cmd/age: add support for encrypted SSH key files

internal/age/age.go

@@ -4,6 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 
+// Package age implements age-tool.com file encryption.
 package age
 
 import (
@@ -22,6 +23,13 @@ type Identity interface {
 	Unwrap(block *format.Recipient) (fileKey []byte, err error)
 }
 
+type IdentityMatcher interface {
+	Identity
+	Matches(block *format.Recipient) error
+}
+
+var ErrIncorrectIdentity = errors.New("incorrect identity for recipient block")
+
 type Recipient interface {
 	Type() string
 	Wrap(fileKey []byte) (*format.Recipient, error)
@@ -89,15 +97,29 @@ RecipientsLoop:
 			return nil, errors.New("an scrypt recipient must be the only one")
 		}
 		for _, i := range identities {
-
 			if i.Type() != r.Type {
 				continue
 			}
 
-			fileKey, err = i.Unwrap(r)
-			if err == nil {
-				break RecipientsLoop
+			if i, ok := i.(IdentityMatcher); ok {
+				err := i.Matches(r)
+				if err != nil {
+					if err == ErrIncorrectIdentity {
+						continue
+					}
+					return nil, err
+				}
 			}
+
+			fileKey, err = i.Unwrap(r)
+			if err != nil {
+				if err == ErrIncorrectIdentity {
+					continue
+				}
+				return nil, err
+			}
+
+			break RecipientsLoop
 		}
 	}
 	if fileKey == nil {

internal/age/scrypt.go

@@ -104,7 +104,7 @@ func (i *ScryptIdentity) SetMaxWorkFactor(logN int) {
 
 func (i *ScryptIdentity) Unwrap(block *format.Recipient) ([]byte, error) {
 	if block.Type != "scrypt" {
-		return nil, errors.New("wrong recipient block type")
+		return nil, ErrIncorrectIdentity
 	}
 	if len(block.Args) != 2 {
 		return nil, errors.New("invalid scrypt recipient block")
@@ -134,7 +134,7 @@ func (i *ScryptIdentity) Unwrap(block *format.Recipient) ([]byte, error) {
 
 	fileKey, err := aeadDecrypt(k, block.Body)
 	if err != nil {
-		return nil, fmt.Errorf("failed to decrypt file key: %v", err)
+		return nil, ErrIncorrectIdentity
 	}
 	return fileKey, nil
 }

internal/age/ssh.go

@@ -98,7 +98,7 @@ func NewSSHRSAIdentity(key *rsa.PrivateKey) (*SSHRSAIdentity, error) {
 
 func (i *SSHRSAIdentity) Unwrap(block *format.Recipient) ([]byte, error) {
 	if block.Type != "ssh-rsa" {
-		return nil, errors.New("wrong recipient block type")
+		return nil, ErrIncorrectIdentity
 	}
 	if len(block.Args) != 1 {
 		return nil, errors.New("invalid ssh-rsa recipient block")
@@ -115,7 +115,7 @@ func (i *SSHRSAIdentity) Unwrap(block *format.Recipient) ([]byte, error) {
 	h.Write(i.sshKey.Marshal())
 	hh := h.Sum(nil)
 	if !bytes.Equal(hh[:4], hash) {
-		return nil, errors.New("wrong ssh-rsa key")
+		return nil, ErrIncorrectIdentity
 	}
 
 	fileKey, err := rsa.DecryptOAEP(sha256.New(), rand.Reader, i.k,
@@ -304,7 +304,7 @@ func ed25519PrivateKeyToCurve25519(pk ed25519.PrivateKey) []byte {
 func (i *SSHEd25519Identity) Unwrap(block *format.Recipient) ([]byte, error) {
 	// TODO: DRY this up with the X25519 implementation.
 	if block.Type != "ssh-ed25519" {
-		return nil, errors.New("wrong recipient block type")
+		return nil, ErrIncorrectIdentity
 	}
 	if len(block.Args) != 2 {
 		return nil, errors.New("invalid ssh-ed25519 recipient block")
@@ -328,7 +328,7 @@ func (i *SSHEd25519Identity) Unwrap(block *format.Recipient) ([]byte, error) {
 	sH.Write(i.sshKey.Marshal())
 	hh := sH.Sum(nil)
 	if !bytes.Equal(hh[:4], hash) {
-		return nil, errors.New("wrong ssh-ed25519 key")
+		return nil, ErrIncorrectIdentity
 	}
 
 	var sharedSecret, theirPublicKey, tweak [32]byte

internal/age/x25519.go

@@ -145,7 +145,7 @@ func ParseX25519Identity(s string) (*X25519Identity, error) {
 
 func (i *X25519Identity) Unwrap(block *format.Recipient) ([]byte, error) {
 	if block.Type != "X25519" {
-		return nil, errors.New("wrong recipient block type")
+		return nil, ErrIncorrectIdentity
 	}
 	if len(block.Args) != 1 {
 		return nil, errors.New("invalid X25519 recipient block")
@@ -174,7 +174,7 @@ func (i *X25519Identity) Unwrap(block *format.Recipient) ([]byte, error) {
 
 	fileKey, err := aeadDecrypt(wrappingKey, block.Body)
 	if err != nil {
-		return nil, fmt.Errorf("failed to decrypt file key: %v", err)
+		return nil, ErrIncorrectIdentity
 	}
 	return fileKey, nil
 }

internal/format/format.go

@@ -4,6 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 
+// Package format implements the age file format.
 package format
 
 import (