internal/agessh: move EncryptedSSHIdentity out of cmd/age

cmd/age/encrypted_keys.go

@@ -7,97 +7,14 @@
 package main
 
 import (
-	"crypto/ed25519"
-	"crypto/rsa"
-	"crypto/sha256"
 	"fmt"
 	"os"
 
 	"filippo.io/age/internal/age"
-	"filippo.io/age/internal/agessh"
 	"filippo.io/age/internal/format"
-	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/terminal"
 )
 
-type EncryptedSSHIdentity struct {
-	pubKey     ssh.PublicKey
-	pemBytes   []byte
-	passphrase func() ([]byte, error)
-
-	decrypted age.Identity
-}
-
-func NewEncryptedSSHIdentity(pubKey ssh.PublicKey, pemBytes []byte, passphrase func() ([]byte, error)) (*EncryptedSSHIdentity, error) {
-	switch t := pubKey.Type(); t {
-	case "ssh-ed25519", "ssh-rsa":
-	default:
-		return nil, fmt.Errorf("unsupported SSH key type: %v", t)
-	}
-	return &EncryptedSSHIdentity{
-		pubKey:     pubKey,
-		pemBytes:   pemBytes,
-		passphrase: passphrase,
-	}, nil
-}
-
-var _ age.IdentityMatcher = &EncryptedSSHIdentity{}
-
-func (i *EncryptedSSHIdentity) Type() string {
-	return i.pubKey.Type()
-}
-
-func (i *EncryptedSSHIdentity) Unwrap(block *format.Recipient) (fileKey []byte, err error) {
-	if i.decrypted != nil {
-		return i.decrypted.Unwrap(block)
-	}
-
-	passphrase, err := i.passphrase()
-	if err != nil {
-		return nil, fmt.Errorf("failed to obtain passphrase: %v", err)
-	}
-	k, err := ssh.ParseRawPrivateKeyWithPassphrase(i.pemBytes, passphrase)
-	if err != nil {
-		return nil, fmt.Errorf("failed to decrypt SSH key file: %v", err)
-	}
-
-	switch k := k.(type) {
-	case *ed25519.PrivateKey:
-		i.decrypted, err = agessh.NewEd25519Identity(*k)
-	case *rsa.PrivateKey:
-		i.decrypted, err = agessh.NewRSAIdentity(k)
-	default:
-		return nil, fmt.Errorf("unexpected SSH key type: %T", k)
-	}
-	if err != nil {
-		return nil, fmt.Errorf("invalid SSH key: %v", err)
-	}
-	if i.decrypted.Type() != i.pubKey.Type() {
-		return nil, fmt.Errorf("mismatched SSH key type: got %q, expected %q", i.decrypted.Type(), i.pubKey.Type())
-	}
-
-	return i.decrypted.Unwrap(block)
-}
-
-func sshFingerprint(pk ssh.PublicKey) string {
-	h := sha256.Sum256(pk.Marshal())
-	return format.EncodeToString(h[:4])
-}
-
-func (i *EncryptedSSHIdentity) Match(block *format.Recipient) error {
-	if block.Type != i.Type() {
-		return age.ErrIncorrectIdentity
-	}
-	if len(block.Args) < 1 {
-		return fmt.Errorf("invalid %v recipient block", i.Type())
-	}
-
-	if block.Args[0] != sshFingerprint(i.pubKey) {
-		return age.ErrIncorrectIdentity
-	}
-	return nil
-}
-
 type LazyScryptIdentity struct {
 	Passphrase func() (string, error)
 }

cmd/age/parse.go

@@ -100,7 +100,7 @@ func parseSSHIdentity(name string, pemBytes []byte) ([]age.Identity, error) {
 			}
 			return pass, nil
 		}
-		i, err := NewEncryptedSSHIdentity(pubKey, pemBytes, passphrasePrompt)
+		i, err := agessh.NewEncryptedSSHIdentity(pubKey, pemBytes, passphrasePrompt)
 		if err != nil {
 			return nil, err
 		}

