ssh: add support for extension negotiation (rfc 8308)

This is a rebase of the following PR

golang#197

with some changes and improvements:

- added support for client certificate authentication
- removed read loop from server handshake
- adapted extInfoMsg to upstream changes

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

ssh/client_auth_test.go

@@ -132,9 +132,7 @@ func TestClientAuthPublicKey(t *testing.T) {
 	if err := tryAuth(t, config); err != nil {
 		t.Fatalf("unable to dial remote side: %s", err)
 	}
-	// Once the server implements the server-sig-algs extension, this will turn
-	// into KeyAlgoRSASHA256.
-	if len(signer.used) != 1 || signer.used[0] != KeyAlgoRSA {
+	if len(signer.used) != 1 || signer.used[0] != KeyAlgoRSASHA256 {
 		t.Errorf("unexpected Sign/SignWithAlgorithm calls: %q", signer.used)
 	}
 }

ssh/common.go

@@ -24,6 +24,18 @@ const (
 	serviceSSH      = "ssh-connection"
 )
 
+// These are string constants related to extensions and extension negotiation
+const (
+	extInfoServer    = "ext-info-s"
+	extInfoClient    = "ext-info-c"
+	ExtServerSigAlgs = "server-sig-algs"
+)
+
+// defaultExtensions lists extensions enabled by default.
+var defaultExtensions = []string{
+	ExtServerSigAlgs,
+}
+
 // supportedCiphers lists ciphers we support but might not recommend.
 var supportedCiphers = []string{
 	"aes128-ctr", "aes192-ctr", "aes256-ctr",
@@ -89,6 +101,15 @@ var supportedMACs = []string{
 
 var supportedCompressions = []string{compressionNone}
 
+// supportedServerSigAlgs defines the algorithms supported for pubkey authentication
+// in no particular order.
+var supportedServerSigAlgs = []string{KeyAlgoRSASHA256,
+	KeyAlgoRSASHA512, KeyAlgoRSA,
+	KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521,
+	KeyAlgoSKECDSA256, KeyAlgoED25519, KeyAlgoSKED25519,
+	KeyAlgoDSA,
+}
+
 // hashFuncs keeps the mapping of supported signature algorithms to their
 // respective hashes needed for signing and verification.
 var hashFuncs = map[string]crypto.Hash{
@@ -180,6 +201,10 @@ func findAgreedAlgorithms(isClient bool, clientKexInit, serverKexInit *kexInitMs
 	result.kex, err = findCommon("key exchange", clientKexInit.KexAlgos, serverKexInit.KexAlgos)
 	if err != nil {
 		return
+	} else if result.kex == extInfoClient || result.kex == extInfoServer {
+		// According to RFC8308 section 2.2 if either the client or server extension signal
+		// is chosen as the kex algorithm the parties must disconnect.
+		return result, fmt.Errorf("ssh: invalid kex algorithm chosen: %s", result.kex)
 	}
 
 	result.hostKey, err = findCommon("host key", clientKexInit.ServerHostKeyAlgos, serverKexInit.ServerHostKeyAlgos)
@@ -257,6 +282,10 @@ type Config struct {
 	// The allowed MAC algorithms. If unspecified then a sensible default
 	// is used.
 	MACs []string
+
+	// A list of enabled extensions. If unspecified then a sensible
+	// default is used
+	Extensions []string
 }
 
 // SetDefaults sets sensible values for unset fields in config. This is
@@ -286,6 +315,10 @@ func (c *Config) SetDefaults() {
 		c.MACs = supportedMACs
 	}
 
+	if c.Extensions == nil {
+		c.Extensions = defaultExtensions
+	}
+
 	if c.RekeyThreshold == 0 {
 		// cipher specific default
 	} else if c.RekeyThreshold < minRekeyThreshold {

ssh/handshake.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"log"
 	"net"
+	"strings"
 	"sync"
 )
 
@@ -94,6 +95,10 @@ type handshakeTransport struct {
 
 	// The session ID or nil if first kex did not complete yet.
 	sessionID []byte
+
+	// True if the first ext info message has been sent immediately following
+	// SSH_MSG_NEWKEYS, false otherwise.
+	extInfoSent bool
 }
 
 type pendingKex struct {
@@ -474,6 +479,9 @@ func (t *handshakeTransport) sendKexInit() error {
 				msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, keyFormat)
 			}
 		}
+		if contains(t.config.Extensions, ExtServerSigAlgs) {
+			msg.KexAlgos = append(msg.KexAlgos, extInfoServer)
+		}
 	} else {
 		msg.ServerHostKeyAlgos = t.hostKeyAlgorithms
 
@@ -483,7 +491,7 @@ func (t *handshakeTransport) sendKexInit() error {
 		if firstKeyExchange := t.sessionID == nil; firstKeyExchange {
 			msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1)
 			msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
-			msg.KexAlgos = append(msg.KexAlgos, "ext-info-c")
+			msg.KexAlgos = append(msg.KexAlgos, extInfoClient)
 		}
 	}
 
@@ -632,6 +640,33 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
 		return unexpectedMessageError(msgNewKeys, packet[0])
 	}
 
+	if !isClient {
+		// We're on the server side, see if the client sent the extension signal
+		if !t.extInfoSent && contains(clientInit.KexAlgos, extInfoClient) && contains(t.config.Extensions, ExtServerSigAlgs) {
+			// The other side supports ext info, an ext info message hasn't been sent this session,
+			// and we have at least one extension enabled, so send an SSH_MSG_EXT_INFO message.
+			extensions := map[string][]byte{}
+			// We're the server, the client supports SSH_MSG_EXT_INFO and server-sig-algs
+			// is enabled. Prepare the server-sig-algos extension message to send.
+			extensions[ExtServerSigAlgs] = []byte(strings.Join(supportedServerSigAlgs, ","))
+			var payload []byte
+			for k, v := range extensions {
+				payload = appendInt(payload, len(k))
+				payload = append(payload, k...)
+				payload = appendInt(payload, len(v))
+				payload = append(payload, v...)
+			}
+			extInfo := extInfoMsg{
+				NumExtensions: uint32(len(extensions)),
+				Payload:       payload,
+			}
+			if err := t.conn.writePacket(Marshal(&extInfo)); err != nil {
+				return err
+			}
+			t.extInfoSent = true
+		}
+	}
+
 	return nil
 }
 

ssh/server.go

@@ -274,11 +274,41 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 	// We just did the key change, so the session ID is established.
 	s.sessionID = s.transport.getSessionID()
 
+	// the client could send a SSH_MSG_EXT_INFO before SSH_MSG_SERVICE_REQUEST
 	var packet []byte
 	if packet, err = s.transport.readPacket(); err != nil {
 		return nil, err
 	}
 
+	// be permissive and don't add contains(s.transport.config.Extensions, ExtServerSigAlgs)
+	if len(packet) > 0 && packet[0] == msgExtInfo {
+		// read SSH_MSG_EXT_INFO
+		var extInfo extInfoMsg
+		extensions := make(map[string][]byte)
+		if err := Unmarshal(packet, &extInfo); err != nil {
+			return nil, err
+		}
+		payload := extInfo.Payload
+		for i := uint32(0); i < extInfo.NumExtensions; i++ {
+			name, rest, ok := parseString(payload)
+			if !ok {
+				return nil, parseError(msgExtInfo)
+			}
+			value, rest, ok := parseString(rest)
+			if !ok {
+				return nil, parseError(msgExtInfo)
+			}
+			extensions[string(name)] = value
+			payload = rest
+		}
+
+		// read the next packet
+		packet = nil
+		if packet, err = s.transport.readPacket(); err != nil {
+			return nil, err
+		}
+	}
+
 	var serviceRequest serviceRequestMsg
 	if err = Unmarshal(packet, &serviceRequest); err != nil {
 		return nil, err
@@ -304,7 +334,8 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 func isAcceptableAlgo(algo string) bool {
 	switch algo {
 	case KeyAlgoRSA, KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoDSA, KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521, KeyAlgoSKECDSA256, KeyAlgoED25519, KeyAlgoSKED25519,
-		CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01, CertAlgoECDSA384v01, CertAlgoECDSA521v01, CertAlgoSKECDSA256v01, CertAlgoED25519v01, CertAlgoSKED25519v01:
+		CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01, CertAlgoECDSA384v01, CertAlgoECDSA521v01, CertAlgoSKECDSA256v01, CertAlgoED25519v01, CertAlgoSKED25519v01,
+		CertAlgoRSASHA256v01, CertAlgoRSASHA512v01:
 		return true
 	}
 	return false