feat: allow backups to be encrypted with age

GPG is known to have usability issues and is generally cumbersome to
use. age [0] is a modern alternative to GPG that is designed by a
cryptographer that has worked and continues to work on Golang's crypto
packages for years.

Allowing age to be used to encrypt backups dramatically simplifies the
backup process.

[0]: https://age-encryption.org/

cmd/backup/config.go

@@ -48,6 +48,8 @@ type Config struct {
 	BackupSkipBackendsFromPrune   []string        `split_words:"true"`
 	GpgPassphrase                 string          `split_words:"true"`
 	GpgPublicKeyRing              string          `split_words:"true"`
+	AgePassphrase                 string          `split_words:"true"`
+	AgePublicKeys                 []string        `split_words:"true"`
 	NotificationURLs              []string        `envconfig:"NOTIFICATION_URLS"`
 	NotificationLevel             string          `split_words:"true" default:"error"`
 	EmailNotificationRecipient    string          `split_words:"true"`

cmd/backup/encrypt_archive.go

@@ -11,119 +11,202 @@ import (
 	"os"
 	"path"
 
+	"filippo.io/age"
 	"github.com/ProtonMail/go-crypto/openpgp/armor"
 	openpgp "github.com/ProtonMail/go-crypto/openpgp/v2"
 	"github.com/offen/docker-volume-backup/internal/errwrap"
 )
 
-func (s *script) encryptAsymmetrically(outFile *os.File) (io.WriteCloser, func() error, error) {
+func countTrue(b ...bool) int {
+	c := int(0)
+	for _, v := range b {
+		if v {
+			c++
+		}
+	}
+	return c
+}
 
-	entityList, err := openpgp.ReadArmoredKeyRing(bytes.NewReader([]byte(s.c.GpgPublicKeyRing)))
-	if err != nil {
-		return nil, nil, errwrap.Wrap(err, "error parsing armored keyring")
+// 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 {
+	useGPGSymmetric := s.c.GpgPassphrase != ""
+	useGPGAsymmetric := s.c.GpgPublicKeyRing != ""
+	useAgeSymmetric := s.c.AgePassphrase != ""
+	useAgeAsymmetric := len(s.c.AgePublicKeys) > 0
+	switch nconfigured := countTrue(
+		useGPGSymmetric,
+		useGPGAsymmetric,
+		useAgeSymmetric,
+		useAgeAsymmetric,
+	); nconfigured {
+	case 0:
+		return nil
+	case 1:
+		// ok!
+	default:
+		return fmt.Errorf(
+			"error in selecting archive encryption method: expected 0 or 1 to be configured, %d methods are configured",
+			nconfigured,
+		)
 	}
 
-	armoredWriter, err := armor.Encode(outFile, "PGP MESSAGE", nil)
-	if err != nil {
-		return nil, nil, errwrap.Wrap(err, "error preparing encryption")
+	if useGPGSymmetric {
+		return s.encryptWithGPGSymmetric()
+	} else if useGPGAsymmetric {
+		return s.encryptWithGPGAsymmetric()
+	} else if useAgeSymmetric || useAgeAsymmetric {
+		ar, err := s.getConfiguredAgeRecipients()
+		if err != nil {
+			return errwrap.Wrap(err, "failed to get configured age recipients")
+		}
+		return s.encryptWithAge(ar)
 	}
+	return nil
+}
 
-	_, 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
+func (s *script) getConfiguredAgeRecipients() ([]age.Recipient, error) {
+	if s.c.AgePassphrase == "" && len(s.c.AgePublicKeys) == 0 {
+		return nil, fmt.Errorf("no age recipients configured")
+	}
+	recipients := []age.Recipient{}
+	if len(s.c.AgePublicKeys) > 0 {
+		for _, pk := range s.c.AgePublicKeys {
+			pkr, err := age.ParseX25519Recipient(pk)
+			if err != nil {
+				return nil, errwrap.Wrap(err, "failed to parse age public key")
+			}
+			recipients = append(recipients, pkr)
+		}
 	}
+	if s.c.AgePassphrase != "" {
+		if len(recipients) != 0 {
+			return nil, fmt.Errorf("age encryption must only be enabled via passphrase or public key, not both")
+		}
 
