 	return h.Sum(nil)
 }
	// Copyright 2016 The Upspin Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package rpc

	import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/binary"
	"fmt"
	"io/ioutil"
	"math/big"
	"net/http"
	"strings"
	"time"

	pb "github.com/golang/protobuf/proto"

	"upspin.io/errors"
	"upspin.io/factotum"
	"upspin.io/log"
	"upspin.io/upspin"
	"upspin.io/valid"
	)

	// Errors returned in case of various authentication failure scenarios.
	var (
	errUnauthenticated = errors.Str("user not authenticated")
	errExpired = errors.Str("auth token expired")
	errMissingSignature = errors.Str("missing or invalid signature")

	authTokenDuration = 20 * time.Hour // Max duration an auth token lasts.
	)

	const (
	// authTokenHeader is the key in the context's metadata for the auth token.
	authTokenHeader = "Upspin-Auth-Token"

	// authRequestHeader is the key for inline user authentication.
	authRequestHeader = "Upspin-Auth-Request"

	// authErrorHeader is the key for inline user authentication errors.
	authErrorHeader = "Upspin-Auth-Error"

	// proxyRequestHeader key is for inline proxy configuration requests.
	proxyRequestHeader = "Upspin-Proxy-Request"

	// authTokenEntropyLen is the size of random bytes in an auth token.
	authTokenEntropyLen = 16

	// clientAuthMagic is a string used in validating the client's user name.
	clientAuthMagic = " Authenticate "

	// serverAuthMagic is a string used in validating the server's user name.
	serverAuthMagic = " AuthenticateServer "
	)

	// Service describes an RPC service.
	type Service struct {
	// The name of the service, which forms the first path component of any
	// HTTP request.
	Name string

	// The RPC methods to serve.
	Methods map[string]Method

	// The RPC methods to serve without validating sessions.
	UnauthenticatedMethods map[string]UnauthenticatedMethod

	// The streaming RPC methods to serve.
	Streams map[string]Stream

	// Lookup is KeyServer.Lookup function that should be used for key
	// lookups during authentication.
	// If nil, PublicUserKeyService will be used.
	Lookup func(userName upspin.UserName) (upspin.PublicKey, error)
	}

	// Method describes an authenticated RPC method.
	type Method func(s Session, reqBytes []byte) (pb.Message, error)

	// UnauthenticatedMethod describes an RPC method that does not require
	// server-side authentication.
	type UnauthenticatedMethod func(reqBytes []byte) (pb.Message, error)

	// Stream describes an authenticated streaming RPC method.
	type Stream func(s Session, reqBytes []byte, done <-chan struct{}) (<-chan pb.Message, error)

	// NewServer returns a new Server that uses the given ServerConfig.
	func NewServer(cfg upspin.Config, svc Service) http.Handler {
	// Validate Service.
	if svc.Name == "" {
	panic("ServerConfig provided with empty Name")
	}
	for name := range svc.Methods {
	if _, ok := svc.UnauthenticatedMethods[name]; ok {
	panic(fmt.Sprintf("Method %q also specified as UnauthenticatedMethod", name))
	}
	if _, ok := svc.Streams[name]; ok {
	panic(fmt.Sprintf("Method %q also specified as Stream", name))
	}
	}
	for name := range svc.Streams {
	if _, ok := svc.UnauthenticatedMethods[name]; ok {
	panic(fmt.Sprintf("Stream %q also specified as UnauthenticatedMethod", name))
	}
	}

	return &serverImpl{
	config: cfg,
	service: svc,
	}
	}

	type serverImpl struct {
	config upspin.Config
	service Service
	}

	func (s *serverImpl) lookup(u upspin.UserName) (upspin.PublicKey, error) {
	if s.service.Lookup == nil {
	return PublicUserKeyService(s.config)(u)
	}
	return s.service.Lookup(u)
	}

	func generateRandomToken() (string, error) {
	var buf [authTokenEntropyLen]byte
	n, err := rand.Read(buf[:])
	if err != nil {
	return "", err
	}
	if n != len(buf) {
	return "", errors.Str("random bytes too short")
	}
	return fmt.Sprintf("%X", buf), nil
	}

