ssh: add server side support for Diffie Hellman Group Exchange

We add this support for the following reasons:

- We are planning to expose recommended (secure) vs. supported (works,
  not necessarily recommended) algorithms. The DHGEX kex is currently
  only exposed as a client-side kex. To simplify the calling convention
  for this follow-on, we expose the server side too.
- Some clients are quite inflexible with reference to kex algorithms
  choice, for example they offer:

  diffie-hellman-group-exchange-sha256, diffie-hellman-group-exchange-sha1,
  diffie-hellman-group14-sha1, diffie-hellman-group1-sha1

  therefore DHGEX helps interoperability.

We do not recommend the DHGEX kex as a whole:

- the negotiation requires an extra round trip
- the server must generate parameters (slow) or hardcode them, which
  defeats the security benefit over traditional DH.

In this implementation we hardcode sending Oakley Group 14, Oakley
Group 15 or Oakley Group 16 based on the requested max size.

Users that are concerned with security of classical DH kex should
migrate to kex based on EC or Ed25519.

Fixes golang/go#54743

Change-Id: I127822e90efc36821af4aca679931f40a2023021
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/532415
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

5 files changed, 118 insertions, 52 deletions

diff --git a/ssh/common.go b/ssh/common.go
index b98b51c..0415d33 100644
--- a/ssh/common.go
+++ b/ssh/common.go
@@ -279,13 +279,6 @@ func InsecureAlgorithms() Algorithms {
 	}
 }
 
-// serverForbiddenKexAlgos contains key exchange algorithms, that are forbidden
-// for the server half.
-var serverForbiddenKexAlgos = map[string]struct{}{
-	InsecureKeyExchangeDHGEXSHA1: {}, // server half implementation is only minimal to satisfy the automated tests
-	KeyExchangeDHGEXSHA256:       {}, // server half implementation is only minimal to satisfy the automated tests
-}
-
 var supportedCompressions = []string{compressionNone}
 
 // hashFuncs keeps the mapping of supported signature algorithms to their
diff --git a/ssh/kex.go b/ssh/kex.go
index 184544a..cf388a9 100644
--- a/ssh/kex.go
+++ b/ssh/kex.go
@@ -19,6 +19,21 @@ import (
 	"golang.org/x/crypto/curve25519"
 )
 
+const (
+	// This is the group called diffie-hellman-group1-sha1 in RFC 4253 and
+	// Oakley Group 2 in RFC 2409.
+	oakleyGroup2 = "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF"
+	// This is the group called diffie-hellman-group14-sha1 in RFC 4253 and
+	// Oakley Group 14 in RFC 3526.
+	oakleyGroup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
+	// This is the group called diffie-hellman-group15-sha512 in RFC 8268 and
+	// Oakley Group 15 in RFC 3526.
+	oakleyGroup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
+	// This is the group called diffie-hellman-group16-sha512 in RFC 8268 and
+	// Oakley Group 16 in RFC 3526.
+	oakleyGroup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
+)
+
 // kexResult captures the outcome of a key exchange.
 type kexResult struct {
 	// Session hash. See also RFC 4253, section 8.
@@ -384,9 +399,7 @@ func ecHash(curve elliptic.Curve) crypto.Hash {
 var kexAlgoMap = map[string]kexAlgorithm{}
 
 func init() {
-	// This is the group called diffie-hellman-group1-sha1 in
-	// RFC 4253 and Oakley Group 2 in RFC 2409.
-	p, _ := new(big.Int).SetString("FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF", 16)
+	p, _ := new(big.Int).SetString(oakleyGroup2, 16)
 	kexAlgoMap[InsecureKeyExchangeDH1SHA1] = &dhGroup{
 		g:        new(big.Int).SetInt64(2),
 		p:        p,
@@ -394,10 +407,7 @@ func init() {
 		hashFunc: crypto.SHA1,
 	}
 
-	// This are the groups called diffie-hellman-group14-sha1 and
-	// diffie-hellman-group14-sha256 in RFC 4253 and RFC 8268,
-	// and Oakley Group 14 in RFC 3526.
-	p, _ = new(big.Int).SetString("FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF6955817183995497CEA956AE515D2261898FA051015728E5A8AACAA68FFFFFFFFFFFFFFFF", 16)
+	p, _ = new(big.Int).SetString(oakleyGroup14, 16)
 	group14 := &dhGroup{
 		g:       new(big.Int).SetInt64(2),
 		p:       p,
@@ -413,9 +423,7 @@ func init() {
 		hashFunc: crypto.SHA256,
 	}
 
