mox!

scram/scram.go

@@ -0,0 +1,368 @@
+// Package scram implements the SCRAM-SHA256 SASL authentication mechanism, RFC 7677.
+//
+// SCRAM-SHA256 allows a client to authenticate to a server using a password
+// without handing plaintext password over to the server. The client also
+// verifies the server knows (a derivative of) the password.
+package scram
+
+// todo: test with messages that contains extensions
+// todo: some tests for the parser
+// todo: figure out how invalid parameters etc should be handled. just abort? perhaps mostly a problem for imap.
+
+import (
+	"bytes"
+	"crypto/hmac"
+	cryptorand "crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"strings"
+
+	"golang.org/x/crypto/pbkdf2"
+	"golang.org/x/text/unicode/norm"
+)
+
+// Errors at scram protocol level. Can be exchanged between client and server.
+var (
+	ErrInvalidEncoding                 Error = "invalid-encoding"
+	ErrExtensionsNotSupported          Error = "extensions-not-supported"
+	ErrInvalidProof                    Error = "invalid-proof"
+	ErrChannelBindingsDontMatch        Error = "channel-bindings-dont-match"
+	ErrServerDoesSupportChannelBinding Error = "server-does-support-channel-binding"
+	ErrChannelBindingNotSupported      Error = "channel-binding-not-supported"
+	ErrUnsupportedChannelBindingType   Error = "unsupported-channel-binding-type"
+	ErrUnknownUser                     Error = "unknown-user"
+	ErrNoResources                     Error = "no-resources"
+	ErrOtherError                      Error = "other-error"
+)
+
+var scramErrors = makeErrors()
+
+func makeErrors() map[string]Error {
+	l := []Error{
+		ErrInvalidEncoding,
+		ErrExtensionsNotSupported,
+		ErrInvalidProof,
+		ErrChannelBindingsDontMatch,
+		ErrServerDoesSupportChannelBinding,
+		ErrChannelBindingNotSupported,
+		ErrUnsupportedChannelBindingType,
+		ErrUnknownUser,
+		ErrNoResources,
+		ErrOtherError,
+	}
+	m := map[string]Error{}
+	for _, e := range l {
+		m[string(e)] = e
+	}
+	return m
+}
+
+var (
+	ErrNorm     = errors.New("parameter not unicode normalized") // E.g. if client sends non-normalized username or authzid.
+	ErrUnsafe   = errors.New("unsafe parameter")                 // E.g. salt, nonce too short, or too few iterations.
+	ErrProtocol = errors.New("protocol error")                   // E.g. server responded with a nonce not prefixed by the client nonce.
+)
+
+type Error string
+
+func (e Error) Error() string {
+	return string(e)
+}
+
+// MakeRandom returns a cryptographically random buffer for use as salt or as
+// nonce.
+func MakeRandom() []byte {
+	buf := make([]byte, 12)
+	_, err := cryptorand.Read(buf)
+	if err != nil {
+		panic("generate random")
+	}
+	return buf
+}
+
+// SaltPassword returns a salted password.
+func SaltPassword(password string, salt []byte, iterations int) []byte {
+	password = norm.NFC.String(password)
+	return pbkdf2.Key([]byte(password), salt, iterations, sha256.Size, sha256.New)
+}
+
+// HMAC returns the hmac with key over msg.
+func HMAC(key []byte, msg string) []byte {
+	mac := hmac.New(sha256.New, key)
+	mac.Write([]byte(msg))
+	return mac.Sum(nil)
+}
+
+func xor(a, b []byte) {
+	for i := range a {
+		a[i] ^= b[i]
+	}
+}
+
+// Server represents the server-side of a SCRAM-SHA-256 authentication.
+type Server struct {
+	Authentication string // Username for authentication, "authc". Always set and non-empty.
+	Authorization  string // If set, role of user to assume after authentication, "authz".
+
+	// Messages used in hash calculations.
+	clientFirstBare         string
+	serverFirst             string
+	clientFinalWithoutProof string
+
+	gs2header           string
+	clientNonce         string // Client-part of the nonce.
+	serverNonceOverride string // If set, server does not generate random nonce, but uses this. For tests with the test vector.
+	nonce               string // Full client + server nonce.
+}
+
+// NewServer returns a server given the first SCRAM message from a client.
+//
+// The sequence for data and calls on a server:
+//
+//   - Read initial data from client, call NewServer (this call), then ServerFirst and write to the client.
+//   - Read response from client, call Finish or FinishFinal and write the resulting string.
