Deterministic private key with Go 1.20 #427

commit 234a8f6b8c
Author: cmeng <cmenginnz@gmail.com>
Date:   Tue May 16 10:26:19 2023 +1200

    allow users to specify a private key file

commit 85a7d96b0c
Author: cmeng <cmenginnz@gmail.com>
Date:   Tue May 16 10:25:20 2023 +1200

    generate deterministic key with a given seed

main.go

@@ -3,7 +3,6 @@ package main
 import (
 	"flag"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"net/http"
 	"os"
@@ -87,7 +86,7 @@ var commonHelp = `
 
 func generatePidFile() {
 	pid := []byte(strconv.Itoa(os.Getpid()))
-	if err := ioutil.WriteFile("chisel.pid", pid, 0644); err != nil {
+	if err := os.WriteFile("chisel.pid", pid, 0644); err != nil {
 		log.Fatal(err)
 	}
 }
@@ -109,6 +108,11 @@ var serverHelp = `
     of man-in-the-middle attacks (defaults to the CHISEL_KEY environment
     variable, otherwise a new key is generate each run).
 
+    --key-file, An optional path to a PEM-encoded SSH private key. When 
+    this flag is set, the --key option is ignored. A private key file
+	can be generated by:
+    openssl ecparam -name prime256v1 -genkey -out private_key.pem
+
     --authfile, An optional path to a users.json file. This file should
     be an object with users defined like:
       {
@@ -170,6 +174,7 @@ func server(args []string) {
 
 	config := &chserver.Config{}
 	flags.StringVar(&config.KeySeed, "key", "", "")
+	flags.StringVar(&config.KeyFile, "key-file", "", "")
 	flags.StringVar(&config.AuthFile, "authfile", "", "")
 	flags.StringVar(&config.Auth, "auth", "", "")
 	flags.DurationVar(&config.KeepAlive, "keepalive", 25*time.Second, "")

server/server.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"os"
 	"regexp"
 	"time"
 
@@ -23,6 +24,7 @@ import (
 // Config is the configuration for the chisel service
 type Config struct {
 	KeySeed   string
+	KeyFile   string
 	AuthFile  string
 	Auth      string
 	Proxy     string
@@ -73,11 +75,23 @@ func NewServer(c *Config) (*Server, error) {
 			server.users.AddUser(u)
 		}
 	}
-	//generate private key (optionally using seed)
-	key, err := ccrypto.GenerateKey(c.KeySeed)
-	if err != nil {
-		log.Fatal("Failed to generate key")
+
+	var key []byte
+	var err error
+	if c.KeyFile != "" {
+		//read private key from the file specified path
+		key, err = os.ReadFile(c.KeyFile)
+		if err != nil {
+			log.Fatal("Failed to read private key from disk")
+		}
+	} else {
+		//generate private key (optionally using seed)
+		key, err = ccrypto.GenerateKey(c.KeySeed)
+		if err != nil {
+			log.Fatal("Failed to generate key")
+		}
 	}
+
 	//convert into ssh.PrivateKey
 	private, err := ssh.ParsePrivateKey(key)
 	if err != nil {

share/ccrypto/determ_rand.go

@@ -34,7 +34,14 @@ func (d *determRand) Read(b []byte) (int, error) {
 	for n < l {
 		next, out := hash(d.next)
 		n += copy(b[n:], out)
-		d.next = next
+		// In Golang 1.20, ecdsa.GenerateKey() introduced a function called
+		// MaybeReadRand() which reads 1 byte from the determRand reader
+		// with 50% chance. As a result, GenerateKey() generates
+		// nondeterministic keys.
+		// The following conditional check neutralizes this effect.
+		if l > 1 {
+			d.next = next
+		}
 	}
 	return n, nil
 }