	// ServeHTTP exposes the configured Service as an HTTP API.
	func (s serverImpl) ServeHTTP(w http.ResponseWriter, r http.Request) {
	d := &s.service
	prefix := "/api/" + d.Name + "/"
	if !strings.HasPrefix(r.URL.Path, prefix) {
	http.NotFound(w, r)
	return
	}
	name := strings.TrimPrefix(r.URL.Path, prefix)

	method := d.Methods[name]
	umethod := d.UnauthenticatedMethods[name]
	stream := d.Streams[name]
	if method == nil && umethod == nil && stream == nil {
	http.NotFound(w, r)
	return
	}

	var session Session
	if umethod == nil {
	var err error
	session, err = s.SessionForRequest(w, r)
	if err != nil {
	http.Error(w, err.Error(), http.StatusUnauthorized)
	return
	}
	}

	body, err := ioutil.ReadAll(r.Body)
	r.Body.Close()
	if err != nil {
	http.Error(w, err.Error(), http.StatusBadRequest)
	return
	}

	switch {
	case method != nil:
	resp, err := method(session, body)
	sendResponse(w, resp, err)
	case umethod != nil:
	resp, err := umethod(body)
	sendResponse(w, resp, err)
	case stream != nil:
	serveStream(stream, session, w, body)
	default:
	panic("this should never happen")
	}
	}

	func sendResponse(w http.ResponseWriter, resp pb.Message, err error) {
	if err != nil {
	sendError(w, err)
	return
	}
	payload, err := pb.Marshal(resp)
	if err != nil {
	log.Error.Printf("error encoding response: %v", err)
	http.Error(w, err.Error(), http.StatusInternalServerError)
	return
	}
	w.Write(payload)
	}

	func sendError(w http.ResponseWriter, err error) {
	h := w.Header()
	h.Set("Content-type", "application/octet-stream")
	w.WriteHeader(http.StatusInternalServerError)
	w.Write(errors.MarshalError(err))
	}

	func serveStream(s Stream, sess Session, w http.ResponseWriter, body []byte) {
	done := make(chan struct{})
	msgs, err := s(sess, body, done)
	if err != nil {
	sendError(w, err)
	return
	}

	connClosed := w.(http.CloseNotifier).CloseNotify()
	go func() {
	<-connClosed
	close(done)
	}()

	// Write the headers, beginning the stream.
	w.Write([]byte("OK"))
	w.(http.Flusher).Flush()

	var lenBytes [4]byte // stores a uint32, the length of each output message
	for {
	select {
	case msg, ok := <-msgs:
	if !ok {
	return
	}
	if done == nil {
	// Drop this message as there's nobody to deliver to.
	continue
	}

	b, err := pb.Marshal(msg)
	if err != nil {
	log.Error.Printf("rpc/auth: error encoding proto in stream: %v", err)
	return
	}

	binary.BigEndian.PutUint32(lenBytes[:], uint32(len(b)))
	if _, err := w.Write(lenBytes[:]); err != nil {
	return
	}
	if _, err := w.Write(b); err != nil {
	return
	}
	w.(http.Flusher).Flush()

	case <-done:
	done = nil
	}
	}
	}

	func (s serverImpl) SessionForRequest(w http.ResponseWriter, r http.Request) (session Session, err error) {
	const op errors.Op = "rpc.SessionForRequest"

	defer func() {
	if err == nil {
	return
	}
	// Capture session setup errors and
	// send them to the client in the HTTP response Header.
	w.Header().Set(authErrorHeader, err.Error())
	// Attach the op to the error here, because the client doesn't
	// care that this error originated in this function.
	err = errors.E(op, err)
	}()

	if tok, ok := r.Header[authTokenHeader]; ok && len(tok) == 1 {
	return s.validateToken(tok[0])
	}

	proxyRequest, ok := r.Header[proxyRequestHeader]
	if ok && len(proxyRequest) != 1 {
	return nil, errors.E(errors.Invalid, "invalid proxy request in header")
	}

	// Clients send a single header line with comma-separated values.
	authRequest, ok := r.Header[authRequestHeader]
	if !ok {
	return nil, errors.E(errors.Invalid, "missing auth request header")
	} else if len(authRequest) == 5 {
	// Old-style authentication tokens should now fail,
	// but provide an informative error message when they do.
	// TODO(adg): Remove this if/else block on April 15.
	return nil, errors.E(errors.Invalid, "invalid auth request header (please update your Upspin clients and servers)")
	} else if len(authRequest) != 1 {
	return nil, errors.E(errors.Invalid, "invalid auth request header")
	}
	authRequest = strings.Split(authRequest[0], ",")

	return s.handleSessionRequest(w, authRequest, proxyRequest, r.Host)
	}

	func (s *serverImpl) validateToken(authToken string) (Session, error) {
	if len(authToken) < authTokenEntropyLen {
	return nil, errors.E(errors.Invalid, "invalid auth token")
	}

	// Get the session for this authToken
	session := GetSession(authToken)
	if session == nil {
	// We don't know this client or have forgotten about it. We must authenticate.
	// Log it so we can track how often this happens. Maybe we need to increase the session cache size.
	log.Debug.Printf("Got token from user but there's no session for it.")
	return nil, errors.E(errors.Permission, errUnauthenticated)
	}