+func NewServer(clientFirst []byte) (server *Server, rerr error) {
+	p := newParser(clientFirst)
+	defer p.recover(&rerr)
+
+	server = &Server{}
+
+	// ../rfc/5802:949 ../rfc/5802:910
+	gs2cbindFlag := p.xbyte()
+	switch gs2cbindFlag {
+	case 'n', 'y':
+	case 'p':
+		p.xerrorf("gs2 header with p: %w", ErrChannelBindingNotSupported)
+	}
+	p.xtake(",")
+	if !p.take(",") {
+		server.Authorization = p.xauthzid()
+		if norm.NFC.String(server.Authorization) != server.Authorization {
+			return nil, fmt.Errorf("%w: authzid", ErrNorm)
+		}
+		p.xtake(",")
+	}
+	server.gs2header = p.s[:p.o]
+	server.clientFirstBare = p.s[p.o:]
+
+	// ../rfc/5802:945
+	if p.take("m=") {
+		p.xerrorf("unexpected mandatory extension: %w", ErrExtensionsNotSupported)
+	}
+	server.Authentication = p.xusername()
+	if norm.NFC.String(server.Authentication) != server.Authentication {
+		return nil, fmt.Errorf("%w: username", ErrNorm)
+	}
+	p.xtake(",")
+	server.clientNonce = p.xnonce()
+	if len(server.clientNonce) < 8 {
+		return nil, fmt.Errorf("%w: client nonce too short", ErrUnsafe)
+	}
+	// Extensions, we don't recognize them.
+	for p.take(",") {
+		p.xattrval()
+	}
+	p.xempty()
+	return server, nil
+}
+
+// ServerFirst returns the string to send back to the client. To be called after NewServer.
+func (s *Server) ServerFirst(iterations int, salt []byte) (string, error) {
+	// ../rfc/5802:959
+	serverNonce := s.serverNonceOverride
+	if serverNonce == "" {
+		serverNonce = base64.StdEncoding.EncodeToString(MakeRandom())
+	}
+	s.nonce = s.clientNonce + serverNonce
+	s.serverFirst = fmt.Sprintf("r=%s,s=%s,i=%d", s.nonce, base64.StdEncoding.EncodeToString(salt), iterations)
+	return s.serverFirst, nil
+}
+
+// Finish takes the final client message, and the salted password (probably
+// from server storage), verifies the client, and returns a message to return
+// to the client. If err is nil, authentication was successful. If the
+// authorization requested is not acceptable, the server should call
+// FinishError instead.
+func (s *Server) Finish(clientFinal []byte, saltedPassword []byte) (serverFinal string, rerr error) {
+	p := newParser(clientFinal)
+	defer p.recover(&rerr)
+
+	cbind := p.xchannelBinding()
+	if cbind != s.gs2header {
+		return "e=" + string(ErrChannelBindingsDontMatch), ErrChannelBindingsDontMatch
+	}
+	p.xtake(",")
+	nonce := p.xnonce()
+	if nonce != s.nonce {
+		return "e=" + string(ErrInvalidProof), ErrInvalidProof
+	}
+	for !p.peek(",p=") {
+		p.xtake(",")
+		p.xattrval() // Ignored.
+	}
+	s.clientFinalWithoutProof = p.s[:p.o]
+	p.xtake(",")
+	proof := p.xproof()
+	p.xempty()
+
+	msg := s.clientFirstBare + "," + s.serverFirst + "," + s.clientFinalWithoutProof
+
+	clientKey := HMAC(saltedPassword, "Client Key")
+	storedKey0 := sha256.Sum256(clientKey)
+	storedKey := storedKey0[:]
+
+	clientSig := HMAC(storedKey, msg)
+	xor(clientSig, clientKey) // Now clientProof.
+	if !bytes.Equal(clientSig, proof) {
+		return "e=" + string(ErrInvalidProof), ErrInvalidProof
+	}
+
+	serverKey := HMAC(saltedPassword, "Server Key")
+	serverSig := HMAC(serverKey, msg)
+	return fmt.Sprintf("v=%s", base64.StdEncoding.EncodeToString(serverSig)), nil
+}
+
+// FinishError returns an error message to write to the client for the final
+// server message.
+func (s *Server) FinishError(err Error) string {
+	return "e=" + string(err)
+}
+
+// Client represents the client-side of a SCRAM-SHA-256 authentication.
+type Client struct {
+	authc string
+	authz string
+
+	// Messages used in hash calculations.
+	clientFirstBare         string
+	serverFirst             string
+	clientFinalWithoutProof string
+	authMessage             string
+
+	gs2header      string
+	clientNonce    string
+	nonce          string // Full client + server nonce.
