add flag to customize kex algos

Adds a flag `ssh-kex-algos` which configures the gogit and libgit2
managed clients to use the specified list of kex algos for ssh. If not
used the default list in `golang/x/crypto/ssh` is used.

Signed-off-by: Sanskar Jaiswal <jaiswalsanskar078@gmail.com>

main.go

@@ -46,6 +46,7 @@ import (
 	"github.com/fluxcd/source-controller/controllers"
 	"github.com/fluxcd/source-controller/internal/cache"
 	"github.com/fluxcd/source-controller/internal/helm"
+	"github.com/fluxcd/source-controller/pkg/git"
 	"github.com/fluxcd/source-controller/pkg/git/libgit2/managed"
 	// +kubebuilder:scaffold:imports
 )
@@ -90,6 +91,7 @@ func main() {
 		helmCacheMaxSize       int
 		helmCacheTTL           string
 		helmCachePurgeInterval string
+		kexAlgos               []string
 	)
 
 	flag.StringVar(&metricsAddr, "metrics-addr", envOrDefault("METRICS_ADDR", ":8080"),
@@ -120,6 +122,8 @@ func main() {
 		"The TTL of an index in the cache. Valid time units are ns, us (or µs), ms, s, m, h.")
 	flag.StringVar(&helmCachePurgeInterval, "helm-cache-purge-interval", "1m",
 		"The interval at which the cache is purged. Valid time units are ns, us (or µs), ms, s, m, h.")
+	flag.StringSliceVar(&kexAlgos, "ssh-kex-algos", []string{},
+		"The list of key exchange algorithms to use for ssh connections, arranged from most preferred to the least.")
 
 	clientOptions.BindFlags(flag.CommandLine)
 	logOptions.BindFlags(flag.CommandLine)
@@ -174,6 +178,7 @@ func main() {
 		storageAdvAddr = determineAdvStorageAddr(storageAddr, setupLog)
 	}
 	storage := mustInitStorage(storagePath, storageAdvAddr, setupLog)
+	setPreferredKexAlgos(kexAlgos)
 
 	if err = (&controllers.GitRepositoryReconciler{
 		Client:         mgr.GetClient(),
@@ -333,3 +338,7 @@ func envOrDefault(envName, defaultValue string) string {
 
 	return defaultValue
 }
+
+func setPreferredKexAlgos(algos []string) {
+	git.KexAlgos = algos
+}

pkg/git/gogit/transport.go

@@ -26,6 +26,8 @@ import (
 	"github.com/fluxcd/pkg/ssh/knownhosts"
 
 	"github.com/fluxcd/source-controller/pkg/git"
+
+	gossh "golang.org/x/crypto/ssh"
 )
 
 // transportAuth constructs the transport.AuthMethod for the git.Transport of
@@ -58,7 +60,10 @@ func transportAuth(opts *git.AuthOptions) (transport.AuthMethod, error) {
 				}
 				pk.HostKeyCallback = callback
 			}
-			return pk, nil
+			customPK := &CustomPublicKeys{
+				pk: pk,
+			}
+			return customPK, nil
 		}
 	case "":
 		return nil, fmt.Errorf("no transport type set")
@@ -75,3 +80,28 @@ func caBundle(opts *git.AuthOptions) []byte {
 	}
 	return opts.CAFile
 }
+
+// CustomPublicKeys is a wrapper around ssh.PublicKeys to help us
+// customize the ssh config. It implements ssh.AuthMethod.
+type CustomPublicKeys struct {
+	pk *ssh.PublicKeys
+}
+
+func (a *CustomPublicKeys) Name() string {
+	return a.pk.Name()
+}
+
+func (a *CustomPublicKeys) String() string {
+	return a.pk.String()
+}
+
+func (a *CustomPublicKeys) ClientConfig() (*gossh.ClientConfig, error) {
+	config, err := a.pk.ClientConfig()
+	if err != nil {
+		return nil, err
+	}
+	if len(git.KexAlgos) > 0 {
+		config.Config.KeyExchanges = git.KexAlgos
+	}
+	return config, nil
+}

pkg/git/gogit/transport_test.go

@@ -22,7 +22,6 @@ import (
 
 	"github.com/go-git/go-git/v5/plumbing/transport"
 	"github.com/go-git/go-git/v5/plumbing/transport/http"
-	"github.com/go-git/go-git/v5/plumbing/transport/ssh"
 	. "github.com/onsi/gomega"
 
