main, sshd: Refactor authentication, add IP throttling, improve passphrase auth

* Move password authentication handling into sshd/auth (fixes #394).

Password authentication is now completely handeled in Auth. The normal
keyboard-interactive handler checks if passwords are supported and asks
for them, removing the need to override the callbacks.

Brute force throttling is removed; I'd like to base it on IP address
banning, which requires changes to the checks.

I'm not sure, but I think timing attacks against the password are fixed:
- The hashing of the real password happens only at startup.
- The hashing of a provided password is something an attacker can do
themselves; It doesn't leak anything about the real password.
- The hash comparison is constant-time.

* refactor checks, IP-ban incorrect passphrases, renames

- s/assword/assphrase/, typo fixes
- bans are checked separately from public keys
- an incorrect passphrase results in a one-minute IP ban
- whitelists no longer override bans (i.e. you can get banned if you're 
whitelisted)

* (hopefully) final changes

Author: mikitsu

auth.go

@@ -1,6 +1,8 @@
 package sshchat
 
 import (
+	"crypto/sha256"
+	"crypto/subtle"
 	"encoding/csv"
 	"errors"
 	"fmt"
@@ -17,9 +19,12 @@ import (
 // when whitelisting is enabled.
 var ErrNotWhitelisted = errors.New("not whitelisted")
 
-// ErrBanned is the error returned when a key is checked that is banned.
+// ErrBanned is the error returned when a client is banned.
 var ErrBanned = errors.New("banned")
 
+// ErrIncorrectPassphrase is the error returned when a provided passphrase is incorrect.
+var ErrIncorrectPassphrase = errors.New("incorrect passphrase")
+
 // newAuthKey returns string from an ssh.PublicKey used to index the key in our lookup.
 func newAuthKey(key ssh.PublicKey) string {
 	if key == nil {
@@ -43,12 +48,14 @@ func newAuthAddr(addr net.Addr) string {
 }
 
 // Auth stores lookups for bans, whitelists, and ops. It implements the sshd.Auth interface.
+// If the contained passphrase is not empty, it complements a whitelist.
 type Auth struct {
-	bannedAddr   *set.Set
-	bannedClient *set.Set
-	banned       *set.Set
-	whitelist    *set.Set
-	ops          *set.Set
+	passphraseHash []byte
+	bannedAddr     *set.Set
+	bannedClient   *set.Set
+	banned         *set.Set
+	whitelist      *set.Set
+	ops            *set.Set
 }
 
 // NewAuth creates a new empty Auth.
@@ -62,23 +69,30 @@ func NewAuth() *Auth {
 	}
 }
 
+// SetPassphrase enables passphrase authentication with the given passphrase.
+// If an empty passphrase is given, disable passphrase authentication.
+func (a *Auth) SetPassphrase(passphrase string) {
+	if passphrase == "" {
+		a.passphraseHash = nil
+	} else {
+		hashArray := sha256.Sum256([]byte(passphrase))
+		a.passphraseHash = hashArray[:]
+	}
+}
+
 // AllowAnonymous determines if anonymous users are permitted.
 func (a *Auth) AllowAnonymous() bool {
-	return a.whitelist.Len() == 0
+	return a.whitelist.Len() == 0 && a.passphraseHash == nil
 }
 
-// Check determines if a pubkey fingerprint is permitted.
-func (a *Auth) Check(addr net.Addr, key ssh.PublicKey, clientVersion string) error {
-	authkey := newAuthKey(key)
+// AcceptPassphrase determines if passphrase authentication is accepted.
+func (a *Auth) AcceptPassphrase() bool {
+	return a.passphraseHash != nil
+}
 
-	if a.whitelist.Len() != 0 {
-		// Only check whitelist if there is something in it, otherwise it's disabled.
-		whitelisted := a.whitelist.In(authkey)
-		if !whitelisted {
-			return ErrNotWhitelisted
-		}
-		return nil
-	}
+// CheckBans checks IP, key and client bans.
+func (a *Auth) CheckBans(addr net.Addr, key ssh.PublicKey, clientVersion string) error {
+	authkey := newAuthKey(key)
 
 	var banned bool
 	if authkey != "" {
@@ -98,6 +112,29 @@ func (a *Auth) Check(addr net.Addr, key ssh.PublicKey, clientVersion string) err
 	return nil
 }
 
