Restrict sshd public_key_algorithms

Since https://github.com/golang/crypto/commit/eb61739cd99fb244c7cd188d3c5bae54824e781d
it is now possible to restrict the host_key_algorithms.
This commit allows sshd to restrict them in the server config.

config.yml.example

@@ -98,6 +98,8 @@ sshd:
   kex_algorithms: [curve25519-sha256, curve25519-sha256@libssh.org, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group14-sha256, diffie-hellman-group14-sha1]
   # Specified the ciphers allowed
   ciphers: [aes128-gcm@openssh.com, chacha20-poly1305@openssh.com, aes256-gcm@openssh.com, aes128-ctr, aes192-ctr,aes256-ctr]
+  # Specified the available Host Key algorithms
+  host_key_algorithms: [ssh-rsa, ssh-dss, ecdsa-sha2-nistp256, sk-ecdsa-sha2-nistp256@openssh.com, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521, ssh-ed25519, sk-ssh-ed25519@openssh.com, rsa-sha2-256, rsa-sha2-512]
   # SSH host key files.
   host_key_files:
     - /run/secrets/ssh-hostkeys/ssh_host_rsa_key

internal/config/config.go

@@ -47,6 +47,7 @@ type ServerConfig struct {
 	HostCertFiles           []string     `yaml:"host_cert_files,omitempty"`
 	MACs                    []string     `yaml:"macs"`
 	KexAlgorithms           []string     `yaml:"kex_algorithms"`
+	HostKeyAlgorithms       []string     `yaml:"host_key_algorithms"`
 	Ciphers                 []string     `yaml:"ciphers"`
 	GSSAPI                  GSSAPIConfig `yaml:"gssapi,omitempty"`
 }

internal/sshd/server_config.go

@@ -36,6 +36,19 @@ var (
 		"diffie-hellman-group14-sha256",
 		"diffie-hellman-group14-sha1",
 	}
+
+	supportedHostKeyAlgorithms = []string{
+		"ssh-rsa",
+		"ssh-dss",
+		"ecdsa-sha2-nistp256",
+		"sk-ecdsa-sha2-nistp256@openssh.com",
+		"ecdsa-sha2-nistp384",
+		"ecdsa-sha2-nistp521",
+		"ssh-ed25519",
+		"sk-ssh-ed25519@openssh.com",
+		"rsa-sha2-256",
+		"rsa-sha2-512",
+	}
 )
 
 type serverConfig struct {
@@ -266,6 +279,12 @@ func (s *serverConfig) get(ctx context.Context) *ssh.ServerConfig {
 		sshCfg.Ciphers = s.cfg.Server.Ciphers
 	}
 
+	if len(s.cfg.Server.HostKeyAlgorithms) > 0 {
+		sshCfg.PublicKeyAuthAlgorithms = s.cfg.Server.HostKeyAlgorithms
+	} else {
+		sshCfg.PublicKeyAuthAlgorithms = supportedHostKeyAlgorithms
+	}
+
 	for _, key := range s.hostKeys {
 		sshCfg.AddHostKey(key)
 	}

internal/sshd/server_config_test.go

@@ -251,6 +251,7 @@ func TestDefaultAlgorithms(t *testing.T) {
 
 	require.Equal(t, supportedMACs, sshServerConfig.MACs)
 	require.Equal(t, supportedKeyExchanges, sshServerConfig.KeyExchanges)
+	require.Equal(t, supportedHostKeyAlgorithms, sshServerConfig.PublicKeyAuthAlgorithms)
 	require.Nil(t, sshServerConfig.Ciphers)
 
 	sshServerConfig.SetDefaults()
@@ -266,6 +267,7 @@ func TestDefaultAlgorithms(t *testing.T) {
 		"aes192-ctr",
 		"aes256-ctr",
 	}
+
 	require.Equal(t, sshServerConfig.Ciphers, defaultCiphers)
 }
 
@@ -273,13 +275,15 @@ func TestCustomAlgorithms(t *testing.T) {
 	customMACs := []string{"hmac-sha2-512-etm@openssh.com"}
 	customKexAlgos := []string{"curve25519-sha256"}
 	customCiphers := []string{"aes256-gcm@openssh.com"}
+	customHostAlgos := []string{"rsa-sha2-256"}
 
 	srvCfg := &serverConfig{
 		cfg: &config.Config{
 			Server: config.ServerConfig{
-				MACs:          customMACs,
-				KexAlgorithms: customKexAlgos,
-				Ciphers:       customCiphers,
+				MACs:              customMACs,
+				KexAlgorithms:     customKexAlgos,
+				Ciphers:           customCiphers,
+				HostKeyAlgorithms: customHostAlgos,
 			},
 		},
 	}
@@ -288,6 +292,7 @@ func TestCustomAlgorithms(t *testing.T) {
 	require.Equal(t, customMACs, sshServerConfig.MACs)
 	require.Equal(t, customKexAlgos, sshServerConfig.KeyExchanges)
 	require.Equal(t, customCiphers, sshServerConfig.Ciphers)
+	require.Equal(t, customHostAlgos, sshServerConfig.PublicKeyAuthAlgorithms)
 
 	sshServerConfig.SetDefaults()