Feature: PGP Asymmetric Encryption (#456)

* feat: asym encryption

* tests

* docs

* refactor

* logs & errs

* comment

* Update docs/reference/index.md

use correct env var in example

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

* Update cmd/backup/encrypt_archive.go

use errwarp for initial error msg

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

* rm orphaned code in encryption functions

* inline readArmoredKeys

* naming -GPG_PUBLIC_KEYS- to GPG_PUBLIC_KEY_RING

* add eror handling for closing func

* use dynamically generated keys for testing

* rm explicit gpg-agent start

* rm unnecessary private_key export

* pass PASSPHRASE correctly to the decryption command

* capture defer errors

* log & err msg

---------

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

cmd/backup/encrypt_archive.go

@@ -4,20 +4,75 @@
 package main
 
 import (
+	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"os"
 	"path"
 
+	"github.com/ProtonMail/go-crypto/openpgp/armor"
 	openpgp "github.com/ProtonMail/go-crypto/openpgp/v2"
 	"github.com/offen/docker-volume-backup/internal/errwrap"
 )
 
-// encryptArchive encrypts the backup file using PGP and the configured passphrase.
-// In case no passphrase is given it returns early, leaving the backup file
+func (s *script) encryptAsymmetrically(outFile *os.File) (io.WriteCloser, func() error, error) {
+
+	entityList, err := openpgp.ReadArmoredKeyRing(bytes.NewReader([]byte(s.c.GpgPublicKeyRing)))
+	if err != nil {
+		return nil, nil, errwrap.Wrap(err, "error parsing armored keyring")
+	}
+
+	armoredWriter, err := armor.Encode(outFile, "PGP MESSAGE", nil)
+	if err != nil {
+		return nil, nil, errwrap.Wrap(err, "error preparing encryption")
+	}
+
+	_, name := path.Split(s.file)
+	dst, err := openpgp.Encrypt(armoredWriter, entityList, nil, nil, &openpgp.FileHints{
+		FileName: name,
+	}, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return dst, func() error {
+		if err := dst.Close(); err != nil {
+			return err
+		}
+		return armoredWriter.Close()
+	}, err
+}
+
+func (s *script) encryptSymmetrically(outFile *os.File) (io.WriteCloser, func() error, error) {
+
+	_, name := path.Split(s.file)
+	dst, err := openpgp.SymmetricallyEncrypt(outFile, []byte(s.c.GpgPassphrase), &openpgp.FileHints{
+		FileName: name,
+	}, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return dst, dst.Close, nil
+}
+
+// encryptArchive encrypts the backup file using PGP and the configured passphrase or publickey(s).
+// In case no passphrase or publickey is given it returns early, leaving the backup file
 // untouched.
 func (s *script) encryptArchive() error {
-	if s.c.GpgPassphrase == "" {
+
+	var encrypt func(outFile *os.File) (io.WriteCloser, func() error, error)
+	var cleanUpErr error
+
+	switch {
+	case s.c.GpgPassphrase != "" && s.c.GpgPublicKeyRing != "":
+		return errwrap.Wrap(nil, "error in selecting asymmetric and symmetric encryption methods: conflicting env vars are set")
+	case s.c.GpgPassphrase != "":
+		encrypt = s.encryptSymmetrically
+	case s.c.GpgPublicKeyRing != "":
+		encrypt = s.encryptAsymmetrically
+	default:
 		return nil
 	}
 
@@ -36,22 +91,31 @@ func (s *script) encryptArchive() error {
 	if err != nil {
 		return errwrap.Wrap(err, "error opening out file")
 	}
-	defer outFile.Close()
+	defer func() {
+		if err := outFile.Close(); err != nil {
+			cleanUpErr = errors.Join(cleanUpErr, errwrap.Wrap(err, "error closing out file"))
+		}
+	}()
 
-	_, name := path.Split(s.file)
-	dst, err := openpgp.SymmetricallyEncrypt(outFile, []byte(s.c.GpgPassphrase), &openpgp.FileHints{
-		FileName: name,
-	}, nil)
+	dst, dstCloseCallback, err := encrypt(outFile)
 	if err != nil {
 		return errwrap.Wrap(err, "error encrypting backup file")
 	}
-	defer dst.Close()
+	defer func() {
+		if err := dstCloseCallback(); err != nil {
+			cleanUpErr = errors.Join(cleanUpErr, errwrap.Wrap(err, "error closing encrypted backup file"))
+		}
+	}()
 
 	src, err := os.Open(s.file)
 	if err != nil {
 		return errwrap.Wrap(err, fmt.Sprintf("error opening backup file `%s`", s.file))
 	}
-	defer src.Close()
+	defer func() {
+		if err := src.Close(); err != nil {
+			cleanUpErr = errors.Join(cleanUpErr, errwrap.Wrap(err, "error closing backup file"))
+		}
+	}()
 
 	if _, err := io.Copy(dst, src); err != nil {
 		return errwrap.Wrap(err, "error writing ciphertext to file")
@@ -59,7 +123,7 @@ func (s *script) encryptArchive() error {
 
 	s.file = gpgFile
 	s.logger.Info(
-		fmt.Sprintf("Encrypted backup using given passphrase, saving as `%s`.", s.file),
+		fmt.Sprintf("Encrypted backup using gpg, saving as `%s`.", s.file),
 	)
-	return nil
+	return cleanUpErr
 }