+// CheckPubkey determines if a pubkey fingerprint is permitted.
+func (a *Auth) CheckPublicKey(key ssh.PublicKey) error {
+	authkey := newAuthKey(key)
+	whitelisted := a.whitelist.In(authkey)
+	if a.AllowAnonymous() || whitelisted {
+		return nil
+	} else {
+		return ErrNotWhitelisted
+	}
+}
+
+// CheckPassphrase determines if a passphrase is permitted.
+func (a *Auth) CheckPassphrase(passphrase string) error {
+	if !a.AcceptPassphrase() {
+		return errors.New("passphrases not accepted") // this should never happen
+	}
+	passedPassphraseHash := sha256.Sum256([]byte(passphrase))
+	if subtle.ConstantTimeCompare(passedPassphraseHash[:], a.passphraseHash) == 0 {
+		return ErrIncorrectPassphrase
+	}
+	return nil
+}
+
 // Op sets a public key as a known operator.
 func (a *Auth) Op(key ssh.PublicKey, d time.Duration) {
 	if key == nil {

auth_test.go

@@ -28,7 +28,7 @@ func TestAuthWhitelist(t *testing.T) {
 	}
 
 	auth := NewAuth()
-	err = auth.Check(nil, key, "")
+	err = auth.CheckPublicKey(key)
 	if err != nil {
 		t.Error("Failed to permit in default state:", err)
 	}
@@ -44,7 +44,7 @@ func TestAuthWhitelist(t *testing.T) {
 		t.Error("Clone key does not match.")
 	}
 
-	err = auth.Check(nil, keyClone, "")
+	err = auth.CheckPublicKey(keyClone)
 	if err != nil {
 		t.Error("Failed to permit whitelisted:", err)
 	}
@@ -54,8 +54,42 @@ func TestAuthWhitelist(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	err = auth.Check(nil, key2, "")
+	err = auth.CheckPublicKey(key2)
 	if err == nil {
 		t.Error("Failed to restrict not whitelisted:", err)
 	}
 }
+
+func TestAuthPassphrases(t *testing.T) {
+	auth := NewAuth()
+
+	if auth.AcceptPassphrase() {
+		t.Error("Doesn't known it won't accept passphrases.")
+	}
+	auth.SetPassphrase("")
+	if auth.AcceptPassphrase() {
+		t.Error("Doesn't known it won't accept passphrases.")
+	}
+
+	err := auth.CheckPassphrase("Pa$$w0rd")
+	if err == nil {
+		t.Error("Failed to deny without passphrase:", err)
+	}
+
+	auth.SetPassphrase("Pa$$w0rd")
+
+	err = auth.CheckPassphrase("Pa$$w0rd")
+	if err != nil {
+		t.Error("Failed to allow vaild passphrase:", err)
+	}
+
+	err = auth.CheckPassphrase("something else")
+	if err == nil {
+		t.Error("Failed to restrict wrong passphrase:", err)
+	}
+
+	auth.SetPassphrase("")
+	if auth.AcceptPassphrase() {
+		t.Error("Didn't clear passphrase.")
+	}
+}

cmd/ssh-chat/cmd.go

@@ -2,15 +2,13 @@ package main
 
 import (
 	"bufio"
-	"errors"
 	"fmt"
 	"io/ioutil"
 	"net/http"
 	"os"
 	"os/signal"
 	"os/user"
 	"strings"
-	"time"
 
 	"github.com/alexcesaro/log"
 	"github.com/alexcesaro/log/golog"
@@ -123,44 +121,6 @@ func main() {
 	config.ServerVersion = "SSH-2.0-Go ssh-chat"
 	// FIXME: Should we be using config.NoClientAuth = true by default?
 
-	if options.Passphrase != "" {
-		if options.Whitelist != "" {
-			logger.Warning("Passphrase is disabled while whitelist is enabled.")
-		}
-		if config.KeyboardInteractiveCallback != nil {
-			fail(1, "Passphrase authentication conflicts with existing KeyboardInteractive setup.") // This should not happen
-		}
-
-		// We use KeyboardInteractiveCallback instead of PasswordCallback to
-		// avoid preventing the client from including a pubkey in the user
-		// identification.
-		config.KeyboardInteractiveCallback = func(conn ssh.ConnMetadata, challenge ssh.KeyboardInteractiveChallenge) (*ssh.Permissions, error) {
-			answers, err := challenge("", "", []string{"Passphrase required to connect: "}, []bool{true})
-			if err != nil {
-				return nil, err
-			}
-			if len(answers) == 1 && answers[0] == options.Passphrase {
-				// Success
-				return nil, nil
-			}
-			// It's not gonna do much but may as well throttle brute force attempts a little
-			time.Sleep(2 * time.Second)
-
-			return nil, errors.New("incorrect passphrase")
-		}
-
-		// We also need to override the PublicKeyCallback to prevent rando pubkeys from bypassing
-		cb := config.PublicKeyCallback
-		config.PublicKeyCallback = func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
-			perms, err := cb(conn, key)
-			if err == nil {
-				err = errors.New("passphrase authentication required")
-			}
-			return perms, err
-		}
-
-	}
-
 	s, err := sshd.ListenSSH(options.Bind, config)
 	if err != nil {
 		fail(4, "Failed to listen on socket: %v\n", err)
@@ -174,6 +134,10 @@ func main() {
 	host.SetTheme(message.Themes[0])
 	host.Version = Version
 
