Update client to fall-back to MD5 fingerprints

Signed-off-by: Simon Rüegg <simon@rueggs.ch>

README.md

@@ -244,9 +244,8 @@ $ chisel client --help
 
   Options:
 
-    --fingerprint, A *strongly recommended* fingerprint string
-    to perform host-key validation against the server's public key.
-    You may provide just a prefix of the key or the entire string.
+    --fingerprint, A *strongly recommended* fingerprint (base64 encoded SHA256)
+    string to perform host-key validation against the server's public key.
     Fingerprint mismatches will close the connection.
 
     --auth, An optional username and password (client authentication)
@@ -319,7 +318,7 @@ $ chisel client --help
 
 ### Security
 
-Encryption is always enabled. When you start up a chisel server, it will generate an in-memory ECDSA public/private key pair. The public key fingerprint will be displayed as the server starts. Instead of generating a random key, the server may optionally specify a key seed, using the `--key` option, which will be used to seed the key generation. When clients connect, they will also display the server's public key fingerprint. The client can force a particular fingerprint using the `--fingerprint` option. See the `--help` above for more information.
+Encryption is always enabled. When you start up a chisel server, it will generate an in-memory ECDSA public/private key pair. The public key fingerprint (base64 encoded SHA256) will be displayed as the server starts. Instead of generating a random key, the server may optionally specify a key seed, using the `--key` option, which will be used to seed the key generation. When clients connect, they will also display the server's public key fingerprint. The client can force a particular fingerprint using the `--fingerprint` option. See the `--help` above for more information.
 
 ### Authentication
 
@@ -341,7 +340,7 @@ docker run \
 2. Connect your chisel client (using server's fingerprint)
 
 ```sh
-chisel client --fingerprint ab:12:34 <server-address>:9312 socks
+chisel client --fingerprint 'rHb55mcxf6vSckL2AezFV09rLs7pfPpavVu++MF7AhQ=' <server-address>:9312 socks
 ```
 
 3. Point your SOCKS5 clients (e.g. OS/Browser) to:

client/client.go

@@ -2,8 +2,10 @@ package chclient
 
 import (
 	"context"
+	"crypto/md5"
 	"crypto/tls"
 	"crypto/x509"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"io/ioutil"
@@ -194,15 +196,40 @@ func (c *Client) Run() error {
 
 func (c *Client) verifyServer(hostname string, remote net.Addr, key ssh.PublicKey) error {
 	expect := c.config.Fingerprint
+	if expect == "" {
+		return nil
+	}
 	got := ccrypto.FingerprintKey(key)
-	if expect != "" && !strings.HasPrefix(got, expect) {
+	_, err := base64.StdEncoding.DecodeString(expect)
+	if _, ok := err.(base64.CorruptInputError); ok {
+		c.Logger.Infof("Specified deprecated MD5 fingerprint (%s), please update to the new SHA256 fingerprint: %s", expect, got)
+		return c.verifyLegacyFingerprint(key)
+	} else if err != nil {
+		return fmt.Errorf("error decoding fingerprint: %w", err)
+	}
+	if got != expect {
 		return fmt.Errorf("Invalid fingerprint (%s)", got)
 	}
 	//overwrite with complete fingerprint
 	c.Infof("Fingerprint %s", got)
 	return nil
 }
 
+//verifyLegacyFingerprint calculates and compares legacy MD5 fingerprints
+func (c *Client) verifyLegacyFingerprint(key ssh.PublicKey) error {
+	bytes := md5.Sum(key.Marshal())
+	strbytes := make([]string, len(bytes))
+	for i, b := range bytes {
+		strbytes[i] = fmt.Sprintf("%02x", b)
+	}
+	got := strings.Join(strbytes, ":")
+	expect := c.config.Fingerprint
+	if !strings.HasPrefix(got, expect) {
+		return fmt.Errorf("Invalid fingerprint (%s)", got)
+	}
+	return nil
+}
+
 //Start client and does not block
 func (c *Client) Start(ctx context.Context) error {
 	ctx, cancel := context.WithCancel(ctx)

client/client_test.go

@@ -1,12 +1,17 @@
 package chclient
 
 import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
 	"log"
 	"net/http"
 	"net/http/httptest"
 	"sync"
 	"testing"
 	"time"
+
+	"github.com/jpillora/chisel/share/ccrypto"
+	"golang.org/x/crypto/ssh"
 )
 
 func TestCustomHeaders(t *testing.T) {
@@ -39,3 +44,72 @@ func TestCustomHeaders(t *testing.T) {
 	wg.Wait()
 	c.Close()
 }
+
+func TestFallbackLegacyFingerprint(t *testing.T) {
+	config := Config{
+		Fingerprint: "a5:32:92:c6:56:7a:9e:61:26:74:1b:81:a6:f5:1b:44",
+	}
+	c, err := NewClient(&config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := ccrypto.NewDetermRand([]byte("test123"))
+	priv, err := ecdsa.GenerateKey(elliptic.P256(), r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pub, err := ssh.NewPublicKey(&priv.PublicKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = c.verifyServer("", nil, pub)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestVerifyLegacyFingerprint(t *testing.T) {
+	config := Config{
+		Fingerprint: "a5:32:92:c6:56:7a:9e:61:26:74:1b:81:a6:f5:1b:44",
+	}
+	c, err := NewClient(&config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := ccrypto.NewDetermRand([]byte("test123"))
+	priv, err := ecdsa.GenerateKey(elliptic.P256(), r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pub, err := ssh.NewPublicKey(&priv.PublicKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = c.verifyLegacyFingerprint(pub)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestVerifyFingerprint(t *testing.T) {
+	config := Config{
+		Fingerprint: "qmrRoo8MIqePv3jC8+wv49gU6uaFgD3FASQx9V8KdmY=",
+	}
+	c, err := NewClient(&config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := ccrypto.NewDetermRand([]byte("test123"))
+	priv, err := ecdsa.GenerateKey(elliptic.P256(), r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pub, err := ssh.NewPublicKey(&priv.PublicKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = c.verifyServer("", nil, pub)
+	if err != nil {
+		t.Fatal(err)
+	}
+}

main.go

@@ -328,9 +328,8 @@ var clientHelp = `
 
   Options:
 
-    --fingerprint, A *strongly recommended* fingerprint string
+    --fingerprint, A *strongly recommended* fingerprint (base64 encoded SHA256) string
     to perform host-key validation against the server's public key.
-    You may provide just a prefix of the key or the entire string.
     Fingerprint mismatches will close the connection.
 
     --auth, An optional username and password (client authentication)

test/bench/main.go

@@ -190,7 +190,7 @@ func main() {
 
 	hf := exec.Command("chisel", "client",
 		// "-v",
-		"--fingerprint", "ed:f2:cf:3c:56",
+		"--fingerprint", "mOz4rg9zlQ409XAhhj6+fDDVwQMY42CL3Zg2W2oTYxA=",
 		"127.0.0.1:2002",
 		"2001:3000")
 	hf.Stdout = os.Stdout