-	return dst, func() error {
-		if err := dst.Close(); err != nil {
-			return err
+		r, err := age.NewScryptRecipient(s.c.AgePassphrase)
+		if err != nil {
+			return nil, errwrap.Wrap(err, "failed to create scrypt identity from age passphrase")
 		}
-		return armoredWriter.Close()
-	}, err
+		recipients = append(recipients, r)
+	}
+	return recipients, nil
 }
 
-func (s *script) encryptSymmetrically(outFile *os.File) (io.WriteCloser, func() error, error) {
+func (s *script) encryptWithAge(rec []age.Recipient) error {
+	return s.doEncrypt("age", func(ciphertextWriter io.Writer) (io.WriteCloser, error) {
+		return age.Encrypt(ciphertextWriter, rec...)
+	})
+}
 
-	_, 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
-	}
+func (s *script) encryptWithGPGSymmetric() error {
+	return s.doEncrypt("gpg", func(ciphertextWriter io.Writer) (io.WriteCloser, error) {
+		_, name := path.Split(s.file)
+		return openpgp.SymmetricallyEncrypt(ciphertextWriter, []byte(s.c.GpgPassphrase), &openpgp.FileHints{
+			FileName: name,
+		}, nil)
+	})
+}
 
-	return dst, dst.Close, nil
+type closeAllWriter struct {
+	io.Writer
+	closers []io.Closer
 }
 
-// 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 {
+func (c *closeAllWriter) Close() (err error) {
+	for _, cl := range c.closers {
+		err = errors.Join(err, cl.Close())
+	}
+	return
+}
 
-	var encrypt func(outFile *os.File) (io.WriteCloser, func() error, error)
-	var cleanUpErr error
+var _ io.WriteCloser = (*closeAllWriter)(nil)
 
-	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
-	}
+func (s *script) encryptWithGPGAsymmetric() error {
+	return s.doEncrypt("gpg", func(ciphertextWriter io.Writer) (_ io.WriteCloser, outerr error) {
+		entityList, err := openpgp.ReadArmoredKeyRing(bytes.NewReader([]byte(s.c.GpgPublicKeyRing)))
+		if err != nil {
+			return nil, errwrap.Wrap(err, "error parsing armored keyring")
+		}
+
+		armoredWriter, err := armor.Encode(ciphertextWriter, "PGP MESSAGE", nil)
+		if err != nil {
+			return nil, errwrap.Wrap(err, "error preparing encryption")
+		}
+		defer func() {
+			if outerr != nil {
+				_ = armoredWriter.Close()
+			}
+		}()
+
+		_, name := path.Split(s.file)
+		encWriter, err := openpgp.Encrypt(armoredWriter, entityList, nil, nil, &openpgp.FileHints{
+			FileName: name,
+		}, nil)
+		if err != nil {
+			return nil, err
+		}
+		return &closeAllWriter{
+			Writer:  encWriter,
+			closers: []io.Closer{encWriter, armoredWriter},
+		}, nil
+	})
+}
 
-	gpgFile := fmt.Sprintf("%s.gpg", s.file)
+func (s *script) doEncrypt(
+	extension string,
+	encryptor func(ciphertextWriter io.Writer) (io.WriteCloser, error),
+) (outerr error) {
+	encFile := fmt.Sprintf("%s.%s", s.file, extension)
 	s.registerHook(hookLevelPlumbing, func(error) error {
-		if err := remove(gpgFile); err != nil {
-			return errwrap.Wrap(err, "error removing gpg file")
+		if err := remove(encFile); err != nil {
+			return errwrap.Wrap(err, "error removing encrypted file")
 		}
 		s.logger.Info(
-			fmt.Sprintf("Removed GPG file `%s`.", gpgFile),
+			fmt.Sprintf("Removed encrypted file `%s`.", encFile),
 		)
 		return nil
 	})
 