+	if options.Passphrase != "" {
+		auth.SetPassphrase(options.Passphrase)
+	}
+
 	err = fromFile(options.Admin, func(line []byte) error {
 		key, _, _, _, err := ssh.ParseAuthorizedKey(line)
 		if err != nil {

sshd/auth.go

@@ -5,17 +5,26 @@ import (
 	"encoding/base64"
 	"errors"
 	"net"
+	"time"
 
 	"github.com/shazow/ssh-chat/internal/sanitize"
 	"golang.org/x/crypto/ssh"
 )
 
-// Auth is used to authenticate connections based on public keys.
+// Auth is used to authenticate connections.
 type Auth interface {
 	// Whether to allow connections without a public key.
 	AllowAnonymous() bool
-	// Given address and public key and client agent string, returns nil if the connection should be allowed.
-	Check(net.Addr, ssh.PublicKey, string) error
+	// If passphrase authentication is accepted
+	AcceptPassphrase() bool
+	// Given address and public key and client agent string, returns nil if the connection is not banned.
+	CheckBans(net.Addr, ssh.PublicKey, string) error
+	// Given a public key, returns nil if the connection should be allowed.
+	CheckPublicKey(ssh.PublicKey) error
+	// Given a passphrase, returns nil if the connection should be allowed.
+	CheckPassphrase(string) error
+	// BanAddr bans an IP address for the specified amount of time.
+	BanAddr(net.Addr, time.Duration)
 }
 
 // MakeAuth makes an ssh.ServerConfig which performs authentication against an Auth implementation.
@@ -25,7 +34,11 @@ func MakeAuth(auth Auth) *ssh.ServerConfig {
 		NoClientAuth: false,
 		// Auth-related things should be constant-time to avoid timing attacks.
 		PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
-			err := auth.Check(conn.RemoteAddr(), key, sanitize.Data(string(conn.ClientVersion()), 64))
+			err := auth.CheckBans(conn.RemoteAddr(), key, sanitize.Data(string(conn.ClientVersion()), 64))
+			if err != nil {
+				return nil, err
+			}
+			err = auth.CheckPublicKey(key)
 			if err != nil {
 				return nil, err
 			}
@@ -34,11 +47,31 @@ func MakeAuth(auth Auth) *ssh.ServerConfig {
 			}}
 			return perm, nil
 		},
+
+		// We use KeyboardInteractiveCallback instead of PasswordCallback to
+		// avoid preventing the client from including a pubkey in the user
+		// identification.
 		KeyboardInteractiveCallback: func(conn ssh.ConnMetadata, challenge ssh.KeyboardInteractiveChallenge) (*ssh.Permissions, error) {
-			if !auth.AllowAnonymous() {
-				return nil, errors.New("public key authentication required")
+			err := auth.CheckBans(conn.RemoteAddr(), nil, sanitize.Data(string(conn.ClientVersion()), 64))
+			if err != nil {
+				return nil, err
+			}
+			if auth.AcceptPassphrase() {
+				var answers []string
+				answers, err = challenge("", "", []string{"Passphrase required to connect: "}, []bool{true})
+				if err == nil {
+					if len(answers) != 1 {
+						err = errors.New("didn't get passphrase")
+					} else {
+						err = auth.CheckPassphrase(answers[0])
+						if err != nil {
+							auth.BanAddr(conn.RemoteAddr(), time.Second*2)
+						}
+					}
+				}
+			} else if !auth.AllowAnonymous() {
+				err = errors.New("public key authentication required")
 			}
-			err := auth.Check(conn.RemoteAddr(), nil, sanitize.Data(string(conn.ClientVersion()), 64))
 			return nil, err
 		},
 	}

sshd/client_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"net"
 	"testing"
+	"time"
 
 	"golang.org/x/crypto/ssh"
 )
@@ -15,9 +16,19 @@ type RejectAuth struct{}
 func (a RejectAuth) AllowAnonymous() bool {
 	return false
 }
-func (a RejectAuth) Check(net.Addr, ssh.PublicKey, string) error {
+func (a RejectAuth) AcceptPassphrase() bool {
+	return false
+}
+func (a RejectAuth) CheckBans(addr net.Addr, key ssh.PublicKey, clientVersion string) error {
+	return errRejectAuth
+}
+func (a RejectAuth) CheckPublicKey(ssh.PublicKey) error {
+	return errRejectAuth
+}
+func (a RejectAuth) CheckPassphrase(string) error {
 	return errRejectAuth
 }
+func (a RejectAuth) BanAddr(net.Addr, time.Duration) {}
 
 func TestClientReject(t *testing.T) {
 	signer, err := NewRandomSigner(512)