internal/agessh/encrypted_keys.go

@@ -0,0 +1,114 @@
+// Copyright 2019 Google LLC
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+package agessh
+
+import (
+	"crypto/ed25519"
+	"crypto/rsa"
+	"fmt"
+
+	"filippo.io/age/internal/age"
+	"filippo.io/age/internal/format"
+	"golang.org/x/crypto/ssh"
+)
+
+// EncryptedSSHIdentity is an age.IdentityMatcher implementation based on a
+// passphrase encrypted SSH private key.
+//
+// It provides public key based matching and deferred decryption so the
+// passphrase is only requested if necessary. If the application knows it will
+// unconditionally have to decrypt the private key, it would be simpler to use
+// ssh.ParseRawPrivateKeyWithPassphrase directly and pass the result to
+// NewEd25519Identity or NewRSAIdentity.
+type EncryptedSSHIdentity struct {
+	pubKey     ssh.PublicKey
+	pemBytes   []byte
+	passphrase func() ([]byte, error)
+
+	decrypted age.Identity
+}
+
+// NewEncryptedSSHIdentity returns a new EncryptedSSHIdentity.
+//
+// pubKey must be the public key associated with the encrypted private key, and
+// it must have type "ssh-ed25519" or "ssh-rsa". For OpenSSH encrypted files it
+// can be extracted from an ssh.PassphraseMissingError, otherwise in can often
+// be found in ".pub" files.
+//
+// pemBytes must be a valid input to ssh.ParseRawPrivateKeyWithPassphrase.
+// passphrase is a callback that will be invoked by Unwrap when the passphrase
+// is necessary.
+func NewEncryptedSSHIdentity(pubKey ssh.PublicKey, pemBytes []byte, passphrase func() ([]byte, error)) (*EncryptedSSHIdentity, error) {
+	switch t := pubKey.Type(); t {
+	case "ssh-ed25519", "ssh-rsa":
+	default:
+		return nil, fmt.Errorf("unsupported SSH key type: %v", t)
+	}
+	return &EncryptedSSHIdentity{
+		pubKey:     pubKey,
+		pemBytes:   pemBytes,
+		passphrase: passphrase,
+	}, nil
+}
+
+var _ age.IdentityMatcher = &EncryptedSSHIdentity{}
+
+// Type returns the type of the underlying private key, "ssh-ed25519" or "ssh-rsa".
+func (i *EncryptedSSHIdentity) Type() string {
+	return i.pubKey.Type()
+}
+
+// Unwrap implements age.Identity. If the private key is still encrypted, it
+// will request the passphrase. The decrypted private key will be cached after
+// the first successful invocation.
+func (i *EncryptedSSHIdentity) Unwrap(block *format.Recipient) (fileKey []byte, err error) {
+	if i.decrypted != nil {
+		return i.decrypted.Unwrap(block)
+	}
+
+	passphrase, err := i.passphrase()
+	if err != nil {
+		return nil, fmt.Errorf("failed to obtain passphrase: %v", err)
+	}
+	k, err := ssh.ParseRawPrivateKeyWithPassphrase(i.pemBytes, passphrase)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decrypt SSH key file: %v", err)
+	}
+
+	switch k := k.(type) {
+	case *ed25519.PrivateKey:
+		i.decrypted, err = NewEd25519Identity(*k)
+	case *rsa.PrivateKey:
+		i.decrypted, err = NewRSAIdentity(k)
+	default:
+		return nil, fmt.Errorf("unexpected SSH key type: %T", k)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("invalid SSH key: %v", err)
+	}
+	if i.decrypted.Type() != i.pubKey.Type() {
+		return nil, fmt.Errorf("mismatched SSH key type: got %q, expected %q", i.decrypted.Type(), i.pubKey.Type())
+	}
+
+	return i.decrypted.Unwrap(block)
+}
+
+// Match implements age.IdentityMatcher without decrypting the private key, to
+// ensure the passphrase is only obtained if necessary.
+func (i *EncryptedSSHIdentity) Match(block *format.Recipient) error {
+	if block.Type != i.Type() {
+		return age.ErrIncorrectIdentity
+	}
+	if len(block.Args) < 1 {
+		return fmt.Errorf("invalid %v recipient block", i.Type())
+	}
+
+	if block.Args[0] != sshFingerprint(i.pubKey) {
+		return age.ErrIncorrectIdentity
+	}
+	return nil
+}