-	outFile, err := os.Create(gpgFile)
+	outFile, err := os.Create(encFile)
 	if err != nil {
 		return errwrap.Wrap(err, "error opening out file")
 	}
 	defer func() {
 		if err := outFile.Close(); err != nil {
-			cleanUpErr = errors.Join(cleanUpErr, errwrap.Wrap(err, "error closing out file"))
+			outerr = errors.Join(outerr, errwrap.Wrap(err, "error closing out file"))
 		}
 	}()
 
-	dst, dstCloseCallback, err := encrypt(outFile)
+	dst, err := encryptor(outFile)
 	if err != nil {
 		return errwrap.Wrap(err, "error encrypting backup file")
 	}
 	defer func() {
-		if err := dstCloseCallback(); err != nil {
-			cleanUpErr = errors.Join(cleanUpErr, errwrap.Wrap(err, "error closing encrypted backup file"))
+		if err := dst.Close(); err != nil {
+			outerr = errors.Join(outerr, 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))
+		return errwrap.Wrap(err, fmt.Sprintf("error opening backup file %q", s.file))
 	}
 	defer func() {
 		if err := src.Close(); err != nil {
-			cleanUpErr = errors.Join(cleanUpErr, errwrap.Wrap(err, "error closing backup file"))
+			outerr = errors.Join(outerr, errwrap.Wrap(err, "error closing backup file"))
 		}
 	}()
 
 	if _, err := io.Copy(dst, src); err != nil {
 		return errwrap.Wrap(err, "error writing ciphertext to file")
 	}
 
-	s.file = gpgFile
+	s.file = encFile
 	s.logger.Info(
-		fmt.Sprintf("Encrypted backup using gpg, saving as `%s`.", s.file),
+		fmt.Sprintf("Encrypted backup using %q, saving as %q", extension, s.file),
 	)
-	return cleanUpErr
+
+	return
 }

docs/how-tos/encrypt-backups-using-gpg.md

@@ -1,17 +1,28 @@
 ---
-title: Encrypt backups using GPG
+title: Encrypting backups
 layout: default
 parent: How Tos
 nav_order: 7
 ---
 
-# Encrypt backups using GPG
+# Encrypting backups
+
+The image supports encrypting backups using one of two available methods: **GPG** or **[age](https://age-encryption.org/)**
+
+## Using GPG encryption
 
-The image supports encrypting backups using GPG out of the box.
 In case a `GPG_PASSPHRASE` or `GPG_PUBLIC_KEY_RING` environment variable is set, the backup archive will be encrypted using the given key and saved as a `.gpg` file instead.
 
 Assuming you have `gpg` installed, you can decrypt such a backup using (your OS will prompt for the passphrase before decryption can happen):
 
 ```console
 gpg -o backup.tar.gz -d backup.tar.gz.gpg
 ```
+
+## Using age encryption
+
+age allows backups to be encrypted with either a symmetric key (password) or a public key. One of those options are available for use.
+
+Given `AGE_PASSPHRASE` being provided, the backup archive will be encrypted with the passphrase and saved as a `.age` file instead. Refer to age documentation for how to properly decrypt.
+
+Given `AGE_PUBLIC_KEYS` being provided (allowing multiple by separating each public key with `,`), the backup archive will be encrypted with the provided public keys. It will also result in the archive being saved as a `.age` file.

go.mod

@@ -3,6 +3,7 @@ module github.com/offen/docker-volume-backup
 go 1.22
 
 require (
+	filippo.io/age v1.2.0
 	github.com/Azure/azure-sdk-for-go/sdk/azidentity v1.7.0
 	github.com/Azure/azure-sdk-for-go/sdk/storage/azblob v1.4.0
 	github.com/containrrr/shoutrrr v0.8.0

