age: remove IdentityMatcher

It was completely useless: the same checks in Match could be implemented
in Unwrap, returning an early ErrIncorrectIdentity.

Not sure why I added it. It felt clever at the time.

age.go

@@ -57,18 +57,6 @@ type Identity interface {
 	Unwrap(block *Stanza) (fileKey []byte, err error)
 }
 
-// IdentityMatcher can be optionally implemented by an Identity that can
-// communicate whether it can decrypt a recipient stanza without decrypting it.
-//
-// If an Identity implements IdentityMatcher, its Unwrap method will only be
-// invoked on blocks for which Match returned nil. Match must return
-// ErrIncorrectIdentity for recipient blocks that don't match the identity, any
-// other error might be considered fatal.
-type IdentityMatcher interface {
-	Identity
-	Match(block *Stanza) error
-}
-
 var ErrIncorrectIdentity = errors.New("incorrect identity for recipient block")
 
 // A Recipient is a public key or other value that can encrypt an opaque file
@@ -161,24 +149,11 @@ RecipientsLoop:
 			return nil, errors.New("an scrypt recipient must be the only one")
 		}
 		for _, i := range identities {
-			if i, ok := i.(IdentityMatcher); ok {
-				err := i.Match((*Stanza)(r))
-				if err != nil {
-					if err == ErrIncorrectIdentity {
-						continue
-					}
-					return nil, err
-				}
-			}
-
 			fileKey, err = i.Unwrap((*Stanza)(r))
+			if errors.Is(err, ErrIncorrectIdentity) {
+				continue
+			}
 			if err != nil {
-				if err == ErrIncorrectIdentity {
-					// TODO: we should collect these errors and return them as an
-					// []error type with an Error method. That will require turning
-					// ErrIncorrectIdentity into an interface or wrapper error.
-					continue
-				}
 				return nil, err
 			}
 
@@ -186,7 +161,7 @@ RecipientsLoop:
 		}
 	}
 	if fileKey == nil {
-		return nil, errors.New("no identity matched a recipient")
+		return nil, errors.New("no identity matched any of the recipients")
 	}
 
 	if mac, err := headerMAC(fileKey, hdr); err != nil {

agessh/encrypted_keys.go

@@ -15,14 +15,13 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
-// EncryptedSSHIdentity is an age.IdentityMatcher implementation based on a
-// passphrase encrypted SSH private key.
+// EncryptedSSHIdentity is an age.Identity 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.
+// It requests the passphrase only if the public key matches a recipient stanza.
+// If the application knows it will always 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
@@ -35,7 +34,7 @@ type EncryptedSSHIdentity struct {
 //
 // 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
+// can be extracted from an ssh.PassphraseMissingError, otherwise it can often
 // be found in ".pub" files.
 //
 // pemBytes must be a valid input to ssh.ParseRawPrivateKeyWithPassphrase.
@@ -54,16 +53,26 @@ func NewEncryptedSSHIdentity(pubKey ssh.PublicKey, pemBytes []byte, passphrase f
 	}, nil
 }
 
-var _ age.IdentityMatcher = &EncryptedSSHIdentity{}
+var _ age.Identity = &EncryptedSSHIdentity{}
 
-// 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.
+// Unwrap implements age.Identity. If the private key is still encrypted, and
+// the block matches the public key, it will request the passphrase. The
+// decrypted private key will be cached after the first successful invocation.
 func (i *EncryptedSSHIdentity) Unwrap(block *age.Stanza) (fileKey []byte, err error) {
 	if i.decrypted != nil {
 		return i.decrypted.Unwrap(block)
 	}
 
+	if block.Type != i.pubKey.Type() {
+		return nil, age.ErrIncorrectIdentity
+	}
+	if len(block.Args) < 1 {
+		return nil, fmt.Errorf("invalid %v recipient block", i.pubKey.Type())
+	}
+	if block.Args[0] != sshFingerprint(i.pubKey) {
+		return nil, age.ErrIncorrectIdentity
+	}
+
 	passphrase, err := i.passphrase()
 	if err != nil {
 		return nil, fmt.Errorf("failed to obtain passphrase: %v", err)
@@ -77,12 +86,12 @@ func (i *EncryptedSSHIdentity) Unwrap(block *age.Stanza) (fileKey []byte, err er
 	case *ed25519.PrivateKey:
 		i.decrypted, err = NewEd25519Identity(*k)
 		if i.pubKey.Type() != ssh.KeyAlgoED25519 {
-			return nil, fmt.Errorf("mismatched SSH key type: got %q, expected %q", ssh.KeyAlgoED25519, i.pubKey.Type())
+			return nil, fmt.Errorf("mismatched private (%s) and public (%s) SSH key types", ssh.KeyAlgoED25519, i.pubKey.Type())
 		}
 	case *rsa.PrivateKey:
 		i.decrypted, err = NewRSAIdentity(k)
 		if i.pubKey.Type() != ssh.KeyAlgoRSA {
-			return nil, fmt.Errorf("mismatched SSH key type: got %q, expected %q", ssh.KeyAlgoRSA, i.pubKey.Type())
+			return nil, fmt.Errorf("mismatched private (%s) and public (%s) SSH key types", ssh.KeyAlgoRSA, i.pubKey.Type())
 		}
 	default:
 		return nil, fmt.Errorf("unexpected SSH key type: %T", k)
@@ -93,19 +102,3 @@ func (i *EncryptedSSHIdentity) Unwrap(block *age.Stanza) (fileKey []byte, err er
 
 	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 *age.Stanza) error {
-	if block.Type != i.pubKey.Type() {
-		return age.ErrIncorrectIdentity
-	}
-	if len(block.Args) < 1 {
-		return fmt.Errorf("invalid %v recipient block", i.pubKey.Type())
-	}
-
-	if block.Args[0] != sshFingerprint(i.pubKey) {
-		return age.ErrIncorrectIdentity
-	}
-	return nil
-}