-	// This is the group called diffie-hellman-group16-sha512 in RFC
-	// 8268 and Oakley Group 16 in RFC 3526.
-	p, _ = new(big.Int).SetString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
+	p, _ = new(big.Int).SetString(oakleyGroup16, 16)
 
 	kexAlgoMap[KeyExchangeDH16SHA512] = &dhGroup{
 		g:        new(big.Int).SetInt64(2),
@@ -583,9 +591,9 @@ const (
 func (gex *dhGEXSHA) Client(c packetConn, randSource io.Reader, magics *handshakeMagics) (*kexResult, error) {
 	// Send GexRequest
 	kexDHGexRequest := kexDHGexRequestMsg{
-		MinBits:      dhGroupExchangeMinimumBits,
-		PreferedBits: dhGroupExchangePreferredBits,
-		MaxBits:      dhGroupExchangeMaximumBits,
+		MinBits:       dhGroupExchangeMinimumBits,
+		PreferredBits: dhGroupExchangePreferredBits,
+		MaxBits:       dhGroupExchangeMaximumBits,
 	}
 	if err := c.writePacket(Marshal(&kexDHGexRequest)); err != nil {
 		return nil, err
@@ -672,9 +680,7 @@ func (gex *dhGEXSHA) Client(c packetConn, randSource io.Reader, magics *handshak
 }
 
 // Server half implementation of the Diffie Hellman Key Exchange with SHA1 and SHA256.
-//
-// This is a minimal implementation to satisfy the automated tests.
-func (gex dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (result *kexResult, err error) {
+func (gex *dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (result *kexResult, err error) {
 	// Receive GexRequest
 	packet, err := c.readPacket()
 	if err != nil {
@@ -684,13 +690,32 @@ func (gex dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshake
 	if err = Unmarshal(packet, &kexDHGexRequest); err != nil {
 		return
 	}
+	// We check that the request received is valid and that the MaxBits
+	// requested are at least equal to our supported minimum. This is the same
+	// check done in OpenSSH:
+	// https://github.com/openssh/openssh-portable/blob/80a2f64b/kexgexs.c#L94
+	//
+	// Furthermore, we also check that the required MinBits are less than or
+	// equal to 4096 because we can use up to Oakley Group 16.
+	if kexDHGexRequest.MaxBits < kexDHGexRequest.MinBits || kexDHGexRequest.PreferredBits < kexDHGexRequest.MinBits ||
+		kexDHGexRequest.MaxBits < kexDHGexRequest.PreferredBits || kexDHGexRequest.MaxBits < dhGroupExchangeMinimumBits ||
+		kexDHGexRequest.MinBits > 4096 {
+		return nil, fmt.Errorf("ssh: DH GEX request out of range, min: %d, max: %d, preferred: %d", kexDHGexRequest.MinBits,
+			kexDHGexRequest.MaxBits, kexDHGexRequest.PreferredBits)
+	}
 
-	// Send GexGroup
-	// This is the group called diffie-hellman-group14-sha1 in RFC
-	// 4253 and Oakley Group 14 in RFC 3526.
-	p, _ := new(big.Int).SetString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
-	g := big.NewInt(2)
+	var p *big.Int
+	// We hardcode sending Oakley Group 14 (2048 bits), Oakley Group 15 (3072
+	// bits) or Oakley Group 16 (4096 bits), based on the requested max size.
+	if kexDHGexRequest.MaxBits < 3072 {
+		p, _ = new(big.Int).SetString(oakleyGroup14, 16)
+	} else if kexDHGexRequest.MaxBits < 4096 {
+		p, _ = new(big.Int).SetString(oakleyGroup15, 16)
+	} else {
+		p, _ = new(big.Int).SetString(oakleyGroup16, 16)
+	}
 
+	g := big.NewInt(2)
 	msg := &kexDHGexGroupMsg{
 		P: p,
 		G: g,
@@ -728,9 +753,9 @@ func (gex dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshake
 	h := gex.hashFunc.New()
 	magics.write(h)
 	writeString(h, hostKeyBytes)
-	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMinimumBits))
-	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangePreferredBits))
-	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMaximumBits))
+	binary.Write(h, binary.BigEndian, kexDHGexRequest.MinBits)
+	binary.Write(h, binary.BigEndian, kexDHGexRequest.PreferredBits)
+	binary.Write(h, binary.BigEndian, kexDHGexRequest.MaxBits)
 	writeInt(h, p)
 	writeInt(h, g)
 	writeInt(h, kexDHGexInit.X)
diff --git a/ssh/messages.go b/ssh/messages.go
index 118427b..251b9d0 100644
--- a/ssh/messages.go
+++ b/ssh/messages.go
@@ -122,9 +122,9 @@ type kexDHGexReplyMsg struct {
 const msgKexDHGexRequest = 34
 
 type kexDHGexRequestMsg struct {
-	MinBits      uint32 `sshtype:"34"`
-	PreferedBits uint32
-	MaxBits      uint32
+	MinBits       uint32 `sshtype:"34"`
+	PreferredBits uint32
+	MaxBits       uint32
 }
 