go.sum

@@ -1,3 +1,5 @@
+c2sp.org/CCTV/age v0.0.0-20240306222714-3ec4d716e805 h1:u2qwJeEvnypw+OCPUHmoZE3IqwfuN5kgDfo5MLzpNM0=
+c2sp.org/CCTV/age v0.0.0-20240306222714-3ec4d716e805/go.mod h1:FomMrUJ2Lxt5jCLmZkG3FHa72zUprnhd3v/Z18Snm4w=
 cloud.google.com/go v0.26.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
 cloud.google.com/go v0.34.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
 cloud.google.com/go v0.38.0/go.mod h1:990N+gfupTy94rShfmMCWGDn0LpTmnzTp2qbd1dvSRU=
@@ -31,6 +33,8 @@ cloud.google.com/go/storage v1.6.0/go.mod h1:N7U0C8pVQ/+NIKOBQyamJIeKQKkZ+mxpohl
 cloud.google.com/go/storage v1.8.0/go.mod h1:Wv1Oy7z6Yz3DshWRJFhqM/UCfaWIRTdp0RXyy7KQOVs=
 cloud.google.com/go/storage v1.10.0/go.mod h1:FLPqc6j+Ki4BU591ie1oL6qBQGu2Bl/tZ9ullr3+Kg0=
 dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
+filippo.io/age v1.2.0 h1:vRDp7pUMaAJzXNIWJVAZnEf/Dyi4Vu4wI8S1LBzufhE=
+filippo.io/age v1.2.0/go.mod h1:JL9ew2lTN+Pyft4RiNGguFfOpewKwSHm5ayKD/A4004=
 github.com/Azure/azure-sdk-for-go/sdk/azcore v1.13.0 h1:GJHeeA2N7xrG3q30L2UXDyuWRzDM900/65j70wcM4Ww=
 github.com/Azure/azure-sdk-for-go/sdk/azcore v1.13.0/go.mod h1:l38EPgmsp71HHLq9j7De57JcKOWPyhrsW1Awm1JS6K0=
 github.com/Azure/azure-sdk-for-go/sdk/azidentity v1.7.0 h1:tfLQ34V6F7tVSwoTf/4lH5sE0o6eCJuNDTmH09nDpbc=
@@ -485,8 +489,8 @@ golang.org/x/tools v0.0.0-20200804011535-6c149bb5ef0d/go.mod h1:njjCfa9FT2d7l9Bc
 golang.org/x/tools v0.0.0-20200825202427-b303f430e36d/go.mod h1:njjCfa9FT2d7l9Bc6FUM5FLjQPp3cFF28FI3qnDFljA=
 golang.org/x/tools v0.0.0-20210106214847-113979e3529a/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
-golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d h1:vU5i/LfpvrRCpgM/VPfJLg5KjxD3E+hfT1SH+d9zLwg=
-golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d/go.mod h1:aiJjzUbINMkxbQROHiO6hDPo2LHcIPhhQsa9DLh0yGk=
+golang.org/x/tools v0.22.0 h1:gqSGLZqv+AI9lIQzniJ0nZDRG5GBPsSi+DRNHWNz6yA=
+golang.org/x/tools v0.22.0/go.mod h1:aCwcsjqvq7Yqt6TNyX7QMU2enbQ/Gt0bo6krSeEri+c=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

test/Dockerfile

@@ -1,8 +1,10 @@
 FROM docker:27-dind
 
 RUN apk add \
+  age \
   coreutils \
   curl \
+  expect \
   gpg \
   gpg-agent \
   jq \

test/age-passphrase/docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  backup:
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
+    restart: always
+    environment:
+      BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
+      BACKUP_FILENAME: test.tar.gz
+      BACKUP_LATEST_SYMLINK: test-latest.tar.gz.age
+      BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-7}
+      AGE_PASSPHRASE: "Dance.0Tonight.Go.Typical"
+    volumes:
+      - ${LOCAL_DIR:-./local}:/archive
+      - app_data:/backup/app_data:ro
+      - /var/run/docker.sock:/var/run/docker.sock
+
+  offen:
+    image: offen/offen:latest
+    labels:
+      - docker-volume-backup.stop-during-backup=true
+    volumes:
+      - app_data:/var/opt/offen
+
+volumes:
+  app_data: