NetBird SSH (#361)

This PR adds support for SSH access through the NetBird network
without managing SSH skeys.
NetBird client app has an embedded SSH server (Linux/Mac only) 
and a netbird ssh command.

client/ssh/util.go

@@ -0,0 +1,86 @@
+package ssh
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"golang.org/x/crypto/ed25519"
+	gossh "golang.org/x/crypto/ssh"
+	"strings"
+)
+
+// KeyType is a type of SSH key
+type KeyType string
+
+// ED25519 is key of type ed25519
+const ED25519 KeyType = "ed25519"
+
+// ECDSA is key of type ecdsa
+const ECDSA KeyType = "ecdsa"
+
+// RSA is key of type rsa
+const RSA KeyType = "rsa"
+
+// RSAKeySize is a size of newly generated RSA key
+const RSAKeySize = 2048
+
+// GeneratePrivateKey creates RSA Private Key of specified byte size
+func GeneratePrivateKey(keyType KeyType) ([]byte, error) {
+
+	var key crypto.Signer
+	var err error
+	switch keyType {
+	case ED25519:
+		_, key, err = ed25519.GenerateKey(rand.Reader)
+	case ECDSA:
+		key, err = ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	case RSA:
+		key, err = rsa.GenerateKey(rand.Reader, RSAKeySize)
+	default:
+		return nil, fmt.Errorf("unsupported ket type %s", keyType)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	pemBytes, err := EncodePrivateKeyToPEM(key)
+	if err != nil {
+		return nil, err
+	}
+
+	return pemBytes, nil
+}
+
+// GeneratePublicKey returns the public part of the private key
+func GeneratePublicKey(key []byte) ([]byte, error) {
+	signer, err := gossh.ParsePrivateKey(key)
+	if err != nil {
+		return nil, err
+	}
+
+	strKey := strings.TrimSpace(string(gossh.MarshalAuthorizedKey(signer.PublicKey())))
+	return []byte(strKey), nil
+}
+
+// EncodePrivateKeyToPEM encodes Private Key from RSA to PEM format
+func EncodePrivateKeyToPEM(privateKey crypto.Signer) ([]byte, error) {
+	mk, err := x509.MarshalPKCS8PrivateKey(privateKey)
+	if err != nil {
+		return nil, err
+	}
+
+	// pem.Block
+	privBlock := pem.Block{
+		Type:  "PRIVATE KEY",
+		Bytes: mk,
+	}
+
+	// Private key in PEM format
+	privatePEM := pem.EncodeToMemory(&privBlock)
+	return privatePEM, nil
+}