 // See RFC 4253, section 10.
diff --git a/ssh/server.go b/ssh/server.go
index f912bfb..98679ba 100644
--- a/ssh/server.go
+++ b/ssh/server.go
@@ -252,13 +252,6 @@ func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewCha
 			}
 		}
 	}
-	// Check if the config contains any unsupported key exchanges
-	for _, kex := range fullConf.KeyExchanges {
-		if _, ok := serverForbiddenKexAlgos[kex]; ok {
-			c.Close()
-			return nil, nil, nil, fmt.Errorf("ssh: unsupported key exchange %s for server", kex)
-		}
-	}
 
 	s := &connection{
 		sshConn: sshConn{conn: c},
diff --git a/ssh/test/sshcli_test.go b/ssh/test/sshcli_test.go
index ac2f7c1..6648067 100644
--- a/ssh/test/sshcli_test.go
+++ b/ssh/test/sshcli_test.go
@@ -34,23 +34,29 @@ func sshClient(t *testing.T) string {
 	return sshCLI
 }
 
+// setupSSHCLIKeys writes the provided key files to a temporary directory and
+// returns the path to the private key.
+func setupSSHCLIKeys(t *testing.T, keyFiles map[string][]byte, privKeyName string) string {
+	tmpDir := t.TempDir()
+	for fn, content := range keyFiles {
+		if err := os.WriteFile(filepath.Join(tmpDir, fn), content, 0600); err != nil {
+			t.Fatalf("WriteFile(%q): %v", fn, err)
+		}
+	}
+	return filepath.Join(tmpDir, privKeyName)
+}
+
 func TestSSHCLIAuth(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		t.Skipf("always fails on Windows, see #64403")
 	}
 	sshCLI := sshClient(t)
-	dir := t.TempDir()
-	keyPrivPath := filepath.Join(dir, "rsa")
-
-	for fn, content := range map[string][]byte{
-		keyPrivPath:                        testdata.PEMBytes["rsa"],
-		keyPrivPath + ".pub":               ssh.MarshalAuthorizedKey(testPublicKeys["rsa"]),
-		filepath.Join(dir, "rsa-cert.pub"): testdata.SSHCertificates["rsa-user-testcertificate"],
-	} {
-		if err := os.WriteFile(fn, content, 0600); err != nil {
-			t.Fatalf("WriteFile(%q): %v", fn, err)
-		}
+	keyFiles := map[string][]byte{
+		"rsa":          testdata.PEMBytes["rsa"],
+		"rsa.pub":      ssh.MarshalAuthorizedKey(testPublicKeys["rsa"]),
+		"rsa-cert.pub": testdata.SSHCertificates["rsa-user-testcertificate"],
 	}
+	keyPrivPath := setupSSHCLIKeys(t, keyFiles, "rsa")
 
 	certChecker := ssh.CertChecker{
 		IsUserAuthority: func(k ssh.PublicKey) bool {
@@ -98,3 +104,52 @@ func TestSSHCLIAuth(t *testing.T) {
 		t.Fatalf("user certificate authentication failed, error: %v, command output %q", err, string(out))
 	}
 }
+
+func TestSSHCLIKeyExchanges(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("always fails on Windows, see #64403")
+	}
+	sshCLI := sshClient(t)
+	keyFiles := map[string][]byte{
+		"rsa":     testdata.PEMBytes["rsa"],
+		"rsa.pub": ssh.MarshalAuthorizedKey(testPublicKeys["rsa"]),
+	}
+	keyPrivPath := setupSSHCLIKeys(t, keyFiles, "rsa")
+
+	keyExchanges := append(ssh.SupportedAlgorithms().KeyExchanges, ssh.InsecureAlgorithms().KeyExchanges...)
+	for _, kex := range keyExchanges {
+		t.Run(kex, func(t *testing.T) {
+			config := &ssh.ServerConfig{
+				Config: ssh.Config{
+					KeyExchanges: []string{kex},
+				},
+				PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+					if conn.User() == "testpubkey" && bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+						return nil, nil
+					}
+
+					return nil, fmt.Errorf("pubkey for %q not acceptable", conn.User())
+				},
+			}
+			config.AddHostKey(testSigners["rsa"])
+
+			server, err := newTestServer(config)
+			if err != nil {
+				t.Fatalf("unable to start test server: %v", err)
+			}
+			defer server.Close()
+
+			port, err := server.port()
+			if err != nil {
+				t.Fatalf("unable to get server port: %v", err)
+			}
+
+			cmd := testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+				"-o", fmt.Sprintf("KexAlgorithms=%s", kex), "-p", port, "testpubkey@127.0.0.1", "true")
+			out, err := cmd.CombinedOutput()
+			if err != nil {
+				t.Fatalf("%s failed, error: %v, command output %q", kex, err, string(out))
+			}
+		})
+	}
+}