From 1e624a4cc3aafa57b5fa213c88bcd3689cefd1c3 Mon Sep 17 00:00:00 2001
From: Pavol Rusnak
Date: Sat, 5 Sep 2020 22:42:15 +0200
Subject: Add support for FIDO2 tokens

---
 internal/configfile/config_file.go   |  20 ++++++-
 internal/configfile/config_test.go   |   8 +--
 internal/configfile/feature_flags.go |   4 ++
 internal/exitcodes/exitcodes.go      |   2 +
 internal/fido2/fido2.go              | 110 +++++++++++++++++++++++++++++++++++
 5 files changed, 138 insertions(+), 6 deletions(-)
 create mode 100644 internal/fido2/fido2.go

(limited to 'internal')

diff --git a/internal/configfile/config_file.go b/internal/configfile/config_file.go
index c27ecd4..e4921f7 100644
--- a/internal/configfile/config_file.go
+++ b/internal/configfile/config_file.go
@@ -10,12 +10,13 @@ import (
 	"log"
 	"syscall"
 
+	"os"
+
 	"github.com/rfjakob/gocryptfs/internal/contentenc"
 	"github.com/rfjakob/gocryptfs/internal/cryptocore"
 	"github.com/rfjakob/gocryptfs/internal/exitcodes"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
-import "os"
 
 const (
 	// ConfDefaultName is the default configuration file name.
@@ -28,6 +29,14 @@ const (
 	ConfReverseName = ".gocryptfs.reverse.conf"
 )
 
+// FIDO2Params is a structure for storing FIDO2 parameters.
+type FIDO2Params struct {
+	// FIDO2 credential
+	CredentialID []byte
+	// FIDO2 hmac-secret salt
+	HMACSalt []byte
+}
+
 // ConfFile is the content of a config file.
 type ConfFile struct {
 	// Creator is the gocryptfs version string.
@@ -46,6 +55,8 @@ type ConfFile struct {
 	// mounting. This mechanism is analogous to the ext4 feature flags that are
 	// stored in the superblock.
 	FeatureFlags []string
+	// FIDO2 parameters
+	FIDO2 FIDO2Params
 	// Filename is the name of the config file. Not exported to JSON.
 	filename string
 }
@@ -69,7 +80,7 @@ func randBytesDevRandom(n int) []byte {
 // "password" and write it to "filename".
 // Uses scrypt with cost parameter logN.
 func Create(filename string, password []byte, plaintextNames bool,
-	logN int, creator string, aessiv bool, devrandom bool) error {
+	logN int, creator string, aessiv bool, devrandom bool, fido2CredentialID []byte, fido2HmacSalt []byte) error {
 	var cf ConfFile
 	cf.filename = filename
 	cf.Creator = creator
@@ -89,6 +100,11 @@ func Create(filename string, password []byte, plaintextNames bool,
 	if aessiv {
 		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagAESSIV])
 	}
+	if len(fido2CredentialID) > 0 {
+		cf.FeatureFlags = append(cf.FeatureFlags, knownFlags[FlagFIDO2])
+		cf.FIDO2.CredentialID = fido2CredentialID
+		cf.FIDO2.HMACSalt = fido2HmacSalt
+	}
 	{
 		// Generate new random master key
 		var key []byte
diff --git a/internal/configfile/config_test.go b/internal/configfile/config_test.go
index 832867c..ce35531 100644
--- a/internal/configfile/config_test.go
+++ b/internal/configfile/config_test.go
@@ -62,7 +62,7 @@ func TestLoadV2StrangeFeature(t *testing.T) {
 }
 
 func TestCreateConfDefault(t *testing.T) {
-	err := Create("config_test/tmp.conf", testPw, false, 10, "test", false, false)
+	err := Create("config_test/tmp.conf", testPw, false, 10, "test", false, false, nil, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -83,14 +83,14 @@ func TestCreateConfDefault(t *testing.T) {
 }
 
 func TestCreateConfDevRandom(t *testing.T) {
-	err := Create("config_test/tmp.conf", testPw, false, 10, "test", false, true)
+	err := Create("config_test/tmp.conf", testPw, false, 10, "test", false, true, nil, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
 }
 
 func TestCreateConfPlaintextnames(t *testing.T) {
-	err := Create("config_test/tmp.conf", testPw, true, 10, "test", false, false)
+	err := Create("config_test/tmp.conf", testPw, true, 10, "test", false, false, nil, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -111,7 +111,7 @@ func TestCreateConfPlaintextnames(t *testing.T) {
 
 // Reverse mode uses AESSIV
 func TestCreateConfFileAESSIV(t *testing.T) {
-	err := Create("config_test/tmp.conf", testPw, false, 10, "test", true, false)
+	err := Create("config_test/tmp.conf", testPw, false, 10, "test", true, false, nil, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/internal/configfile/feature_flags.go b/internal/configfile/feature_flags.go
index 2d609f2..5964a53 100644
--- a/internal/configfile/feature_flags.go
+++ b/internal/configfile/feature_flags.go
@@ -25,6 +25,9 @@ const (
 	// Note that this flag does not change the password hashing algorithm
 	// which always is scrypt.
 	FlagHKDF
+	// FlagFIDO2 means that "-fido2" was used when creating the filesystem.
+	// The masterkey is protected using a FIDO2 token instead of a password.
+	FlagFIDO2
 )
 
 // knownFlags stores the known feature flags and their string representation
@@ -37,6 +40,7 @@ var knownFlags = map[flagIota]string{
 	FlagAESSIV:         "AESSIV",
 	FlagRaw64:          "Raw64",
 	FlagHKDF:           "HKDF",
+	FlagFIDO2:          "FIDO2",
 }
 
 // Filesystems that do not have these feature flags set are deprecated.
diff --git a/internal/exitcodes/exitcodes.go b/internal/exitcodes/exitcodes.go
index b876333..508ba38 100644
--- a/internal/exitcodes/exitcodes.go
+++ b/internal/exitcodes/exitcodes.go
@@ -70,6 +70,8 @@ const (
 	ExcludeError = 29
 	// DevNull means that /dev/null could not be opened
 	DevNull = 30
+	// FIDO2Error - an error was encountered while interacting with a FIDO2 token
+	FIDO2Error = 31
 )
 
 // Err wraps an error with an associated numeric exit code
diff --git a/internal/fido2/fido2.go b/internal/fido2/fido2.go
new file mode 100644
index 0000000..ea8ffd8
--- /dev/null
+++ b/internal/fido2/fido2.go
@@ -0,0 +1,110 @@
+package fido2
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+
+	"github.com/rfjakob/gocryptfs/internal/cryptocore"
+	"github.com/rfjakob/gocryptfs/internal/exitcodes"
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+type fidoCommand int
+
+const (
+	cred          fidoCommand = iota
+	assert        fidoCommand = iota
+	assertWithPIN fidoCommand = iota
+)
+
+const relyingPartyID = "gocryptfs"
+
+func callFidoCommand(command fidoCommand, device string, stdin []string) ([]string, error) {
+	var cmd *exec.Cmd
+	switch command {
+	case cred:
+		cmd = exec.Command("fido2-cred", "-M", "-h", "-v", device)
+	case assert:
+		cmd = exec.Command("fido2-assert", "-G", "-h", device)
+	case assertWithPIN:
+		cmd = exec.Command("fido2-assert", "-G", "-h", "-v", device)
+	}
+	tlog.Debug.Printf("callFidoCommand: executing %q with args %v", cmd.Path, cmd.Args)
+	cmd.Stderr = os.Stderr
+	in, err := cmd.StdinPipe()
+	if err != nil {
+		return nil, err
+	}
+	for _, s := range stdin {
+		// This does not deadlock because the pipe buffer is big enough (64kiB on Linux)
+		io.WriteString(in, s+"\n")
+	}
+	in.Close()
+	out, err := cmd.Output()
+	if err != nil {
+		return nil, fmt.Errorf("%s failed with %v", cmd.Args[0], err)
+	}
+	return strings.Split(string(out), "\n"), nil
+}
+
+// Register registers a credential using a FIDO2 token
+func Register(device string, userName string) (credentialID []byte) {
+	tlog.Info.Printf("FIDO2 Register: interact with your device ...")
+	cdh := base64.StdEncoding.EncodeToString(cryptocore.RandBytes(32))
+	userID := base64.StdEncoding.EncodeToString(cryptocore.RandBytes(32))
+	stdin := []string{cdh, relyingPartyID, userName, userID}
+	out, err := callFidoCommand(cred, device, stdin)
+	if err != nil {
+		tlog.Fatal.Println(err)
+		os.Exit(exitcodes.FIDO2Error)
+	}
+	credentialID, err = base64.StdEncoding.DecodeString(out[4])
+	if err != nil {
+		tlog.Fatal.Println(err)
+		os.Exit(exitcodes.FIDO2Error)
+	}
+	return credentialID
+}
+
+// Secret generates a HMAC secret using a FIDO2 token
+func Secret(device string, credentialID []byte, salt []byte) (secret []byte) {
+	tlog.Info.Printf("FIDO2 Secret: interact with your device ...")
+	cdh := base64.StdEncoding.EncodeToString(cryptocore.RandBytes(32))
+	crid := base64.StdEncoding.EncodeToString(credentialID)
+	hmacsalt := base64.StdEncoding.EncodeToString(salt)
+	stdin := []string{cdh, relyingPartyID, crid, hmacsalt}
+	// try asserting without PIN first
+	out, err := callFidoCommand(assert, device, stdin)
+	if err != nil {
+		// if that fails, let's assert with PIN
+		out, err = callFidoCommand(assertWithPIN, device, stdin)
+		if err != nil {
+			tlog.Fatal.Println(err)
+			os.Exit(exitcodes.FIDO2Error)
+		}
+	}
+	secret, err = base64.StdEncoding.DecodeString(out[4])
+	if err != nil {
+		tlog.Fatal.Println(err)
+		os.Exit(exitcodes.FIDO2Error)
+	}
+
+	// sanity checks
+	secretLen := len(secret)
+	if secretLen < 32 {
+		tlog.Fatal.Printf("FIDO2 HMACSecret too short (%d)!\n", secretLen)
+		os.Exit(exitcodes.FIDO2Error)
+	}
+	zero := make([]byte, secretLen)
+	if bytes.Equal(zero, secret) {
+		tlog.Fatal.Printf("FIDO2 HMACSecret is all zero!")
+		os.Exit(exitcodes.FIDO2Error)
+	}
+
+	return secret
+}
-- 
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