	// If session has expired, forcibly remove it from the cache and return an error.
	if session.Expires().Before(time.Now()) {
	ClearSession(authToken)
	return nil, errors.E(errors.Permission, errExpired)
	}

	return session, nil
	}

	func (s *serverImpl) handleSessionRequest(w http.ResponseWriter, authRequest []string, proxyRequest []string, host string) (Session, error) {
	// Validate the username.
	user := upspin.UserName(authRequest[0])
	if err := valid.UserName(user); err != nil {
	return nil, errors.E(user, err)
	}

	// Get user's public key.
	key, err := s.lookup(user)
	if err != nil {
	return nil, errors.E(user, err)
	}

	// If this is a proxy request, extract the endpoint and
	// set the signed host to that endpoint.
	ep := &upspin.Endpoint{}
	if len(proxyRequest) == 1 {
	if pUser := s.config.UserName(); user != pUser {
	return nil, errors.E(errors.Permission, errors.Errorf("client %q and proxy %q users mismatched", user, pUser))
	}
	ep, err = upspin.ParseEndpoint(proxyRequest[0])
	if err != nil {
	return nil, errors.E(errors.Invalid, errors.Errorf("invalid proxy endpoint: %v", err))
	}
	host = string(ep.NetAddr)
	}

	now := time.Now()

	// Validate signature.
	if err := verifyUser(key, authRequest, clientAuthMagic, host, now); err != nil {
	return nil, errors.E(errors.Permission, user, errors.Errorf("invalid signature: %v", err))
	}

	// Generate an auth token and bind it to a session for the client.
	expiration := now.Add(authTokenDuration)
	authToken, err := generateRandomToken()
	if err != nil {
	return nil, err
	}
	w.Header().Set(authTokenHeader, authToken)

	// If there is a proxy request, authenticate server to client.
	if len(proxyRequest) == 1 {
	// Authenticate the server to the user.
	authMsg, err := signUser(s.config, serverAuthMagic, "[localproxy]")
	if err != nil {
	return nil, errors.E(errors.Permission, err)
	}
	w.Header()[authRequestHeader] = authMsg
	}

	return NewSession(user, expiration, authToken, ep, nil), nil
	}

	// verifyUser authenticates the remote user.
	// msg is a slice of strings: user, host, time, sig.R, sig.S
	func verifyUser(key upspin.PublicKey, msg []string, magic, host string, now time.Time) error {
	if len(msg) != 5 {
	return errors.Str("bad header")
	}

	if host != msg[1] \|\| len(host) == 0 {
	return errors.Errorf("signature was for host %q but this is %q", msg[1], host)
	}

	// Make sure the challenge time is sane.
	msgNow, err := time.Parse(time.ANSIC, msg[2])
	if err != nil {
	return err
	}
	// Currently just print a message if the time is too far off.
	// TODO(p): we have to do better than this.
	if msgNow.After(now.Add(30time.Second)) \|\| msgNow.Before(now.Add(-45time.Second)) {
	log.Info.Printf("verifying %s: timestamp is far wrong (%v); proceeding anyway", msg[0], now.Sub(msgNow))
	}

	// Parse signature
	var rs, ss big.Int
	if _, ok := rs.SetString(msg[3], 10); !ok {
	return errMissingSignature
	}
	if _, ok := ss.SetString(msg[4], 10); !ok {
	return errMissingSignature
	}

	// Validate signature.
	hash := hashUser(magic, msg[0], msg[1], msg[2])
	err = factotum.Verify(hash, upspin.Signature{R: &rs, S: &ss}, key)
	if err != nil {
	shortKey := string(key)
	if len(shortKey) > 16 {
	shortKey = shortKey[:16] + "..."
	}
	user := msg[0]
	log.Debug.Printf("rpc/server: signature fails to validate using key %q for %q: %s", shortKey, user, err)
	return err
	}
	return nil
	}

	// signUser creates a header authenticating the local user.
	func signUser(cfg upspin.Config, magic, host string) ([]string, error) {
	if cfg == nil {
	return nil, errors.Str("nil config")
	}
	f := cfg.Factotum()
	if f == nil {
	return nil, errors.E(cfg.UserName(), "no factotum available")
	}

	user := string(cfg.UserName())
	if len(host) == 0 {
	return nil, errors.Str("unset host")
	}
	now := time.Now().UTC().Format(time.ANSIC)
	sig, err := f.Sign(hashUser(magic, user, host, now))
	if err != nil {
	log.Error.Printf("proxyRequest signing server user: %v", err)
	return nil, err
	}
	return []string{
	user,
	host,
	now,
	sig.R.String(),
	sig.S.String(),
	}, nil
	}

	func hashUser(magic, user, host, now string) []byte {
	h := sha256.New()
	h.Write([]byte(magic))
	w := func(s string) {
	var l [4]byte
	binary.BigEndian.PutUint32(l[:], uint32(len(s)))
	h.Write(l[:])
	h.Write([]byte(s))
	}
	w(user)
	w(host)
	w(now)
	return h.Sum(nil)
	}