 	"github.com/fluxcd/source-controller/pkg/git"
@@ -72,6 +71,7 @@ func Test_transportAuth(t *testing.T) {
 		name     string
 		opts     *git.AuthOptions
 		wantFunc func(g *WithT, t transport.AuthMethod, opts *git.AuthOptions)
+		kexAlgos []string
 		wantErr  error
 	}{
 		{
@@ -128,10 +128,10 @@ func Test_transportAuth(t *testing.T) {
 				Identity:  []byte(privateKeyFixture),
 			},
 			wantFunc: func(g *WithT, t transport.AuthMethod, opts *git.AuthOptions) {
-				tt, ok := t.(*ssh.PublicKeys)
+				tt, ok := t.(*CustomPublicKeys)
 				g.Expect(ok).To(BeTrue())
-				g.Expect(tt.User).To(Equal(opts.Username))
-				g.Expect(tt.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
+				g.Expect(tt.pk.User).To(Equal(opts.Username))
+				g.Expect(tt.pk.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
 			},
 		},
 		{
@@ -143,10 +143,31 @@ func Test_transportAuth(t *testing.T) {
 				Identity:  []byte(privateKeyPassphraseFixture),
 			},
 			wantFunc: func(g *WithT, t transport.AuthMethod, opts *git.AuthOptions) {
-				tt, ok := t.(*ssh.PublicKeys)
+				tt, ok := t.(*CustomPublicKeys)
 				g.Expect(ok).To(BeTrue())
-				g.Expect(tt.User).To(Equal(opts.Username))
-				g.Expect(tt.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
+				g.Expect(tt.pk.User).To(Equal(opts.Username))
+				g.Expect(tt.pk.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
+			},
+		},
+		{
+			name: "SSH with custom key exchanges",
+			opts: &git.AuthOptions{
+				Transport:  git.SSH,
+				Username:   "example",
+				Identity:   []byte(privateKeyFixture),
+				KnownHosts: []byte(knownHostsFixture),
+			},
+			kexAlgos: []string{"curve25519-sha256", "diffie-hellman-group-exchange-sha256"},
+			wantFunc: func(g *WithT, t transport.AuthMethod, opts *git.AuthOptions) {
+				tt, ok := t.(*CustomPublicKeys)
+				g.Expect(ok).To(BeTrue())
+				g.Expect(tt.pk.User).To(Equal(opts.Username))
+				g.Expect(tt.pk.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
+				config, err := tt.ClientConfig()
+				g.Expect(err).ToNot(HaveOccurred())
+				g.Expect(config.Config.KeyExchanges).To(Equal(
+					[]string{"curve25519-sha256", "diffie-hellman-group-exchange-sha256"}),
+				)
 			},
 		},
 		{
@@ -168,11 +189,11 @@ func Test_transportAuth(t *testing.T) {
 				KnownHosts: []byte(knownHostsFixture),
 			},
 			wantFunc: func(g *WithT, t transport.AuthMethod, opts *git.AuthOptions) {
-				tt, ok := t.(*ssh.PublicKeys)
+				tt, ok := t.(*CustomPublicKeys)
 				g.Expect(ok).To(BeTrue())
-				g.Expect(tt.User).To(Equal(opts.Username))
-				g.Expect(tt.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
-				g.Expect(tt.HostKeyCallback).ToNot(BeNil())
+				g.Expect(tt.pk.User).To(Equal(opts.Username))
+				g.Expect(tt.pk.Signer.PublicKey().Type()).To(Equal("ssh-rsa"))
+				g.Expect(tt.pk.HostKeyCallback).ToNot(BeNil())
 			},
 		},
 		{
@@ -202,6 +223,10 @@ func Test_transportAuth(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			g := NewWithT(t)
 
+			if len(tt.kexAlgos) > 0 {
+				git.KexAlgos = tt.kexAlgos
+			}
+
 			got, err := transportAuth(tt.opts)
 			if tt.wantErr != nil {
 				g.Expect(err).To(Equal(tt.wantErr))

pkg/git/libgit2/managed/ssh.go

@@ -58,6 +58,7 @@ import (
 
 	"golang.org/x/crypto/ssh"
 
+	"github.com/fluxcd/source-controller/pkg/git"
 	git2go "github.com/libgit2/git2go/v33"
 )
 
@@ -344,6 +345,9 @@ func cacheKeyAndConfig(remoteAddress string, cred *git2go.Credential) (string, *
 		Auth:    []ssh.AuthMethod{ssh.PublicKeys(key)},
 		Timeout: sshConnectionTimeOut,
 	}
+	if len(git.KexAlgos) > 0 {
+		cfg.Config.KeyExchanges = git.KexAlgos
+	}
 
 	return ck, cfg, nil
 }

pkg/git/options.go

@@ -70,6 +70,9 @@ type AuthOptions struct {
 	CAFile     []byte
 }
 
+// List of custom key exchange algorithms to be used for ssh connections.
+var KexAlgos []string
+
 // Validate the AuthOptions against the defined Transport.
 func (o AuthOptions) Validate() error {
 	switch o.Transport {