+	saltedPassword []byte
+}
+
+// NewClient returns a client for authentication authc, optionally for
+// authorization with role authz.
+//
+// The sequence for data and calls on a client:
+//
+//   - ClientFirst, write result to server.
+//   - Read response from server, feed to ServerFirst, write response to server.
+//   - Read response from server, feed to ServerFinal.
+func NewClient(authc, authz string) *Client {
+	authc = norm.NFC.String(authc)
+	authz = norm.NFC.String(authz)
+	return &Client{authc: authc, authz: authz}
+}
+
+// ClientFirst returns the first client message to write to the server.
+// No channel binding is done/supported.
+// A random nonce is generated.
+func (c *Client) ClientFirst() (clientFirst string, rerr error) {
+	c.gs2header = fmt.Sprintf("n,%s,", saslname(c.authz))
+	if c.clientNonce == "" {
+		c.clientNonce = base64.StdEncoding.EncodeToString(MakeRandom())
+	}
+	c.clientFirstBare = fmt.Sprintf("n=%s,r=%s", saslname(c.authc), c.clientNonce)
+	return c.gs2header + c.clientFirstBare, nil
+}
+
+// ServerFirst processes the first response message from the server. The
+// provided nonce, salt and iterations are checked. If valid, a final client
+// message is calculated and returned. This message must be written to the
+// server. It includes proof that the client knows the password.
+func (c *Client) ServerFirst(serverFirst []byte, password string) (clientFinal string, rerr error) {
+	c.serverFirst = string(serverFirst)
+	p := newParser(serverFirst)
+	defer p.recover(&rerr)
+
+	// ../rfc/5802:959
+	if p.take("m=") {
+		p.xerrorf("unsupported mandatory extension: %w", ErrExtensionsNotSupported)
+	}
+
+	c.nonce = p.xnonce()
+	p.xtake(",")
+	salt := p.xsalt()
+	p.xtake(",")
+	iterations := p.xiterations()
+	// We ignore extensions that we don't know about.
+	for p.take(",") {
+		p.xattrval()
+	}
+	p.xempty()
+
+	if !strings.HasPrefix(c.nonce, c.clientNonce) {
+		return "", fmt.Errorf("%w: server dropped our nonce", ErrProtocol)
+	}
+	if len(c.nonce)-len(c.clientNonce) < 8 {
+		return "", fmt.Errorf("%w: server nonce too short", ErrUnsafe)
+	}
+	if len(salt) < 8 {
+		return "", fmt.Errorf("%w: salt too short", ErrUnsafe)
+	}
+	if iterations < 2048 {
+		return "", fmt.Errorf("%w: too few iterations", ErrUnsafe)
+	}
+
+	c.clientFinalWithoutProof = fmt.Sprintf("c=%s,r=%s", base64.StdEncoding.EncodeToString([]byte(c.gs2header)), c.nonce)
+
+	c.authMessage = c.clientFirstBare + "," + c.serverFirst + "," + c.clientFinalWithoutProof
+
+	c.saltedPassword = SaltPassword(password, salt, iterations)
+	clientKey := HMAC(c.saltedPassword, "Client Key")
+	storedKey0 := sha256.Sum256(clientKey)
+	storedKey := storedKey0[:]
+	clientSig := HMAC(storedKey, c.authMessage)
+	xor(clientSig, clientKey) // Now clientProof.
+	clientProof := clientSig
+
+	r := c.clientFinalWithoutProof + ",p=" + base64.StdEncoding.EncodeToString(clientProof)
+	return r, nil
+}
+
+// ServerFinal processes the final message from the server, verifying that the
+// server knows the password.
+func (c *Client) ServerFinal(serverFinal []byte) (rerr error) {
+	p := newParser(serverFinal)
+	defer p.recover(&rerr)
+
+	if p.take("e=") {
+		errstr := p.xvalue()
+		var err error = scramErrors[errstr]
+		if err == Error("") {
+			err = errors.New(errstr)
+		}
+		return fmt.Errorf("error from server: %w", err)
+	}
+	p.xtake("v=")
+	verifier := p.xbase64()
+
+	serverKey := HMAC(c.saltedPassword, "Server Key")
+	serverSig := HMAC(serverKey, c.authMessage)
+	if !bytes.Equal(verifier, serverSig) {
+		return fmt.Errorf("incorrect server signature")
+	}
+	return nil
+}
+
+// Convert "," to =2C and "=" to =3D.
+func saslname(s string) string {
+	var r string
+	for _, c := range s {
+		if c == ',' {
+			r += "=2C"
+		} else if c == '=' {
+			r += "=3D"
+		} else {
+			r += string(c)
+		}
+	}
+	return r
+}