Implement function to verify index signature with custom key (#1304)

* add gpg public key to verify board_index.json

* add new function to allow the usage of another gpg key

* add new test and enhance existing ones

* apply suggestions by @cmaglie

* move `module_firmware_index.gpg.key` under `testdata/`

arduino/security/signature_test.go

@@ -22,14 +22,36 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestSignatureVerification(t *testing.T) {
-	res, signer, err := VerifyArduinoDetachedSignature(paths.New("testdata/package_index.json"), paths.New("testdata/package_index.json.sig"))
+var (
+	PackageIndexPath     = paths.New("testdata/package_index.json")
+	PackageSignaturePath = paths.New("testdata/package_index.json.sig")
+	BoardIndexPath       = paths.New("testdata/module_firmware_index.json")
+	BoardSignaturePath   = paths.New("testdata/module_firmware_index.json.sig")
+	BoardKey             = paths.New("testdata/module_firmware_index_public.gpg.key")
+	InvalidIndexPath     = paths.New("testdata/invalid_file.json")
+)
+
+func TestVerifyArduinoDetachedSignature(t *testing.T) {
+	res, signer, err := VerifyArduinoDetachedSignature(PackageIndexPath, PackageSignaturePath)
 	require.NoError(t, err)
 	require.NotNil(t, signer)
 	require.True(t, res)
 	require.Equal(t, uint64(0x7baf404c2dfab4ae), signer.PrimaryKey.KeyId)
 
-	res, signer, err = VerifyArduinoDetachedSignature(paths.New("testdata/invalid_file.json"), paths.New("testdata/package_index.json.sig"))
+	res, signer, err = VerifyArduinoDetachedSignature(InvalidIndexPath, PackageSignaturePath)
+	require.False(t, res)
+	require.Nil(t, signer)
+	require.Error(t, err)
+}
+
+func TestVerifyDetachedSignature(t *testing.T) {
+	res, signer, err := VerifyDetachedSignature(BoardIndexPath, BoardSignaturePath, BoardKey)
+	require.NoError(t, err)
+	require.NotNil(t, signer)
+	require.True(t, res)
+	require.Equal(t, uint64(0x82f2d7c7c5a22a73), signer.PrimaryKey.KeyId)
+
+	res, signer, err = VerifyDetachedSignature(InvalidIndexPath, PackageSignaturePath, BoardKey)
 	require.False(t, res)
 	require.Nil(t, signer)
 	require.Error(t, err)

arduino/security/signatures.go

@@ -17,6 +17,8 @@ package security
 
 import (
 	"fmt"
+	"io"
+	"os"
 
 	"github.com/arduino/go-paths-helper"
 	rice "github.com/cmaglie/go.rice"
@@ -37,11 +39,33 @@ func VerifyArduinoDetachedSignature(targetPath *paths.Path, signaturePath *paths
 	if err != nil {
 		panic("could not find bundled signature keys")
 	}
+	return verifySignature(targetPath, signaturePath, arduinoKeyringFile)
+}
+
+// VerifyDetachedSignature checks that the detached GPG signature (in the
+// signaturePath file) matches the given targetPath file and is an authentic
+// signature from the bundled trusted keychain. The keyPath is the path of the public key used.
+// This function allows to specify the path of the key to use.
+// If any of the above conditions fails this function returns false.
+// The PGP entity in the trusted keychain that produced the signature is returned too.
+func VerifyDetachedSignature(targetPath *paths.Path, signaturePath *paths.Path, keyPath *paths.Path) (bool, *openpgp.Entity, error) {
+	arduinoKeyringFile, err := os.Open(keyPath.String())
+	if err != nil {
+		panic("could not open signature keys")
+	}
+	defer arduinoKeyringFile.Close()
+	return verifySignature(targetPath, signaturePath, arduinoKeyringFile)
+}
+
+//verifySignature is an helper function that checks that the detached GPG signature (in the
+// signaturePath file) matches the given targetPath file and is an authentic
+// signature. If any of the above conditions fails this function returns false.
+// The PGP entity in the trusted keychain that produced the signature is returned too.
+func verifySignature(targetPath *paths.Path, signaturePath *paths.Path, arduinoKeyringFile io.Reader) (bool, *openpgp.Entity, error) {
 	keyRing, err := openpgp.ReadKeyRing(arduinoKeyringFile)
 	if err != nil {
 		return false, nil, fmt.Errorf("retrieving Arduino public keys: %s", err)
 	}
-
 	target, err := targetPath.Open()
 	if err != nil {
 		return false, nil, fmt.Errorf("opening target file: %s", err)